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Effect of hydrogen enrichment on combustion characteristics of methane swirling and non-swirling inverse diffusion flame
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Effect of hydrogen enrichment on combustion characteristics of methane swirling and nonswirling inverse diffusion flame Vipul Patel, Rupesh Shah* Department of Mechanical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat, 395007, India
highlights Higher combustibility of hydrogen improves combustion of methane in IDF. Hydrogen blending with methane decreases flame length. Hydrogen concentration on mass basis reduces CO emission and increases NOx. Combustion noise increases with the hydrogen addition.
article info
abstract
Article history:
Influence of hydrogen addition on appearance of swirling and non-swirling inverse diffu-
Received 26 June 2019
sion flame (IDF) along with emissions characteristics are investigated experimentally. The
Received in revised form
combustion characteristics including flame length, axial and radial temperature variation,
25 August 2019
and noise level are analysed for hydrogen addition in methane by mass basis for constant
Accepted 9 September 2019
energy input and by volume basis for constant volumetric fuel flow rate. Hydrogen addition
Available online 3 October 2019
in methane IDF produces shorter flame by compressing entrainment zone, mixing zone, reaction zone, and post-combustion zone. Hydrogen addition shift these zones towards
Keywords:
fuel and air exit from the burner. Enrichment of methane with hydrogen on a mass basis
Inverse diffusion flame
up to 6% reduces CO emission considerably and increases NOx emission moderately. Effect
Hydrogen enrichment
of H2 addition on combustion and emission characteristics is more prominent in non-
Combustion characteristics
swirling IDF. Combustion noise is augmented with the hydrogen addition and the
Sound level
magnitude of sound level depends on the hydrogen concentration. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The requirement of environment-friendly fuels has been increasing day by day to meet stringent emission regulations. In order to increase the availability of such clean energy resources, alternative fuel needs to be burned with conventional fuel. One way of using alternative fuel is to complement
hydrogen (H2) with hydrocarbon gaseous fuel [1e5]. Greater molecular diffusivity, higher burning speed, and wider flammability limits favour hydrogen blending with a conventional fuel [6e10]. Addition of hydrogen increases adiabatic flame temperature and enhances the heat release rate [11e15]. Combustion characteristics of hydrogen-enriched low calorific value coal gases improves considerably as the coal gases
* Corresponding author. E-mail addresses: [email protected] (V. Patel), [email protected] (R. Shah). https://doi.org/10.1016/j.ijhydene.2019.09.076 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature ∅ Sn m_ a m_ f
Equivalence ratio Swirl number Air mass flow rate, kg/s Fuel mass flow rate, kg/s Energy input rate, kW Qenergy Hydrogen mass flow rate, kg/s m_ H2 Methane mass flow rate, kg/s m_ CH4 Z Axial distance, mm IDF Inverse Diffusion Flame Abbreviations LCV Lower Calorific Value SCADA Supervisory Control and Data Acquisition
are enriched by hydrogen [16]. Moderate addition of hydrogen in Internal Combustion engine at low energy level reduces NOX emissions [17]. Kumar and Mishra [18] characterised liquefied petroleum gas (LPG)eH2 diffusion flame in terms of flame length. They reported that the addition of hydrogen results in a reduction in flame length. Wu et al. [19] observed that flame length reduces when hydrogen added in methane. Kumar and Mishra [20] investigated the effects of bluff-body lip thickness on flame length in LPGeH2 combustion process. The results show a reduction in the flame height by adding H2. Zheng et al. [21] conducted a numerical study to investigate the flame propagation of a premixed methane-hydrogen flame. They observed that as hydrogen content increases from 0% to 50%, the flame develops in “tulip” shape. Liu et al. [22] numerically studied thermal and chemical effects of hydrogen adding on methane-air mixture flame. The result indicated that laminar flame velocity increases because of the chemical effect due to increased active radicals. Li et al. [23] found that the addition of hydrogen has a greater impact on the flame propagation speed of methane compared to that of propene and ethane. Li et al. [24] investigated the methane-hydrogen flame characteristics by varying the initial pressure and hydrogen content in the rich, stoichiometric and lean fuel mixture conditions. It was observed that diffusional-thermal and hydrodynamic instability on flame destabilization are improved with hydrogen addition. Afarin and Tabejamaat [25] numerically analysed the influence of hydrogen content in methane on the flame temperature. According to their results, an increase in hydrogen content increases hydroxyl (OH) mass fraction and flame temperature. Bouras et al. [26] show that as hydrogen in the fuel mixture increases the peak value of flame temperature in the diffusion flame. Several attempts have been made to envisage the change of emission behaviour due to hydrogen enrichment. Gulder et al. [27] examined the effect of hydrogen enrichment on soot in the ethylene jet diffusion flame and observed reduction in the soot formation. Ezenwajiaku et al. [28] analysed the formation characteristics of polycyclic aromatic hydrocarbons in a hydrogen-enriched methane flame. They observed reduction in formation of polycyclic aromatic hydrocarbons by hydrogen addition. Miao et al. [29] found that hydrogen
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addition promotes the conversion of CO into CO2 and a significant reduction of CO and HC occurs. Burbano et al. [30] also noticed a considerable decrease in the CO emissions for natural gas combustion with hydrogen addition. Tseng [31] numerically investigated the mixing of hydrogen/methane on premixed combustion process in porous medium burners. Kumar et al. [32] showed a reduction in CO emissions using small quantities of hydrogen in a compression ignition engine primarily fuelled with a Jatropha oil. Sun et al. [33] revealed that the addition of hydrogen to ethylene/air diffusion flame could influence the soot volume fraction. Kahraman et al. [34] numerically investigated the combustion characteristics in the gas turbine combustion chamber fuelled by hydrogen. Wang et al. [35] numerical analysed the potential of reducing emission with hydrogen enrichment. Rortveit and Hustad [36] showed that prompt NOx mechanism leads to the formation of total NOx when hydrogen is added to a natural gas flame. In contrast, Naha and Aggarwal [37] reported that for partially premixed combustion, the thermal NOx mechanism is a major factor in the formation of total NOx. In a similar study, Mishra and Kumar [38] revealed that the formation of thermal NOx by Zeldovich mechanism at high flame temperatures results in increased NOx level. Miao et al. [39] studied the flame stability of LPG IDF with hydrogen enrichment. In addition, Miao et al. [40] claimed that hydrogen addition can significantly improve combustion of LPG IDF. Restricted studies have been conducted on swirl-stabilized flame using hydrogen as a fuel additive. Tong et al. [41] experimentally investigated the influence of swirl flow on the hydrogen-enriched methane flame. It turns out that the flame dynamics and the flame shape were strongly determined by the swirl flow. Kim et al. [42] reported that hydrogen enrichment can decrease NOx emissions for premixed swirl flames compared to diffusion flames. Khalil and Gupta [43] observed a reduction in CO and increase in NO using hydrogen in a swirlstabilized non-premixed gas turbine fuelled with methane. Shanbhogue [44] reported transitions in flame microstructures under an acoustically coupled and uncoupled condition in swirl-stabilized premixed methane-hydrogen combustion. Schefer et al. [45] investigated the emission behaviour in a swirl-stabilized premixed burner using hydrogen as an additive to methane. They found a reduction in CO emission, without adversely affecting NO emission. Kim et al. [46] studied the effect of swirl on combustion characteristics of hydrogen added methane premixed flames. Emadi et al. [47] examined how the flame dynamics change in response to increase in pressure and hydrogen enrichment in a low-swirl methane-air burner. Kashir et al. [48] numerically investigated flame characteristics of methane hydrogen flames under distinct swirl numbers. IDFs are distinct kind of diffusion flames because the position of air and fuel is reversed compared to the normal diffusion flames. The diffusion flames (non-premixed) have a wide operating range due to their non-premixed nature. However, the high soot loading limits the use of these flames in domestic applications where clean combustion is required. Premixed flames burn more intense and are relatively clean. However, the flammability range is narrow due to the lift-off of the flame. IDFs have a wide operating range with less soot loading and no lift-off, allowing IDF burners to be used in
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industrial applications. Although many researchers have studied the role of hydrogen enrichment in a diffusion flame, few researchers have experimentally studied the combustion characteristics of swirling IDF with hydrogen enrichment. This research aims to examine the role of hydrogen enrichment in non-premixed methane IDF. In this regard, combustion characteristics including flame length, temperature profile, emission, and noise level are analysed for various methane-hydrogen hybrid flames.
Experimental facility and test conditions Schematics of the experimental setup and coaxial burner along with developed test facility and actual burner configuration are shown in Fig. 1 and Fig. 2 respectively. Combustion air is delivered by a blower. The air and fuel are supplied and burned into the burner at atmospheric pressure. Fuel flow is regulated by a mass flow controller (MCR 50 slpm, Alicat Scientific) with an accuracy ± 0.4% of reading. Air flow is metered by a thermal mass flow meter (4040, TSI Incorporated). The accuracy of the thermal mass flow meter is ±2% of reading. Flame images are captured with a digital SLR camera (D5300, Nikon) through a glass window of the enclosure. Fig. 2 demonstrates the photographic view of the experimental facility and burner used in this study. The flame length reported is derived by ImageJ (image processing software). The flame
length is calculated by taking the average of 10 images [38]. To study the effect of swirl on combustion characteristics, 30 swirler with 6 flat vanes (Sn ¼ 0.6, numerically derived) is used (Fig. 2). Swirl in flow is characterised by dimensionless swirl number (Sn). Swirl number is a ratio of the axial flux of angular momentum to the axial flux of axial momentum [49]. Influence of swirl on flame characteristics is analysed by measuring radial and axial temperature profiles. Type R thermocouple (bead diameter ¼ 1.5 mm, wire diameter of 0.3 mm) is used to measure flame temperature. The thermocouple is held horizontal and thus normal to the flow. Correction for flame temperatures is made for the radiative loss [50]. The maximum eccentricity in flame temperature measurement is ±50 K. Emission from flame is measured in terms of. Emission species are detected by flue gas analyser (Testo 340, accuracy ± 2 ppm). The emission level of CO and NOx are recorded along the centerline of IDF at approximately 2 times the flame length from the burner exit [51]. Effect of swirl and hydrogen addition on the sound generated in IDF is measured using a sound level meter (Testo 816-1) with an accuracy ± 1.4 dB. A microphone is mounted on a tripod with its sensor surface facing the IDF for accurate measurement. The distance from the IDF axis to the microphone is 500 mm, while the azimuth angle, the angle from the microphone to the flame axis is 90 [52]. The microphone is connected to a PC through Testo 816-1 software’s user interface and signals are logged. One hundred number of readings are sampled with a
Fig. 1 e Schematic of (a) experimental setup and (b) coaxial burner.
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Fig. 2 e Photographic view of (a) experimental facility and (b) coaxial burner. sampling rate of 0.5 s for each configuration and the average value is recorded as the final reading of the sound level. All experimental data is collected at least three times to ensure good repeatability. The error bars of data are estimated from the standard deviation of measurements. In this study, experiments are carried out for two test conditions, the hydrogen concentration is varied in methane by mass basis (constant energy input rate, 7.43 kW) and hydrogen concentration is varied in methane by volume basis (constant volume fuel flow rate, 13.5 lpm). The air flow rate is fixed at 125 lpm at the inner tube of the coaxial burner. Some of the important properties of hydrogen and methane are listed in Table 1. In the first test condition, the energy input rate to the burner is calculated using the equation given below Qenergy ¼ m_ CH4 LCVCH4 þ m_ H2 LCVH2
(1)
where, Qenergy is the energy input rate, m_ H2 and m_ CH4 are the mass flow rate of hydrogen and methane respectively, while LCVCH4 and LCVH2 are the lower calorific value of methane and hydrogen respectively. The hydrogen concentration on a mass basis (0%e10%) with various combination of flow rates of hydrogen and methane for the case of constant energy input rate, are presented in Table 2. In the second test condition, the constant volumetric fuel flow rate of the mixture (hydrogen and methane) is fed to the burner. The hydrogen concentration on a volume basis (0%e80%) with a different combination of flow rates of hydrogen and methane for the case of constant volume fuel flow rate, are given in Table 3. In both test conditions, 0% hydrogen refers to pure methane IDF. The equivalence ratio∅, indicating relative richness and leanness of fuel in air-fuel mixture is defined as
Table 1 e Properties of hydrogen and methane. Properties Lower Calorific value (LCV) (MJ/kg) Density (kg/m3) Mol. weight Gas constant (J/kgK)
H2
CH4
121 0.082 2.016 4124
50 0.65 16.04 518.3
∅ ¼ m_ a m_ f stoich m_ a m_ f actual
(2)
where ðm_ a =m_ f Þstoich is stoichiometric air-fuel ratio and ðm_ a =m_ f Þactual is the actual air-fuel ratio. H2 concentration on mass basis is varied to maintain energy input rate constant. In this case as shown in Table 2 fuel mixture flow rate reduces. This in turn decreases mixture fuel velocity. Equivalence ratio remains in narrow band (0.999e0.960). Thus H2 addition on mass basis indicates effect of H2 addition and mixture fuel velocity. H2 addition at constant volumetric rate is the case of constant air and fuel mixture velocity at burner exit. Therefore, H2 concentration variation on volume basis shows effect of H2 addition, energy input and equivalence ratio. To gain insight of effect of H2 addition along with mixture fuel velocity, energy input and equivalence ratio performance parameters and flow physics are compared for H2 concentration variation in methane by mass basis and that by volume basis.
Results and discussion Effect of H2 addition on flame length Fig. 3 shows the images of both IDFs at various hydrogen concentration (0%e10%) on a mass basis. From Fig. 3 it is seen that for both IDFs, flame length decreases with an increase in hydrogen concentration. Swirl results in shorter IDF for same mixture strength. Fig. 3 indicates that IDFs comprise a bluish reaction zone followed by a purple-orange luminous zone. The bluish zone of IDFs results from greater entrainment of fuel through a central jet of air that burns fuel in premixed combustion mode. While the purple-orange zone indicates the presence of soot particles due to fuel burns with air entrained from its surroundings. It is found from Fig. 3 that at a lower percentage of hydrogen, the bluish reaction zones are longer. The blue zone inside the flame shows the presence of partially premixed burning of fuel in the flame. The entrainment of the fuel in the air jet depends on the velocity difference between air and fuel jets. This difference in velocity creates a sufficient pressure difference between fuel/air
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Table 2 e Test conditions for constant energy input rate. Cases
H2 by mass basis
Energy input rate (kW)
CH4 (lpm)
H2 (lpm)
Total flow rate of fuel mixture (kg/s)
Equivalence ratio (∅)
0% 2% 4% 6% 8% 10%
7.43 7.43 7.43 7.43 7.43 7.43
13.50 12.89 12.23 11.58 10.92 10.27
0.00 2.18 4.36 6.54 8.72 10.90
0.000149 0.000145 0.000141 0.000136 0.000132 0.000128
0.999 0.994 0.985 0.977 0.969 0.960
1 2 3 4 5 6
Table 3 e Test conditions for constant volume fuel flow rate. Cases 1 2 3 4 5
H2 by volume basis
Total flow rate of fuel mixture (lpm)
CH4 (lpm)
H2 (lpm)
Energy input rate (kW)
Total flow rate of fuel mixture (kg/s)
Equivalence ratio (∅)
0% 20% 40% 60% 80%
13.50 13.50 13.50 13.50 13.50
13.50 10.80 8.10 5.40 2.70
0.00 2.70 5.40 8.10 10.80
7.43 6.39 5.35 4.31 3.27
0.000149 0.000122 0.000096 0.000070 0.000044
0.999 0.849 0.699 0.549 0.399
streams to divert the fuel jet and redirect it to the central air jet. The fuel eventually impinges on the central air jet, resulting in a premix of air and fuel near the flame neck. In other words, the difference in the momentums of the fuel and air jet increases, leading to entrainment of fuel into the central air jet. As the hydrogen content increases, the bluish zones become shorter and shift upstream. Since the energy input rate is fixed, the hydrogen percentage growth means an increase in the fuel velocity, whereby the velocity difference decreases. Thus, the difference in the momentums of two jets decreases, thereby reducing the entrainment of fuel. Therefore, the fuel partially burns in the premixed mode and partially carries downstream along with the flame torch. Therefore, with higher hydrogen content blue zones appear shorter. The longer luminous zones at a higher hydrogen content reflect that the fuel that is not entrained by the central air burns with ambient air at the periphery of the flame torch.
In addition, higher flammability of hydrogen, which supports the faster chemical reaction and hence enhances the combustion rate. Flame images are shown in Fig. 3 indicates that higher hydrogen content in IDF reduces the size of the reaction zone and shifts the reaction zone upstream. Fig. 4 shows the measured flame length of swirling and non-swirling IDFs at various hydrogen concentration on a mass basis. The length of both IDFs decrease with hydrogen enrichment. When hydrogen is less than 2%, the rate of decrease is slight. However, when hydrogen is above 2%, the rate of decrease is considerable, indicating a strong dependency of flame length on hydrogen enrichment. At 10% hydrogen enrichment, flame length of swirling and nonswirling IDFs reduces approximately 17.5% and 16.1% respectively. The decrease in flame length is due to a higher molecular diffusivity of the hydrogen [19]. Increased molecular diffusivity results in rapid combustion through greater air/
Fig. 3 e Flame photographs of IDFs with variation in H2 by mass.
Fig. 4 e Flame length profiles of IDFs with variation in H2 by mass.
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fuel mixing. This is due to the increase in OH and H radicals associated with the addition of hydrogen which improves the burning rate and thereby reducing the flame [11]. In addition, the total mixture mass flow rate of fuel is also reduced (Table 2), which also contribute to flame length reduction. This is in good agreement with the observation made by Choudhary et al. [6] and Francis et al. [53] in a hydrogen-hydrocarbon diffusion flame. Fig. 5 shows the swirling and non-swirling IDFs for hydrogen concentration (0%e80%) by volume basis. For each flame, there is a purple-orange luminous zone overlapping an intense bluish premixed combustion zone. The blue zone within the flame indicates the presence of partially premixed combustion of fuel in the flame. In the present study, it is found that with increasing hydrogen content, the blue zones become shorter, as fewer fuel burns in the premixed mode of combustion. As hydrogen content increases, fuel velocity increases as hydrogen is less dense than methane. A higher fuel velocity influences the difference in the momentums of two jets and thus less fuel is entrained. Therefore, blue zones appear shorter with hydrogen addition. In Fig. 6, it is observed that the length of both IDFs decreases considerably with hydrogen. Cozzi and Coghe [7] also found a decrease in the flame length with hydrogen blending for a swirl stabilized the non-premixed flame. There is a progressive shortening of the bluish zone indicating a decrease in equivalence ratio. The length of non-swirling IDF decreased by approximately 58.8%, while the length of swirling IDF decreased approximately 63.1% respectively, as compared with the pure methane IDF (0% hydrogen). This is due to the reduced mass flow rate of the total fuel mixture with increasing hydrogen concentration (Table 3). Hydrogen addition at a constant volumetric flow rate reduces heat input rate (Table 3) due to low density compared to methane. Also, higher combustibility of hydrogen supports the chemical conversion of fuel into molecular diffusivity of the flame and
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Fig. 6 e Flame length profiles of IDFs with variation in H2 by volume.
accelerates the exothermic reaction [42]. In addition, at low equivalence ratio, low fuel concentration and excess air are not favouring the generation of the larger reaction zone, and the high air/fuel ratio consequences in a high strain rate on the flame fronts, resulting in a short and open IDF. Consequently, there is a reduction in flame length.
Effect of H2 addition on flame temperature In order to interpret the emission level and noise level in flames, the thermal behaviour of both IDFs is examined by flame temperature measurement. Both IDFs are experimentally investigated under two different hydrogen addition cases, 10% hydrogen (by mass basis) and 80% hydrogen (by volume basis) and compared with results obtained for pure methane combustion. The effect of hydrogen addition on the temperature distribution along the centerline of the flame is investigated first. Then, the effect of hydrogen addition on the radial temperature distribution at a different axial distance is analysed.
Centerline temperature distribution
Fig. 5 e Flame photographs of IDFs with variation in H2 by volume.
Fig. 7 shows flame temperature distribution along the flame centerline of non-swirling IDF with varying hydrogen concentration. The centerline temperature profiles of pure methane and 0% hydrogen (by mass basis) IDFs are comparable. It is seen that the temperature increases at a fast pace in the 10% hydrogen (by mass basis) IDF with increasing axial distance up to 120 mm as compared with the pure methane IDF. This is because of higher combustibility of hydrogen, which enhances the exothermic chemical reaction of fuel and shifts the reaction zone in upstream. The highest temperatures in the pure methane IDF and 10% hydrogen (by mass basis) IDF are 1447 K, and 1311 K, respectively. The highest temperatures in both IDFs are observed at approximately 230 mm away from the burner outlet. Hence, the position of the highest temperature thus remains the same from the burner outlet with 10% hydrogen addition (by mass basis).
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Fig. 7 e Centerline temperature distribution of nonswirling IDFs.
However, the highest temperature is decreased by 136 K (approximately 9.4%) as compared with pure methane IDF. While hydrogen is blended to the methane stream at a constant energy input rate, the mass flow rate of the total fuel mixture is decreased slightly as the density of hydrogen is lower than that of methane, hence decrease in an equivalence ratio (Table 2). This results in a decrease in the highest temperature of 10% hydrogen (by mass basis) IDF. The centerline temperature then drops steadily once the peak value is reached because no major reaction occurs in the postcombustion zone. Nearly all fuel is consumed in the main reaction zone and leaves the post-combustion zone only by the entrainment and dilution. This results in a gradual reduction of the flame temperature. By adding 80% hydrogen (by volume basis) to methane stream, the highest temperature, 770 K is observed at approximately 260 mm away from the burner outlet. The reduction in the highest flame temperature is attributed to the low equivalence ratio which is resulted because of the reduced mass flow rate of a total fuel mixture when 80% hydrogen is added by volume basis (Table 3). In addition, reduced energy input also influences the flame temperature. Fig. 8 shows the temperature profiles along with the flame centerline of swirling IDFs with varying hydrogen concentration. Trend of the flame temperature distribution of pure methane and 10% hydrogen (by mass basis) IDFs is comparable. The highest temperatures in pure methane and 10% hydrogen (by mass basis) IDFs are measured 1644 K and 1684 K, respectively. With 10% hydrogen (by mass basis) addition to methane stream, the highest temperature is increased by 40 K (approximately 2%) compared to pure methane flame. Positions of the highest temperatures of the pure methane and 10% hydrogen (by mass basis) IDFs are observed at approximately 80 mm and 100 mm from the burner outlet, respectively. The position of the highest temperature thus moved away from the burner outlet in the swirling IDF with the addition of 10% hydrogen (by mass basis). This is due to the higher combustibility of hydrogen which promotes the exothermic reaction and expands
Fig. 8 e Centerline temperature distribution of swirling IDFs. combustion products speedily. Therefore, a large amount of heat energy is released during the combustion which enhances the flow velocity and thus reduces the recirculation flow in the internal recirculation zone of the swirling IDF. The reduction in the recirculation flow leads to an increase in flame temperature of the reaction zone with the addition of hydrogen to the methane fuel [46]. Besides, the higher combustibility of hydrogen shifts the reaction zone upstream [54]. With 80% hydrogen (by volume basis) addition to the methane stream, the highest 1017 K temperature is observed at approximately 80 mm from the burner outlet. This decrease in the highest flame temperature is because of the low equivalence ratio and reduced energy input rate (Table 3).
Radial temperature distribution Fig. 9 presents the comparison of flame images and radial temperature profiles of non-swirling IDFs. The images of IDF represent the cases of pure methane (0% hydrogen), 10% hydrogen addition to methane by mass basis and 80% hydrogen addition to methane by volume basis. Generally, the flame structure of the IDF comprises four zones according to temperature distribution [55]. Zone 1 is a base zone in which fuel is entrained by central air with high velocity. Zone 2 is a mixing zone where the mixing of fuel and air is completed. Zone 3 is a reaction zone that has the highest flame temperature due to intense heat release by the reaction. Zone 4 is a post-combustion zone associated with the cooling of the hot gases due to the entrainment of ambient air. The pure methane IDF is approximated in four different zones. The radial temperature profiles of 10% hydrogen (by mass basis) and 80% hydrogen (by volume basis) IDFs are compared with pure methane IDF at five axial locations. For pure methane IDF, the temperature profile at a low elevation of Z ¼ 5 mm passes through the base zone. The two temperature profiles at the near-burner outlet of Z ¼ 25 and 50 mm traverse through the mixing zone. The temperature profile at mid-flame of Z ¼ 75 mm represents the reaction zone. The other one at the far-burner outlet of Z ¼ 125 mm passes through the postcombustion zone.
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Fig. 9 e Comparison of flame images and radial temperature profiles at five axial locations of non-swirling IDFs.
Radial temperature distribution at these locations is shown in Fig. 9. For pure methane case at Z ¼ 5 mm maximum temperature is recorded at approximately 15 mm. In the present case, the outer radius of the burner is 18.5 mm. The occurrence of maximum temperature at 15 mm is the indication of burning of fuel due to the presence of outside air in diffusion mode. At Z ¼ 25 mm maximum temperature location shifts inward at a radial position around 10 mm. In the current study, inner tube radius is 9.5 mm. Happening of maximum temperature at 10 mm radial position indicates burning of fuel due to entrainment within air jet. At Z ¼ 5 mm and Z ¼ 25 mm near flame axis temperature gradient is low. In mixing zone at Z ¼ 50 mm peak of the temperature profile is recorded at the same radial location that of at Z ¼ 25 mm however at flame axis higher temperature gradient is recorded. At Z ¼ 75 mm location of maximum flame temperature shifts radially outward while the temperature gradient at axis becomes steeper than at Z ¼ 50 mm. This steep gradient is due to the presence of a reaction zone at this axial location. Combustion releases a large amount of energy which in turn
expands domain in all direction thus there is a radial outward shift in the location of maximum temperature. At Z ¼ 125 mm in the post-combustion zone near flame axis steepness of temperature gradient reduces compare to that of at Z ¼ 75 mm. This is due to the absence of major energy release phenomena in this zone. Addition of 10% H2 by mass in methane alters flow and reaction mechanism. At all axial positions, except at Z ¼ 125 mm, near flame axis higher temperature and greater gradients of temperature are observed due to the addition of H2. For axial location Z ¼ 5 and Z ¼ 25 mm maximum temperature is comparable in both cases. At Z ¼ 50 and Z ¼ 75 mm at all radial position, measured temperature with 10% H2 is on the higher side compared to that of pure methane case. Near the flame axis, the steeper temperature gradient can be observed at these locations due to the addition of H2. Presence of steeper temperature gradient and higher temperature in the vicinity of axis indicates that the strength of the reaction zone increases due to the addition of H2. Temperature profile at Z ¼ 125 shifts towards the lower value of
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temperature compares to that of pure methane case. This indicates compression and shift of reaction and post-reaction zone towards the flame base. Thus the addition of H2 accelerates fuel burning rate. This results in an intense and shorter reaction zone. Overall flame becomes shorter compared to pure methane case due to the addition of H2 on mass basis. In case when 80% H2 is added on volume basis, total energy to burner reduces to 3.27 kW 55% of this total energy input is due to H2. Thus flow physics and reaction mechanism are dominated by H2. Maximum temperature at Z ¼ 5 mm and Z ¼ 50 mm are comparable. At Z ¼ 25 mm maximum flame temperature is measured compared to Z ¼ 75 mm in pure methane case. This indicates the presence of a reaction zone at axial location Z ¼ 25 mm. Thus Reaction zone shifts towards the flame base in case of 80% H2 on volume basis compare to pure methane case. Temperature profile at Z ¼ 125 mm indicates the flow of hot products only. This results in a shorter flame length for 80% H2 by volume case. Fig. 10 shows the comparison of flame images and radial temperature profiles of swirling IDFs. The appearance of the
pure methane swirling IDF is observed different compares to the pure methane non-swirling IDF. The flame torch and flame base shorten in the pure methane swirling IDF because of the formation of an internal recirculation zone [56]. As previously discussed, the pure methane swirling IDF is characterised in four zones, as shown in the image of the pure methane swirling IDF. Hence, the position of the five profiles is slightly changed in pure methane swirling IDF. The profile of Z ¼ 5 mm passes through the base zone. The one profiles of Z ¼ 25 mm approximate cross through the mixing zone. The one profile at mid-flame of Z ¼ 50 mm approximate penetrates reaction zone. The two profiles of Z ¼ 75 and 125 mm approximate passes through the post-combustion zone. With swirl also when H2 is added at 10% on mass basis at the same energy rate at axial location 5 mm, 25 mm and 50 mm near flame axis temperature increases. Rate of temperature increase at 25 mm and 50 mm is higher compared to that for 5 mm. Also, the maximum flame temperature recorded at 50 mm is higher than that of maximum flame temperature recorded at 75 mm. This shows the presence of a
Fig. 10 e Comparison of flame images and radial temperature profiles of swirling IDFs.
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reaction zone at 50 mm. Thus due to the addition of H2 stronger and shorter flame is produced. Non-swirling IDF with 80% H2 by volume has the highest flame temperature (1657 K) at Z ¼ 25 mm. This highest flame temperature is lower than that of pure methane (1716 K, Z ¼ 125 mm) and 10% H2 by mass (1874 K, Z ¼ 75 mm) cases. This is due to reduce energy input in case of 80% H2 by volume (3.27 kW) compare to other cases (7.43 kW). Swirling IDF with 80% H2 by volume attains highest flame temperature (1723 K) at Z ¼ 25 mm. This highest flame temperature is lower than that of pure methane (1864 K, Z ¼ 75 mm) and 10% H2 by mass (1907 K, Z ¼ 50 mm) cases with the swirl. Maximum temperature values of swirling IDF without and with 10% H2 are comparable. Effect H2 addition on reaction and flow mechanism is more prominent in non-swirling IDF than swirling IDF. Early occurrence of maximum temperature compared to pure methane case indicates the shift of reaction zone towards the flame base and thus generates a shorter flame. Higher molecular diffusivity and higher flammability of hydrogen result in a faster chemical reaction through improved air/fuel mixture. Also due to the increase in OH and H radicals associated with the addition of hydrogen, the rate of combustion improves. The results obtained for the radial temperature distribution in both IDFs shows that increasing the hydrogen concentration on the basis of mass (10% hydrogen) considerably increases the flame temperature, which is more desirable for heating purpose. Also, the position of the highest temperature moves near the burner outlet. However, when hydrogen is added on the basis of volume (80% hydrogen), the highest flame temperature is decreased due to the reduced energy input rate. Wu et al. [19] and Choudhuri et al. [6] reported that the flame temperature increases due to the adding of hydrogen in a diffusion flame. Therefore, the observed tendency of increasing the flame temperature with hydrogen enrichment is consistent with previous experimental studies. When hydrogen burns with methane-air IDF, the radical OH in the reaction zone increases, which promotes the reaction rate by increasing the chain-branching process [19]. In addition,
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while hydrogen is blended to methane stream, the rate of carbon input decreases, resulting in a reduction in radiation heat loss [11]. Also, higher combustibility of hydrogen rapidly expands the combustion product, thereby reducing the recirculation flow. The decline of the recirculation flow plays an important role in the temperature rise of the reaction zone [46]. And that increases the flame temperature.
Effect of H2 addition on CO and NOx Fig. 11(a) demonstrates the variation of CO and NOx emissions with increasing hydrogen concentration at constant energy input rate (by mass basis). It is observed that the CO emission decreases with increasing hydrogen in swirling and nonswirling IDFs. While the NOx emission follows the reverse trend in both IDFs. The CO emission in pure methane (0% hydrogen) non-swirling and swirling IDFs is around 35 ppm and 43 ppm, respectively. As hydrogen addition (by mass basis) increases from 0% to 10%, CO is decreased by around 21 ppm for non-swirling IDF and 11 ppm for swirling IDF. The CO emission in the swirling IDF decreases quickly as the hydrogen increase from 0% to 4% and then gradually decrease with hydrogen addition up to 10%. The decrease of CO in the non-swirling and swirling IDFs is approximately 40% and 74%, respectively, compared to the pure methane flame. When hydrogen is blended to the methane, the amount of methane decrease hence, carbon content decreases accordingly in total fuel mixture flow rate. Increasing hydrogen concentration at constant energy input rate (by mass basis) increases the H, O and OH radicals during combustion. The same can be explained with following elementary reactions: OH þ H2 4H þ H2 O
(3)
H þ O2 4O þ OH
(4)
These OH radicals promote the speedy transformation of CO into CO2 by the reaction mechanism defined as OH þ CO4H þ CO2
Fig. 11 e Hydrogen addition effect on emission level.
(5)
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Thus, the decrease of CO emission is partly because of the chemical kinetic effect and partially caused by the reduction in carbon content [35]. Rajpara et al. [57] observed that the increase of hydrogen in methane decreases CO emission with minimal increase in NOx emission. Similarly, Miao et al. [29] found that the hydrogen addition reduced CO emission into LPG IDF. In Fig. 11(a), trends of NOx emissions with hydrogen addition by mass basis are opposite to those of CO. The NOx emission of pure methane (0% hydrogen) non-swirling IDF is around 4 ppm. It is seen that as hydrogen addition (by mass basis) increases from 0% to 6%, NOx increase from around 7 ppm; as hydrogen addition further increases, NOx stabilize at around 7 ppm. For the pure methane swirling IDF, NOx is around 3 ppm. With 2% hydrogen addition to the methane swirling IDF, NOx increased around 4 ppm. As hydrogen addition increases from 2% to 10%, NOx stabilizes at around 4 ppm. The emission level of NOx thus increases with the addition of hydrogen for both IDFs. The prompt and thermal mechanisms are accountable for most of NO formed in the non-premixed combustion [58]. In general, the formation of NOx follows very well the temperature change in the reaction zone. Hence, the production of thermal NO in the reaction zone plays a key role in the NOx emission. Fig. 9 (b) and Fig. 10 (b) show that the temperature inside the IDFs with hydrogen addition (by mass bass) is greater than 1800 K, which provides adequate energy to trigger the chain reactions in the Zeldovich mechanism [29]. Hence there is an increase in the NOx emission level. Thus increasing hydrogen concentration at constant energy input rate (by mass basis) reduces CO emission and increases NOx emission. Similarly, Zhen et al. [59] also observed the increase in NOx for a swirl-stabilized LPG IDF. Fig. 11(b) depicts the measured emission of CO and NOx with increasing hydrogen concentration at constant volume fuel flow rate (by volume). The results show that the CO emission increases with increasing hydrogen content for both IDFs. This is due to the quenching effect. In IDF, when a fuel mixture impinges on central air jet in the flame neck zone, part of the fuel mixture cools down by the cold air jet,
which reduces the OH concentration [39]. Also, a high strain rate and local air/fuel ratio in the flame neck zone extinguish the flame locally and contribute to the formation of CO [60]. Furthermore, the reduced flame length of IDFs (Figs. 3 and 5) offers shorter residence time for the oxidation of CO into CO2. Quenching effect, inadequate reaction time and higher strain rate are reasons for the increase of CO emission. Fig. 11(b) also presents the variation in NOx emission with hydrogen enrichment (by volume basis). It turns out that with increasing hydrogen concentration, the NOx emission decreases. This is in good agreement with the observation made by Kim et al. [42]. Also, Figs. 9 (c) and Fig. 10 (c) depict that the temperature within the IDFs is less than 1800 K, which does not favour the chain reactions in the Zeldovich mechanism, subsequently the decrease of NOx emission is observed. Therefore, we conclude that hydrogen enrichment at a constant volume flow rate (by volume basis) increases CO emission and decreases NOx emission. Addition of hydrogen results in shorter flame, higher flame temperature and low CO emission. Low CO emission indicates better fuel burning and higher burner efficiency. Thus H2 addition can result in reduction of space requirement if IDFs are used for heating purpose. Enhancement in maximum flame temperature increases rate of heat transfer. Therefore, H2 addition in IFDs flame results in enhanced, efficient and cleaner combustion phenomena along with enhanced heat transfer rate.
Effect of H2 addition on combustion noise Experiments are carried out to ascertain the effect of hydrogen addition on the behaviour of noise radiation from swirling and non-swirling IDFs. The combustion noise characteristics are firstly examined at increasing hydrogen concentration while the energy input rate is kept constant (by mass basis). Secondly, the hydrogen concentration is varied while the volume fuel flow rate is kept constant (by volume basis). Sound level measurements are performed to measure the flame noise. The flame noise is the sound radiated from the burning air/fuel jets.
Fig. 12 e Hydrogen addition effect on combustion noise.
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Variation of sound level for both test conditions is demonstrated in Fig. 12. It is seen that the hydrogen addition increases the noise level under both test conditions. While hydrogen is blended to methane stream at constant energy input rate (Fig. 12(a)), the sound level of pure methane (0% hydrogen) non-swirling IDF and swirling IDF is around 69.3 dB and 78.8 dB, respectively. As hydrogen addition increases from 0% to 10%, the sound level increases by around 4.5 dB (6.1%) for non-swirling IDF and 3 dB (3.67%) for swirling IDF, respectively, compared to that of pure methane flame. Similarly, Fig. 12(b) shows that, when hydrogen addition increases from 0% to 80% (by volume basis), the increase in sound level in the non-swirling and swirling IDFs is approximately 10.23% and 5.52%, respectively, compared to pure methane flame (0% hydrogen). Combustion noise increases with hydrogen addition and the magnitude of sound level depend on hydrogen concentration. In general, sound radiated during the combustion process is due to the fluctuation of the heat release rate at the flame front [61]. Besides this, the change in heat release rate associated with the exothermic chemical reactions in the reaction zone also contributes to the generation of combustion noise. As the amount of hydrogen added to the methane fuel increases, higher OH radical formation promotes the higher rate of reaction. This results in an increase in the sound level. It is noticed that the sound level of pure methane non-swirling IDF is well below (69.3 dB), while the sound level of swirling IDF with 80% hydrogen addition (by volume basis) is much higher (83.40 dB). This is because higher combustibility of hydrogen than methane results in rapid combustion due to faster heat release rate. Addition of H2 addition on a mass basis for constant energy input rate reduces flame length without affecting flame diameter as shown in Fig. 4. Same energy release in smaller length indicates higher energy release rate per unit length. This higher energy rate of energy release increases sound level radiated from flame. This finding is in line with research, which states that the acoustic power is proportional to the burning speed of the flame [62]. For constant volume flow rate as H2% increases flame dynamics and reaction mechanisms are controlled by H2 as it becomes the main species in fuel. Thus, with an increase in the % volume of H2, the sound level increases.
Conclusions The role of hydrogen addition in non-premixed methane IDF is examined. The combustion characteristics of methanehydrogen hybrid flames are investigated and analysed at various hydrogen concentration on the basis of mass and volume. Measurements for flame length, radial temperature profile, emission, and noise level are carried out in swirling and non-swirling IDFs. Major findings of the present investigation are: 1. Hydrogen blending reduces flame length for both cases. Reduction in flame length for H2 addition on a mass basis for constant energy input is due to enhanced fuel burning rate and shorter reaction zone near the flame axis. For a
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constant volume flow rate, the reduced flame length is due to a higher percentage of hydrogen and reducing energy input rate with an increasing percentage of hydrogen. 2. In non-swirling IDF at 10% H2 addition on mass bases case rise in maximum temperature is 158 K compare to 43 K rise in case of swirling case. Effect of H2 addition on reaction and flow mechanism is more prominent in non-swirling IDF than swirling IDF. 3. It is observed that increasing hydrogen concentration at constant energy input rate (by mass basis) reduces CO emission and increases NOx emission. While the hydrogen enrichment at a constant volume flow rate (by volume basis) increases CO emission and decreases NOx emission. 4. Combustion noise increases with hydrogen addition and the magnitude of sound level depend on the hydrogen concentration. Swirling IDF with 80% hydrogen addition (by volume basis) generates maximum sound level (83.40 dB).
references
[1] Zohra KF, Mounir A, Salah C. Numerical simulation of CH4H2-AIR non-premixed flame stabilized by a bluff body. Energy Procedia 2017;139:530e6. [2] Zhao K, Cui D, Xu T, Zhou Q, Hui S, Hu H. Effects of hydrogen addition on methane combustion. Fuel Process Technol 2008;89:1142e7. [3] Schefer RW, Oefelein J. Reduced turbine emissions using hydrogen-enriched fuels. In: Proceedings of the 2002 US DOE hydrogen program review; 2002. p. 1e16. NREL/CP-610-32405. [4] Juste GL. Hydrogen injection as additional fuel in gas turbine combustor. Evaluation of effects. Int J Hydrogen Energy 2006;31:2112e21. _ _ Effect of swirl number on [5] Ilbas ‚ M, Karyeyen S, Yilmaz I. combustion characteristics of hydrogen-containing fuels in a combustor. Int J Hydrogen Energy 2016;41:7185e91. [6] Choudhuri AR, Gollahalli SR. Combustion characteristics of hydrogenehydrocarbon hybrid fuels. Int J Hydrogen Energy 2000;25:451e62. [7] Cozzi F, Coghe A. Behavior of hydrogen-enriched nonpremixed swirled natural gas flames. Int J Hydrogen Energy 2006;31:669e77. € kalp I. Characterization of the [8] Halter F, Chauveau C, Go effects of hydrogen addition in premixed methane/air flames. Int J Hydrogen Energy 2007;32:2585e92. [9] Hu E, Huang Z, He J, Jin C, Zheng J. Experimental and numerical study on laminar burning characteristics of premixed methaneehydrogeneair flames. Int J Hydrogen Energy 2009;34:4876e88. [10] Okafor EC, Hayakawa A, Nagano Y, Kitagawa T. Effects of hydrogen concentration on premixed laminar flames of hydrogenemethaneeair. Int J Hydrogen Energy 2014;39:2409e17. [11] Choudhuri AR, Gollahalli SR. Characteristics of hydrogenehydrocarbon composite fuel turbulent jet flames. Int J Hydrogen Energy 2003;28:445e54. [12] El-Ghafour SAA, El-dein AHE, Aref AAR. Combustion characteristics of natural gasehydrogen hybrid fuel turbulent diffusion flame. Int J Hydrogen Energy 2010;35:2556e65.
28328
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 3 1 6 e2 8 3 2 9
[13] Fukuda M, Korematsu K, Sakamoto M. On quenching distance of mixtures of methane and hydrogen with air. Bulletin of JSME 1981;24:1192e7. [14] Lafay Y, Renou B, Cabot G, Boukhalfa M. Experimental and numerical investigation of the effect of H2 enrichment on laminar methaneeair flame thickness. Combust Flame 2008;153:540e61. [15] Mishra SK, Dahiya RP. Adiabatic flame temperature of hydrogen in combination with gaseous fuels. Int J Hydrogen Energy 1989;14:839e44. _ [16] Ilbas ‚ M, Karyeyen S. A numerical study on combustion behaviours of hydrogen-enriched low calorific value coal gases. Int J Hydrogen Energy 2015;40:15218e26. _ Sandalcı T, Yu¨ksek L, Dalkılıc¸ AS, € z Y, Gu¨ler I, [17] Karago Wongwises S. Effects of hydrogen and methane addition on combustion characteristics, emissions, and performance of a CI engine. Int J Hydrogen Energy 2016;41:1313e25. [18] Kumar P, Mishra DP. Experimental investigation of laminar LPGeH2 jet diffusion flame. Int J Hydrogen Energy 2008;33:225e31. [19] Wu L, Kobayashi N, Li Z, Huang H, Li J. Emission and heat transfer characteristics of methaneehydrogen hybrid fuel laminar diffusion flame. Int J Hydrogen Energy 2015;40:9579e89. [20] Kumar P, Mishra DP. Effects of bluff-body shape on LPGeH2 jet diffusion flame. Int J Hydrogen Energy 2008;33:2578e85. [21] Zheng K, Yu M, Liang Y, Zheng L, Wen X. Large eddy simulation of premixed hydrogen/methane/air flame propagation in a closed duct. Int J Hydrogen Energy 2018;43:3871e84. [22] Liu J, Zhang X, Wang T, Hou X, Zhang J, Zheng S. Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methaneeair mixture. Int J Hydrogen Energy 2015;40:8475e83. [23] Li Z, Cheng X, Wei W, Qiu L, Wu H. Effects of hydrogen addition on laminar flame speeds of methane, ethane and propane: experimental and numerical analysis. Int J Hydrogen Energy 2017;42:24055e66. [24] Li Y, Bi M, Li B, Zhou Y, Gao W. Effects of hydrogen and initial pressure on flame characteristics and explosion pressure of methane/hydrogen fuels. Fuel 2018;233:269e82. [25] Afarin Y, Tabejamaat S. Effect of hydrogen on H2/CH4 flame structure of MILD combustion using the LES method. Int J Hydrogen Energy 2013;38:3447e58. [26] Bouras F, El Hadi Attia M, Khaldi F, Si-Ameur M. Control of methane flame properties by hydrogen fuel addition: application to power plant combustion chamber. Int J Hydrogen Energy 2017;42:8932e9. € Snelling DR, Sawchuk RA. Influence of hydrogen [27] Gu¨lder OL, addition to fuel on temperature field and soot formation in diffusion flames. Symposium (International) on Combustion 1996;26:2351e8. [28] Ezenwajiaku C, Talibi M, Doan NAK, Swaminathan N, Balachandran R. Study of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched methane diffusion flames. Int J Hydrogen Energy 2019;44:7642e55. [29] Miao J, Leung CW, Cheung CS, Huang ZH, Zhen HS. Effect of hydrogen addition on overall pollutant emissions of inverse diffusion flame. Energy 2016;104:284e94. [30] Burbano HJ, Amell AA, Garcı´a JM. Effects of hydrogen addition to methane on the flame structure and CO emissions in atmospheric burners. Int J Hydrogen Energy 2008;33:3410e5. [31] Tseng CJ. Effects of hydrogen addition on methane combustion in a porous medium burner. Int J Hydrogen Energy 2002;27:699e707. [32] Kumar MS, Ramesh A, Nagalingam B. Use of hydrogen to enhance the performance of a vegetable oil fuelled
[33]
[34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49] [50]
[51]
[52]
compression ignition engine. Int J Hydrogen Energy 2003;28:1143e54. Sun Z, Dally B, Nathan G, Alwahabi Z. Effects of hydrogen and nitrogen on soot volume fraction, primary particle diameter and temperature in laminar ethylene/air diffusion flames. Combust Flame 2017;175:270e82. € z S, Akansu SO. Numerical analysis of a Kahraman N, Tango gas turbine combustor fueled by hydrogen in comparison with jet-A fuel. Fuel 2018;217:66e77. Wang J, Huang Z, Tang C, Miao H, Wang X. Numerical study of the effect of hydrogen addition on methaneeair mixtures combustion. Int J Hydrogen Energy 2009;34:1084e96. Rortveit G, Hustad J. NO x formations in diluted CH 4/H 2 counterflow diffusion flames. Int J Energy a Clean Environ (IJECE) 2003;4. Naha S, Aggarwal SK. Fuel effects on NOx emissions in partially premixed flames. Combust Flame 2004;139:90e105. Mishra DP, Kumar P. Experimental investigation of laminar LPGeH2 jet diffusion flame with preheated reactants. Fuel 2008;87:3091e5. Miao J, Leung C, Cheung C. Effect of hydrogen percentage and air jet Reynolds number on fuel lean flame stability of LPGfired inverse diffusion flame with hydrogen enrichment. Int J Hydrogen Energy 2014;39:602e9. Miao J, Leung C, Cheung C, Huang Z, Jin W. Effect of H2 addition on OH distribution of LPG/Air circumferential inverse diffusion flame. Int J Hydrogen Energy 2016;41:9653e63. Tong Y, Yu S, Liu X, Klingmann J. Experimental study on dynamics of a confined low swirl partially premixed methane-hydrogen-air flame. Int J Hydrogen Energy 2017;42:27400e15. Kim HS, Arghode VK, Linck MB, Gupta AK. Hydrogen addition effects in a confined swirl-stabilized methane-air flame. Int J Hydrogen Energy 2009;34:1054e62. Khalil AEE, Gupta AK. Hydrogen addition effects on high intensity distributed combustion. Appl Energy 2013;104:71e8. Shanbhogue SJ, Sanusi YS, Taamallah S, Habib MA, Mokheimer EMA, Ghoniem AF. Flame macrostructures, combustion instability and extinction strain scaling in swirlstabilized premixed CH4/H2 combustion. Combust Flame 2016;163:494e507. Schefer RW, Wicksall DM, Agrawal AK. Combustion of hydrogen-enriched methane in a lean premixed swirlstabilized burner. In: Presented at the proceedings of the combustion institute; 2002. Kim HS, Arghode VK, Gupta AK. Flame characteristics of hydrogen-enriched methaneeair premixed swirling flames. Int J Hydrogen Energy 2009;34:1063e73. Emadi M, Karkow D, Salameh T, Gohil A, Ratner A. Flame structure changes resulting from hydrogen-enrichment and pressurization for low-swirl premixed methaneeair flames. Int J Hydrogen Energy 2012;37:10397e404. Kashir B, Tabejamaat S, Jalalatian N. A numerical study on combustion characteristics of blended methane-hydrogen bluff-body stabilized swirl diffusion flames. Int J Hydrogen Energy 2015;40:6243e58. Lilley DG. Swirl flows in combustion: a review. AIAA J 1977;15:1063e78. Brohez S, Delvosalle C, Marlair G. A two-thermocouples probe for radiation corrections of measured temperatures in compartment fires. Fire Saf J 2004;39:399e411. Mahesh S, Mishra D. Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner. Fuel 2008;87:2614e9. Choy YS, Zhen HS, Leung CW, Cheung CS, Leung RCK. Noise generation by open inverse diffusion flames. J Vib Control 2014;20:1671e81.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 3 1 6 e2 8 3 2 9
[53] Alex Francis K, Sreenivasan R, Raghavan V. Investigation of structures and reaction zones of methaneehydrogen laminar jet diffusion flames. Int J Hydrogen Energy 2011;36:11183e94. [54] Gupta AK, Lilley DG, Syred N. Swirl flows. Tunbridge wells. Kent, England: Abacus Press; 1984. p. 488. 1984. [55] Zhen H, Cheung C, Leung C, Li H. Thermal and heat transfer behaviors of an inverse diffusion flame with induced swirl. Fuel 2013;103:212e9. [56] Patel V, Shah R. Effect of swirl and number of swirler vanes on combustion characteristics of methane inverse diffusion flame. J Mech Sci Technol 2019;33:1947e58. [57] Rajpara P, Shah R, Banerjee J. Effect of hydrogen addition on combustion and emission characteristics of methane fuelled upward swirl can combustor. Int J Hydrogen Energy 2018;43:17505e19.
28329
[58] Turns SR. Understanding NOx formation in nonpremixed flames: experiments and modeling. Prog Energy Combust Sci 1995;21:361e85. [59] Zhen H, Leung C, Cheung C. A comparison of the thermal, emission and heat transfer characteristics of swirl-stabilized premixed and inverse diffusion flames. Energy Convers Manag 2011;52:1263e71. [60] Elbaz AM, Roberts WL. Flame structure of methane inverse diffusion flame. Exp Therm Fluid Sci 2014;56:23e32. [61] Candel S, Durox D, Ducruix S, Birbaud A-L, Noiray N, Schuller T. Flame dynamics and combustion noise: progress and challenges. Int J Aeroacoustics 2009;8:1e56. [62] Smith T, Kilham J. Noise generation by open turbulent flames. J Acoust Soc Am 1963;35:715e24.
Available online at www.sciencedirect.com
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Effect of hydrogen enrichment on combustion characteristics of methane swirling and nonswirling inverse diffusion flame Vipul Patel, Rupesh Shah* Department of Mechanical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat, 395007, India
highlights Higher combustibility of hydrogen improves combustion of methane in IDF. Hydrogen blending with methane decreases flame length. Hydrogen concentration on mass basis reduces CO emission and increases NOx. Combustion noise increases with the hydrogen addition.
article info
abstract
Article history:
Influence of hydrogen addition on appearance of swirling and non-swirling inverse diffu-
Received 26 June 2019
sion flame (IDF) along with emissions characteristics are investigated experimentally. The
Received in revised form
combustion characteristics including flame length, axial and radial temperature variation,
25 August 2019
and noise level are analysed for hydrogen addition in methane by mass basis for constant
Accepted 9 September 2019
energy input and by volume basis for constant volumetric fuel flow rate. Hydrogen addition
Available online 3 October 2019
in methane IDF produces shorter flame by compressing entrainment zone, mixing zone, reaction zone, and post-combustion zone. Hydrogen addition shift these zones towards
Keywords:
fuel and air exit from the burner. Enrichment of methane with hydrogen on a mass basis
Inverse diffusion flame
up to 6% reduces CO emission considerably and increases NOx emission moderately. Effect
Hydrogen enrichment
of H2 addition on combustion and emission characteristics is more prominent in non-
Combustion characteristics
swirling IDF. Combustion noise is augmented with the hydrogen addition and the
Sound level
magnitude of sound level depends on the hydrogen concentration. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The requirement of environment-friendly fuels has been increasing day by day to meet stringent emission regulations. In order to increase the availability of such clean energy resources, alternative fuel needs to be burned with conventional fuel. One way of using alternative fuel is to complement
hydrogen (H2) with hydrocarbon gaseous fuel [1e5]. Greater molecular diffusivity, higher burning speed, and wider flammability limits favour hydrogen blending with a conventional fuel [6e10]. Addition of hydrogen increases adiabatic flame temperature and enhances the heat release rate [11e15]. Combustion characteristics of hydrogen-enriched low calorific value coal gases improves considerably as the coal gases
* Corresponding author. E-mail addresses: [email protected] (V. Patel), [email protected] (R. Shah). https://doi.org/10.1016/j.ijhydene.2019.09.076 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature ∅ Sn m_ a m_ f
Equivalence ratio Swirl number Air mass flow rate, kg/s Fuel mass flow rate, kg/s Energy input rate, kW Qenergy Hydrogen mass flow rate, kg/s m_ H2 Methane mass flow rate, kg/s m_ CH4 Z Axial distance, mm IDF Inverse Diffusion Flame Abbreviations LCV Lower Calorific Value SCADA Supervisory Control and Data Acquisition
are enriched by hydrogen [16]. Moderate addition of hydrogen in Internal Combustion engine at low energy level reduces NOX emissions [17]. Kumar and Mishra [18] characterised liquefied petroleum gas (LPG)eH2 diffusion flame in terms of flame length. They reported that the addition of hydrogen results in a reduction in flame length. Wu et al. [19] observed that flame length reduces when hydrogen added in methane. Kumar and Mishra [20] investigated the effects of bluff-body lip thickness on flame length in LPGeH2 combustion process. The results show a reduction in the flame height by adding H2. Zheng et al. [21] conducted a numerical study to investigate the flame propagation of a premixed methane-hydrogen flame. They observed that as hydrogen content increases from 0% to 50%, the flame develops in “tulip” shape. Liu et al. [22] numerically studied thermal and chemical effects of hydrogen adding on methane-air mixture flame. The result indicated that laminar flame velocity increases because of the chemical effect due to increased active radicals. Li et al. [23] found that the addition of hydrogen has a greater impact on the flame propagation speed of methane compared to that of propene and ethane. Li et al. [24] investigated the methane-hydrogen flame characteristics by varying the initial pressure and hydrogen content in the rich, stoichiometric and lean fuel mixture conditions. It was observed that diffusional-thermal and hydrodynamic instability on flame destabilization are improved with hydrogen addition. Afarin and Tabejamaat [25] numerically analysed the influence of hydrogen content in methane on the flame temperature. According to their results, an increase in hydrogen content increases hydroxyl (OH) mass fraction and flame temperature. Bouras et al. [26] show that as hydrogen in the fuel mixture increases the peak value of flame temperature in the diffusion flame. Several attempts have been made to envisage the change of emission behaviour due to hydrogen enrichment. Gulder et al. [27] examined the effect of hydrogen enrichment on soot in the ethylene jet diffusion flame and observed reduction in the soot formation. Ezenwajiaku et al. [28] analysed the formation characteristics of polycyclic aromatic hydrocarbons in a hydrogen-enriched methane flame. They observed reduction in formation of polycyclic aromatic hydrocarbons by hydrogen addition. Miao et al. [29] found that hydrogen
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addition promotes the conversion of CO into CO2 and a significant reduction of CO and HC occurs. Burbano et al. [30] also noticed a considerable decrease in the CO emissions for natural gas combustion with hydrogen addition. Tseng [31] numerically investigated the mixing of hydrogen/methane on premixed combustion process in porous medium burners. Kumar et al. [32] showed a reduction in CO emissions using small quantities of hydrogen in a compression ignition engine primarily fuelled with a Jatropha oil. Sun et al. [33] revealed that the addition of hydrogen to ethylene/air diffusion flame could influence the soot volume fraction. Kahraman et al. [34] numerically investigated the combustion characteristics in the gas turbine combustion chamber fuelled by hydrogen. Wang et al. [35] numerical analysed the potential of reducing emission with hydrogen enrichment. Rortveit and Hustad [36] showed that prompt NOx mechanism leads to the formation of total NOx when hydrogen is added to a natural gas flame. In contrast, Naha and Aggarwal [37] reported that for partially premixed combustion, the thermal NOx mechanism is a major factor in the formation of total NOx. In a similar study, Mishra and Kumar [38] revealed that the formation of thermal NOx by Zeldovich mechanism at high flame temperatures results in increased NOx level. Miao et al. [39] studied the flame stability of LPG IDF with hydrogen enrichment. In addition, Miao et al. [40] claimed that hydrogen addition can significantly improve combustion of LPG IDF. Restricted studies have been conducted on swirl-stabilized flame using hydrogen as a fuel additive. Tong et al. [41] experimentally investigated the influence of swirl flow on the hydrogen-enriched methane flame. It turns out that the flame dynamics and the flame shape were strongly determined by the swirl flow. Kim et al. [42] reported that hydrogen enrichment can decrease NOx emissions for premixed swirl flames compared to diffusion flames. Khalil and Gupta [43] observed a reduction in CO and increase in NO using hydrogen in a swirlstabilized non-premixed gas turbine fuelled with methane. Shanbhogue [44] reported transitions in flame microstructures under an acoustically coupled and uncoupled condition in swirl-stabilized premixed methane-hydrogen combustion. Schefer et al. [45] investigated the emission behaviour in a swirl-stabilized premixed burner using hydrogen as an additive to methane. They found a reduction in CO emission, without adversely affecting NO emission. Kim et al. [46] studied the effect of swirl on combustion characteristics of hydrogen added methane premixed flames. Emadi et al. [47] examined how the flame dynamics change in response to increase in pressure and hydrogen enrichment in a low-swirl methane-air burner. Kashir et al. [48] numerically investigated flame characteristics of methane hydrogen flames under distinct swirl numbers. IDFs are distinct kind of diffusion flames because the position of air and fuel is reversed compared to the normal diffusion flames. The diffusion flames (non-premixed) have a wide operating range due to their non-premixed nature. However, the high soot loading limits the use of these flames in domestic applications where clean combustion is required. Premixed flames burn more intense and are relatively clean. However, the flammability range is narrow due to the lift-off of the flame. IDFs have a wide operating range with less soot loading and no lift-off, allowing IDF burners to be used in
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industrial applications. Although many researchers have studied the role of hydrogen enrichment in a diffusion flame, few researchers have experimentally studied the combustion characteristics of swirling IDF with hydrogen enrichment. This research aims to examine the role of hydrogen enrichment in non-premixed methane IDF. In this regard, combustion characteristics including flame length, temperature profile, emission, and noise level are analysed for various methane-hydrogen hybrid flames.
Experimental facility and test conditions Schematics of the experimental setup and coaxial burner along with developed test facility and actual burner configuration are shown in Fig. 1 and Fig. 2 respectively. Combustion air is delivered by a blower. The air and fuel are supplied and burned into the burner at atmospheric pressure. Fuel flow is regulated by a mass flow controller (MCR 50 slpm, Alicat Scientific) with an accuracy ± 0.4% of reading. Air flow is metered by a thermal mass flow meter (4040, TSI Incorporated). The accuracy of the thermal mass flow meter is ±2% of reading. Flame images are captured with a digital SLR camera (D5300, Nikon) through a glass window of the enclosure. Fig. 2 demonstrates the photographic view of the experimental facility and burner used in this study. The flame length reported is derived by ImageJ (image processing software). The flame
length is calculated by taking the average of 10 images [38]. To study the effect of swirl on combustion characteristics, 30 swirler with 6 flat vanes (Sn ¼ 0.6, numerically derived) is used (Fig. 2). Swirl in flow is characterised by dimensionless swirl number (Sn). Swirl number is a ratio of the axial flux of angular momentum to the axial flux of axial momentum [49]. Influence of swirl on flame characteristics is analysed by measuring radial and axial temperature profiles. Type R thermocouple (bead diameter ¼ 1.5 mm, wire diameter of 0.3 mm) is used to measure flame temperature. The thermocouple is held horizontal and thus normal to the flow. Correction for flame temperatures is made for the radiative loss [50]. The maximum eccentricity in flame temperature measurement is ±50 K. Emission from flame is measured in terms of. Emission species are detected by flue gas analyser (Testo 340, accuracy ± 2 ppm). The emission level of CO and NOx are recorded along the centerline of IDF at approximately 2 times the flame length from the burner exit [51]. Effect of swirl and hydrogen addition on the sound generated in IDF is measured using a sound level meter (Testo 816-1) with an accuracy ± 1.4 dB. A microphone is mounted on a tripod with its sensor surface facing the IDF for accurate measurement. The distance from the IDF axis to the microphone is 500 mm, while the azimuth angle, the angle from the microphone to the flame axis is 90 [52]. The microphone is connected to a PC through Testo 816-1 software’s user interface and signals are logged. One hundred number of readings are sampled with a
Fig. 1 e Schematic of (a) experimental setup and (b) coaxial burner.
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Fig. 2 e Photographic view of (a) experimental facility and (b) coaxial burner. sampling rate of 0.5 s for each configuration and the average value is recorded as the final reading of the sound level. All experimental data is collected at least three times to ensure good repeatability. The error bars of data are estimated from the standard deviation of measurements. In this study, experiments are carried out for two test conditions, the hydrogen concentration is varied in methane by mass basis (constant energy input rate, 7.43 kW) and hydrogen concentration is varied in methane by volume basis (constant volume fuel flow rate, 13.5 lpm). The air flow rate is fixed at 125 lpm at the inner tube of the coaxial burner. Some of the important properties of hydrogen and methane are listed in Table 1. In the first test condition, the energy input rate to the burner is calculated using the equation given below Qenergy ¼ m_ CH4 LCVCH4 þ m_ H2 LCVH2
(1)
where, Qenergy is the energy input rate, m_ H2 and m_ CH4 are the mass flow rate of hydrogen and methane respectively, while LCVCH4 and LCVH2 are the lower calorific value of methane and hydrogen respectively. The hydrogen concentration on a mass basis (0%e10%) with various combination of flow rates of hydrogen and methane for the case of constant energy input rate, are presented in Table 2. In the second test condition, the constant volumetric fuel flow rate of the mixture (hydrogen and methane) is fed to the burner. The hydrogen concentration on a volume basis (0%e80%) with a different combination of flow rates of hydrogen and methane for the case of constant volume fuel flow rate, are given in Table 3. In both test conditions, 0% hydrogen refers to pure methane IDF. The equivalence ratio∅, indicating relative richness and leanness of fuel in air-fuel mixture is defined as
Table 1 e Properties of hydrogen and methane. Properties Lower Calorific value (LCV) (MJ/kg) Density (kg/m3) Mol. weight Gas constant (J/kgK)
H2
CH4
121 0.082 2.016 4124
50 0.65 16.04 518.3
∅ ¼ m_ a m_ f stoich m_ a m_ f actual
(2)
where ðm_ a =m_ f Þstoich is stoichiometric air-fuel ratio and ðm_ a =m_ f Þactual is the actual air-fuel ratio. H2 concentration on mass basis is varied to maintain energy input rate constant. In this case as shown in Table 2 fuel mixture flow rate reduces. This in turn decreases mixture fuel velocity. Equivalence ratio remains in narrow band (0.999e0.960). Thus H2 addition on mass basis indicates effect of H2 addition and mixture fuel velocity. H2 addition at constant volumetric rate is the case of constant air and fuel mixture velocity at burner exit. Therefore, H2 concentration variation on volume basis shows effect of H2 addition, energy input and equivalence ratio. To gain insight of effect of H2 addition along with mixture fuel velocity, energy input and equivalence ratio performance parameters and flow physics are compared for H2 concentration variation in methane by mass basis and that by volume basis.
Results and discussion Effect of H2 addition on flame length Fig. 3 shows the images of both IDFs at various hydrogen concentration (0%e10%) on a mass basis. From Fig. 3 it is seen that for both IDFs, flame length decreases with an increase in hydrogen concentration. Swirl results in shorter IDF for same mixture strength. Fig. 3 indicates that IDFs comprise a bluish reaction zone followed by a purple-orange luminous zone. The bluish zone of IDFs results from greater entrainment of fuel through a central jet of air that burns fuel in premixed combustion mode. While the purple-orange zone indicates the presence of soot particles due to fuel burns with air entrained from its surroundings. It is found from Fig. 3 that at a lower percentage of hydrogen, the bluish reaction zones are longer. The blue zone inside the flame shows the presence of partially premixed burning of fuel in the flame. The entrainment of the fuel in the air jet depends on the velocity difference between air and fuel jets. This difference in velocity creates a sufficient pressure difference between fuel/air
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Table 2 e Test conditions for constant energy input rate. Cases
H2 by mass basis
Energy input rate (kW)
CH4 (lpm)
H2 (lpm)
Total flow rate of fuel mixture (kg/s)
Equivalence ratio (∅)
0% 2% 4% 6% 8% 10%
7.43 7.43 7.43 7.43 7.43 7.43
13.50 12.89 12.23 11.58 10.92 10.27
0.00 2.18 4.36 6.54 8.72 10.90
0.000149 0.000145 0.000141 0.000136 0.000132 0.000128
0.999 0.994 0.985 0.977 0.969 0.960
1 2 3 4 5 6
Table 3 e Test conditions for constant volume fuel flow rate. Cases 1 2 3 4 5
H2 by volume basis
Total flow rate of fuel mixture (lpm)
CH4 (lpm)
H2 (lpm)
Energy input rate (kW)
Total flow rate of fuel mixture (kg/s)
Equivalence ratio (∅)
0% 20% 40% 60% 80%
13.50 13.50 13.50 13.50 13.50
13.50 10.80 8.10 5.40 2.70
0.00 2.70 5.40 8.10 10.80
7.43 6.39 5.35 4.31 3.27
0.000149 0.000122 0.000096 0.000070 0.000044
0.999 0.849 0.699 0.549 0.399
streams to divert the fuel jet and redirect it to the central air jet. The fuel eventually impinges on the central air jet, resulting in a premix of air and fuel near the flame neck. In other words, the difference in the momentums of the fuel and air jet increases, leading to entrainment of fuel into the central air jet. As the hydrogen content increases, the bluish zones become shorter and shift upstream. Since the energy input rate is fixed, the hydrogen percentage growth means an increase in the fuel velocity, whereby the velocity difference decreases. Thus, the difference in the momentums of two jets decreases, thereby reducing the entrainment of fuel. Therefore, the fuel partially burns in the premixed mode and partially carries downstream along with the flame torch. Therefore, with higher hydrogen content blue zones appear shorter. The longer luminous zones at a higher hydrogen content reflect that the fuel that is not entrained by the central air burns with ambient air at the periphery of the flame torch.
In addition, higher flammability of hydrogen, which supports the faster chemical reaction and hence enhances the combustion rate. Flame images are shown in Fig. 3 indicates that higher hydrogen content in IDF reduces the size of the reaction zone and shifts the reaction zone upstream. Fig. 4 shows the measured flame length of swirling and non-swirling IDFs at various hydrogen concentration on a mass basis. The length of both IDFs decrease with hydrogen enrichment. When hydrogen is less than 2%, the rate of decrease is slight. However, when hydrogen is above 2%, the rate of decrease is considerable, indicating a strong dependency of flame length on hydrogen enrichment. At 10% hydrogen enrichment, flame length of swirling and nonswirling IDFs reduces approximately 17.5% and 16.1% respectively. The decrease in flame length is due to a higher molecular diffusivity of the hydrogen [19]. Increased molecular diffusivity results in rapid combustion through greater air/
Fig. 3 e Flame photographs of IDFs with variation in H2 by mass.
Fig. 4 e Flame length profiles of IDFs with variation in H2 by mass.
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fuel mixing. This is due to the increase in OH and H radicals associated with the addition of hydrogen which improves the burning rate and thereby reducing the flame [11]. In addition, the total mixture mass flow rate of fuel is also reduced (Table 2), which also contribute to flame length reduction. This is in good agreement with the observation made by Choudhary et al. [6] and Francis et al. [53] in a hydrogen-hydrocarbon diffusion flame. Fig. 5 shows the swirling and non-swirling IDFs for hydrogen concentration (0%e80%) by volume basis. For each flame, there is a purple-orange luminous zone overlapping an intense bluish premixed combustion zone. The blue zone within the flame indicates the presence of partially premixed combustion of fuel in the flame. In the present study, it is found that with increasing hydrogen content, the blue zones become shorter, as fewer fuel burns in the premixed mode of combustion. As hydrogen content increases, fuel velocity increases as hydrogen is less dense than methane. A higher fuel velocity influences the difference in the momentums of two jets and thus less fuel is entrained. Therefore, blue zones appear shorter with hydrogen addition. In Fig. 6, it is observed that the length of both IDFs decreases considerably with hydrogen. Cozzi and Coghe [7] also found a decrease in the flame length with hydrogen blending for a swirl stabilized the non-premixed flame. There is a progressive shortening of the bluish zone indicating a decrease in equivalence ratio. The length of non-swirling IDF decreased by approximately 58.8%, while the length of swirling IDF decreased approximately 63.1% respectively, as compared with the pure methane IDF (0% hydrogen). This is due to the reduced mass flow rate of the total fuel mixture with increasing hydrogen concentration (Table 3). Hydrogen addition at a constant volumetric flow rate reduces heat input rate (Table 3) due to low density compared to methane. Also, higher combustibility of hydrogen supports the chemical conversion of fuel into molecular diffusivity of the flame and
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Fig. 6 e Flame length profiles of IDFs with variation in H2 by volume.
accelerates the exothermic reaction [42]. In addition, at low equivalence ratio, low fuel concentration and excess air are not favouring the generation of the larger reaction zone, and the high air/fuel ratio consequences in a high strain rate on the flame fronts, resulting in a short and open IDF. Consequently, there is a reduction in flame length.
Effect of H2 addition on flame temperature In order to interpret the emission level and noise level in flames, the thermal behaviour of both IDFs is examined by flame temperature measurement. Both IDFs are experimentally investigated under two different hydrogen addition cases, 10% hydrogen (by mass basis) and 80% hydrogen (by volume basis) and compared with results obtained for pure methane combustion. The effect of hydrogen addition on the temperature distribution along the centerline of the flame is investigated first. Then, the effect of hydrogen addition on the radial temperature distribution at a different axial distance is analysed.
Centerline temperature distribution
Fig. 5 e Flame photographs of IDFs with variation in H2 by volume.
Fig. 7 shows flame temperature distribution along the flame centerline of non-swirling IDF with varying hydrogen concentration. The centerline temperature profiles of pure methane and 0% hydrogen (by mass basis) IDFs are comparable. It is seen that the temperature increases at a fast pace in the 10% hydrogen (by mass basis) IDF with increasing axial distance up to 120 mm as compared with the pure methane IDF. This is because of higher combustibility of hydrogen, which enhances the exothermic chemical reaction of fuel and shifts the reaction zone in upstream. The highest temperatures in the pure methane IDF and 10% hydrogen (by mass basis) IDF are 1447 K, and 1311 K, respectively. The highest temperatures in both IDFs are observed at approximately 230 mm away from the burner outlet. Hence, the position of the highest temperature thus remains the same from the burner outlet with 10% hydrogen addition (by mass basis).
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Fig. 7 e Centerline temperature distribution of nonswirling IDFs.
However, the highest temperature is decreased by 136 K (approximately 9.4%) as compared with pure methane IDF. While hydrogen is blended to the methane stream at a constant energy input rate, the mass flow rate of the total fuel mixture is decreased slightly as the density of hydrogen is lower than that of methane, hence decrease in an equivalence ratio (Table 2). This results in a decrease in the highest temperature of 10% hydrogen (by mass basis) IDF. The centerline temperature then drops steadily once the peak value is reached because no major reaction occurs in the postcombustion zone. Nearly all fuel is consumed in the main reaction zone and leaves the post-combustion zone only by the entrainment and dilution. This results in a gradual reduction of the flame temperature. By adding 80% hydrogen (by volume basis) to methane stream, the highest temperature, 770 K is observed at approximately 260 mm away from the burner outlet. The reduction in the highest flame temperature is attributed to the low equivalence ratio which is resulted because of the reduced mass flow rate of a total fuel mixture when 80% hydrogen is added by volume basis (Table 3). In addition, reduced energy input also influences the flame temperature. Fig. 8 shows the temperature profiles along with the flame centerline of swirling IDFs with varying hydrogen concentration. Trend of the flame temperature distribution of pure methane and 10% hydrogen (by mass basis) IDFs is comparable. The highest temperatures in pure methane and 10% hydrogen (by mass basis) IDFs are measured 1644 K and 1684 K, respectively. With 10% hydrogen (by mass basis) addition to methane stream, the highest temperature is increased by 40 K (approximately 2%) compared to pure methane flame. Positions of the highest temperatures of the pure methane and 10% hydrogen (by mass basis) IDFs are observed at approximately 80 mm and 100 mm from the burner outlet, respectively. The position of the highest temperature thus moved away from the burner outlet in the swirling IDF with the addition of 10% hydrogen (by mass basis). This is due to the higher combustibility of hydrogen which promotes the exothermic reaction and expands
Fig. 8 e Centerline temperature distribution of swirling IDFs. combustion products speedily. Therefore, a large amount of heat energy is released during the combustion which enhances the flow velocity and thus reduces the recirculation flow in the internal recirculation zone of the swirling IDF. The reduction in the recirculation flow leads to an increase in flame temperature of the reaction zone with the addition of hydrogen to the methane fuel [46]. Besides, the higher combustibility of hydrogen shifts the reaction zone upstream [54]. With 80% hydrogen (by volume basis) addition to the methane stream, the highest 1017 K temperature is observed at approximately 80 mm from the burner outlet. This decrease in the highest flame temperature is because of the low equivalence ratio and reduced energy input rate (Table 3).
Radial temperature distribution Fig. 9 presents the comparison of flame images and radial temperature profiles of non-swirling IDFs. The images of IDF represent the cases of pure methane (0% hydrogen), 10% hydrogen addition to methane by mass basis and 80% hydrogen addition to methane by volume basis. Generally, the flame structure of the IDF comprises four zones according to temperature distribution [55]. Zone 1 is a base zone in which fuel is entrained by central air with high velocity. Zone 2 is a mixing zone where the mixing of fuel and air is completed. Zone 3 is a reaction zone that has the highest flame temperature due to intense heat release by the reaction. Zone 4 is a post-combustion zone associated with the cooling of the hot gases due to the entrainment of ambient air. The pure methane IDF is approximated in four different zones. The radial temperature profiles of 10% hydrogen (by mass basis) and 80% hydrogen (by volume basis) IDFs are compared with pure methane IDF at five axial locations. For pure methane IDF, the temperature profile at a low elevation of Z ¼ 5 mm passes through the base zone. The two temperature profiles at the near-burner outlet of Z ¼ 25 and 50 mm traverse through the mixing zone. The temperature profile at mid-flame of Z ¼ 75 mm represents the reaction zone. The other one at the far-burner outlet of Z ¼ 125 mm passes through the postcombustion zone.
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Fig. 9 e Comparison of flame images and radial temperature profiles at five axial locations of non-swirling IDFs.
Radial temperature distribution at these locations is shown in Fig. 9. For pure methane case at Z ¼ 5 mm maximum temperature is recorded at approximately 15 mm. In the present case, the outer radius of the burner is 18.5 mm. The occurrence of maximum temperature at 15 mm is the indication of burning of fuel due to the presence of outside air in diffusion mode. At Z ¼ 25 mm maximum temperature location shifts inward at a radial position around 10 mm. In the current study, inner tube radius is 9.5 mm. Happening of maximum temperature at 10 mm radial position indicates burning of fuel due to entrainment within air jet. At Z ¼ 5 mm and Z ¼ 25 mm near flame axis temperature gradient is low. In mixing zone at Z ¼ 50 mm peak of the temperature profile is recorded at the same radial location that of at Z ¼ 25 mm however at flame axis higher temperature gradient is recorded. At Z ¼ 75 mm location of maximum flame temperature shifts radially outward while the temperature gradient at axis becomes steeper than at Z ¼ 50 mm. This steep gradient is due to the presence of a reaction zone at this axial location. Combustion releases a large amount of energy which in turn
expands domain in all direction thus there is a radial outward shift in the location of maximum temperature. At Z ¼ 125 mm in the post-combustion zone near flame axis steepness of temperature gradient reduces compare to that of at Z ¼ 75 mm. This is due to the absence of major energy release phenomena in this zone. Addition of 10% H2 by mass in methane alters flow and reaction mechanism. At all axial positions, except at Z ¼ 125 mm, near flame axis higher temperature and greater gradients of temperature are observed due to the addition of H2. For axial location Z ¼ 5 and Z ¼ 25 mm maximum temperature is comparable in both cases. At Z ¼ 50 and Z ¼ 75 mm at all radial position, measured temperature with 10% H2 is on the higher side compared to that of pure methane case. Near the flame axis, the steeper temperature gradient can be observed at these locations due to the addition of H2. Presence of steeper temperature gradient and higher temperature in the vicinity of axis indicates that the strength of the reaction zone increases due to the addition of H2. Temperature profile at Z ¼ 125 shifts towards the lower value of
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temperature compares to that of pure methane case. This indicates compression and shift of reaction and post-reaction zone towards the flame base. Thus the addition of H2 accelerates fuel burning rate. This results in an intense and shorter reaction zone. Overall flame becomes shorter compared to pure methane case due to the addition of H2 on mass basis. In case when 80% H2 is added on volume basis, total energy to burner reduces to 3.27 kW 55% of this total energy input is due to H2. Thus flow physics and reaction mechanism are dominated by H2. Maximum temperature at Z ¼ 5 mm and Z ¼ 50 mm are comparable. At Z ¼ 25 mm maximum flame temperature is measured compared to Z ¼ 75 mm in pure methane case. This indicates the presence of a reaction zone at axial location Z ¼ 25 mm. Thus Reaction zone shifts towards the flame base in case of 80% H2 on volume basis compare to pure methane case. Temperature profile at Z ¼ 125 mm indicates the flow of hot products only. This results in a shorter flame length for 80% H2 by volume case. Fig. 10 shows the comparison of flame images and radial temperature profiles of swirling IDFs. The appearance of the
pure methane swirling IDF is observed different compares to the pure methane non-swirling IDF. The flame torch and flame base shorten in the pure methane swirling IDF because of the formation of an internal recirculation zone [56]. As previously discussed, the pure methane swirling IDF is characterised in four zones, as shown in the image of the pure methane swirling IDF. Hence, the position of the five profiles is slightly changed in pure methane swirling IDF. The profile of Z ¼ 5 mm passes through the base zone. The one profiles of Z ¼ 25 mm approximate cross through the mixing zone. The one profile at mid-flame of Z ¼ 50 mm approximate penetrates reaction zone. The two profiles of Z ¼ 75 and 125 mm approximate passes through the post-combustion zone. With swirl also when H2 is added at 10% on mass basis at the same energy rate at axial location 5 mm, 25 mm and 50 mm near flame axis temperature increases. Rate of temperature increase at 25 mm and 50 mm is higher compared to that for 5 mm. Also, the maximum flame temperature recorded at 50 mm is higher than that of maximum flame temperature recorded at 75 mm. This shows the presence of a
Fig. 10 e Comparison of flame images and radial temperature profiles of swirling IDFs.
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reaction zone at 50 mm. Thus due to the addition of H2 stronger and shorter flame is produced. Non-swirling IDF with 80% H2 by volume has the highest flame temperature (1657 K) at Z ¼ 25 mm. This highest flame temperature is lower than that of pure methane (1716 K, Z ¼ 125 mm) and 10% H2 by mass (1874 K, Z ¼ 75 mm) cases. This is due to reduce energy input in case of 80% H2 by volume (3.27 kW) compare to other cases (7.43 kW). Swirling IDF with 80% H2 by volume attains highest flame temperature (1723 K) at Z ¼ 25 mm. This highest flame temperature is lower than that of pure methane (1864 K, Z ¼ 75 mm) and 10% H2 by mass (1907 K, Z ¼ 50 mm) cases with the swirl. Maximum temperature values of swirling IDF without and with 10% H2 are comparable. Effect H2 addition on reaction and flow mechanism is more prominent in non-swirling IDF than swirling IDF. Early occurrence of maximum temperature compared to pure methane case indicates the shift of reaction zone towards the flame base and thus generates a shorter flame. Higher molecular diffusivity and higher flammability of hydrogen result in a faster chemical reaction through improved air/fuel mixture. Also due to the increase in OH and H radicals associated with the addition of hydrogen, the rate of combustion improves. The results obtained for the radial temperature distribution in both IDFs shows that increasing the hydrogen concentration on the basis of mass (10% hydrogen) considerably increases the flame temperature, which is more desirable for heating purpose. Also, the position of the highest temperature moves near the burner outlet. However, when hydrogen is added on the basis of volume (80% hydrogen), the highest flame temperature is decreased due to the reduced energy input rate. Wu et al. [19] and Choudhuri et al. [6] reported that the flame temperature increases due to the adding of hydrogen in a diffusion flame. Therefore, the observed tendency of increasing the flame temperature with hydrogen enrichment is consistent with previous experimental studies. When hydrogen burns with methane-air IDF, the radical OH in the reaction zone increases, which promotes the reaction rate by increasing the chain-branching process [19]. In addition,
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while hydrogen is blended to methane stream, the rate of carbon input decreases, resulting in a reduction in radiation heat loss [11]. Also, higher combustibility of hydrogen rapidly expands the combustion product, thereby reducing the recirculation flow. The decline of the recirculation flow plays an important role in the temperature rise of the reaction zone [46]. And that increases the flame temperature.
Effect of H2 addition on CO and NOx Fig. 11(a) demonstrates the variation of CO and NOx emissions with increasing hydrogen concentration at constant energy input rate (by mass basis). It is observed that the CO emission decreases with increasing hydrogen in swirling and nonswirling IDFs. While the NOx emission follows the reverse trend in both IDFs. The CO emission in pure methane (0% hydrogen) non-swirling and swirling IDFs is around 35 ppm and 43 ppm, respectively. As hydrogen addition (by mass basis) increases from 0% to 10%, CO is decreased by around 21 ppm for non-swirling IDF and 11 ppm for swirling IDF. The CO emission in the swirling IDF decreases quickly as the hydrogen increase from 0% to 4% and then gradually decrease with hydrogen addition up to 10%. The decrease of CO in the non-swirling and swirling IDFs is approximately 40% and 74%, respectively, compared to the pure methane flame. When hydrogen is blended to the methane, the amount of methane decrease hence, carbon content decreases accordingly in total fuel mixture flow rate. Increasing hydrogen concentration at constant energy input rate (by mass basis) increases the H, O and OH radicals during combustion. The same can be explained with following elementary reactions: OH þ H2 4H þ H2 O
(3)
H þ O2 4O þ OH
(4)
These OH radicals promote the speedy transformation of CO into CO2 by the reaction mechanism defined as OH þ CO4H þ CO2
Fig. 11 e Hydrogen addition effect on emission level.
(5)
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Thus, the decrease of CO emission is partly because of the chemical kinetic effect and partially caused by the reduction in carbon content [35]. Rajpara et al. [57] observed that the increase of hydrogen in methane decreases CO emission with minimal increase in NOx emission. Similarly, Miao et al. [29] found that the hydrogen addition reduced CO emission into LPG IDF. In Fig. 11(a), trends of NOx emissions with hydrogen addition by mass basis are opposite to those of CO. The NOx emission of pure methane (0% hydrogen) non-swirling IDF is around 4 ppm. It is seen that as hydrogen addition (by mass basis) increases from 0% to 6%, NOx increase from around 7 ppm; as hydrogen addition further increases, NOx stabilize at around 7 ppm. For the pure methane swirling IDF, NOx is around 3 ppm. With 2% hydrogen addition to the methane swirling IDF, NOx increased around 4 ppm. As hydrogen addition increases from 2% to 10%, NOx stabilizes at around 4 ppm. The emission level of NOx thus increases with the addition of hydrogen for both IDFs. The prompt and thermal mechanisms are accountable for most of NO formed in the non-premixed combustion [58]. In general, the formation of NOx follows very well the temperature change in the reaction zone. Hence, the production of thermal NO in the reaction zone plays a key role in the NOx emission. Fig. 9 (b) and Fig. 10 (b) show that the temperature inside the IDFs with hydrogen addition (by mass bass) is greater than 1800 K, which provides adequate energy to trigger the chain reactions in the Zeldovich mechanism [29]. Hence there is an increase in the NOx emission level. Thus increasing hydrogen concentration at constant energy input rate (by mass basis) reduces CO emission and increases NOx emission. Similarly, Zhen et al. [59] also observed the increase in NOx for a swirl-stabilized LPG IDF. Fig. 11(b) depicts the measured emission of CO and NOx with increasing hydrogen concentration at constant volume fuel flow rate (by volume). The results show that the CO emission increases with increasing hydrogen content for both IDFs. This is due to the quenching effect. In IDF, when a fuel mixture impinges on central air jet in the flame neck zone, part of the fuel mixture cools down by the cold air jet,
which reduces the OH concentration [39]. Also, a high strain rate and local air/fuel ratio in the flame neck zone extinguish the flame locally and contribute to the formation of CO [60]. Furthermore, the reduced flame length of IDFs (Figs. 3 and 5) offers shorter residence time for the oxidation of CO into CO2. Quenching effect, inadequate reaction time and higher strain rate are reasons for the increase of CO emission. Fig. 11(b) also presents the variation in NOx emission with hydrogen enrichment (by volume basis). It turns out that with increasing hydrogen concentration, the NOx emission decreases. This is in good agreement with the observation made by Kim et al. [42]. Also, Figs. 9 (c) and Fig. 10 (c) depict that the temperature within the IDFs is less than 1800 K, which does not favour the chain reactions in the Zeldovich mechanism, subsequently the decrease of NOx emission is observed. Therefore, we conclude that hydrogen enrichment at a constant volume flow rate (by volume basis) increases CO emission and decreases NOx emission. Addition of hydrogen results in shorter flame, higher flame temperature and low CO emission. Low CO emission indicates better fuel burning and higher burner efficiency. Thus H2 addition can result in reduction of space requirement if IDFs are used for heating purpose. Enhancement in maximum flame temperature increases rate of heat transfer. Therefore, H2 addition in IFDs flame results in enhanced, efficient and cleaner combustion phenomena along with enhanced heat transfer rate.
Effect of H2 addition on combustion noise Experiments are carried out to ascertain the effect of hydrogen addition on the behaviour of noise radiation from swirling and non-swirling IDFs. The combustion noise characteristics are firstly examined at increasing hydrogen concentration while the energy input rate is kept constant (by mass basis). Secondly, the hydrogen concentration is varied while the volume fuel flow rate is kept constant (by volume basis). Sound level measurements are performed to measure the flame noise. The flame noise is the sound radiated from the burning air/fuel jets.
Fig. 12 e Hydrogen addition effect on combustion noise.
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Variation of sound level for both test conditions is demonstrated in Fig. 12. It is seen that the hydrogen addition increases the noise level under both test conditions. While hydrogen is blended to methane stream at constant energy input rate (Fig. 12(a)), the sound level of pure methane (0% hydrogen) non-swirling IDF and swirling IDF is around 69.3 dB and 78.8 dB, respectively. As hydrogen addition increases from 0% to 10%, the sound level increases by around 4.5 dB (6.1%) for non-swirling IDF and 3 dB (3.67%) for swirling IDF, respectively, compared to that of pure methane flame. Similarly, Fig. 12(b) shows that, when hydrogen addition increases from 0% to 80% (by volume basis), the increase in sound level in the non-swirling and swirling IDFs is approximately 10.23% and 5.52%, respectively, compared to pure methane flame (0% hydrogen). Combustion noise increases with hydrogen addition and the magnitude of sound level depend on hydrogen concentration. In general, sound radiated during the combustion process is due to the fluctuation of the heat release rate at the flame front [61]. Besides this, the change in heat release rate associated with the exothermic chemical reactions in the reaction zone also contributes to the generation of combustion noise. As the amount of hydrogen added to the methane fuel increases, higher OH radical formation promotes the higher rate of reaction. This results in an increase in the sound level. It is noticed that the sound level of pure methane non-swirling IDF is well below (69.3 dB), while the sound level of swirling IDF with 80% hydrogen addition (by volume basis) is much higher (83.40 dB). This is because higher combustibility of hydrogen than methane results in rapid combustion due to faster heat release rate. Addition of H2 addition on a mass basis for constant energy input rate reduces flame length without affecting flame diameter as shown in Fig. 4. Same energy release in smaller length indicates higher energy release rate per unit length. This higher energy rate of energy release increases sound level radiated from flame. This finding is in line with research, which states that the acoustic power is proportional to the burning speed of the flame [62]. For constant volume flow rate as H2% increases flame dynamics and reaction mechanisms are controlled by H2 as it becomes the main species in fuel. Thus, with an increase in the % volume of H2, the sound level increases.
Conclusions The role of hydrogen addition in non-premixed methane IDF is examined. The combustion characteristics of methanehydrogen hybrid flames are investigated and analysed at various hydrogen concentration on the basis of mass and volume. Measurements for flame length, radial temperature profile, emission, and noise level are carried out in swirling and non-swirling IDFs. Major findings of the present investigation are: 1. Hydrogen blending reduces flame length for both cases. Reduction in flame length for H2 addition on a mass basis for constant energy input is due to enhanced fuel burning rate and shorter reaction zone near the flame axis. For a
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constant volume flow rate, the reduced flame length is due to a higher percentage of hydrogen and reducing energy input rate with an increasing percentage of hydrogen. 2. In non-swirling IDF at 10% H2 addition on mass bases case rise in maximum temperature is 158 K compare to 43 K rise in case of swirling case. Effect of H2 addition on reaction and flow mechanism is more prominent in non-swirling IDF than swirling IDF. 3. It is observed that increasing hydrogen concentration at constant energy input rate (by mass basis) reduces CO emission and increases NOx emission. While the hydrogen enrichment at a constant volume flow rate (by volume basis) increases CO emission and decreases NOx emission. 4. Combustion noise increases with hydrogen addition and the magnitude of sound level depend on the hydrogen concentration. Swirling IDF with 80% hydrogen addition (by volume basis) generates maximum sound level (83.40 dB).
references
[1] Zohra KF, Mounir A, Salah C. Numerical simulation of CH4H2-AIR non-premixed flame stabilized by a bluff body. Energy Procedia 2017;139:530e6. [2] Zhao K, Cui D, Xu T, Zhou Q, Hui S, Hu H. Effects of hydrogen addition on methane combustion. Fuel Process Technol 2008;89:1142e7. [3] Schefer RW, Oefelein J. Reduced turbine emissions using hydrogen-enriched fuels. In: Proceedings of the 2002 US DOE hydrogen program review; 2002. p. 1e16. NREL/CP-610-32405. [4] Juste GL. Hydrogen injection as additional fuel in gas turbine combustor. Evaluation of effects. Int J Hydrogen Energy 2006;31:2112e21. _ _ Effect of swirl number on [5] Ilbas ‚ M, Karyeyen S, Yilmaz I. combustion characteristics of hydrogen-containing fuels in a combustor. Int J Hydrogen Energy 2016;41:7185e91. [6] Choudhuri AR, Gollahalli SR. Combustion characteristics of hydrogenehydrocarbon hybrid fuels. Int J Hydrogen Energy 2000;25:451e62. [7] Cozzi F, Coghe A. Behavior of hydrogen-enriched nonpremixed swirled natural gas flames. Int J Hydrogen Energy 2006;31:669e77. € kalp I. Characterization of the [8] Halter F, Chauveau C, Go effects of hydrogen addition in premixed methane/air flames. Int J Hydrogen Energy 2007;32:2585e92. [9] Hu E, Huang Z, He J, Jin C, Zheng J. Experimental and numerical study on laminar burning characteristics of premixed methaneehydrogeneair flames. Int J Hydrogen Energy 2009;34:4876e88. [10] Okafor EC, Hayakawa A, Nagano Y, Kitagawa T. Effects of hydrogen concentration on premixed laminar flames of hydrogenemethaneeair. Int J Hydrogen Energy 2014;39:2409e17. [11] Choudhuri AR, Gollahalli SR. Characteristics of hydrogenehydrocarbon composite fuel turbulent jet flames. Int J Hydrogen Energy 2003;28:445e54. [12] El-Ghafour SAA, El-dein AHE, Aref AAR. Combustion characteristics of natural gasehydrogen hybrid fuel turbulent diffusion flame. Int J Hydrogen Energy 2010;35:2556e65.
28328
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 3 1 6 e2 8 3 2 9
[13] Fukuda M, Korematsu K, Sakamoto M. On quenching distance of mixtures of methane and hydrogen with air. Bulletin of JSME 1981;24:1192e7. [14] Lafay Y, Renou B, Cabot G, Boukhalfa M. Experimental and numerical investigation of the effect of H2 enrichment on laminar methaneeair flame thickness. Combust Flame 2008;153:540e61. [15] Mishra SK, Dahiya RP. Adiabatic flame temperature of hydrogen in combination with gaseous fuels. Int J Hydrogen Energy 1989;14:839e44. _ [16] Ilbas ‚ M, Karyeyen S. A numerical study on combustion behaviours of hydrogen-enriched low calorific value coal gases. Int J Hydrogen Energy 2015;40:15218e26. _ Sandalcı T, Yu¨ksek L, Dalkılıc¸ AS, € z Y, Gu¨ler I, [17] Karago Wongwises S. Effects of hydrogen and methane addition on combustion characteristics, emissions, and performance of a CI engine. Int J Hydrogen Energy 2016;41:1313e25. [18] Kumar P, Mishra DP. Experimental investigation of laminar LPGeH2 jet diffusion flame. Int J Hydrogen Energy 2008;33:225e31. [19] Wu L, Kobayashi N, Li Z, Huang H, Li J. Emission and heat transfer characteristics of methaneehydrogen hybrid fuel laminar diffusion flame. Int J Hydrogen Energy 2015;40:9579e89. [20] Kumar P, Mishra DP. Effects of bluff-body shape on LPGeH2 jet diffusion flame. Int J Hydrogen Energy 2008;33:2578e85. [21] Zheng K, Yu M, Liang Y, Zheng L, Wen X. Large eddy simulation of premixed hydrogen/methane/air flame propagation in a closed duct. Int J Hydrogen Energy 2018;43:3871e84. [22] Liu J, Zhang X, Wang T, Hou X, Zhang J, Zheng S. Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methaneeair mixture. Int J Hydrogen Energy 2015;40:8475e83. [23] Li Z, Cheng X, Wei W, Qiu L, Wu H. Effects of hydrogen addition on laminar flame speeds of methane, ethane and propane: experimental and numerical analysis. Int J Hydrogen Energy 2017;42:24055e66. [24] Li Y, Bi M, Li B, Zhou Y, Gao W. Effects of hydrogen and initial pressure on flame characteristics and explosion pressure of methane/hydrogen fuels. Fuel 2018;233:269e82. [25] Afarin Y, Tabejamaat S. Effect of hydrogen on H2/CH4 flame structure of MILD combustion using the LES method. Int J Hydrogen Energy 2013;38:3447e58. [26] Bouras F, El Hadi Attia M, Khaldi F, Si-Ameur M. Control of methane flame properties by hydrogen fuel addition: application to power plant combustion chamber. Int J Hydrogen Energy 2017;42:8932e9. € Snelling DR, Sawchuk RA. Influence of hydrogen [27] Gu¨lder OL, addition to fuel on temperature field and soot formation in diffusion flames. Symposium (International) on Combustion 1996;26:2351e8. [28] Ezenwajiaku C, Talibi M, Doan NAK, Swaminathan N, Balachandran R. Study of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched methane diffusion flames. Int J Hydrogen Energy 2019;44:7642e55. [29] Miao J, Leung CW, Cheung CS, Huang ZH, Zhen HS. Effect of hydrogen addition on overall pollutant emissions of inverse diffusion flame. Energy 2016;104:284e94. [30] Burbano HJ, Amell AA, Garcı´a JM. Effects of hydrogen addition to methane on the flame structure and CO emissions in atmospheric burners. Int J Hydrogen Energy 2008;33:3410e5. [31] Tseng CJ. Effects of hydrogen addition on methane combustion in a porous medium burner. Int J Hydrogen Energy 2002;27:699e707. [32] Kumar MS, Ramesh A, Nagalingam B. Use of hydrogen to enhance the performance of a vegetable oil fuelled
[33]
[34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49] [50]
[51]
[52]
compression ignition engine. Int J Hydrogen Energy 2003;28:1143e54. Sun Z, Dally B, Nathan G, Alwahabi Z. Effects of hydrogen and nitrogen on soot volume fraction, primary particle diameter and temperature in laminar ethylene/air diffusion flames. Combust Flame 2017;175:270e82. € z S, Akansu SO. Numerical analysis of a Kahraman N, Tango gas turbine combustor fueled by hydrogen in comparison with jet-A fuel. Fuel 2018;217:66e77. Wang J, Huang Z, Tang C, Miao H, Wang X. Numerical study of the effect of hydrogen addition on methaneeair mixtures combustion. Int J Hydrogen Energy 2009;34:1084e96. Rortveit G, Hustad J. NO x formations in diluted CH 4/H 2 counterflow diffusion flames. Int J Energy a Clean Environ (IJECE) 2003;4. Naha S, Aggarwal SK. Fuel effects on NOx emissions in partially premixed flames. Combust Flame 2004;139:90e105. Mishra DP, Kumar P. Experimental investigation of laminar LPGeH2 jet diffusion flame with preheated reactants. Fuel 2008;87:3091e5. Miao J, Leung C, Cheung C. Effect of hydrogen percentage and air jet Reynolds number on fuel lean flame stability of LPGfired inverse diffusion flame with hydrogen enrichment. Int J Hydrogen Energy 2014;39:602e9. Miao J, Leung C, Cheung C, Huang Z, Jin W. Effect of H2 addition on OH distribution of LPG/Air circumferential inverse diffusion flame. Int J Hydrogen Energy 2016;41:9653e63. Tong Y, Yu S, Liu X, Klingmann J. Experimental study on dynamics of a confined low swirl partially premixed methane-hydrogen-air flame. Int J Hydrogen Energy 2017;42:27400e15. Kim HS, Arghode VK, Linck MB, Gupta AK. Hydrogen addition effects in a confined swirl-stabilized methane-air flame. Int J Hydrogen Energy 2009;34:1054e62. Khalil AEE, Gupta AK. Hydrogen addition effects on high intensity distributed combustion. Appl Energy 2013;104:71e8. Shanbhogue SJ, Sanusi YS, Taamallah S, Habib MA, Mokheimer EMA, Ghoniem AF. Flame macrostructures, combustion instability and extinction strain scaling in swirlstabilized premixed CH4/H2 combustion. Combust Flame 2016;163:494e507. Schefer RW, Wicksall DM, Agrawal AK. Combustion of hydrogen-enriched methane in a lean premixed swirlstabilized burner. In: Presented at the proceedings of the combustion institute; 2002. Kim HS, Arghode VK, Gupta AK. Flame characteristics of hydrogen-enriched methaneeair premixed swirling flames. Int J Hydrogen Energy 2009;34:1063e73. Emadi M, Karkow D, Salameh T, Gohil A, Ratner A. Flame structure changes resulting from hydrogen-enrichment and pressurization for low-swirl premixed methaneeair flames. Int J Hydrogen Energy 2012;37:10397e404. Kashir B, Tabejamaat S, Jalalatian N. A numerical study on combustion characteristics of blended methane-hydrogen bluff-body stabilized swirl diffusion flames. Int J Hydrogen Energy 2015;40:6243e58. Lilley DG. Swirl flows in combustion: a review. AIAA J 1977;15:1063e78. Brohez S, Delvosalle C, Marlair G. A two-thermocouples probe for radiation corrections of measured temperatures in compartment fires. Fire Saf J 2004;39:399e411. Mahesh S, Mishra D. Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner. Fuel 2008;87:2614e9. Choy YS, Zhen HS, Leung CW, Cheung CS, Leung RCK. Noise generation by open inverse diffusion flames. J Vib Control 2014;20:1671e81.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 3 1 6 e2 8 3 2 9
[53] Alex Francis K, Sreenivasan R, Raghavan V. Investigation of structures and reaction zones of methaneehydrogen laminar jet diffusion flames. Int J Hydrogen Energy 2011;36:11183e94. [54] Gupta AK, Lilley DG, Syred N. Swirl flows. Tunbridge wells. Kent, England: Abacus Press; 1984. p. 488. 1984. [55] Zhen H, Cheung C, Leung C, Li H. Thermal and heat transfer behaviors of an inverse diffusion flame with induced swirl. Fuel 2013;103:212e9. [56] Patel V, Shah R. Effect of swirl and number of swirler vanes on combustion characteristics of methane inverse diffusion flame. J Mech Sci Technol 2019;33:1947e58. [57] Rajpara P, Shah R, Banerjee J. Effect of hydrogen addition on combustion and emission characteristics of methane fuelled upward swirl can combustor. Int J Hydrogen Energy 2018;43:17505e19.
28329
[58] Turns SR. Understanding NOx formation in nonpremixed flames: experiments and modeling. Prog Energy Combust Sci 1995;21:361e85. [59] Zhen H, Leung C, Cheung C. A comparison of the thermal, emission and heat transfer characteristics of swirl-stabilized premixed and inverse diffusion flames. Energy Convers Manag 2011;52:1263e71. [60] Elbaz AM, Roberts WL. Flame structure of methane inverse diffusion flame. Exp Therm Fluid Sci 2014;56:23e32. [61] Candel S, Durox D, Ducruix S, Birbaud A-L, Noiray N, Schuller T. Flame dynamics and combustion noise: progress and challenges. Int J Aeroacoustics 2009;8:1e56. [62] Smith T, Kilham J. Noise generation by open turbulent flames. J Acoust Soc Am 1963;35:715e24.