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Laminar burning characteristics of premixed methane-dissociated methanol-diluent mixtures
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Laminar burning characteristics of premixed methane-dissociated methanol-diluent mixtures Lili Lu a, Yiqiang Pei a,*, Jing Qin a,b, Zinong Zuo a, He Xu b a b
State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China Internal Combustion Engine Research Institute, Tianjin University, Tianjin 300072, China
article info
abstract
Article history:
The influence of dissociated methanol (DM) and diluent (CO2 and N2) addition on methane
Received 5 December 2018
was investigated in a constant volume chamber under initial conditions of 3 bar and 343 K.
Received in revised form
CO was also added in separate proportions instead of DM under the same conditions to
8 February 2019
assess its effect. The laminar burning velocity, Markstein length and flame instability were
Accepted 9 February 2019
analyzed systematically under various equivalence ratios (0.8e1.4), dissociated methanol
Available online 21 March 2019
gas ratios (40 and 80%), CO ratios (40 and 80%) and dilution ratios (0e15%). Furthermore, the flame speed of the fuel mixture and the production rate of key reactants were analyzed
Keywords:
based on the calculation results of the Aramco Mech 2.0 mechanism to determine the
Dissociated methanol
influence principles of dilution. The results show that dissociated methanol gas increases
Methane
the flame speed of the mixtures and promotes instability of the flame, and H2 is the
Diluent
dominant component in enhancing the combustion process. Within the dilution ratio
Carbon monoxide
range of this study, the diluents decrease the laminar burning velocity of the mixtures
Laminar burning velocity
since the addition of diluent gas decreases the concentration of key reactants, such as H
Flame stability
and OH. The addition of diluent gas can inhibit the flame instability, but the effect is not clear. Compared with N2, the effect of CO2 is more significant. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the fossil fuel shortages and stringent emission regulations, research on alternative fuels has gradually become a hot topic [1e4]. Within all kinds of alternative fuels, natural gas [5] (mainly composed of methane, CH4) has been extensively studied for its advantages. First, the reserves of natural gas are much larger than those of crude oil. Second, due to the high knock resistance of CH4, it can be used as engine fuel under higher working compression ratios, which leads to higher thermal efficiency. Moreover, CH4 does not contain sulfur, and its combustion emits less CO2 than gasoline due to
the high H/C ratio of CH4 [6]. However, CH4 burns slowly and requires a large amount of ignition energy. The latter will reduce the thermal efficiency of the engine and promote postcombustion phenomena. One effective method to improve the burning velocity of CH4 is to add hydrogen (H2). H2 has a high burning velocity that is approximately six times larger than that of CH4 under stoichiometric conditions; consequently, the lean burn limit can be extended, and engine emissions will be decreased by mixing CH4 and H2 [7,8]. Nevertheless, the high production, storage and transportation cost of H2 impede its wide application in vehicles.
* Corresponding author. E-mail address: [email protected] (Y. Pei). https://doi.org/10.1016/j.ijhydene.2019.02.080 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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As fuel reforming techniques developed, synthesis gas (syngas) has attracted significant interest as a promising alternative and environmentally clean fuel for various power devices, and it is a feasible solution to improve the engine performance. The composition of syngas depends on the fuel source and processing technique. However, H2 and carbon monoxide (CO) are always the main components. Syngas can be derived from numerous sources, such as coal, natural gas, coke, and heavy oil. Methanol is also an important source of syngas by dissociation at relatively low reaction temperatures, so the exhaust heat recovery from engines can be used. China is rich in coal reserves, and more than 90% of the methanol produced in China comes from coal [9]. Through the dissociative reaction, methanol vapor is reformed into dissociated methanol (DM), and the main reaction is shown in Eq. (1), [10]. DM is mainly composed of H2 and CO. However, the components of DM vary with different dissociative reaction temperatures and catalyst types in practical applications, and a small amount of substances such as methanol steam will exist. To accurately control the equivalence ratio and ensure the repeatability of the tests, DM is simulated by H2 and CO with a volumetric ratio equal to 2:1 in this study, as previously published studies have suggested [11,12]. This is actually a kind of syngas. catalyzer
CH3 OH!2H2 þ CO
(1)
In addition to the application of cleaner energy, many other measures have been taken to reduce engine emissions. Mixture dilution technology is one of the most effective ways to reduce NOx formation during engine combustion. Exhaust gas recirculation (EGR) is the most popular dilution method used in internal combustion engines [9], which introduces combustion exhaust gas from the tail pipe into the cylinders through the intake ports and valves. Lower levels of NOx can be achieved with decreasing combustion temperature and oxygen concentration by EGR [13]. Combined with turbocharging technology, engines can operate with relatively higher compression ratios and increase the thermal efficiency. A decrease in the knocking tendency is another advantage of the use of EGR. Carbon dioxide (CO2) is the main gas for EGR, and nitrogen (N2) is a common inert gas. In this paper, CO2 and N2 are used as diluents to explore how they affect the premixed laminar flame combustion characteristics when they are added to the fuel. During the last few decades, numerous studies have reported the combustion characteristics of CH4, H2 and syngas from experimental investigations [11,14e17]. Extensive attention has been paid to the basic combustion characteristics of methane fuel, including the laminar burning rate and flame stability. However, there are few studies on the characteristics of methane combustion under dilution and DM addition combustion conditions. This study aims to investigate the laminar burning characteristics of CH4-DM-diluent blended fuel through the outwardly propagating spherical flame method. Moreover, considering that the reaction of COeO2 in a dry atmosphere has a high activation energy, the combustion characteristics of CO are different from those of H2 and CH4. As an important component of DM, CO is not only the initial fuel in the
combustion of CH4-DM-diluent-air but also the participant of many intermediate reactions [18,19]. Therefore, it is of great significance to explore the effects of CO on the combustion separately. This paper not only comprehensively studies the effects of DM on the premixed laminar combustion characteristics of CH4 but also analyzes the effects of CO addition alone. The laminar burning velocity and flame stability are fundamental parameters for studying the propagation characteristics of laminar flames. Laminar premixed combustion is the basis for the study of combustion. For CH4eDM-diluent mixed fuel, it is necessary to carry out a systematic and indepth study of its laminar burning velocity and intrinsic flame instability. The data gained from this work can contribute to the optimization of the combustion process and alternative fuel utilization, which is urgently required by engine researchers.
Experimental procedure Experimental setup Fig. 1 shows the experimental setup. The experimental device consisted of a spherical constant volume chamber (CVC), high-speed schlieren shooting system, temperature control system, trigger control system and ignition system. The inner diameter of the CVC used in this test was 350 mm, and the volume was 22.4 L. Two quartz windows with an efficient diameter of 100 mm and a thickness of 40 mm were mounted on two sides of the chamber for optical access. Two extended spark plugs were centrally installed in the CVC and controlled by a capacitor discharge ignition unit. The ignition pulse width in the experiment was 2 ms. The CVC was wrapped by electrical heating tape and by thermal insulation material to decrease the heat loss. The temperature controlling unit was equipped to maintain the test temperature within the range of ±3 K through a proportional-integral-derivative (PID) controller. The triggering control unit (EcuTek Calibration V2) was used to control the ignition and to start the camera. The flame propagation process was recorded by a high-speed digital camera (Phantom V7.3) with a frequency of 10000 Hz; the resolution was 512 512 pixels. After the combustion, the process of exhausting, scavenging and vacuuming was achieved through the intake and exhaust system. Table 1 lists the experimental conditions. The purity of all the gases used in this study was 99.9%. In the experiment, the parameter a is defined as the volume fraction of the DM in the CH4-DM mixed fuel, and the parameter is defined as the volume fraction of the CO in the CH4eCO mixed fuel. The dilution ratio 4r is defined as the volumetric fraction of the diluent gas addition in the total blends. That is: a¼
VH2 þ VCO VH2 þ VCO þ VCH4
(2)
b¼
VCO VCO þ VCH4
(3)
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Fig. 1 e Experimental setup.
fr ¼
VDilution Vfuel þ VDilution þ VAir
(4)
Data processing Laminar burning velocity and markstein length The laminar burning velocity and Markstein length are fundamental parameters for the study of laminar combustion [20]. In this paper, the schlieren photographs were processed by the MATLAB program to calculate the radius (R) of the outwardly propagating spherical flame. Fig. 2 shows the diagram of the flame image processing flow of the MATLAB code. To avoid the possible distortion effect caused by the spark ignition and pressure rise, the flame photos during the range of 7e24 mm were selected [8]. Markstein length expresses the influence of the flame speed on the flame stretch rate. It is usually defined by the negative value of the slope of the stretched flame velocity (Sn)eflame stretch rate curve [21]. However, this method has
been proved not to be the most accurate. Based on the suggestion of Chen [22], when the mixed fuel has a value Lewis number Le < 1 or close to 1, the nonlinear calculation method proposed by Kelley and Law [23] is used: Sn Sn 2Lb ln ¼ Sl Sl R
(5)
SL is the unstretched flame propagation speed of flame. In the case of Le > 1, the following nonlinear formula is used: Sn ¼ Sl Sl Lb :
2 R
(6)
The laminar burning velocity (UL) is calculated by the following formula:
Table 1 e Experimental conditions. Parameters Equivalence ratio (4) DM fraction (a) CO fraction (b) Diluent gas Dilution ratio (4r ) Initial temperature (K) Initial pressure (MPa)
Approximately 0.8e1.4 (interval 0.1） 0, 40, 80% 0, 40, 80% CO2, N2 0, 5, 10, 15% 343 0.3
Fig. 2 e Diagram of the flame image processing flow of the MATLAB program.
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r UL ¼ Sl u rb
(7)
where rb is the density of the gas burned and ru is the density of the unburned gas, calculated by the equilibrium module of the CHEMKIN software package. This method has been widely used and validated from the calculation methodology adopted by Salzano et al. [24].
Flame instability The premixed laminar flow flame instability is divided into the buoyancy instability, diffusive-thermal instability and hydrodynamic instability. The buoyancy instability can be neglected when the laminar flame combustion speed is high enough [25]. Therefore, only the diffusive-thermal instability and hydrodynamic instability are considered in this study. For a spherical diffusion flame, subject to instability, the flame front will appear as a cellular structure during the propagation process [30]. The hydrodynamic instability is the inherent instability of the flame. When the flame thickness decreases, the hydrodynamic instability increases. In this study, the flame thickness (Lf) is calculated using the formula proposed by Law [33]: Lf ¼
l=CP ru UL
(8)
where l is the unburned gas thermal conductivity and Cp is the constant pressure specific heat capacity. The thermal expansion ratio is the ratio of the density of the unburned and burned gas mixtures. When the thermal expansion ratio (s) increases, the hydrodynamic instability of combustion is enhanced. It is calculated as follows: s ¼ ru =rb
(9)
The diffusive-thermal instability is usually characterized by the Lewis number, Le, as shown in Equation (9). Le is defined as the ratio of the heat diffusivity and mass diffusivity: Le ¼
DT Dm
(10)
The flame thermal diffusion tends to be stable when Le > 1. As Le < 1, the thermal dispersion of the flame tends to be unstable. In this study, the volume-weighted method was used to calculate the effective Lewis number of the mixtures [34]: Le ¼
n X
xi Lei
(11)
i¼1
where xi is the volume fraction corresponding to the ith substance of the mixture composition and Lei is the corresponding Lewis number of the ith substance.
Numerical simulation process For the premixed laminar combustion process, the laminar burning velocity of the mixture is limited by the rate of the reaction. The chemical reaction kinetics are an effective way to analyze the combustion process [25e27]. CHEMKIN software was used to study the effects of diluent addition on the primitive reaction rate of the mixed fuel in this study.
Therefore, the impact mechanism of the addition of diluted gas on the combustion speed of mixed fuel laminar flames can be explored. When performing chemical reaction kinetics analysis, the choice of the combustion reaction mechanism is particularly important. At present, GRI-Mech 3.0, USC-Mech 2.0 and AramcoMech 2.0 are the most widely used chemical mechanisms of methane combustion around the world [28e30]. Researchers have also improved and verified more complete alkane mechanisms based on these widely used mechanisms [31,32]. The AramcoMech 2.0 mechanism was developed in 2016 by NUI Galway. This mechanism has been validated over a wide range of initial conditions and experimental devices, including flow reactors, shock tubes, jetstirred reactors, and flame studies. It has now become the base model for methane combustion and has been systematically modified and validated for various experimental purposes. In this paper, the flame speed and the rate of production (ROP) of CH4, H2, CO were calculated by the CHEMKIN software premixed laminar-flame speed calculation model and the AramcoMech 2.0 mechanism.
Experimental uncertainty analysis and system validation The factors that affected the accuracy of the test results were derived in part from the test equipment and the extraction method of the unstretched flame propagation speed Sl. The maximum error generated by the program when identifying the radius in the schlieren image was 1 pixel, and the maximum error of the corresponding actual radius was 0.25 mm, while the minimum time interval corresponding to the selected point in the calculation was 0.2 ms, that is, the corresponding maximum Sn error was 1.25 m/s. Thus, the estimation indirectly led to a maximum UL error of 0.25 cm/s due to the pixel limitation during the radius recognition, which was the most extreme error case. It has been documented [35] that Sn is more susceptible to stretching (especially at lower initial pressures) in smaller volume combustion bombs and is prone to computational errors. Moreover, the cylindrical combustion bomb can increase the flame velocity in the axial direction compared to the spherical combustion bomb and can decrease the radial direction. The bulky spherical shells used in this experiment reduced these effects. The relatively large spherical constant-volume combustion chamber used in this study reduced these effects to improve the accuracy of the test results. In this paper, two nonlinear methods were selected to calculate Sl and Lb. Combined with the initial work, it was found that the nonlinear method did not improve UL significantly (less than 3 cm/s) compared with the linear method, while Lb changed more than 100% under certain very aggressive conditions (e.g., 4 ¼ 1.8). Other factors, such as the influence of the ignition energy, are described in the previous section. What also needs to be considered is the effect of flame radiation, which can be evaluated by the following formula [36]: UL;RCFS UL;EXP ¼ 0:82UL;EXP
1:14 0:3 UL;EXP Tu Pu S0 T0 P0
(12)
where UL;RCFS and are the radiation-corrected laminar burning velocity and experimental laminar burning velocity,
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respectively. Under this test condition, the influence of the radiation on UL was not more than 0.08 cm/s and could be neglected. Considering the actual operation of the partial pressure test and other error effects, each condition test was repeated at least twice to ensure that the calculated UL error was less than 5%. Fig. 3 shows a comparison of the laminar burning velocities obtained in the present study with previous data from the literature [36]. The initial pressure and initial temperature were 0.3 MPa, 353 K, respectively. The experiment was conducted three times to validate the measured experimental system of this work. As seen from the figure, the experimental results were in good agreement with the data in a previous paper. Therefore, the laminar burning velocity measured in this work could be considered reasonable and correct.
Comparing the data of a ¼ 40% with those of a ¼ 80%, the UL increased with a larger DM addition ratio. The peak value of the laminar burning velocity of pure CH4 in air was at a ratio value of 4 ¼ 1.1 [37], the peak of the DM laminar burning velocity was at 4 ¼ 1.8 [11], and the peak value of the laminar burning velocity of pure CO in air was between the equivalence ratio 4 ¼ 2 and 3 [38]. When a ¼ 40%, the peak laminar burning velocity appeared at an equivalence ratio 4 of approximately 1.1, while with a larger value of a, the equivalence ratio of the peak laminar burning velocity increased. When a ¼ 80%, the equivalence ratio reached a value of 1.3. Theoretically, the peak laminar burning velocity of DM and
Results and discussion The development of a laminar flame Fig. 4 shows the schlieren images of the laminar flames of seven kinds of fuel. After ignition, the premixed laminar flow flame front had a smooth spherical surface that propagated from the burned area to the unburned area. Comparing the flame images, the speed of flame propagation was increased with the addition of DM, and when the proportion of DM was 80%, the flame propagated the most rapidly. The addition of two kinds of diluent gases, CO2 and N2, could reduce the propagation speed of the flame, and the dilution effects of the two kinds of dilution gas were also different; the dilution effects of CO2 were more notable than those of N2. Compared with the pictures under different equivalence ratios, the flame propagation was the most rapid at the stoichiometric equivalence ratio.
Laminar burning velocity and the markstein length Fig. 5 illustrates the relationship between the laminar burning velocity UL and equivalence ratio 4 of the mixed gas at different values of a or b, and a dilution ratio 4r of 10%.
Fig. 3 e Comparison between the results of the present work and published literature data of CH4-air mixtures.
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Fig. 4 e Flame spread schlieren images.
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Fig. 5 e Laminar burning velocity versus the equivalence ratio.
CH4 mixed fuel should be between 4 ¼ 1.1 and 1.8, but this value is relatively lower due to the addition of diluent gas. Comparing graphs a and b under the same test conditions, the addition of DM caused a greater increase in the laminar burning velocity of the premixed flame than the addition of CO alone. The H2 component in DM played a leading role in the change in the flame morphology due to the extremely strong diffusion capacity and fast burning rate of H2 [39]. Based on the experimental results, the relationship between the laminar burning velocity and equivalence ratio for the CH4-
Fig. 6 e Laminar burning velocity versus the dilution ratio. DM-air-dilution mixtures and CH4eCO-air-dilution mixtures at 343 K and 0.3 MPa under different alternative gas fractions was fitted through a three-order polynomial function as given by Eq. (13) [11]. The fitting curves are shown in Fig. 5. Table 2 provides the coefficients of the fitting curve. The laminar burning velocity can be predicted using the curves given a specific initial condition. UL ¼ A þ B4 þ C42 þ D43
(13)
Fig. 6 depicts the relationship between the laminar burning velocity UL and the dilution ratio 4r of the mixed gas at
Table 2 e Coefficients of the third-order polynomials fitted to the data. Tu ¼ 343 K, Pu ¼ 0.3 MPa 4r ¼ 10% A B C D
a ¼ 40%
a ¼ 40%
a ¼ 80%
a ¼ 80%
b ¼ 40%
b ¼ 40%
b ¼ 80%
b ¼ 80%
CO2 2.928 7.721 6.07 1.501
N2 2.206 5.518 3.513 0.546
CO2 1.387 4.833 6.199 2.322
N2 0.191 2.061 4.580 2.001
CO2 3.024 8.250 6.772 1.756
N2 0.628 2.422 3.595 1.489
CO2 0.996 2.750 1.898 0.429
N2 2.787 7.058 5.712 1.451
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Fig. 7 e The rate of production under no dilution.
different values of a or b and an equivalence ratio 4 ¼ 1. As shown in the figure, the laminar burning velocity of the mixed gas decreased with the addition of diluent gas. It can be inferred that the addition of diluent gas reduced the volume fraction of the reactants, thereby reducing the chances of the reactants and other important groups to come into contact with oxygen, which reduced the amount of heat released by the combustion. In addition, the diluent gas absorbed a portion of the reaction heat and reduced the combustion temperature. Moreover, the addition of diluent gas would
cause a certain offset in the flame front position. These factors led to a decrease in UL. When CO2 was used as the diluent gas, the reduction effect on UL was more notable than that of N2. Because CO2 has a larger heat capacity than N2, the reduction in the combustion temperature became more pronounced. In addition, the presence of CO2 produced thermal radiation and made the shift in the flame front position more pronounced [40]. Therefore, when CO2 was used as diluent, the reduction in the laminar burning velocity of the CH4-DM-air mixed fuel was more complicated than that of N2. Comparing graphs a
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Fig. 8 e The rate of production under a 15% CO2 dilution ratio.
and b, when CO was added by itself, the trend of the changes in UL with the dilution ratio was the same as the trend when DM was added, but the overall UL value was decreased. The results of the laminar burning velocity calculation by the AramcoMech 2.0 mechanism are also shown in Fig. 6, which were in good agreement with the experimental results. The overall deviation between the calculated and experimental values was less than 10%. Therefore, this model could
be used to calculate the rate of production (ROP) of the key reactants. Considering that the influence of CO2 dilution on the laminar burning velocity of the CH4-DM-air mixture is more complex, mixtures under different CO2 dilution ratios were selected for the chemical reaction kinetics analysis in this paper. Figs. 7 and 8 show plots of the relationship between the ROP of the three reactants, the content of CH4, H2, and CO and
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Table 3 e The main reaction selected by ROP analysis. Reaction number 43 44 45 46 49 158 168 191 203 210 97 2 3 5 28 60 115 155
Reaction formula
Reaction number
Reaction formula
CH3þH<¼>CH4 CH4þH<¼>CH3þH2 CH4þO<¼>CH3þOH CH4þOH<¼>CH3þH2O CH3þHO2<¼>CH4þO2 CH2O þ CH3<¼>HCO þ CH4 HCO þ CH3 <¼>CH4þCO HOCHO þ CH3¼>CH4þCO þ OH C2H6þCH3<¼>C2H5þCH4 C2H5þCH3<¼>CH4þC2H4 CH3þOH<¼>HCOH þ H2 H2þO<¼>H þ OH H2þOH<¼>H þ H20 O2þH<¼>O þ OH HO2þH<¼> H2þO2 CH2þH2<¼>CH3þH CH3OH þ H<¼>CH2OH þ H2 CH2O þ H<¼>HCO þ H2
387 36 166 56 63 163 164 187 198 390 66 143 257 27 68 67 165 167
HCCO þ OH¼>H2þCO þ CO CO þ OH<¼>CO2þH HCO þ H <¼>CO þ H2 CH2þO2<¼>H þ OH þ CO CH2þCO2<¼>CH2O þ CO HCO þ M<¼>H þ CO þ M HCO þ O2<¼>CO þ HO2 HOCHO þ H¼>h2þCO þ OH C2H6þH<¼>C2H5þH2 HCCO þ H<¼>CH2þCO CH2þO2<¼>CO2þH þ H CH3O þ CH3<¼>CH2O þ CH4 CH þ CH4<¼>C2H4þH HO2þH<¼>OH þ OH CH2þH<¼>CH þ H2 CH2þO<¼> CO þ H þ H HCO þ O<¼>CO þ OH HCO þ OH <¼>CO þ H2O
the distance from the burner. The CO2 dilution ratios of Figs. 7 and 8 were 0 and 15%, respectively. When the substance was generated, the rate of change was positive, and it was opposite when the substance was consumed. For the sake of clarity, Table 3 lists the reactions involved in the ROP analysis. Fig. 7 shows that the main elementary reactions under no dilution were R46 consuming CH4 (CH4þOH<¼>CH3þH2O), R44(CH4þH<¼>CH3þH2). The main elementary reaction to generate CH4 is R43(CH3þH<¼>CH4). Therefore, the OH and H a groups had a great influence on the combustion reaction of CH4. The reaction rate of CH4 consumption increased with the addition of DM. The main elementary reactions that consumed H2 were R3 (OH þ H2<¼>H þ H2O) and R2 (O þ H2<¼>H þ OH). The main were elementary reactions that generated H2 R44(CH4þH<¼>CH3þH2) and R155(CH2O þ H<¼>HCO þ H2). Therefore, the OH and H groups also had a great influence on the combustion reaction of H2. Both the reaction rates of H2 generation and consumption increase with the addition of DM, and the consumption reaction rate increases more. The main element reaction that consumed CO was R36 (CO þ OH<¼>CO2þH). The OH group had a significant influence on the combustion reaction of CO. It can be found that the consumption of CO is mainly involved in the reaction with OH, and the main reaction of OH radicals generation are R2(H2þO<¼>H þ OH), R5(O2þH<¼>O þ OH) and R27(HO2þH<¼>OH þ OH). All the three reactions require the participation of H or H2. The main reaction of H radicals generation is R3(OH þ H2<¼>H þ H2O), and this reaction also requires the participation of H2. Because of the presence of H2, the addition of DM has a very important role in promoting the consumption reaction of CO. The laminar combustion became more rapid as the content of active radicals increased. As a result, adding DM in CH4 would increase the laminar burning velocity of the mixed gas, and the effect was more noteworthy than that when CO alone was added. H2 has a great effect on promoting combustion reaction. Comparing Figs. 7 and 8, it was found that when the CO2 dilution gas was added, the reaction rates of CH4, H2 and CO
Fig. 9 e Markstein length versus the equivalence ratio.
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were significantly reduced, resulting in a decrease in the burning velocity, which was consistent with the experimental results. The addition of the diluent gas reduced the concentration of each reactant important groups OH and H, so that the consumption rate of CH4, H2, CO and important reactive groups was reduced. It is worth noting that some dilution gases are not just inert gases that do not participate in the reaction. In the elementary reaction R36 (CO þ OH<¼>CO2þH), as a reactant, the diluent gas CO2 participated in the chemical reaction. This is the main elementary reaction that consumes CO and OH. When CO2 was added, the rate of the reverse reaction was accelerated, and the oxidation of CO was inhibited, thereby reducing the burning velocity of the fuel mixture. The Markstein length Lb characterizes the sensitivity of the premixed laminar flame to stretching, reflecting the stability of the flame. When Lb < 0, the flame will spread more rapidly with increasing flame tensile rate. The surface of the flame is prone to produce a bulge, and the spreading velocity of the bulging part will increase, which will lead to instability of the flame. When Lb > 0, the flame instability can be suppressed. Fig. 9 shows the Lb variation of the mixed fuel under different equivalence ratios 4. The experimental data published by Zhang et al. [11] for COeH2-air mixtures (CO:H2 ¼ 33:67) at 358 K and 0.1 MPa is also plotted in Fig. 9. The trend in the Markstein length for the COeH2-air mixtures obtained in this study showed a reasonable agreement with the previous research. From this figure, Lb was positive when 4r was 10%, which means that the flame was stable in the range of the research conditions. Lb increased monotonically with increasing equivalence ratio. Comparing Lb of the mixed fuel under different values of a or, the mixtures with a or b ¼ 80% had less variation in Lb with variation in the equivalence ratio. This implies that the mixtures with more DM or CO were less sensitive to the flame stretch effects. Under the same equivalence ratio, Lb decreased as more DM or CO was added. That is, the addition of DM made flame tend to be unstable, and this trend was clear, specifically in the case of 4 > 1. Fig. 10 shows the relation between Lb of the premixed laminar flame and the dilution ratio. It can be seen from the figure that for the two kinds of diluent gases, CO2 and N2, Lb increased with the increase in 4r when 4 ¼ 1, but the increment was small. This indicates that the addition of diluent gas to methane fuel had little effect on the Markstein length of the flame.
stretching effect of the flame, it appeared almost simultaneously on the flame front, and the corresponding flame radius at this time is called the critical radius Rcr. In this paper, the critical state is determined by the flame stretch rate and flame propagation speed. The pressure rising inside the chamber has influences on the flame stability characteristics [20]. In this experiment, it is observed that the pressure has almost no fluctuation during the very short period of laminar combustion. The pressure inside the chamber is constant for the determination of Rcr. We can see from the flame spread schlieren images of Fig. 4 that the flame instability tended to be promoted by an increasing DM fraction. For a given equivalence ratio, only a few wrinkles appeared at the flame surface when a ¼ 40%, and the wrinkles developed into clear cellular structures when a ¼ 80% at the same time. The flame tended to be more unstable, and the strong cellular structures decreased and became denser with the addition of DM. The equivalence ratio
Flame instability The diffusive-thermal instability is the combination of the non-equidiffusive effect and the pure curvature instability effect, and the diffusive-thermal instability can be evaluated by the Lewis number [27]. As an intrinsic instability of the flame, the hydrodynamic instability is one of the most important factors affecting the flame instability, which is caused by a density jump across the flame front and can be evaluated by the laminar flame thickness and thermodynamic expansion ratio [8,27]. It can be seen from the flame pattern images that the smooth surface of the flame began to exhibit folds, followed by distinct cellular structures with the flame development. When the cellular structure broke free from the
Fig. 10 e Markstein length versus the dilution ratio.
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had less effect on the flame instability and cellular structure than the DM addition ratio. The flame tended to be more stable with increasing equivalence ratios. From the schlieren images, the effect of dilution gas on flame stability was not significant within the scope of this study. This is consistent with the conclusion in Fig. 10. Fig. 11 shows the Lewis number, Le, of the premixed laminar flame under different equivalent ratios, diluent gas types and dilution ratios. The addition ratio a of DM in Figures (a) and (b) is 40 and 80%, respectively. From this figure, Le is less than 1 when DM was added to CH4, which indicates that the thermal diffusion instability of the flame was relatively strong at this time. Le increased with a larger equivalence ratio, which is consistent with the schlieren images in Fig. 4. Le decreased with the addition of diluent gases. This trend was more notable under the condition of N2 dilution. The addition of diluents reduced the thermal diffusion rate of the mixed fuel, resulting in a reduction in Le [21]. However, Le had a low sensitivity to the dilution gas addition. Comparing Figures (a) and (b), it can be found that Le decreased with increasing DM fraction for a given equivalence ratio, and the decreasing Le promoted the thermal diffusion instability.
Figs. 12 and 13 plot the flame thickness, Lf and the thermodynamic expansion ratio s of the mixture under different equivalent ratios, and the addition ratios of DM in Figures (a) and (b) were 40 and 80%, respectively. It can be seen from the graph that Lf tended to decrease followed by an increase as the equivalent ratio, 4, was increased. Lf reached a minimum near the stoichiometric equivalence ratio. In contrast, the tendency of the thermodynamic expansion ratio, s, was to increase followed by a decrease with the increase in 4. The thermodynamic expansion ratio reached a maximum near the stoichiometric equivalence ratio, which indicates that the hydrodynamic instability was the most significant near the stoichiometric equivalence ratio. The flame thickness, Lf, increased with increasing 4r, while the thermodynamic expansion ratio, s, showed the opposite trend. The effect was more noteworthy under CO2 dilution. This result indicates that the hydrodynamic instability of the premixed laminar flame was suppressed with the addition of the diluent gas. Comparing Figures (a) and (b), it can be found that Lf decreased with an increasing DM fraction for a given equivalence ratio,
Fig. 11 e Lewis number of the mixtures.
Fig. 12 e Flame thickness of the mixture.
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while the thermal expansion ratio was insensitive to the changes in the DM addition fraction. Overall, the addition of DM promoted the hydrodynamic instability of the flame. Fig. 14 shows the critical radius of the flame at different equivalence ratios, dilution gases and dilution ratios. The DM addition ratio a is 80%, and at that ratio value a flame critical radius can be obtained under more operating conditions in the observation window (when a is 40%, the critical radius in most conditions exceeded the observation window scope of this study). As shown in this figure, Rcr increased with increasing equivalent ratio. The addition of diluent gases had an increasing effect on Rcr, but the effect was not significant. Compared with N2, the effect of adding CO2 on Rcr was more noteworthy. Meanwhile, it can be seen from this figure that when CO was added alone, the critical radius of the flame exceeded 40 mm under most test conditions, which indicates that CO had less effect on the instability of the cellular structure of the flame. The conclusion shown in Fig. 14 is in accordance with the results from the schlieren images.
Fig. 14 e Flame critical radius of the mixture. The Lewis number of the premixed laminar flame of the CH4-DM-air mixture decreased as the dilution ratio increased, indicating that the thermal diffusion instability of the mixture increased with the addition of diluent gas. As the dilution ratio increased, the thermodynamic expansion ratio decreased and the flame thickness increased, indicating that the hydrodynamic instability of the mixture was suppressed. Overall, the mixture flame tended to be more unstable with the addition of DM, while the addition of diluent gas promoted the stability of the flame. This verifies the conclusion that hydrodynamic instability was the dominant factor of flame instability when the flame had developed to a certain extent, and the thermodynamic instability had the leading role only within a small flame radius after the ignition [41].
Conclusion Measurement of the laminar burning velocity and flame stability analysis were conducted using the outwardly spherical laminar premixed flame of CH4-DMeair-dilution mixtures. The laminar burning velocity, Markstein length and flame instability were analyzed at different equivalence ratios, DM addition ratios and dilution levels. The main conclusions are summarized as follows.
Fig. 13 e Thermodynamic expansion ratio of the mixture.
(1) The peak laminar burning velocity value of the CH4-DM mixed fuel is reached close to an equivalence ratio of 1.1. The addition of DM increases the laminar burning velocity of CH4, and with the increase in the addition proportion, the peak value of the laminar burning velocity shifted to the fuel rich area. The effect of adding CO alone is far less pronounced than that of adding DM. The flame propagation speed and laminar burning velocity decrease with an increase in the dilution ratio. (2) The Markstein length of the CH4-DM mixed fuel flame increases monotonically with the increase in the equivalence ratio. (3) Within the range of the dilution ratio in this study (approximately 0e15%), the addition of diluent gas
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 ) 1 1 0 9 7 e1 1 1 1 0
decreases the mole fraction of the reactants and certain important groups. The diluent gas, CO2, also participates in intermediate reactions as a reactant, leading to a decrease in the mixture laminar burning velocity. (4) The addition of diluent gas has no significant effect on the flame stability. (5) The influence of CO2 dilution on the burning velocity of the mixture and stability of flame is more notable than that of N2 dilution. (6) H2 is the important component of DM which leads to a high burning velocity and flame stability.
[12]
[13]
[14]
[15]
Acknowledgments [16]
This work was supported by the National Natural Science Foundation of China (50806051). The experimental facilities were supported by the State Key Laboratory of Engines, Tianjin University. The authors would like to express sincere thanks to Zhaoxu Chen, Xiang Li, Yi Liu for their assistance.
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Available online at www.sciencedirect.com
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Laminar burning characteristics of premixed methane-dissociated methanol-diluent mixtures Lili Lu a, Yiqiang Pei a,*, Jing Qin a,b, Zinong Zuo a, He Xu b a b
State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China Internal Combustion Engine Research Institute, Tianjin University, Tianjin 300072, China
article info
abstract
Article history:
The influence of dissociated methanol (DM) and diluent (CO2 and N2) addition on methane
Received 5 December 2018
was investigated in a constant volume chamber under initial conditions of 3 bar and 343 K.
Received in revised form
CO was also added in separate proportions instead of DM under the same conditions to
8 February 2019
assess its effect. The laminar burning velocity, Markstein length and flame instability were
Accepted 9 February 2019
analyzed systematically under various equivalence ratios (0.8e1.4), dissociated methanol
Available online 21 March 2019
gas ratios (40 and 80%), CO ratios (40 and 80%) and dilution ratios (0e15%). Furthermore, the flame speed of the fuel mixture and the production rate of key reactants were analyzed
Keywords:
based on the calculation results of the Aramco Mech 2.0 mechanism to determine the
Dissociated methanol
influence principles of dilution. The results show that dissociated methanol gas increases
Methane
the flame speed of the mixtures and promotes instability of the flame, and H2 is the
Diluent
dominant component in enhancing the combustion process. Within the dilution ratio
Carbon monoxide
range of this study, the diluents decrease the laminar burning velocity of the mixtures
Laminar burning velocity
since the addition of diluent gas decreases the concentration of key reactants, such as H
Flame stability
and OH. The addition of diluent gas can inhibit the flame instability, but the effect is not clear. Compared with N2, the effect of CO2 is more significant. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the fossil fuel shortages and stringent emission regulations, research on alternative fuels has gradually become a hot topic [1e4]. Within all kinds of alternative fuels, natural gas [5] (mainly composed of methane, CH4) has been extensively studied for its advantages. First, the reserves of natural gas are much larger than those of crude oil. Second, due to the high knock resistance of CH4, it can be used as engine fuel under higher working compression ratios, which leads to higher thermal efficiency. Moreover, CH4 does not contain sulfur, and its combustion emits less CO2 than gasoline due to
the high H/C ratio of CH4 [6]. However, CH4 burns slowly and requires a large amount of ignition energy. The latter will reduce the thermal efficiency of the engine and promote postcombustion phenomena. One effective method to improve the burning velocity of CH4 is to add hydrogen (H2). H2 has a high burning velocity that is approximately six times larger than that of CH4 under stoichiometric conditions; consequently, the lean burn limit can be extended, and engine emissions will be decreased by mixing CH4 and H2 [7,8]. Nevertheless, the high production, storage and transportation cost of H2 impede its wide application in vehicles.
* Corresponding author. E-mail address: [email protected] (Y. Pei). https://doi.org/10.1016/j.ijhydene.2019.02.080 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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As fuel reforming techniques developed, synthesis gas (syngas) has attracted significant interest as a promising alternative and environmentally clean fuel for various power devices, and it is a feasible solution to improve the engine performance. The composition of syngas depends on the fuel source and processing technique. However, H2 and carbon monoxide (CO) are always the main components. Syngas can be derived from numerous sources, such as coal, natural gas, coke, and heavy oil. Methanol is also an important source of syngas by dissociation at relatively low reaction temperatures, so the exhaust heat recovery from engines can be used. China is rich in coal reserves, and more than 90% of the methanol produced in China comes from coal [9]. Through the dissociative reaction, methanol vapor is reformed into dissociated methanol (DM), and the main reaction is shown in Eq. (1), [10]. DM is mainly composed of H2 and CO. However, the components of DM vary with different dissociative reaction temperatures and catalyst types in practical applications, and a small amount of substances such as methanol steam will exist. To accurately control the equivalence ratio and ensure the repeatability of the tests, DM is simulated by H2 and CO with a volumetric ratio equal to 2:1 in this study, as previously published studies have suggested [11,12]. This is actually a kind of syngas. catalyzer
CH3 OH!2H2 þ CO
(1)
In addition to the application of cleaner energy, many other measures have been taken to reduce engine emissions. Mixture dilution technology is one of the most effective ways to reduce NOx formation during engine combustion. Exhaust gas recirculation (EGR) is the most popular dilution method used in internal combustion engines [9], which introduces combustion exhaust gas from the tail pipe into the cylinders through the intake ports and valves. Lower levels of NOx can be achieved with decreasing combustion temperature and oxygen concentration by EGR [13]. Combined with turbocharging technology, engines can operate with relatively higher compression ratios and increase the thermal efficiency. A decrease in the knocking tendency is another advantage of the use of EGR. Carbon dioxide (CO2) is the main gas for EGR, and nitrogen (N2) is a common inert gas. In this paper, CO2 and N2 are used as diluents to explore how they affect the premixed laminar flame combustion characteristics when they are added to the fuel. During the last few decades, numerous studies have reported the combustion characteristics of CH4, H2 and syngas from experimental investigations [11,14e17]. Extensive attention has been paid to the basic combustion characteristics of methane fuel, including the laminar burning rate and flame stability. However, there are few studies on the characteristics of methane combustion under dilution and DM addition combustion conditions. This study aims to investigate the laminar burning characteristics of CH4-DM-diluent blended fuel through the outwardly propagating spherical flame method. Moreover, considering that the reaction of COeO2 in a dry atmosphere has a high activation energy, the combustion characteristics of CO are different from those of H2 and CH4. As an important component of DM, CO is not only the initial fuel in the
combustion of CH4-DM-diluent-air but also the participant of many intermediate reactions [18,19]. Therefore, it is of great significance to explore the effects of CO on the combustion separately. This paper not only comprehensively studies the effects of DM on the premixed laminar combustion characteristics of CH4 but also analyzes the effects of CO addition alone. The laminar burning velocity and flame stability are fundamental parameters for studying the propagation characteristics of laminar flames. Laminar premixed combustion is the basis for the study of combustion. For CH4eDM-diluent mixed fuel, it is necessary to carry out a systematic and indepth study of its laminar burning velocity and intrinsic flame instability. The data gained from this work can contribute to the optimization of the combustion process and alternative fuel utilization, which is urgently required by engine researchers.
Experimental procedure Experimental setup Fig. 1 shows the experimental setup. The experimental device consisted of a spherical constant volume chamber (CVC), high-speed schlieren shooting system, temperature control system, trigger control system and ignition system. The inner diameter of the CVC used in this test was 350 mm, and the volume was 22.4 L. Two quartz windows with an efficient diameter of 100 mm and a thickness of 40 mm were mounted on two sides of the chamber for optical access. Two extended spark plugs were centrally installed in the CVC and controlled by a capacitor discharge ignition unit. The ignition pulse width in the experiment was 2 ms. The CVC was wrapped by electrical heating tape and by thermal insulation material to decrease the heat loss. The temperature controlling unit was equipped to maintain the test temperature within the range of ±3 K through a proportional-integral-derivative (PID) controller. The triggering control unit (EcuTek Calibration V2) was used to control the ignition and to start the camera. The flame propagation process was recorded by a high-speed digital camera (Phantom V7.3) with a frequency of 10000 Hz; the resolution was 512 512 pixels. After the combustion, the process of exhausting, scavenging and vacuuming was achieved through the intake and exhaust system. Table 1 lists the experimental conditions. The purity of all the gases used in this study was 99.9%. In the experiment, the parameter a is defined as the volume fraction of the DM in the CH4-DM mixed fuel, and the parameter is defined as the volume fraction of the CO in the CH4eCO mixed fuel. The dilution ratio 4r is defined as the volumetric fraction of the diluent gas addition in the total blends. That is: a¼
VH2 þ VCO VH2 þ VCO þ VCH4
(2)
b¼
VCO VCO þ VCH4
(3)
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Fig. 1 e Experimental setup.
fr ¼
VDilution Vfuel þ VDilution þ VAir
(4)
Data processing Laminar burning velocity and markstein length The laminar burning velocity and Markstein length are fundamental parameters for the study of laminar combustion [20]. In this paper, the schlieren photographs were processed by the MATLAB program to calculate the radius (R) of the outwardly propagating spherical flame. Fig. 2 shows the diagram of the flame image processing flow of the MATLAB code. To avoid the possible distortion effect caused by the spark ignition and pressure rise, the flame photos during the range of 7e24 mm were selected [8]. Markstein length expresses the influence of the flame speed on the flame stretch rate. It is usually defined by the negative value of the slope of the stretched flame velocity (Sn)eflame stretch rate curve [21]. However, this method has
been proved not to be the most accurate. Based on the suggestion of Chen [22], when the mixed fuel has a value Lewis number Le < 1 or close to 1, the nonlinear calculation method proposed by Kelley and Law [23] is used: Sn Sn 2Lb ln ¼ Sl Sl R
(5)
SL is the unstretched flame propagation speed of flame. In the case of Le > 1, the following nonlinear formula is used: Sn ¼ Sl Sl Lb :
2 R
(6)
The laminar burning velocity (UL) is calculated by the following formula:
Table 1 e Experimental conditions. Parameters Equivalence ratio (4) DM fraction (a) CO fraction (b) Diluent gas Dilution ratio (4r ) Initial temperature (K) Initial pressure (MPa)
Approximately 0.8e1.4 (interval 0.1） 0, 40, 80% 0, 40, 80% CO2, N2 0, 5, 10, 15% 343 0.3
Fig. 2 e Diagram of the flame image processing flow of the MATLAB program.
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r UL ¼ Sl u rb
(7)
where rb is the density of the gas burned and ru is the density of the unburned gas, calculated by the equilibrium module of the CHEMKIN software package. This method has been widely used and validated from the calculation methodology adopted by Salzano et al. [24].
Flame instability The premixed laminar flow flame instability is divided into the buoyancy instability, diffusive-thermal instability and hydrodynamic instability. The buoyancy instability can be neglected when the laminar flame combustion speed is high enough [25]. Therefore, only the diffusive-thermal instability and hydrodynamic instability are considered in this study. For a spherical diffusion flame, subject to instability, the flame front will appear as a cellular structure during the propagation process [30]. The hydrodynamic instability is the inherent instability of the flame. When the flame thickness decreases, the hydrodynamic instability increases. In this study, the flame thickness (Lf) is calculated using the formula proposed by Law [33]: Lf ¼
l=CP ru UL
(8)
where l is the unburned gas thermal conductivity and Cp is the constant pressure specific heat capacity. The thermal expansion ratio is the ratio of the density of the unburned and burned gas mixtures. When the thermal expansion ratio (s) increases, the hydrodynamic instability of combustion is enhanced. It is calculated as follows: s ¼ ru =rb
(9)
The diffusive-thermal instability is usually characterized by the Lewis number, Le, as shown in Equation (9). Le is defined as the ratio of the heat diffusivity and mass diffusivity: Le ¼
DT Dm
(10)
The flame thermal diffusion tends to be stable when Le > 1. As Le < 1, the thermal dispersion of the flame tends to be unstable. In this study, the volume-weighted method was used to calculate the effective Lewis number of the mixtures [34]: Le ¼
n X
xi Lei
(11)
i¼1
where xi is the volume fraction corresponding to the ith substance of the mixture composition and Lei is the corresponding Lewis number of the ith substance.
Numerical simulation process For the premixed laminar combustion process, the laminar burning velocity of the mixture is limited by the rate of the reaction. The chemical reaction kinetics are an effective way to analyze the combustion process [25e27]. CHEMKIN software was used to study the effects of diluent addition on the primitive reaction rate of the mixed fuel in this study.
Therefore, the impact mechanism of the addition of diluted gas on the combustion speed of mixed fuel laminar flames can be explored. When performing chemical reaction kinetics analysis, the choice of the combustion reaction mechanism is particularly important. At present, GRI-Mech 3.0, USC-Mech 2.0 and AramcoMech 2.0 are the most widely used chemical mechanisms of methane combustion around the world [28e30]. Researchers have also improved and verified more complete alkane mechanisms based on these widely used mechanisms [31,32]. The AramcoMech 2.0 mechanism was developed in 2016 by NUI Galway. This mechanism has been validated over a wide range of initial conditions and experimental devices, including flow reactors, shock tubes, jetstirred reactors, and flame studies. It has now become the base model for methane combustion and has been systematically modified and validated for various experimental purposes. In this paper, the flame speed and the rate of production (ROP) of CH4, H2, CO were calculated by the CHEMKIN software premixed laminar-flame speed calculation model and the AramcoMech 2.0 mechanism.
Experimental uncertainty analysis and system validation The factors that affected the accuracy of the test results were derived in part from the test equipment and the extraction method of the unstretched flame propagation speed Sl. The maximum error generated by the program when identifying the radius in the schlieren image was 1 pixel, and the maximum error of the corresponding actual radius was 0.25 mm, while the minimum time interval corresponding to the selected point in the calculation was 0.2 ms, that is, the corresponding maximum Sn error was 1.25 m/s. Thus, the estimation indirectly led to a maximum UL error of 0.25 cm/s due to the pixel limitation during the radius recognition, which was the most extreme error case. It has been documented [35] that Sn is more susceptible to stretching (especially at lower initial pressures) in smaller volume combustion bombs and is prone to computational errors. Moreover, the cylindrical combustion bomb can increase the flame velocity in the axial direction compared to the spherical combustion bomb and can decrease the radial direction. The bulky spherical shells used in this experiment reduced these effects. The relatively large spherical constant-volume combustion chamber used in this study reduced these effects to improve the accuracy of the test results. In this paper, two nonlinear methods were selected to calculate Sl and Lb. Combined with the initial work, it was found that the nonlinear method did not improve UL significantly (less than 3 cm/s) compared with the linear method, while Lb changed more than 100% under certain very aggressive conditions (e.g., 4 ¼ 1.8). Other factors, such as the influence of the ignition energy, are described in the previous section. What also needs to be considered is the effect of flame radiation, which can be evaluated by the following formula [36]: UL;RCFS UL;EXP ¼ 0:82UL;EXP
1:14 0:3 UL;EXP Tu Pu S0 T0 P0
(12)
where UL;RCFS and are the radiation-corrected laminar burning velocity and experimental laminar burning velocity,
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 ) 1 1 0 9 7 e1 1 1 1 0
respectively. Under this test condition, the influence of the radiation on UL was not more than 0.08 cm/s and could be neglected. Considering the actual operation of the partial pressure test and other error effects, each condition test was repeated at least twice to ensure that the calculated UL error was less than 5%. Fig. 3 shows a comparison of the laminar burning velocities obtained in the present study with previous data from the literature [36]. The initial pressure and initial temperature were 0.3 MPa, 353 K, respectively. The experiment was conducted three times to validate the measured experimental system of this work. As seen from the figure, the experimental results were in good agreement with the data in a previous paper. Therefore, the laminar burning velocity measured in this work could be considered reasonable and correct.
Comparing the data of a ¼ 40% with those of a ¼ 80%, the UL increased with a larger DM addition ratio. The peak value of the laminar burning velocity of pure CH4 in air was at a ratio value of 4 ¼ 1.1 [37], the peak of the DM laminar burning velocity was at 4 ¼ 1.8 [11], and the peak value of the laminar burning velocity of pure CO in air was between the equivalence ratio 4 ¼ 2 and 3 [38]. When a ¼ 40%, the peak laminar burning velocity appeared at an equivalence ratio 4 of approximately 1.1, while with a larger value of a, the equivalence ratio of the peak laminar burning velocity increased. When a ¼ 80%, the equivalence ratio reached a value of 1.3. Theoretically, the peak laminar burning velocity of DM and
Results and discussion The development of a laminar flame Fig. 4 shows the schlieren images of the laminar flames of seven kinds of fuel. After ignition, the premixed laminar flow flame front had a smooth spherical surface that propagated from the burned area to the unburned area. Comparing the flame images, the speed of flame propagation was increased with the addition of DM, and when the proportion of DM was 80%, the flame propagated the most rapidly. The addition of two kinds of diluent gases, CO2 and N2, could reduce the propagation speed of the flame, and the dilution effects of the two kinds of dilution gas were also different; the dilution effects of CO2 were more notable than those of N2. Compared with the pictures under different equivalence ratios, the flame propagation was the most rapid at the stoichiometric equivalence ratio.
Laminar burning velocity and the markstein length Fig. 5 illustrates the relationship between the laminar burning velocity UL and equivalence ratio 4 of the mixed gas at different values of a or b, and a dilution ratio 4r of 10%.
Fig. 3 e Comparison between the results of the present work and published literature data of CH4-air mixtures.
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Fig. 4 e Flame spread schlieren images.
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Fig. 5 e Laminar burning velocity versus the equivalence ratio.
CH4 mixed fuel should be between 4 ¼ 1.1 and 1.8, but this value is relatively lower due to the addition of diluent gas. Comparing graphs a and b under the same test conditions, the addition of DM caused a greater increase in the laminar burning velocity of the premixed flame than the addition of CO alone. The H2 component in DM played a leading role in the change in the flame morphology due to the extremely strong diffusion capacity and fast burning rate of H2 [39]. Based on the experimental results, the relationship between the laminar burning velocity and equivalence ratio for the CH4-
Fig. 6 e Laminar burning velocity versus the dilution ratio. DM-air-dilution mixtures and CH4eCO-air-dilution mixtures at 343 K and 0.3 MPa under different alternative gas fractions was fitted through a three-order polynomial function as given by Eq. (13) [11]. The fitting curves are shown in Fig. 5. Table 2 provides the coefficients of the fitting curve. The laminar burning velocity can be predicted using the curves given a specific initial condition. UL ¼ A þ B4 þ C42 þ D43
(13)
Fig. 6 depicts the relationship between the laminar burning velocity UL and the dilution ratio 4r of the mixed gas at
Table 2 e Coefficients of the third-order polynomials fitted to the data. Tu ¼ 343 K, Pu ¼ 0.3 MPa 4r ¼ 10% A B C D
a ¼ 40%
a ¼ 40%
a ¼ 80%
a ¼ 80%
b ¼ 40%
b ¼ 40%
b ¼ 80%
b ¼ 80%
CO2 2.928 7.721 6.07 1.501
N2 2.206 5.518 3.513 0.546
CO2 1.387 4.833 6.199 2.322
N2 0.191 2.061 4.580 2.001
CO2 3.024 8.250 6.772 1.756
N2 0.628 2.422 3.595 1.489
CO2 0.996 2.750 1.898 0.429
N2 2.787 7.058 5.712 1.451
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Fig. 7 e The rate of production under no dilution.
different values of a or b and an equivalence ratio 4 ¼ 1. As shown in the figure, the laminar burning velocity of the mixed gas decreased with the addition of diluent gas. It can be inferred that the addition of diluent gas reduced the volume fraction of the reactants, thereby reducing the chances of the reactants and other important groups to come into contact with oxygen, which reduced the amount of heat released by the combustion. In addition, the diluent gas absorbed a portion of the reaction heat and reduced the combustion temperature. Moreover, the addition of diluent gas would
cause a certain offset in the flame front position. These factors led to a decrease in UL. When CO2 was used as the diluent gas, the reduction effect on UL was more notable than that of N2. Because CO2 has a larger heat capacity than N2, the reduction in the combustion temperature became more pronounced. In addition, the presence of CO2 produced thermal radiation and made the shift in the flame front position more pronounced [40]. Therefore, when CO2 was used as diluent, the reduction in the laminar burning velocity of the CH4-DM-air mixed fuel was more complicated than that of N2. Comparing graphs a
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Fig. 8 e The rate of production under a 15% CO2 dilution ratio.
and b, when CO was added by itself, the trend of the changes in UL with the dilution ratio was the same as the trend when DM was added, but the overall UL value was decreased. The results of the laminar burning velocity calculation by the AramcoMech 2.0 mechanism are also shown in Fig. 6, which were in good agreement with the experimental results. The overall deviation between the calculated and experimental values was less than 10%. Therefore, this model could
be used to calculate the rate of production (ROP) of the key reactants. Considering that the influence of CO2 dilution on the laminar burning velocity of the CH4-DM-air mixture is more complex, mixtures under different CO2 dilution ratios were selected for the chemical reaction kinetics analysis in this paper. Figs. 7 and 8 show plots of the relationship between the ROP of the three reactants, the content of CH4, H2, and CO and
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Table 3 e The main reaction selected by ROP analysis. Reaction number 43 44 45 46 49 158 168 191 203 210 97 2 3 5 28 60 115 155
Reaction formula
Reaction number
Reaction formula
CH3þH<¼>CH4 CH4þH<¼>CH3þH2 CH4þO<¼>CH3þOH CH4þOH<¼>CH3þH2O CH3þHO2<¼>CH4þO2 CH2O þ CH3<¼>HCO þ CH4 HCO þ CH3 <¼>CH4þCO HOCHO þ CH3¼>CH4þCO þ OH C2H6þCH3<¼>C2H5þCH4 C2H5þCH3<¼>CH4þC2H4 CH3þOH<¼>HCOH þ H2 H2þO<¼>H þ OH H2þOH<¼>H þ H20 O2þH<¼>O þ OH HO2þH<¼> H2þO2 CH2þH2<¼>CH3þH CH3OH þ H<¼>CH2OH þ H2 CH2O þ H<¼>HCO þ H2
387 36 166 56 63 163 164 187 198 390 66 143 257 27 68 67 165 167
HCCO þ OH¼>H2þCO þ CO CO þ OH<¼>CO2þH HCO þ H <¼>CO þ H2 CH2þO2<¼>H þ OH þ CO CH2þCO2<¼>CH2O þ CO HCO þ M<¼>H þ CO þ M HCO þ O2<¼>CO þ HO2 HOCHO þ H¼>h2þCO þ OH C2H6þH<¼>C2H5þH2 HCCO þ H<¼>CH2þCO CH2þO2<¼>CO2þH þ H CH3O þ CH3<¼>CH2O þ CH4 CH þ CH4<¼>C2H4þH HO2þH<¼>OH þ OH CH2þH<¼>CH þ H2 CH2þO<¼> CO þ H þ H HCO þ O<¼>CO þ OH HCO þ OH <¼>CO þ H2O
the distance from the burner. The CO2 dilution ratios of Figs. 7 and 8 were 0 and 15%, respectively. When the substance was generated, the rate of change was positive, and it was opposite when the substance was consumed. For the sake of clarity, Table 3 lists the reactions involved in the ROP analysis. Fig. 7 shows that the main elementary reactions under no dilution were R46 consuming CH4 (CH4þOH<¼>CH3þH2O), R44(CH4þH<¼>CH3þH2). The main elementary reaction to generate CH4 is R43(CH3þH<¼>CH4). Therefore, the OH and H a groups had a great influence on the combustion reaction of CH4. The reaction rate of CH4 consumption increased with the addition of DM. The main elementary reactions that consumed H2 were R3 (OH þ H2<¼>H þ H2O) and R2 (O þ H2<¼>H þ OH). The main were elementary reactions that generated H2 R44(CH4þH<¼>CH3þH2) and R155(CH2O þ H<¼>HCO þ H2). Therefore, the OH and H groups also had a great influence on the combustion reaction of H2. Both the reaction rates of H2 generation and consumption increase with the addition of DM, and the consumption reaction rate increases more. The main element reaction that consumed CO was R36 (CO þ OH<¼>CO2þH). The OH group had a significant influence on the combustion reaction of CO. It can be found that the consumption of CO is mainly involved in the reaction with OH, and the main reaction of OH radicals generation are R2(H2þO<¼>H þ OH), R5(O2þH<¼>O þ OH) and R27(HO2þH<¼>OH þ OH). All the three reactions require the participation of H or H2. The main reaction of H radicals generation is R3(OH þ H2<¼>H þ H2O), and this reaction also requires the participation of H2. Because of the presence of H2, the addition of DM has a very important role in promoting the consumption reaction of CO. The laminar combustion became more rapid as the content of active radicals increased. As a result, adding DM in CH4 would increase the laminar burning velocity of the mixed gas, and the effect was more noteworthy than that when CO alone was added. H2 has a great effect on promoting combustion reaction. Comparing Figs. 7 and 8, it was found that when the CO2 dilution gas was added, the reaction rates of CH4, H2 and CO
Fig. 9 e Markstein length versus the equivalence ratio.
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were significantly reduced, resulting in a decrease in the burning velocity, which was consistent with the experimental results. The addition of the diluent gas reduced the concentration of each reactant important groups OH and H, so that the consumption rate of CH4, H2, CO and important reactive groups was reduced. It is worth noting that some dilution gases are not just inert gases that do not participate in the reaction. In the elementary reaction R36 (CO þ OH<¼>CO2þH), as a reactant, the diluent gas CO2 participated in the chemical reaction. This is the main elementary reaction that consumes CO and OH. When CO2 was added, the rate of the reverse reaction was accelerated, and the oxidation of CO was inhibited, thereby reducing the burning velocity of the fuel mixture. The Markstein length Lb characterizes the sensitivity of the premixed laminar flame to stretching, reflecting the stability of the flame. When Lb < 0, the flame will spread more rapidly with increasing flame tensile rate. The surface of the flame is prone to produce a bulge, and the spreading velocity of the bulging part will increase, which will lead to instability of the flame. When Lb > 0, the flame instability can be suppressed. Fig. 9 shows the Lb variation of the mixed fuel under different equivalence ratios 4. The experimental data published by Zhang et al. [11] for COeH2-air mixtures (CO:H2 ¼ 33:67) at 358 K and 0.1 MPa is also plotted in Fig. 9. The trend in the Markstein length for the COeH2-air mixtures obtained in this study showed a reasonable agreement with the previous research. From this figure, Lb was positive when 4r was 10%, which means that the flame was stable in the range of the research conditions. Lb increased monotonically with increasing equivalence ratio. Comparing Lb of the mixed fuel under different values of a or, the mixtures with a or b ¼ 80% had less variation in Lb with variation in the equivalence ratio. This implies that the mixtures with more DM or CO were less sensitive to the flame stretch effects. Under the same equivalence ratio, Lb decreased as more DM or CO was added. That is, the addition of DM made flame tend to be unstable, and this trend was clear, specifically in the case of 4 > 1. Fig. 10 shows the relation between Lb of the premixed laminar flame and the dilution ratio. It can be seen from the figure that for the two kinds of diluent gases, CO2 and N2, Lb increased with the increase in 4r when 4 ¼ 1, but the increment was small. This indicates that the addition of diluent gas to methane fuel had little effect on the Markstein length of the flame.
stretching effect of the flame, it appeared almost simultaneously on the flame front, and the corresponding flame radius at this time is called the critical radius Rcr. In this paper, the critical state is determined by the flame stretch rate and flame propagation speed. The pressure rising inside the chamber has influences on the flame stability characteristics [20]. In this experiment, it is observed that the pressure has almost no fluctuation during the very short period of laminar combustion. The pressure inside the chamber is constant for the determination of Rcr. We can see from the flame spread schlieren images of Fig. 4 that the flame instability tended to be promoted by an increasing DM fraction. For a given equivalence ratio, only a few wrinkles appeared at the flame surface when a ¼ 40%, and the wrinkles developed into clear cellular structures when a ¼ 80% at the same time. The flame tended to be more unstable, and the strong cellular structures decreased and became denser with the addition of DM. The equivalence ratio
Flame instability The diffusive-thermal instability is the combination of the non-equidiffusive effect and the pure curvature instability effect, and the diffusive-thermal instability can be evaluated by the Lewis number [27]. As an intrinsic instability of the flame, the hydrodynamic instability is one of the most important factors affecting the flame instability, which is caused by a density jump across the flame front and can be evaluated by the laminar flame thickness and thermodynamic expansion ratio [8,27]. It can be seen from the flame pattern images that the smooth surface of the flame began to exhibit folds, followed by distinct cellular structures with the flame development. When the cellular structure broke free from the
Fig. 10 e Markstein length versus the dilution ratio.
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 ) 1 1 0 9 7 e1 1 1 1 0
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had less effect on the flame instability and cellular structure than the DM addition ratio. The flame tended to be more stable with increasing equivalence ratios. From the schlieren images, the effect of dilution gas on flame stability was not significant within the scope of this study. This is consistent with the conclusion in Fig. 10. Fig. 11 shows the Lewis number, Le, of the premixed laminar flame under different equivalent ratios, diluent gas types and dilution ratios. The addition ratio a of DM in Figures (a) and (b) is 40 and 80%, respectively. From this figure, Le is less than 1 when DM was added to CH4, which indicates that the thermal diffusion instability of the flame was relatively strong at this time. Le increased with a larger equivalence ratio, which is consistent with the schlieren images in Fig. 4. Le decreased with the addition of diluent gases. This trend was more notable under the condition of N2 dilution. The addition of diluents reduced the thermal diffusion rate of the mixed fuel, resulting in a reduction in Le [21]. However, Le had a low sensitivity to the dilution gas addition. Comparing Figures (a) and (b), it can be found that Le decreased with increasing DM fraction for a given equivalence ratio, and the decreasing Le promoted the thermal diffusion instability.
Figs. 12 and 13 plot the flame thickness, Lf and the thermodynamic expansion ratio s of the mixture under different equivalent ratios, and the addition ratios of DM in Figures (a) and (b) were 40 and 80%, respectively. It can be seen from the graph that Lf tended to decrease followed by an increase as the equivalent ratio, 4, was increased. Lf reached a minimum near the stoichiometric equivalence ratio. In contrast, the tendency of the thermodynamic expansion ratio, s, was to increase followed by a decrease with the increase in 4. The thermodynamic expansion ratio reached a maximum near the stoichiometric equivalence ratio, which indicates that the hydrodynamic instability was the most significant near the stoichiometric equivalence ratio. The flame thickness, Lf, increased with increasing 4r, while the thermodynamic expansion ratio, s, showed the opposite trend. The effect was more noteworthy under CO2 dilution. This result indicates that the hydrodynamic instability of the premixed laminar flame was suppressed with the addition of the diluent gas. Comparing Figures (a) and (b), it can be found that Lf decreased with an increasing DM fraction for a given equivalence ratio,
Fig. 11 e Lewis number of the mixtures.
Fig. 12 e Flame thickness of the mixture.
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while the thermal expansion ratio was insensitive to the changes in the DM addition fraction. Overall, the addition of DM promoted the hydrodynamic instability of the flame. Fig. 14 shows the critical radius of the flame at different equivalence ratios, dilution gases and dilution ratios. The DM addition ratio a is 80%, and at that ratio value a flame critical radius can be obtained under more operating conditions in the observation window (when a is 40%, the critical radius in most conditions exceeded the observation window scope of this study). As shown in this figure, Rcr increased with increasing equivalent ratio. The addition of diluent gases had an increasing effect on Rcr, but the effect was not significant. Compared with N2, the effect of adding CO2 on Rcr was more noteworthy. Meanwhile, it can be seen from this figure that when CO was added alone, the critical radius of the flame exceeded 40 mm under most test conditions, which indicates that CO had less effect on the instability of the cellular structure of the flame. The conclusion shown in Fig. 14 is in accordance with the results from the schlieren images.
Fig. 14 e Flame critical radius of the mixture. The Lewis number of the premixed laminar flame of the CH4-DM-air mixture decreased as the dilution ratio increased, indicating that the thermal diffusion instability of the mixture increased with the addition of diluent gas. As the dilution ratio increased, the thermodynamic expansion ratio decreased and the flame thickness increased, indicating that the hydrodynamic instability of the mixture was suppressed. Overall, the mixture flame tended to be more unstable with the addition of DM, while the addition of diluent gas promoted the stability of the flame. This verifies the conclusion that hydrodynamic instability was the dominant factor of flame instability when the flame had developed to a certain extent, and the thermodynamic instability had the leading role only within a small flame radius after the ignition [41].
Conclusion Measurement of the laminar burning velocity and flame stability analysis were conducted using the outwardly spherical laminar premixed flame of CH4-DMeair-dilution mixtures. The laminar burning velocity, Markstein length and flame instability were analyzed at different equivalence ratios, DM addition ratios and dilution levels. The main conclusions are summarized as follows.
Fig. 13 e Thermodynamic expansion ratio of the mixture.
(1) The peak laminar burning velocity value of the CH4-DM mixed fuel is reached close to an equivalence ratio of 1.1. The addition of DM increases the laminar burning velocity of CH4, and with the increase in the addition proportion, the peak value of the laminar burning velocity shifted to the fuel rich area. The effect of adding CO alone is far less pronounced than that of adding DM. The flame propagation speed and laminar burning velocity decrease with an increase in the dilution ratio. (2) The Markstein length of the CH4-DM mixed fuel flame increases monotonically with the increase in the equivalence ratio. (3) Within the range of the dilution ratio in this study (approximately 0e15%), the addition of diluent gas
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decreases the mole fraction of the reactants and certain important groups. The diluent gas, CO2, also participates in intermediate reactions as a reactant, leading to a decrease in the mixture laminar burning velocity. (4) The addition of diluent gas has no significant effect on the flame stability. (5) The influence of CO2 dilution on the burning velocity of the mixture and stability of flame is more notable than that of N2 dilution. (6) H2 is the important component of DM which leads to a high burning velocity and flame stability.
[12]
[13]
[14]
[15]
Acknowledgments [16]
This work was supported by the National Natural Science Foundation of China (50806051). The experimental facilities were supported by the State Key Laboratory of Engines, Tianjin University. The authors would like to express sincere thanks to Zhaoxu Chen, Xiang Li, Yi Liu for their assistance.
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