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Appearance, temperature, and NOx emission of two inverse diffusion flames with different port design
Combustion and Flame 144 (2006) 237–248 www.elsevier.com/locate/combustflame
Appearance, temperature, and NOx emission of two inverse diffusion flames with different port design L.K. Sze ∗ , C.S. Cheung, C.W. Leung Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China Received 11 October 2004; received in revised form 1 July 2005; accepted 12 July 2005 Available online 12 September 2005
Abstract Experiments were carried out to investigate the appearance, temperature distribution, and NOx emission index of two inverse diffusion flames, one with circumferentially arranged ports (CAPs) and the other with co-axial (CoA) jets, both burning LPG with 70% butane and 30% propane. Flame appearances were investigated first with a fixed fueling rate at different airflow rates equivalent to air jet Reynolds numbers (Re) of 1000 to 4500; and then at a fixed airflow rate with different fueling rates equivalent to overall equivalence ratios (Φ) of 1.0 to 2.0. The CAP flame is found to consist of two zones: a lower entrainment zone and an upper mixing and combustion zone. The CoA flame in most cases is similar to a diffusion flame. The two-zone structure can be observed only at Re larger than 2500. The temperature distributions of the flames are similar at overall equivalence ratios of 1.0 and 1.2 for Re = 2500, except that the corresponding CoA flame is longer. The flame temperature is higher in the CAP flame than the CoA flame at higher overall equivalence ratios. A measurement of centerline oxygen concentrations shows that the oxygen concentration reaches a minimum value at a flame height of 50 mm in the CAP flame but decreases more gradually in the CoA flame. It can be concluded that there is more intense air–fuel mixing in a CAP flame than the CoA flame. Investigation of the emission index of NOx (EINOx ) for both flames at Re = 2500 and overall equivalence ratios of 1.0 to 6.0 reveals that the EINOx curve of each flame is bell-shaped, with a maximum value of 3.2 g/kg at Φ = 1.2 for the CAP flame and 3 g/kg at Φ = 2.2 for the CoA flame. 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Circumferentially arranged ports; Co-axial; Inverse diffusion flame; Partially premixed flame; Emission index of NOx ; Reynolds number; LPG
1. Introduction Diffusion flames constitute one of the most basic flame configurations in the combustion process. Numerous investigations have already been carried out to gain an understanding of the characteristics of the flame. The inverse diffusion flame (IDF) is a kind of * Corresponding author. Fax: +852 23654703.
E-mail address: [email protected] (L.K. Sze).
flame with an inner air jet surrounded by an outer fuel jet(s). If the air jet velocity is high enough, the fuel at the outer jet(s) is entrained inward and mixes with the air to form a partially premixed flame. Otherwise, it might burn mainly in the diffusion mode. The IDF is a combination of a premixed flame and a diffusion flame that can have a larger flammability range than the premixed flame and is cleaner than the diffusion flame. Therefore, the feasibility of applying IDFs in industrial and domestic heating processes is of interest.
0010-2180/$ – see front matter 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2005.07.008
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In previous investigations, IDFs were obtained using coaxial jet (CoA) burners. A CoA burner consists of two concentric tubes, with the inner tube supplying air and the outer annular tube supplying fuel. Most of these studies were carried out under confined conditions. The most rigorous research on a confined IDF was carried out by Wu and Essenhigh [1,2], who mapped out six different types of IDFs according to their appearance and stability. They found that in a confined laminar condition, the appearance of the IDF was similar to that of a premixed flame, consisting of a bell-shaped bluish nonluminous reaction zone stabilized on the air jet. As the fuel flow rate was increased, a luminous yellowish wing with soot formation appeared on the bluish reaction zone. Takagi et al. [3] carried out experiments on a laminar IDF without confinement. They did not report the flame shape; however, they found that the flame temperature of the IDF was higher than that of the corresponding normal diffusion flame at the same flame height. Sidebotham and Glassman [4] investigated the soot formation characteristics of IDFs under the effects of temperature, fuel structure, and fuel concentration, and discovered that IDFs had lower soot loading than normal diffusion flames. Wentzell [5] reported the temperature distribution and flame length of an IDF at different air and fuel jet diameters and at different air and fuel Reynolds numbers. The IDF studied showed a dual flame structure composed of a premixed region located on the upstream portion of the flame centerline and a diffusion flame envelope. He suggested that the IDF could be classified as a partially premixed flame, and the degree of partial premixing on the flame centerline depended on the nozzle geometry and the flow conditions. Fleck [6] investigated the IDF formed in a staged air burner. For this burner, the high-velocity air jet entrained the fuel from the surrounding fuel jets, which were diluted by the surrounding flue gas. Because of the diluted fuel effect, the burner generated a large combustion zone of relatively low temperature that was not favorable to the formation of NOx in the flame. Recently, Bindar and Irawan [7] investigated a confined IDF at a high level of fuel excess. The flame shape they reported was different from that of Wu [2]. They proposed that the shape of the IDF was affected by both the inlet air momentum and the inlet fuel momentum. At low air jet velocity, the flame shape was similar to that of a normal diffusion flame. At higher inlet air velocity, usually 5–16 times higher than the fuel velocity, the flame shape was composed of two flame regions: a base and a tower on top of the base. They showed that the IDFs’ highest temperature zone was located at the center of the flame. As the supplied
fuel fraction in the mixture increased, the highest temperature zone would shift closer to the flame tip. In general, previous investigations were concerned mainly with flame temperature, flame appearance, and soot formation characteristics of CoA IDFs. There have been very few investigations on emission characteristics. Wu [2] plotted a simple emission species contour; however, there was lack of detailed analysis of the emissions. The low NOx emission characteristics of IDFs had been reported [6,8,9] in relation to staged air burners. Partridge and Laurendeau [8] studied NO in the IDFs of staged air burners, using different diameters of the central oxidizer jet, different angles of secondary air injection, and different overall equivalence ratios, and found that, in general, the emission index of NOx (EINOx ) for a staged air burner would achieve a maximum value at Φ = 1. More recently, Partridge et al. [9] reported the temperature distribution and NO concentration in a laminar IDF of a staged-air burner. The concentration of NO was found to increase through the IDF, and peaked just above the outer edge of the flame tip. Research work on IDFs is limited to coaxial jets and there is very little information concerning their emission characteristics. In this article, newly developed IDF burners with circumferentially arranged fuel ports (CAPs), as well as an IDF with a CoA fuel jet, are tested in the unconfined condition. Flame shape, temperature distribution, and NOx emissions are reported and compared. The intensities of air–fuel mixing at different air jet Reynolds numbers and fueling rates are also compared. Moreover, NOx emissions are compared with those from partially premixed flames as reported [10,11].
2. Experimental setup and method The CAP and CoA flame holders are shown in Fig. 1. The CAP flame holder used in the present study is the same as that used by Sze et al. [12]. It has an inner air jet of 6-mm diameter, surrounded by 12 outer fuel jets each 2.4 mm in diameter, with a center-to-center distance of 11.5 mm between the air
Fig. 1. Design of the CAP and CoA burners.
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Fig. 2. Schematic arrangement of IDF for pollutant measurements.
jet and the fuel jets. The CoA flame holder is made up of two concentric tubes, one with an internal diameter of 6 mm and the other with an internal diameter of 11.5 mm. The fuel port areas are the same for both flame holders so that the fuel jets have the same velocity for the same fuel consumption rate. Hence, the CoA fuel jet is in direct contact with the air jet, whereas in the CAP, the fuel jets are at a distance from the air jet. It was observed [12] that the air and fuel mixed more intensively than the corresponding CoA flames observed in published literature, which might be a result of the difference in fuel port design. The experimental setup is shown in Fig. 2. LPG (70% butane and 30% propane) and compressed air were metered by rotameters and passed through the corresponding jets. The flame holder was fixed on a positioning system so that the burner could be moved in all directions. Tests were first carried out with a fixed fueling rate while the air jet Re was varied from 1000 to 4500 to observe the flame shapes. Tests were continued with a fixed air jet Re of 2500 while the overall equivalence ratio was varied from 1.0 to 2.0, for observation of both flame shape and flame height. Although the air and fuel jets are initially separated, and hence the concept of equivalence ratio might not be applicable to an IDF, an indication of the relative strength of the air and fuel supply in relation to the stoichiometric air/fuel ratio would be helpful in the analysis. The overall equivalence ratio, Φ, is used in this article for such a purpose. The detailed flow conditions are given in Table 1. Temperature distributions and centerline oxygen concentrations were measured for flames at Re = 2500 and overall equivalence ratios of 1.0 to 2.0 to provide information for analysis of the combustion of the flames. The NOx concentration in the postflame region was measured and the emission index of NOx was calculated at Re = 2500 for overall equivalence
ratios from 1.0 to 6.0. Unless otherwise specified, the Reynolds number refers to that of the air jet alone. The temperature distribution of the flame was measured with an uncoated type B thermocouple with Pt–30%/Rh as the positive lead and Pt–6%/Rh as the negative lead. The wire has a diameter of 0.25 mm. The thermocouple was fixed, and the burner fixed on the positioning system was moved in the r direction (the x direction in Fig. 1), with an interval of 1 mm, from r = 0 mm to r = 30 mm; and moved in the z direction, with an interval of 5 mm, from z = 0 mm to z = 20 mm and, with an interval of 10 mm, from z = 30 mm to z = 160 mm. Temperatures reported in this article have been corrected for radiation loss according to the method of Sato et al. [13]. The maximum temperature correction in the present study was 154 ◦ C. The centerline oxygen concentration was collected by an air-cooled quartz probe with an inner diameter of 1 mm and outer diameter of 2 mm. The probe was inserted into the flame from the top to reduce disturbance of the flame. The gas collected was cooled down to about 60 ◦ C by the cooling air and sent to an oxygen gas analyzer. The concentration was measured from the surface of the flame holder to a flame height of 200 mm. Concentrations of NOx , CO, and CO2 in the postflame region were measured with the flame holder positioned inside a screened enclosure. The screened enclosure regulated airflow from the surroundings to reduce disturbance from surrounding air. Flue gas was collected by a stainless-steel probe in the postflame region. The air-cooled probe has an inner tube diameter of 6.35 mm. The probe was aligned at the same central axis as the air jet of the burner to ensure isokinetic sampling conditions. The flue gas, diluted and well-mixed with regulated atmospheric air, was extracted from the sampling probe and immediately cooled to about 60 ◦ C by the cooling air for the purpose of freezing the reaction and species
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Table 1 Operating conditions for inverse diffusion flames Air jet velocity (m/s)
Fuel jet velocity (m/s)
2.54 3.81 5.08 6.35 7.61 8.88 10.15 11.42
0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17
6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35
0.11 0.13 0.15 0.17 0.19 0.21 0.23 0.26 0.28 0.30 0.32 0.43 0.53 0.64
Reair
Φ
Air momentum/ fuel momentum
Conditions for Fig. 3 with fixed fuel flow rate 0.72 9.26 1.08 9.26 1.44 9.26 1.79 9.26 2.15 9.26 2.51 9.26 2.87 9.26 3.23 9.26
1000 1500 2000 2500 3000 3500 4000 4500
4.0 2.7 2.0 1.6 1.3 1.1 1.0 0.9
1.61 3.63 6.46 10.09 14.52 19.77 25.82 32.68
Conditions for Figs. 4 and 5 with fixed airflow rate 1.79 5.79 1.79 6.95 1.79 8.11 1.79 9.26 1.79 10.42 1.79 11.58 1.79 12.74 1.79 13.90 1.79 15.05 1.79 16.21 1.79 17.37 1.79 23.16 1.79 28.95 1.79 34.74
2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 4.0 5.0 6.0
10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09
Airflow rate (× 10−4 m3 /s)
formation. The sampled gas was then separated into two streams. One stream was further cooled to below 30 ◦ C for condensation and dehydration. The gas was then sent to a three-gas analyzer for O2 (Electrochemical cell), CO (NDIR), and CO2 (NDIR) measurement. Another stream was heated to and maintained above 190 ◦ C before entering a heated flame ionization detector (HFID) for HC measurement and above 60 ◦ C before entering a chemiluminescence analyzer (CLA) for NO/NOx measurement. Emission index, expressed as grams of NOx formed per kilogram of fuel consumed, was used in the analysis.
3. Flame appearance The appearance of a flame depends on the burner design, the fueling rate, and the airflow rate. The fueling rate and the airflow rate determine the overall equivalence ratio, as well as the ratio between the momentums of the jets. At a fixed fueling rate, an increase in the airflow rate decreases the overall equivalence ratio, but increases the ratio of air jet momentum to fuel jet momentum. At a fixed airflow rate, an increase in the fueling rate increases the overall equivalence ratio as well as the fuel jet momentum. The effects of these changes on the appearance of the flame are investigated first.
Total fuel flow rate (× 10−6 m3 /s)
3.1. Effect of airflow rate with fixed fueling rate on flame appearance Figs. 3a and 3b compare the effects of increasing airflow rate on the appearance of the flames. In each flame, the fueling rate was maintained constant at 9.26 × 10−6 m3 /s, whereas the airflow rate was varied from Re = 1000 to Re = 4500, to create flames with different air/fuel ratios varying from overall equivalence ratios of 4.0 to 0.9. For both CAP and CoA flames, flame length decreases with an increase in airflow rate. The corresponding CoA flames are longer than the CAP flames. The color of the flames changes from yellow to blue, indicating a change from fuel-rich to fuel-lean conditions. The dual flame structure (a bluish reaction zone followed by a diffusion combustion zone) as reported by Wu and Essenhigh [1] and Wentzell [5] is observed in the CAP flames at Re = 2000 and Re = 2500 and in the CoA flames at Re = 2500. In a pure diffusion flame, the fuel burns with air entrained from its surroundings. The flame is purely yellow in such case. In an inversion diffusion flame, the central air jet entrains the fuel jet(s). The degree of entrainment of the fuel jet(s) by the air jet determines the extent to which the fuel is burned in the diffusion mode or premixed mode. Change in the color of the
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(a)
(b) Fig. 3. Effect of airflow rate on appearance of (a) CAP flame and (b) CoA flame.
flames is an indication of the extent of fuel entrainment. It can be seen that with an increase in airflow rate, entrainment becomes stronger, leading to more fuel burned in the premixed mode, and at high enough airflow rate, all the fuel burns in the premixed mode. The same effects of airflow rate on flame length and color have been reported [5,7] for CoA flames. In Fig. 3a, it can be observed that the CAP flame consists of two parts, which becomes more obvious at higher airflow rates: a lower zone in which fuel is entrained toward the air jet, and an upper zone in which the fuel is mixed with air and intense combustion occurs. We call the lower part the entrainment zone and the upper part the mixing and combustion zone. The same flame shape was reported [7] for a CoA flame. The height of the entrainment zone decreases as the airflow rate is increased, indicating an increase in the entrainment effect. In Fig. 3b, it can be observed that at low airflow rates, the CoA flame is close to a pure diffusion flame. As the airflow rate increases, the fuel jet is entrained
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toward the air jet. At high enough airflow rates, similar to the CAP flame, the flame consists of two parts: a base and a tower. The two-zone structure starts to be observable at Re = 2500 and becomes more obvious at higher Re. Comparison of the two sets of flames shows that mixing is more intense in the CAP flame than in the CoA flame. At the same Re of 2000 and 2500, the CAP flame has a blue zone inside the flame, indicating the existence of partial premixed combustion in the flame, while partial premixed combustion is not observable in the corresponding CoA flame. At higher Re, turbulence and stronger entrainment aid mixing between air and fuel, so that the flame becomes shorter and more intense in combustion. This is indicated by the height of the entrainment zone of the flames. In the CAP flames, the entrainment zone becomes shorter as the airflow rate is increased; however, the change is not significant. In CoA flames, the entrainment zone is not observed below Re = 2000. At Re = 2500, the fuel jet starts to converge to the centerline of the flame and becomes turbulent above the region of convergence. At still higher airflow rates, the entrainment zone becomes shorter. At Re of 4000 and 4500, the CAP flames are mainly bluish with yellowish entrainment zones but the CoA flames are completely bluish. The distance between the fuel jets and the air jet of the CAP creates this effect. The better fuel entrainment of the CAP flame might be attributed to the separation of the fuel jets from the air jet. For the CoA flame, at lower airflow rates, the fuel basically coflows with the central air jet. Mixing with the inner air jet is limited due to the small difference in the momentums of the jets. At higher airflow rates, the difference in the momentums of the two jets increases, leading to entrainment of some fuel into the central air jet. For the CAP flame, the central air jet and the fuel jets both entrain surrounding quiescent gas. This creates a low-pressure region between the jets, resulting in the fuel jets flowing toward the central air jet within the entrainment zone. A numerical simulation with a commercial CFD code indicates this effect, with the CAP flame having a higher radial velocity than the corresponding CoA flame at the neck of the entrainment zone. 3.2. Effect of fuel flow rate with fixed airflow rate on flame appearance We have chosen a fixed air jet Re of 2500 for investigation of differences between the two flames under different fueling rates. The fueling rate was varied from an overall equivalence ratio of 1.0 to 2.0 so that the effect of varying the fueling rate can be observed. In Fig. 4a are schematic diagrams and photographs of
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(a)
(b) Fig. 4. Schematic diagrams of (a) CAP flame and (b) CoA flame and effect of overall equivalence ratio on flame appearance at Re = 2500.
the CAP flames, and in Fig. 4b are those for the CoA flames. Fig. 4a shows that a change in the fueling rate does not significantly affect the shape of the CAP flames. All CAP flames are composed of two parts. The upper part of the flame is similar to a normal, partially premixed flame and combustion occurs mainly within this zone. At the center of the flame, a thick fluctuating reaction zone can be observed. The visible blue reaction sheet is formed at or close to stoichiometric conditions. In the CAP flame, the local equivalence
ratio varies within the flame as the air and fuel are not uniformly mixed. A yellow soot ring is always observed at the top of the entrainment zone at all equivalence ratios studied, indicating non-premixed combustion of the fuel jets before entrainment into the central air jet. Variation of fueling rate has little effect on the length of the entrainment zone and no effect on the color of this zone. The effect of overall equivalence ratio on the appearance of the mixing and combustion zone is similar to that normally observed in partially pre-
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mixed flames. At Φ = 1.0, the flame is mainly blue and a thick fluctuating reaction zone can be clearly observed, indicating that the combustion is mainly premixed. The air jet in this case is able to penetrate through the flame, creating a hollow flame structure observable from the top of the flame, as shown in the schematic on the left-hand side of Fig. 4a. For 1.2 Φ 1.4, there is a main bluish reaction zone with a weak yellowish tail appearing on top of it, indicating that diffusive combustion with soot formation is occurring under excess fuel conditions at the outer layer of the flame. The hollow flame structure disappears at Φ = 1.4. For Φ 1.6, the bluish reaction zone representing premixed combustion can still be observed, but the flame is mainly diffusive in nature and has a highly fluctuating flame tip, resulting in the kind of flame structure shown in the schematic on the right-hand side of Fig. 4a. The increase in the amount of fuel burning in the diffusion mode leads to an increase in the length of the mixing and combustion zone and, hence, to an increase in flame length. Fig. 4b shows that the corresponding CoA flames are basically diffusive in nature within the range 1.0 Φ 2.0. There is also an increase in flame length with increasing Φ, but the increase is not as great as that observed for CAP flames. At Φ = 1.0 and 1.2, the flame is again hollow in shape with a laminar flow structure, as shown schematically on the left-hand side of Fig. 4b. The hollow structure indicates that the air jet mainly coflows with the fuel jets, with some of the fuel being entrained into the air jet. This results in a diffusion flame rather than a partially premixed flame. The flame again closes at higher fueling rates, resulting in the kind of flame shown in the schematic on the right-hand side of Fig. 4b. A transition neck can be observed when the fueling rate increases to Φ 1.4. Under the transition neck, the flame remains laminar; however, above the neck, it becomes turbulent. The observations show that the central air jet provides little improvement in air and fuel mixing in CoA flames, compared with CAP flames, at Re = 2500. Comparison of the two types of flames reveals that they are significantly different from each other at Re = 2500, with overall equivalence ratios ranging from 1.0 to 2.0. The CAP flame has a dual flame structure, whereas the CoA flame is mainly diffusive in nature. We have explained that CAP flames have better entrainment than CoA flames. In this case, when the airflow is fixed, an increase in the fueling rate has two implications. First, more fuel has to be burned in the diffusion mode. Second, the fuel jet comes out at a higher velocity and less fuel is entrained into the entrainment zone; hence, less fuel is burned in the partially premixed mode. We can observe from the CAP flames that some of the fuel flows along the periphery
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of the flame and burns in the diffusion mode. Excess fuel burned in the diffusion mode results in a longer flame length with soot formation. Hence, there is no change in the basic structure of the flames within the range of fueling rates studied.
4. Flame height analysis Fig. 5 demonstrates the variation of flame height with overall equivalence ratio at Re = 2500 for the CAP flame (Fig. 5a) and the CoA flame (Fig. 5b). In this study, the equivalence ratio is extended to 6.0. In both cases, increasing the overall equivalence ratio leads to an increase in flame height. The flame lengths were obtained by visually averaging 10 images captured by a digital camera. As mentioned before, the CAP flame is composed of two parts. In Fig. 5a the hollow circle represents the height of the entrainment zone, and the inverted triangle represents the total flame height, with the fluctuation in height indicated by the error bars. Fig. 5b shows the height of the CoA
(a)
(b) Fig. 5. Effect of overall equivalence ratio on (a) CAP flame height and (b) CoA flame height at Re = 2500.
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flame. As no entrainment was observed, the triangle represents the total flame height with the flame fluctuation represented by the error bars. In the CAP flame, the height of the entrainment zone is affected very little by the fueling rate. At an overall stoichiometric condition of Φ = 1.0, the height of the entrainment zone is 10 mm, which increases to 25 mm at Φ = 6.0. The increment is small in comparison with total flame height. Moreover, the height of the entrainment zone remains very stable. For both flames, in the range 1.0 Φ 1.4, a somewhat steeper increase in flame height is observed. As the overall equivalence ratio is further increased to 1.4 < Φ 6.0, the increase in flame height becomes more gentle. The flame height of a CoA flame at Φ = 1.0 is 30 mm higher than that of a corresponding CAP flame. The difference in flame height is due mainly to the entrainment effect in the CAP flame. The entrainment improves the mixing between air and fuel and generates an intense combustion zone and, hence, shortens the flame length. However, as the overall equivalence ratio increases, both flames exhibit a diffusion flame nature and the difference in flame height decreases.
The fluctuation in flame height increases with rising overall equivalence ratio. For the CAP flame, at Φ = 1.0 the maximum fluctuation in flame height is ±5 mm, which becomes larger than 20 mm at Φ 4.0. At Φ = 6.0 the maximum fluctuation is as large as ±30 mm. The fluctuation for the CoA flame is large even at low overall equivalence ratios. The maximum fluctuation is larger than 20 mm even at Φ 1.4, whereas in the CAP flame, the same level of fluctuation occurs at Φ 4.0. The flame heights and their fluctuations indicate that there is better air fuel mixing in the CAP flames than the CoA flames and that the former are more stable than the latter.
5. Temperature distribution Figs. 6a–6d are contour plots of flame temperature on the r–z plane. The figures are arranged with the left-hand side showing CAP flame temperatures and the right-hand side showing CoA flame temperatures for easy comparison. The r axis is the radial distance, with r = 0 representing the center of the air jet. The
(a)
(b)
(c)
(d)
Fig. 6. Temperature distribution of CAP and CoA flames at (a) Φ = 1.0, (b) Φ = 1.2, (c) Φ = 1.6, and (d) Φ = 2.0.
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z axis is the height of the flame, with z = 0 indicating the surface of the burner port. The range 5 z 160 is considered in this article because this portion shows the key features of the flame. The maximum temperature obtained under individual conditions is shown in the figures for reference. In analyzing the temperature distribution, the overall equivalence ratio is the main factor of interest. Four flames are shown in each of Figs. 6a–6d, with overall equivalence ratios of 1.0, 1.2, 1.6, and 2.0 and Re = 2500. The adiabatic flame temperature of the fuel is about 1990 K under stoichiometric conditions. Fig. 6a shows the temperature distribution at Φ = 1.0. The temperature distributions of the two flames are quite similar. For both flames, the highest temperature region is located 5–6 mm away from the centerline. In the CAP flame, the maximum temperature zone appears close to the top of the entrainment zone, which is about 20 to 30 mm above the burner port. For the CoA flame, the maximum temperature is located higher, about 30 to 50 mm above the burner port. The maximum temperatures obtained in the two flames, around 1660 ◦ C, are almost the same, taking into consideration fluctuations, which average about 20 ◦ C, in the core of the flames. In Fig. 6b, for Φ = 1.2, the temperature distributions are similar to those obtained at Φ = 1.0 except that the flames have broader 1600 and 1400 ◦ C temperature zones. Moreover, the maximum temperature in both flames is almost the same, around 1690 ◦ C, which is slightly higher than that obtained at Φ = 1.0. For both Φ = 1.0 and Φ = 1.2, the maximum temperatures are offset from the centerline. This result is in line with observations that the flames have hollow structures at these equivalence ratios. The heated air in the hollow core is at a lower temperature than the surrounding flame. At Φ = 1.6, the peak temperature of the CAP flame increases to 1840 ◦ C, as shown in Fig. 6c. The location of the peak temperature zone also shifts toward the center of the flame and it appears higher. The shifting of the peak temperature zone is due to an increase in fuel momentum at higher equivalence ratios and the disappearance of the hollow core. The flame appearance shows that the height of the entrainment zone increases slightly with increased fueling rate. The maximum temperature in the CoA flame is about 170 ◦ C lower than that of the CAP flame. Furthermore, the highest temperature zone of the flame is still located away from the centerline at a height which is less than 80 mm but converges to the centerline at higher flame heights. This behavior indicates that non-premixed combustion is the main mode of combustion in the CoA flame. The higher maximum temperature obtained in the CAP flame is due to the improved mixing between air and fuel and an in-
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tense combustion zone is formed at the center of the flame. Fig. 6d shows the case of Φ = 2.0. Because of the excessive fuel supply, most of the fuel is burned in the diffusion mode in both flames. The maximum temperature of 1790 ◦ C obtained in the CAP flame is lower than the peak temperature obtained at Φ = 1.6. For this condition, the temperature distribution of the CAP flame is quite uniform at the center region and decays gradually in the radial direction, indicating that the energy is released more evenly in the flame. In the CoA flame, the maximum temperature obtained is 140 ◦ C, lower than that of the CAP flame, and the highest-temperature zone is offset from the centerline. The temperature variations indicate that the partial premixed combustion zone in the CAP flame is fuel lean at overall equivalence ratios of 1.0 and 1.2, but converges to a stoichiometric condition at higher overall equivalence ratios. This results in an increase in the peak flame temperature. The maximum temperature of the CoA flame is quite constant for all overall equivalence ratios investigated, indicating that diffusive combustion is prevalent throughout.
6. Centerline oxygen content With the special air and fuel jet arrangement in an IDF, the oxygen concentration at the centerline of the flame becomes an indicator of the level of combustion in the flame. Figs. 7a and 7b show the centerline oxygen concentrations of the CAP and CoA flames at Φ = 1.0 and Φ = 2.0, respectively, at Re = 2500. In Fig. 7a, at z = 0, the location of the burner tip, the oxygen content of both the CAP and CoA flames is 20.9%, indicating that the central stream is purely air. In the CAP flame, oxygen content decreases steeply until reaching a minimum of 5.5% at z = 60 mm. Oxygen content increases subsequently and reaches about 17.7% at z = 200 mm. In the CoA flame, the oxygen concentration remains high until z = 40 mm, drops gradually to a minimum value of 10% at z = 120 mm, and then increases gradually to about 15% at z = 200 mm. The results reflect the hollow structure of both types of flames at Φ = 1.0 and indicate more intense air–fuel mixing in the CAP flame than in the CoA flame, resulting in rapid consumption of oxygen along the centerline of the CAP flame. The mixing also induces turbulence which helps to entrain air into the flame and hence the CAP flame has higher oxygen concentration in the tail of the flame than the CoA flame. The centerline oxygen concentration can be compared with the flame appearances shown in Fig. 4. At
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(a)
trainment of atmospheric air. In the CoA flame, the oxygen consumption rate is slow at z < 40 mm and becomes steep at z > 40 mm. The oxygen concentration reaches a minimum of 1% at 90 < z < 140. Oxygen concentration increases again to about 10.8% at z = 200 mm. At Φ = 2.0, the flames are mainly diffusive in nature, as observed in Figs. 4a and 4b. For the CAP flame, basically, all the oxygen in the central air jet has been consumed at z = 50 mm, and subsequent combustion relies on oxygen from the environment. In the CoA flame, the majority but not all of the oxygen in the air jet is gradually consumed up to z = 90 mm. This shows a great difference between the two flames in the region 0 < z < 100. As observed in Fig. 3a, some premixed combustion occurs in the CAP flame in this region. In both Figs. 7a and 7b, the sharp decrease in oxygen concentration in the CAP flame indicates the intensity of air–fuel mixing and subsequent combustion in a narrow length of the flame. Partially premixed combustion occurs and consumes the oxygen in the air jet. On the other hand, the gradual consumption of oxygen in the CoA flame indicates that oxygen mixes with the fuel slowly, and thus, partially premixed combustion is unimportant, at least, at Re = 2500.
7. Emission analysis
(b) Fig. 7. Oxygen concentration along centerline of CAP and CoA flames at (a) Φ = 1.0 and Re = 2500 and (b) Φ = 2.0 and Re = 2500.
Φ = 1.0, the CAP flame is 60 mm high, whereas the CoA flame is 100 mm high. The flame heights match the height of the minimum oxygen concentration for the flames. This is an indication that oxygen is gradually consumed. The amount consumed is higher in the CAP flame than in the CoA flame. Not all of the oxygen is consumed at Φ = 1.0 for either the CAP or CoA flame; hence some of the air for combustion comes from the atmosphere, in addition to that supplied in the air jet. It is observed that above the visible flame height, the oxygen content increases again due to the entrainment of surrounding air. In Fig. 7b, oxygen consumption starts just above the air jet exit in the CAP flame and the oxygen concentration drops to 0.1% at z = 50 mm and remains at this level until z = 120 mm. The oxygen concentration increases again to 11.6% at z = 200 mm. Although the visible flame height is 195 mm in the CAP flame at Φ = 2.0, the fluctuating flame tip helps en-
The emission index has been generally used for comparison of flame emissions and for jet flames, especially for NOx emissions. Turns and Lovett [14] reported that for turbulent propane diffusion flame jets, both axial and radial EINOx values within the postflame region showed little deviation. However, the flame considered in this article is a different type of flame and is smaller in scale. It is important to find the location of the postflame region to ensure proper sampling. Fig. 8a shows the axial NOx and EINOx profiles of the CAP flame at Φ = 2.0. The x axis of the figure represents the distance between the sampling probe and the flame tip, with H = 0 indicating the flame tip. The sampling probe was moved at 10-mm intervals in the range 0 H 60, and 20-mm intervals at higher heights. The results demonstrate that as sampling height increases, NOx concentration decreases exponentially due to entrainment of air. NOx concentration approaches 42 ppm at the flame tip, but decreases to below 10 ppm at H = 200 mm. Although NOx concentration decays in the downstream direction, EINOx remains quite constant within the region 0 < H < 200 mm and has a value of about 2.5 g/kg fuel. Similar tests were carried out for CoA flames, for which EINOx also remains constant in the same region 0 < H < 200 mm. Hence, sampling within this region ensures consistent results.
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(a)
(b) Fig. 8. (a) Variation of NOx and EINOx in postflame region of CAP flame at Φ = 2.0 and Re = 2500. (b) Comparison of EINOx values for CAP and CoA flames with those for partially premixed flames [10,11].
The NOx emission indices of CAP flames and CoA flames at different overall equivalence ratios, at Re = 2500, are shown in Fig. 8b. The hollow circles represent CAP flames, and the squares represent CoA flames. For 1.0 Φ 3.0, the overall equivalence ratio was adjusted at 0.2 intervals, and the interval was increased to 1.0 at higher overall equivalence ratios until Φ = 6.0. At each overall equivalence ratio, EINOx was obtained by averaging nine measurements at heights below H = 100 mm. The CAP and CoA flames exhibit similar variations in EINOx with overall equivalence ratio. The CAP flame has an EINOx of 2.75 g/kg at Φ = 1.0 which increases to a peak value of 3.2 g/kg at Φ = 1.2. EINOx decays exponentially to below 2 g/kg at Φ = 3.0 and then decreases gradually to 1.75 g/kg at Φ = 6.0. For the CoA flame, after increasing from 2.3 g/kg at Φ = 1.0 to the peak value of 3.0 g/kg at Φ = 2.2, EINOx decreases to 2.4 g/kg at Φ = 6.0.
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The CAP flame has higher EINOx values than the CoA flame under close-to-stoichiometric conditions but lower EINOx values for Φ 1.6. The difference in EINOx between the flames remains quite constant for Φ > 2.0. Both flames show a bell-shaped distribution of EINOx values. The authors also performed EINOx emissions for the CAP and CoA flames at a fixed fueling rate and for different airflow rates, for equivalence ratios ranging from 0.6 to 6. The results also show a bell-shaped distribution, with a maximum value occurring at an overall equivalence ratio around 1.5. In Fig. 8b, the curve formed by the inverted triangles shows EINOx values for the turbulent, partially premixed flame reported in Lyle et al. [10], while the curve formed by the cross symbols shows results for the laminar partially premixed flame reported in Gore and Zhan [11], which are presented here for comparison purposes. In Lyle et al. [10], the fueling rate was fixed and the airflow rate was varied. The EINOx of the turbulent, partially premixed flame has a minimum value of 1.6 g/kg at Φ = 1.5 and increases under fuel-leaner and fuel-richer conditions. Similar results were obtained by Gore and Zhan [11], who tested the laminar, partially premixed flame using the experimental method of Lyle et al. [10], except that the flame was about 10 times less powerful. The laminar, partially premixed flame has a minimum value of about 0.95 g/kg at Φ = 2.0. Hence, the results show that there is a fundamental difference in NOx emissions between the IDF flames and partially premixed flames. The flames discussed in Refs. [10,11] consist of an inner fuel-rich partially premixed flame and an outer diffusion flame. The investigators suggested that NOx is formed by the Fenimore mechanism in the inner flame and by the thermal mechanism in the diffusion flame. The EINOx values converged to that of a pure diffusion flame at equivalence ratios larger than 5.0. The minimum EINOx was attributed by the investigators to the reburn mechanism. In the CAP flames, the increase in EINOx at overall equivalence ratios of 1.0 and 1.2 can be related to the increase in flame temperature, as the flames are mainly premixed in nature. EINOx decreases at higher overall equivalence ratios, up to 3.0. In this range of overall equivalence ratios, the flame consists of an inner fuel-lean partially premixed flame and an outer diffusion flame. Reburning could occur, which might lead to a reduction in NOx formation. At even higher overall equivalence ratios, the flame is mainly diffusion controlled. In the CoA flame, the initial gradual increase in EINOx can be related to the broadening of the hightemperature zones. The central air jet contributes air for some degree of fuel-lean premixed combustion which is not observed to be as obvious as in the CAP
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case. Reburning of NOx formed in the fuel-lean premixed combustion could also occur, which leads to a gradual reduction of EINOx up to an overall equivalence ratio of 4.0. The final EINOx in the CAP and CoA flames is an overall effect of NOx formed in the fuel-lean premixed combustion zone, NOx destruction due to the reburn mechanism, and NOx formed in the diffusion combustion mode. The CAP flame has more fuel in the premixed mode, and thus the reburn effect is more significant, resulting in lower EINOx values at very high overall equivalence ratios.
5. The centerline oxygen concentration is an indication of the combustion process. It drops to a minimum value in the CAP flame at the height of 50–60 mm, whereas in the CoA flame, the minimum value occurs at around 100 mm. This result indicates that the air and fuel mix and burn at around a height of 50 mm in the CAP flame, while mixing is limited in the CoA flame. 6. At Re = 2500 and for overall equivalence ratios of 1.0 to 6.0, the EINOx curve for both flames is bell-shaped, with a maximum value of 3.2 g/kg at Φ = 1.2 for the CAP flame and 3.0 g/kg at Φ = 2.2 for the CoA flame.
8. Conclusion In the present study, flame appearance, temperature distribution, oxygen profile, and NOx emission index are compared for inverse CAP and CoA flames. The following results were obtained. 1. An obvious fuel entrainment effect is observed in the CAP flame. The CAP flame can be divided into two zones separated by a neck: a lower entrainment zone, and an upper mixing and combustion zone. A bluish fluctuating reaction zone is formed in the mixing and combustion zone. In the CoA flame, fuel entrainment is obvious only at high air jet Reynolds numbers. The air jet normally coflows with the fuel jet. 2. At a fixed fueling rate, when the airflow rate is increased from Re = 1000 to Re = 4500, both the CAP and the CoA flames become shorter and change from yellowish to bluish, indicating transition from non-premixed combustion to premixed combustion. The transition occurs at a lower Re in the CAP flame, indicating that mixing is more intense in the CAP flame. 3. At a fixed airflow rate of Re = 2500, with an increase in the fueling rate from an overall equivalence ratio of 1.0 to 2.0, the CAP flame changes from a partially premixed flame to one mainly diffusive in nature. The CoA flame remains diffusive. The CoA flame is higher than the CAP flame at Φ 1.8. At higher overall equivalence ratios, the difference in height of the two flames is small. 4. At low overall equivalence ratios of Φ = 1.0 and 1.2, the temperature distributions of the CAP and CoA flames are similar. The highest temperatures attained in both flames are the same. At higher overall equivalence ratios, the maximum temperature attained in the CAP flame is higher than that of the CoA flame.
Acknowledgment The authors thank The Hong Kong Polytechnic University for financial support of the present study.
References [1] K.T. Wu, R.H. Essenhigh, Proc. Combust. Inst. 20 (1984) 1925–1932. [2] K.T. Wu, The Comparative Structure of Normal and Inverse Diffusion Flames, Ph.D. dissertation, Ohio State University, 1984. [3] T. Takagi, Z. Xu, M. Komiyama, Combust. Flame 106 (1996) 252–260. [4] G.W. Sidebotham, I. Glassman, Combust. Flame 90 (1992) 269–283. [5] J. Wentzell, Characteristics and Structure of Inverse Flames of Natural Gas, M.Sc. thesis, Queen’s University, 1998. [6] B. Fleck, Experimental and Numerical Investigation of the Novel Low-NOx Industrial Burner, Ph.D. thesis, Queen’s University, 1998. [7] Y. Bindar, A. Irawan, in: 6th APISCEU, 2002, pp. 124– 130. [8] W.P. Partridge, N.M. Laurendeau, Fuel 74 (1995) 1424–1430. [9] W.P. Partridge, J.R. Reisel, N.M. Laurendeau, Combust. Flame 116 (1999) 282–290. [10] K.H. Lyle, L.K. Tseng, J.P. Gore, N.M. Laurendeau, Combust. Flame 116 (1999) 627–639. [11] J.P. Gore, N.J. Zhan, Combust. Flame 105 (1996) 414– 427. [12] L.K. Sze, C.S. Cheung, C.W. Leung, Int. J. Heat Mass Transfer 47 (2004) 3119–3129. [13] A. Sato, K. Hashiba, M. Hasatani, S. Sugiyama, J. Kimura, Combust. Flame 24 (1975) 35–41. [14] S.R. Turns, J.A. Lovett, Combust. Sci. Technol. 66 (1989) 233–249.
Appearance, temperature, and NOx emission of two inverse diffusion flames with different port design L.K. Sze ∗ , C.S. Cheung, C.W. Leung Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China Received 11 October 2004; received in revised form 1 July 2005; accepted 12 July 2005 Available online 12 September 2005
Abstract Experiments were carried out to investigate the appearance, temperature distribution, and NOx emission index of two inverse diffusion flames, one with circumferentially arranged ports (CAPs) and the other with co-axial (CoA) jets, both burning LPG with 70% butane and 30% propane. Flame appearances were investigated first with a fixed fueling rate at different airflow rates equivalent to air jet Reynolds numbers (Re) of 1000 to 4500; and then at a fixed airflow rate with different fueling rates equivalent to overall equivalence ratios (Φ) of 1.0 to 2.0. The CAP flame is found to consist of two zones: a lower entrainment zone and an upper mixing and combustion zone. The CoA flame in most cases is similar to a diffusion flame. The two-zone structure can be observed only at Re larger than 2500. The temperature distributions of the flames are similar at overall equivalence ratios of 1.0 and 1.2 for Re = 2500, except that the corresponding CoA flame is longer. The flame temperature is higher in the CAP flame than the CoA flame at higher overall equivalence ratios. A measurement of centerline oxygen concentrations shows that the oxygen concentration reaches a minimum value at a flame height of 50 mm in the CAP flame but decreases more gradually in the CoA flame. It can be concluded that there is more intense air–fuel mixing in a CAP flame than the CoA flame. Investigation of the emission index of NOx (EINOx ) for both flames at Re = 2500 and overall equivalence ratios of 1.0 to 6.0 reveals that the EINOx curve of each flame is bell-shaped, with a maximum value of 3.2 g/kg at Φ = 1.2 for the CAP flame and 3 g/kg at Φ = 2.2 for the CoA flame. 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Circumferentially arranged ports; Co-axial; Inverse diffusion flame; Partially premixed flame; Emission index of NOx ; Reynolds number; LPG
1. Introduction Diffusion flames constitute one of the most basic flame configurations in the combustion process. Numerous investigations have already been carried out to gain an understanding of the characteristics of the flame. The inverse diffusion flame (IDF) is a kind of * Corresponding author. Fax: +852 23654703.
E-mail address: [email protected] (L.K. Sze).
flame with an inner air jet surrounded by an outer fuel jet(s). If the air jet velocity is high enough, the fuel at the outer jet(s) is entrained inward and mixes with the air to form a partially premixed flame. Otherwise, it might burn mainly in the diffusion mode. The IDF is a combination of a premixed flame and a diffusion flame that can have a larger flammability range than the premixed flame and is cleaner than the diffusion flame. Therefore, the feasibility of applying IDFs in industrial and domestic heating processes is of interest.
0010-2180/$ – see front matter 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2005.07.008
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In previous investigations, IDFs were obtained using coaxial jet (CoA) burners. A CoA burner consists of two concentric tubes, with the inner tube supplying air and the outer annular tube supplying fuel. Most of these studies were carried out under confined conditions. The most rigorous research on a confined IDF was carried out by Wu and Essenhigh [1,2], who mapped out six different types of IDFs according to their appearance and stability. They found that in a confined laminar condition, the appearance of the IDF was similar to that of a premixed flame, consisting of a bell-shaped bluish nonluminous reaction zone stabilized on the air jet. As the fuel flow rate was increased, a luminous yellowish wing with soot formation appeared on the bluish reaction zone. Takagi et al. [3] carried out experiments on a laminar IDF without confinement. They did not report the flame shape; however, they found that the flame temperature of the IDF was higher than that of the corresponding normal diffusion flame at the same flame height. Sidebotham and Glassman [4] investigated the soot formation characteristics of IDFs under the effects of temperature, fuel structure, and fuel concentration, and discovered that IDFs had lower soot loading than normal diffusion flames. Wentzell [5] reported the temperature distribution and flame length of an IDF at different air and fuel jet diameters and at different air and fuel Reynolds numbers. The IDF studied showed a dual flame structure composed of a premixed region located on the upstream portion of the flame centerline and a diffusion flame envelope. He suggested that the IDF could be classified as a partially premixed flame, and the degree of partial premixing on the flame centerline depended on the nozzle geometry and the flow conditions. Fleck [6] investigated the IDF formed in a staged air burner. For this burner, the high-velocity air jet entrained the fuel from the surrounding fuel jets, which were diluted by the surrounding flue gas. Because of the diluted fuel effect, the burner generated a large combustion zone of relatively low temperature that was not favorable to the formation of NOx in the flame. Recently, Bindar and Irawan [7] investigated a confined IDF at a high level of fuel excess. The flame shape they reported was different from that of Wu [2]. They proposed that the shape of the IDF was affected by both the inlet air momentum and the inlet fuel momentum. At low air jet velocity, the flame shape was similar to that of a normal diffusion flame. At higher inlet air velocity, usually 5–16 times higher than the fuel velocity, the flame shape was composed of two flame regions: a base and a tower on top of the base. They showed that the IDFs’ highest temperature zone was located at the center of the flame. As the supplied
fuel fraction in the mixture increased, the highest temperature zone would shift closer to the flame tip. In general, previous investigations were concerned mainly with flame temperature, flame appearance, and soot formation characteristics of CoA IDFs. There have been very few investigations on emission characteristics. Wu [2] plotted a simple emission species contour; however, there was lack of detailed analysis of the emissions. The low NOx emission characteristics of IDFs had been reported [6,8,9] in relation to staged air burners. Partridge and Laurendeau [8] studied NO in the IDFs of staged air burners, using different diameters of the central oxidizer jet, different angles of secondary air injection, and different overall equivalence ratios, and found that, in general, the emission index of NOx (EINOx ) for a staged air burner would achieve a maximum value at Φ = 1. More recently, Partridge et al. [9] reported the temperature distribution and NO concentration in a laminar IDF of a staged-air burner. The concentration of NO was found to increase through the IDF, and peaked just above the outer edge of the flame tip. Research work on IDFs is limited to coaxial jets and there is very little information concerning their emission characteristics. In this article, newly developed IDF burners with circumferentially arranged fuel ports (CAPs), as well as an IDF with a CoA fuel jet, are tested in the unconfined condition. Flame shape, temperature distribution, and NOx emissions are reported and compared. The intensities of air–fuel mixing at different air jet Reynolds numbers and fueling rates are also compared. Moreover, NOx emissions are compared with those from partially premixed flames as reported [10,11].
2. Experimental setup and method The CAP and CoA flame holders are shown in Fig. 1. The CAP flame holder used in the present study is the same as that used by Sze et al. [12]. It has an inner air jet of 6-mm diameter, surrounded by 12 outer fuel jets each 2.4 mm in diameter, with a center-to-center distance of 11.5 mm between the air
Fig. 1. Design of the CAP and CoA burners.
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Fig. 2. Schematic arrangement of IDF for pollutant measurements.
jet and the fuel jets. The CoA flame holder is made up of two concentric tubes, one with an internal diameter of 6 mm and the other with an internal diameter of 11.5 mm. The fuel port areas are the same for both flame holders so that the fuel jets have the same velocity for the same fuel consumption rate. Hence, the CoA fuel jet is in direct contact with the air jet, whereas in the CAP, the fuel jets are at a distance from the air jet. It was observed [12] that the air and fuel mixed more intensively than the corresponding CoA flames observed in published literature, which might be a result of the difference in fuel port design. The experimental setup is shown in Fig. 2. LPG (70% butane and 30% propane) and compressed air were metered by rotameters and passed through the corresponding jets. The flame holder was fixed on a positioning system so that the burner could be moved in all directions. Tests were first carried out with a fixed fueling rate while the air jet Re was varied from 1000 to 4500 to observe the flame shapes. Tests were continued with a fixed air jet Re of 2500 while the overall equivalence ratio was varied from 1.0 to 2.0, for observation of both flame shape and flame height. Although the air and fuel jets are initially separated, and hence the concept of equivalence ratio might not be applicable to an IDF, an indication of the relative strength of the air and fuel supply in relation to the stoichiometric air/fuel ratio would be helpful in the analysis. The overall equivalence ratio, Φ, is used in this article for such a purpose. The detailed flow conditions are given in Table 1. Temperature distributions and centerline oxygen concentrations were measured for flames at Re = 2500 and overall equivalence ratios of 1.0 to 2.0 to provide information for analysis of the combustion of the flames. The NOx concentration in the postflame region was measured and the emission index of NOx was calculated at Re = 2500 for overall equivalence
ratios from 1.0 to 6.0. Unless otherwise specified, the Reynolds number refers to that of the air jet alone. The temperature distribution of the flame was measured with an uncoated type B thermocouple with Pt–30%/Rh as the positive lead and Pt–6%/Rh as the negative lead. The wire has a diameter of 0.25 mm. The thermocouple was fixed, and the burner fixed on the positioning system was moved in the r direction (the x direction in Fig. 1), with an interval of 1 mm, from r = 0 mm to r = 30 mm; and moved in the z direction, with an interval of 5 mm, from z = 0 mm to z = 20 mm and, with an interval of 10 mm, from z = 30 mm to z = 160 mm. Temperatures reported in this article have been corrected for radiation loss according to the method of Sato et al. [13]. The maximum temperature correction in the present study was 154 ◦ C. The centerline oxygen concentration was collected by an air-cooled quartz probe with an inner diameter of 1 mm and outer diameter of 2 mm. The probe was inserted into the flame from the top to reduce disturbance of the flame. The gas collected was cooled down to about 60 ◦ C by the cooling air and sent to an oxygen gas analyzer. The concentration was measured from the surface of the flame holder to a flame height of 200 mm. Concentrations of NOx , CO, and CO2 in the postflame region were measured with the flame holder positioned inside a screened enclosure. The screened enclosure regulated airflow from the surroundings to reduce disturbance from surrounding air. Flue gas was collected by a stainless-steel probe in the postflame region. The air-cooled probe has an inner tube diameter of 6.35 mm. The probe was aligned at the same central axis as the air jet of the burner to ensure isokinetic sampling conditions. The flue gas, diluted and well-mixed with regulated atmospheric air, was extracted from the sampling probe and immediately cooled to about 60 ◦ C by the cooling air for the purpose of freezing the reaction and species
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Table 1 Operating conditions for inverse diffusion flames Air jet velocity (m/s)
Fuel jet velocity (m/s)
2.54 3.81 5.08 6.35 7.61 8.88 10.15 11.42
0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17
6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35 6.35
0.11 0.13 0.15 0.17 0.19 0.21 0.23 0.26 0.28 0.30 0.32 0.43 0.53 0.64
Reair
Φ
Air momentum/ fuel momentum
Conditions for Fig. 3 with fixed fuel flow rate 0.72 9.26 1.08 9.26 1.44 9.26 1.79 9.26 2.15 9.26 2.51 9.26 2.87 9.26 3.23 9.26
1000 1500 2000 2500 3000 3500 4000 4500
4.0 2.7 2.0 1.6 1.3 1.1 1.0 0.9
1.61 3.63 6.46 10.09 14.52 19.77 25.82 32.68
Conditions for Figs. 4 and 5 with fixed airflow rate 1.79 5.79 1.79 6.95 1.79 8.11 1.79 9.26 1.79 10.42 1.79 11.58 1.79 12.74 1.79 13.90 1.79 15.05 1.79 16.21 1.79 17.37 1.79 23.16 1.79 28.95 1.79 34.74
2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500 2500
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 4.0 5.0 6.0
10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09 10.09
Airflow rate (× 10−4 m3 /s)
formation. The sampled gas was then separated into two streams. One stream was further cooled to below 30 ◦ C for condensation and dehydration. The gas was then sent to a three-gas analyzer for O2 (Electrochemical cell), CO (NDIR), and CO2 (NDIR) measurement. Another stream was heated to and maintained above 190 ◦ C before entering a heated flame ionization detector (HFID) for HC measurement and above 60 ◦ C before entering a chemiluminescence analyzer (CLA) for NO/NOx measurement. Emission index, expressed as grams of NOx formed per kilogram of fuel consumed, was used in the analysis.
3. Flame appearance The appearance of a flame depends on the burner design, the fueling rate, and the airflow rate. The fueling rate and the airflow rate determine the overall equivalence ratio, as well as the ratio between the momentums of the jets. At a fixed fueling rate, an increase in the airflow rate decreases the overall equivalence ratio, but increases the ratio of air jet momentum to fuel jet momentum. At a fixed airflow rate, an increase in the fueling rate increases the overall equivalence ratio as well as the fuel jet momentum. The effects of these changes on the appearance of the flame are investigated first.
Total fuel flow rate (× 10−6 m3 /s)
3.1. Effect of airflow rate with fixed fueling rate on flame appearance Figs. 3a and 3b compare the effects of increasing airflow rate on the appearance of the flames. In each flame, the fueling rate was maintained constant at 9.26 × 10−6 m3 /s, whereas the airflow rate was varied from Re = 1000 to Re = 4500, to create flames with different air/fuel ratios varying from overall equivalence ratios of 4.0 to 0.9. For both CAP and CoA flames, flame length decreases with an increase in airflow rate. The corresponding CoA flames are longer than the CAP flames. The color of the flames changes from yellow to blue, indicating a change from fuel-rich to fuel-lean conditions. The dual flame structure (a bluish reaction zone followed by a diffusion combustion zone) as reported by Wu and Essenhigh [1] and Wentzell [5] is observed in the CAP flames at Re = 2000 and Re = 2500 and in the CoA flames at Re = 2500. In a pure diffusion flame, the fuel burns with air entrained from its surroundings. The flame is purely yellow in such case. In an inversion diffusion flame, the central air jet entrains the fuel jet(s). The degree of entrainment of the fuel jet(s) by the air jet determines the extent to which the fuel is burned in the diffusion mode or premixed mode. Change in the color of the
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(a)
(b) Fig. 3. Effect of airflow rate on appearance of (a) CAP flame and (b) CoA flame.
flames is an indication of the extent of fuel entrainment. It can be seen that with an increase in airflow rate, entrainment becomes stronger, leading to more fuel burned in the premixed mode, and at high enough airflow rate, all the fuel burns in the premixed mode. The same effects of airflow rate on flame length and color have been reported [5,7] for CoA flames. In Fig. 3a, it can be observed that the CAP flame consists of two parts, which becomes more obvious at higher airflow rates: a lower zone in which fuel is entrained toward the air jet, and an upper zone in which the fuel is mixed with air and intense combustion occurs. We call the lower part the entrainment zone and the upper part the mixing and combustion zone. The same flame shape was reported [7] for a CoA flame. The height of the entrainment zone decreases as the airflow rate is increased, indicating an increase in the entrainment effect. In Fig. 3b, it can be observed that at low airflow rates, the CoA flame is close to a pure diffusion flame. As the airflow rate increases, the fuel jet is entrained
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toward the air jet. At high enough airflow rates, similar to the CAP flame, the flame consists of two parts: a base and a tower. The two-zone structure starts to be observable at Re = 2500 and becomes more obvious at higher Re. Comparison of the two sets of flames shows that mixing is more intense in the CAP flame than in the CoA flame. At the same Re of 2000 and 2500, the CAP flame has a blue zone inside the flame, indicating the existence of partial premixed combustion in the flame, while partial premixed combustion is not observable in the corresponding CoA flame. At higher Re, turbulence and stronger entrainment aid mixing between air and fuel, so that the flame becomes shorter and more intense in combustion. This is indicated by the height of the entrainment zone of the flames. In the CAP flames, the entrainment zone becomes shorter as the airflow rate is increased; however, the change is not significant. In CoA flames, the entrainment zone is not observed below Re = 2000. At Re = 2500, the fuel jet starts to converge to the centerline of the flame and becomes turbulent above the region of convergence. At still higher airflow rates, the entrainment zone becomes shorter. At Re of 4000 and 4500, the CAP flames are mainly bluish with yellowish entrainment zones but the CoA flames are completely bluish. The distance between the fuel jets and the air jet of the CAP creates this effect. The better fuel entrainment of the CAP flame might be attributed to the separation of the fuel jets from the air jet. For the CoA flame, at lower airflow rates, the fuel basically coflows with the central air jet. Mixing with the inner air jet is limited due to the small difference in the momentums of the jets. At higher airflow rates, the difference in the momentums of the two jets increases, leading to entrainment of some fuel into the central air jet. For the CAP flame, the central air jet and the fuel jets both entrain surrounding quiescent gas. This creates a low-pressure region between the jets, resulting in the fuel jets flowing toward the central air jet within the entrainment zone. A numerical simulation with a commercial CFD code indicates this effect, with the CAP flame having a higher radial velocity than the corresponding CoA flame at the neck of the entrainment zone. 3.2. Effect of fuel flow rate with fixed airflow rate on flame appearance We have chosen a fixed air jet Re of 2500 for investigation of differences between the two flames under different fueling rates. The fueling rate was varied from an overall equivalence ratio of 1.0 to 2.0 so that the effect of varying the fueling rate can be observed. In Fig. 4a are schematic diagrams and photographs of
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(a)
(b) Fig. 4. Schematic diagrams of (a) CAP flame and (b) CoA flame and effect of overall equivalence ratio on flame appearance at Re = 2500.
the CAP flames, and in Fig. 4b are those for the CoA flames. Fig. 4a shows that a change in the fueling rate does not significantly affect the shape of the CAP flames. All CAP flames are composed of two parts. The upper part of the flame is similar to a normal, partially premixed flame and combustion occurs mainly within this zone. At the center of the flame, a thick fluctuating reaction zone can be observed. The visible blue reaction sheet is formed at or close to stoichiometric conditions. In the CAP flame, the local equivalence
ratio varies within the flame as the air and fuel are not uniformly mixed. A yellow soot ring is always observed at the top of the entrainment zone at all equivalence ratios studied, indicating non-premixed combustion of the fuel jets before entrainment into the central air jet. Variation of fueling rate has little effect on the length of the entrainment zone and no effect on the color of this zone. The effect of overall equivalence ratio on the appearance of the mixing and combustion zone is similar to that normally observed in partially pre-
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mixed flames. At Φ = 1.0, the flame is mainly blue and a thick fluctuating reaction zone can be clearly observed, indicating that the combustion is mainly premixed. The air jet in this case is able to penetrate through the flame, creating a hollow flame structure observable from the top of the flame, as shown in the schematic on the left-hand side of Fig. 4a. For 1.2 Φ 1.4, there is a main bluish reaction zone with a weak yellowish tail appearing on top of it, indicating that diffusive combustion with soot formation is occurring under excess fuel conditions at the outer layer of the flame. The hollow flame structure disappears at Φ = 1.4. For Φ 1.6, the bluish reaction zone representing premixed combustion can still be observed, but the flame is mainly diffusive in nature and has a highly fluctuating flame tip, resulting in the kind of flame structure shown in the schematic on the right-hand side of Fig. 4a. The increase in the amount of fuel burning in the diffusion mode leads to an increase in the length of the mixing and combustion zone and, hence, to an increase in flame length. Fig. 4b shows that the corresponding CoA flames are basically diffusive in nature within the range 1.0 Φ 2.0. There is also an increase in flame length with increasing Φ, but the increase is not as great as that observed for CAP flames. At Φ = 1.0 and 1.2, the flame is again hollow in shape with a laminar flow structure, as shown schematically on the left-hand side of Fig. 4b. The hollow structure indicates that the air jet mainly coflows with the fuel jets, with some of the fuel being entrained into the air jet. This results in a diffusion flame rather than a partially premixed flame. The flame again closes at higher fueling rates, resulting in the kind of flame shown in the schematic on the right-hand side of Fig. 4b. A transition neck can be observed when the fueling rate increases to Φ 1.4. Under the transition neck, the flame remains laminar; however, above the neck, it becomes turbulent. The observations show that the central air jet provides little improvement in air and fuel mixing in CoA flames, compared with CAP flames, at Re = 2500. Comparison of the two types of flames reveals that they are significantly different from each other at Re = 2500, with overall equivalence ratios ranging from 1.0 to 2.0. The CAP flame has a dual flame structure, whereas the CoA flame is mainly diffusive in nature. We have explained that CAP flames have better entrainment than CoA flames. In this case, when the airflow is fixed, an increase in the fueling rate has two implications. First, more fuel has to be burned in the diffusion mode. Second, the fuel jet comes out at a higher velocity and less fuel is entrained into the entrainment zone; hence, less fuel is burned in the partially premixed mode. We can observe from the CAP flames that some of the fuel flows along the periphery
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of the flame and burns in the diffusion mode. Excess fuel burned in the diffusion mode results in a longer flame length with soot formation. Hence, there is no change in the basic structure of the flames within the range of fueling rates studied.
4. Flame height analysis Fig. 5 demonstrates the variation of flame height with overall equivalence ratio at Re = 2500 for the CAP flame (Fig. 5a) and the CoA flame (Fig. 5b). In this study, the equivalence ratio is extended to 6.0. In both cases, increasing the overall equivalence ratio leads to an increase in flame height. The flame lengths were obtained by visually averaging 10 images captured by a digital camera. As mentioned before, the CAP flame is composed of two parts. In Fig. 5a the hollow circle represents the height of the entrainment zone, and the inverted triangle represents the total flame height, with the fluctuation in height indicated by the error bars. Fig. 5b shows the height of the CoA
(a)
(b) Fig. 5. Effect of overall equivalence ratio on (a) CAP flame height and (b) CoA flame height at Re = 2500.
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flame. As no entrainment was observed, the triangle represents the total flame height with the flame fluctuation represented by the error bars. In the CAP flame, the height of the entrainment zone is affected very little by the fueling rate. At an overall stoichiometric condition of Φ = 1.0, the height of the entrainment zone is 10 mm, which increases to 25 mm at Φ = 6.0. The increment is small in comparison with total flame height. Moreover, the height of the entrainment zone remains very stable. For both flames, in the range 1.0 Φ 1.4, a somewhat steeper increase in flame height is observed. As the overall equivalence ratio is further increased to 1.4 < Φ 6.0, the increase in flame height becomes more gentle. The flame height of a CoA flame at Φ = 1.0 is 30 mm higher than that of a corresponding CAP flame. The difference in flame height is due mainly to the entrainment effect in the CAP flame. The entrainment improves the mixing between air and fuel and generates an intense combustion zone and, hence, shortens the flame length. However, as the overall equivalence ratio increases, both flames exhibit a diffusion flame nature and the difference in flame height decreases.
The fluctuation in flame height increases with rising overall equivalence ratio. For the CAP flame, at Φ = 1.0 the maximum fluctuation in flame height is ±5 mm, which becomes larger than 20 mm at Φ 4.0. At Φ = 6.0 the maximum fluctuation is as large as ±30 mm. The fluctuation for the CoA flame is large even at low overall equivalence ratios. The maximum fluctuation is larger than 20 mm even at Φ 1.4, whereas in the CAP flame, the same level of fluctuation occurs at Φ 4.0. The flame heights and their fluctuations indicate that there is better air fuel mixing in the CAP flames than the CoA flames and that the former are more stable than the latter.
5. Temperature distribution Figs. 6a–6d are contour plots of flame temperature on the r–z plane. The figures are arranged with the left-hand side showing CAP flame temperatures and the right-hand side showing CoA flame temperatures for easy comparison. The r axis is the radial distance, with r = 0 representing the center of the air jet. The
(a)
(b)
(c)
(d)
Fig. 6. Temperature distribution of CAP and CoA flames at (a) Φ = 1.0, (b) Φ = 1.2, (c) Φ = 1.6, and (d) Φ = 2.0.
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z axis is the height of the flame, with z = 0 indicating the surface of the burner port. The range 5 z 160 is considered in this article because this portion shows the key features of the flame. The maximum temperature obtained under individual conditions is shown in the figures for reference. In analyzing the temperature distribution, the overall equivalence ratio is the main factor of interest. Four flames are shown in each of Figs. 6a–6d, with overall equivalence ratios of 1.0, 1.2, 1.6, and 2.0 and Re = 2500. The adiabatic flame temperature of the fuel is about 1990 K under stoichiometric conditions. Fig. 6a shows the temperature distribution at Φ = 1.0. The temperature distributions of the two flames are quite similar. For both flames, the highest temperature region is located 5–6 mm away from the centerline. In the CAP flame, the maximum temperature zone appears close to the top of the entrainment zone, which is about 20 to 30 mm above the burner port. For the CoA flame, the maximum temperature is located higher, about 30 to 50 mm above the burner port. The maximum temperatures obtained in the two flames, around 1660 ◦ C, are almost the same, taking into consideration fluctuations, which average about 20 ◦ C, in the core of the flames. In Fig. 6b, for Φ = 1.2, the temperature distributions are similar to those obtained at Φ = 1.0 except that the flames have broader 1600 and 1400 ◦ C temperature zones. Moreover, the maximum temperature in both flames is almost the same, around 1690 ◦ C, which is slightly higher than that obtained at Φ = 1.0. For both Φ = 1.0 and Φ = 1.2, the maximum temperatures are offset from the centerline. This result is in line with observations that the flames have hollow structures at these equivalence ratios. The heated air in the hollow core is at a lower temperature than the surrounding flame. At Φ = 1.6, the peak temperature of the CAP flame increases to 1840 ◦ C, as shown in Fig. 6c. The location of the peak temperature zone also shifts toward the center of the flame and it appears higher. The shifting of the peak temperature zone is due to an increase in fuel momentum at higher equivalence ratios and the disappearance of the hollow core. The flame appearance shows that the height of the entrainment zone increases slightly with increased fueling rate. The maximum temperature in the CoA flame is about 170 ◦ C lower than that of the CAP flame. Furthermore, the highest temperature zone of the flame is still located away from the centerline at a height which is less than 80 mm but converges to the centerline at higher flame heights. This behavior indicates that non-premixed combustion is the main mode of combustion in the CoA flame. The higher maximum temperature obtained in the CAP flame is due to the improved mixing between air and fuel and an in-
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tense combustion zone is formed at the center of the flame. Fig. 6d shows the case of Φ = 2.0. Because of the excessive fuel supply, most of the fuel is burned in the diffusion mode in both flames. The maximum temperature of 1790 ◦ C obtained in the CAP flame is lower than the peak temperature obtained at Φ = 1.6. For this condition, the temperature distribution of the CAP flame is quite uniform at the center region and decays gradually in the radial direction, indicating that the energy is released more evenly in the flame. In the CoA flame, the maximum temperature obtained is 140 ◦ C, lower than that of the CAP flame, and the highest-temperature zone is offset from the centerline. The temperature variations indicate that the partial premixed combustion zone in the CAP flame is fuel lean at overall equivalence ratios of 1.0 and 1.2, but converges to a stoichiometric condition at higher overall equivalence ratios. This results in an increase in the peak flame temperature. The maximum temperature of the CoA flame is quite constant for all overall equivalence ratios investigated, indicating that diffusive combustion is prevalent throughout.
6. Centerline oxygen content With the special air and fuel jet arrangement in an IDF, the oxygen concentration at the centerline of the flame becomes an indicator of the level of combustion in the flame. Figs. 7a and 7b show the centerline oxygen concentrations of the CAP and CoA flames at Φ = 1.0 and Φ = 2.0, respectively, at Re = 2500. In Fig. 7a, at z = 0, the location of the burner tip, the oxygen content of both the CAP and CoA flames is 20.9%, indicating that the central stream is purely air. In the CAP flame, oxygen content decreases steeply until reaching a minimum of 5.5% at z = 60 mm. Oxygen content increases subsequently and reaches about 17.7% at z = 200 mm. In the CoA flame, the oxygen concentration remains high until z = 40 mm, drops gradually to a minimum value of 10% at z = 120 mm, and then increases gradually to about 15% at z = 200 mm. The results reflect the hollow structure of both types of flames at Φ = 1.0 and indicate more intense air–fuel mixing in the CAP flame than in the CoA flame, resulting in rapid consumption of oxygen along the centerline of the CAP flame. The mixing also induces turbulence which helps to entrain air into the flame and hence the CAP flame has higher oxygen concentration in the tail of the flame than the CoA flame. The centerline oxygen concentration can be compared with the flame appearances shown in Fig. 4. At
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(a)
trainment of atmospheric air. In the CoA flame, the oxygen consumption rate is slow at z < 40 mm and becomes steep at z > 40 mm. The oxygen concentration reaches a minimum of 1% at 90 < z < 140. Oxygen concentration increases again to about 10.8% at z = 200 mm. At Φ = 2.0, the flames are mainly diffusive in nature, as observed in Figs. 4a and 4b. For the CAP flame, basically, all the oxygen in the central air jet has been consumed at z = 50 mm, and subsequent combustion relies on oxygen from the environment. In the CoA flame, the majority but not all of the oxygen in the air jet is gradually consumed up to z = 90 mm. This shows a great difference between the two flames in the region 0 < z < 100. As observed in Fig. 3a, some premixed combustion occurs in the CAP flame in this region. In both Figs. 7a and 7b, the sharp decrease in oxygen concentration in the CAP flame indicates the intensity of air–fuel mixing and subsequent combustion in a narrow length of the flame. Partially premixed combustion occurs and consumes the oxygen in the air jet. On the other hand, the gradual consumption of oxygen in the CoA flame indicates that oxygen mixes with the fuel slowly, and thus, partially premixed combustion is unimportant, at least, at Re = 2500.
7. Emission analysis
(b) Fig. 7. Oxygen concentration along centerline of CAP and CoA flames at (a) Φ = 1.0 and Re = 2500 and (b) Φ = 2.0 and Re = 2500.
Φ = 1.0, the CAP flame is 60 mm high, whereas the CoA flame is 100 mm high. The flame heights match the height of the minimum oxygen concentration for the flames. This is an indication that oxygen is gradually consumed. The amount consumed is higher in the CAP flame than in the CoA flame. Not all of the oxygen is consumed at Φ = 1.0 for either the CAP or CoA flame; hence some of the air for combustion comes from the atmosphere, in addition to that supplied in the air jet. It is observed that above the visible flame height, the oxygen content increases again due to the entrainment of surrounding air. In Fig. 7b, oxygen consumption starts just above the air jet exit in the CAP flame and the oxygen concentration drops to 0.1% at z = 50 mm and remains at this level until z = 120 mm. The oxygen concentration increases again to 11.6% at z = 200 mm. Although the visible flame height is 195 mm in the CAP flame at Φ = 2.0, the fluctuating flame tip helps en-
The emission index has been generally used for comparison of flame emissions and for jet flames, especially for NOx emissions. Turns and Lovett [14] reported that for turbulent propane diffusion flame jets, both axial and radial EINOx values within the postflame region showed little deviation. However, the flame considered in this article is a different type of flame and is smaller in scale. It is important to find the location of the postflame region to ensure proper sampling. Fig. 8a shows the axial NOx and EINOx profiles of the CAP flame at Φ = 2.0. The x axis of the figure represents the distance between the sampling probe and the flame tip, with H = 0 indicating the flame tip. The sampling probe was moved at 10-mm intervals in the range 0 H 60, and 20-mm intervals at higher heights. The results demonstrate that as sampling height increases, NOx concentration decreases exponentially due to entrainment of air. NOx concentration approaches 42 ppm at the flame tip, but decreases to below 10 ppm at H = 200 mm. Although NOx concentration decays in the downstream direction, EINOx remains quite constant within the region 0 < H < 200 mm and has a value of about 2.5 g/kg fuel. Similar tests were carried out for CoA flames, for which EINOx also remains constant in the same region 0 < H < 200 mm. Hence, sampling within this region ensures consistent results.
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(a)
(b) Fig. 8. (a) Variation of NOx and EINOx in postflame region of CAP flame at Φ = 2.0 and Re = 2500. (b) Comparison of EINOx values for CAP and CoA flames with those for partially premixed flames [10,11].
The NOx emission indices of CAP flames and CoA flames at different overall equivalence ratios, at Re = 2500, are shown in Fig. 8b. The hollow circles represent CAP flames, and the squares represent CoA flames. For 1.0 Φ 3.0, the overall equivalence ratio was adjusted at 0.2 intervals, and the interval was increased to 1.0 at higher overall equivalence ratios until Φ = 6.0. At each overall equivalence ratio, EINOx was obtained by averaging nine measurements at heights below H = 100 mm. The CAP and CoA flames exhibit similar variations in EINOx with overall equivalence ratio. The CAP flame has an EINOx of 2.75 g/kg at Φ = 1.0 which increases to a peak value of 3.2 g/kg at Φ = 1.2. EINOx decays exponentially to below 2 g/kg at Φ = 3.0 and then decreases gradually to 1.75 g/kg at Φ = 6.0. For the CoA flame, after increasing from 2.3 g/kg at Φ = 1.0 to the peak value of 3.0 g/kg at Φ = 2.2, EINOx decreases to 2.4 g/kg at Φ = 6.0.
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The CAP flame has higher EINOx values than the CoA flame under close-to-stoichiometric conditions but lower EINOx values for Φ 1.6. The difference in EINOx between the flames remains quite constant for Φ > 2.0. Both flames show a bell-shaped distribution of EINOx values. The authors also performed EINOx emissions for the CAP and CoA flames at a fixed fueling rate and for different airflow rates, for equivalence ratios ranging from 0.6 to 6. The results also show a bell-shaped distribution, with a maximum value occurring at an overall equivalence ratio around 1.5. In Fig. 8b, the curve formed by the inverted triangles shows EINOx values for the turbulent, partially premixed flame reported in Lyle et al. [10], while the curve formed by the cross symbols shows results for the laminar partially premixed flame reported in Gore and Zhan [11], which are presented here for comparison purposes. In Lyle et al. [10], the fueling rate was fixed and the airflow rate was varied. The EINOx of the turbulent, partially premixed flame has a minimum value of 1.6 g/kg at Φ = 1.5 and increases under fuel-leaner and fuel-richer conditions. Similar results were obtained by Gore and Zhan [11], who tested the laminar, partially premixed flame using the experimental method of Lyle et al. [10], except that the flame was about 10 times less powerful. The laminar, partially premixed flame has a minimum value of about 0.95 g/kg at Φ = 2.0. Hence, the results show that there is a fundamental difference in NOx emissions between the IDF flames and partially premixed flames. The flames discussed in Refs. [10,11] consist of an inner fuel-rich partially premixed flame and an outer diffusion flame. The investigators suggested that NOx is formed by the Fenimore mechanism in the inner flame and by the thermal mechanism in the diffusion flame. The EINOx values converged to that of a pure diffusion flame at equivalence ratios larger than 5.0. The minimum EINOx was attributed by the investigators to the reburn mechanism. In the CAP flames, the increase in EINOx at overall equivalence ratios of 1.0 and 1.2 can be related to the increase in flame temperature, as the flames are mainly premixed in nature. EINOx decreases at higher overall equivalence ratios, up to 3.0. In this range of overall equivalence ratios, the flame consists of an inner fuel-lean partially premixed flame and an outer diffusion flame. Reburning could occur, which might lead to a reduction in NOx formation. At even higher overall equivalence ratios, the flame is mainly diffusion controlled. In the CoA flame, the initial gradual increase in EINOx can be related to the broadening of the hightemperature zones. The central air jet contributes air for some degree of fuel-lean premixed combustion which is not observed to be as obvious as in the CAP
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case. Reburning of NOx formed in the fuel-lean premixed combustion could also occur, which leads to a gradual reduction of EINOx up to an overall equivalence ratio of 4.0. The final EINOx in the CAP and CoA flames is an overall effect of NOx formed in the fuel-lean premixed combustion zone, NOx destruction due to the reburn mechanism, and NOx formed in the diffusion combustion mode. The CAP flame has more fuel in the premixed mode, and thus the reburn effect is more significant, resulting in lower EINOx values at very high overall equivalence ratios.
5. The centerline oxygen concentration is an indication of the combustion process. It drops to a minimum value in the CAP flame at the height of 50–60 mm, whereas in the CoA flame, the minimum value occurs at around 100 mm. This result indicates that the air and fuel mix and burn at around a height of 50 mm in the CAP flame, while mixing is limited in the CoA flame. 6. At Re = 2500 and for overall equivalence ratios of 1.0 to 6.0, the EINOx curve for both flames is bell-shaped, with a maximum value of 3.2 g/kg at Φ = 1.2 for the CAP flame and 3.0 g/kg at Φ = 2.2 for the CoA flame.
8. Conclusion In the present study, flame appearance, temperature distribution, oxygen profile, and NOx emission index are compared for inverse CAP and CoA flames. The following results were obtained. 1. An obvious fuel entrainment effect is observed in the CAP flame. The CAP flame can be divided into two zones separated by a neck: a lower entrainment zone, and an upper mixing and combustion zone. A bluish fluctuating reaction zone is formed in the mixing and combustion zone. In the CoA flame, fuel entrainment is obvious only at high air jet Reynolds numbers. The air jet normally coflows with the fuel jet. 2. At a fixed fueling rate, when the airflow rate is increased from Re = 1000 to Re = 4500, both the CAP and the CoA flames become shorter and change from yellowish to bluish, indicating transition from non-premixed combustion to premixed combustion. The transition occurs at a lower Re in the CAP flame, indicating that mixing is more intense in the CAP flame. 3. At a fixed airflow rate of Re = 2500, with an increase in the fueling rate from an overall equivalence ratio of 1.0 to 2.0, the CAP flame changes from a partially premixed flame to one mainly diffusive in nature. The CoA flame remains diffusive. The CoA flame is higher than the CAP flame at Φ 1.8. At higher overall equivalence ratios, the difference in height of the two flames is small. 4. At low overall equivalence ratios of Φ = 1.0 and 1.2, the temperature distributions of the CAP and CoA flames are similar. The highest temperatures attained in both flames are the same. At higher overall equivalence ratios, the maximum temperature attained in the CAP flame is higher than that of the CoA flame.
Acknowledgment The authors thank The Hong Kong Polytechnic University for financial support of the present study.
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