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Investigation of combustion process of a kerosene fueled combustor with air throttling
Combustion and Flame 179 (2017) 74–85
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Investigation of combustion process of a kerosene fueled combustor with air throttling Ye Tian∗, Shunhua Yang, Jialing Le, Fuyu Zhong, Xiaoqiang Tian Science and Technology on Scramjet Laboratory of Hypervelocity Aerodynamics Institute, CARDC, Mianyang 621000, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Revised 14 October 2016 Accepted 24 January 2017
Keywords: Combustion process Flame stabilization Air throttling Kerosene Ignition Blown off
a b s t r a c t An experimental and numerical study was carried out to investigate the combustion process of a kerosene fueled combustor with air throttling. The results were obtained with the inflow conditions of Mach number of 2.0, total temperature of 953 K and total pressure of 0.82 MPa, respectively. The air throttling was located 0.575 m downstream the combustor entrance, and the mass flux of air throttling was 27.2% inflow mass flux. The pilot flame was blown off by the room temperature kerosene when the kerosene supply pressure was 0.25 MPa, but the kerosene was ignited successfully when the throttling air was injected into the combustor, and the flame stabilization was achieved even when the pilot hydrogen was removed. The combustion process could be divided into four parts based on changes in the pressure monitored near the cavity: kerosene was ignited successfully by the pilot flame, and the mixture flame was stable during part-a. As the kerosene supply pressure was increasing, the flame was blown off by the room temperature kerosene in part-b. Successful ignition and flame stabilization had been achieved with the aid of air throttling in part-c, and the combustion mode was subsonic combustion. The flame was stable even after the pilot hydrogen was removed in part-d, but the combustion mode was supersonic combustion. © 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Achieving flame stabilization is a difficult problem in scramjet combustor, because in supersonic combustor, the time available for fuel injected, vaporized, mixed with air, and combustion is very short, of the order of milliseconds [1]. This problem applies especially to hydrocarbon fuels such as kerosene that are often used in the scramjet, Which consists of long chains of hydrogen and carbon molecules with longer reaction times than smaller molecules (such as hydrogen and ethylene) and thus has long ignition delay times, often exceeding a millisecond [2]. So the flameholders should be used in the scramjet combustor in order to achieve flame stabilization. Some different kinds of flame holders, such as cavity [3–5], strut [6–9], step [10] and air throttling [11–14], have been investigated by many researchers. Ignition transients in a scramjet engine with air throttling were investigated by Li and coworkers [13,14]. In their paper, a pre-combustion shock train was generated in the isolator due to the increased back pressure by the throttling air. The resultant increase in the temperature and pressure of the airstream in the combustor, along with the decrease in the flow velocity, lead to smooth and reliable ignition. The incidentally formed separated
∗
Corresponding author. E-mail addresses: [email protected] (Y. Tian), [email protected] (J. Le).
flows adjacent to the combustor sidewall improved fuel/air mixing as a result of enhanced flow distortion and increased residence time. Successful ignition could only be achieved with the aid of air throttling under the present flow conditions. Chemical reactions were intensified and produced sufficient heat release to maintain a flow environment conducive to flame stabilization. A self-sustaining mechanism was thus established between the flow and flame development. Stable flames were achieved even after the deactivation of air throttling. Our previous work studied the effect of air throttling on flow structure, fuel/air mixing, ignition transients and flame stabilization in the scramjet combustor [15–19]. Mathur et al. [20] conducted an experiment using air throttling to initiate combustion in a scramjet combustor. Their results showed that once the air throttling was removed after the flame establishment, the shock train was retained leading to sustained combustion if heat release was sufficient. Conversely, insufficient heat release might result in an unstable shock train and caused flame blowout. Donbar et al. [21] tested the operation sequence of ignition in an ethylene fueled scramjet combustor. Air throttling was used after a stable fuel condition was reached. Once ignition occurred by activating spark igniters, the air throttling was removed, after which sustained combustion proceeded. From the above discussion, we found the effect of air throttling on flame stabilization had been investigated by several researchers, but most of them focused on gas (ethylene or hydrogen) fueled
http://dx.doi.org/10.1016/j.combustflame.2017.01.021 0010-2180/© 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Y. Tian et al. / Combustion and Flame 179 (2017) 74–85
Fig. 1. Photo of the supersonic combustion facility in CARDC.
Nomenclatures Ma ER D L P t x T
Mach number equivalence ratio depth of cavity length of cavity wall pressure of combustor time distance from combustor entrance mass averaged temperature
combustor. Few published papers investigated the effect of air throttling on room temperature liquid (kerosene) fueled combustor, which was our main purpose of the present paper. We used a small scaled cavity as a flameholder in the combustor in order to reduce the cavity drag, also the pilot hydrogen was used to ignite the room temperature kerosene, and air throttling was used to achieve flame stabilization after pilot hydrogen was removed. 2. Experimental and numerical simulation methods 2.1. Facility and combustor configuration Experimental investigations were conducted on a directconnected supersonic combustion facility (Fig. 1) in China Aerodynamics Research and Development Centre (CARDC). Hydrogen fueled heater was used to heat the air up to 10 0 0 K and additional oxygen was added to maintain a 21% O2 mole fraction in the
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vitiated air, and the mole fraction of H2 O and N2 were 12%, and 67%, respectively. A pulse Mach 2.0 airflow was supplied via a two-dimensional nozzle which was connected to the upstream of the combustor. The total temperature and total pressure of the inflow were 953 K and 0.82 MPa, respectively. The lab-scale combustor [19] was made of stainless steel and divided into two sections, which was shown in Fig. 2, the first section was a rectangular isolator with the length of 430 mm (350 mm straight section and 80 mm expansion section with upwall divergent angle being 1.4°) and the cross section area was 30 × 150mm² . The second section was the combustor which included a cavity (D: 11 mm, L/D: 11) and a four- part expansion section (range: 551mm–1070 mm). There were two fuel injected positions shown in Fig. 2, the first injector was designed for injecting room temperature kerosene and the second injector was designed for introducing pilot hydrogen, and the locations of the two injectors were 410 mm and 440 mm from the isolator entrance, respectively. The kerosene was injected at sonic speed at an angle of 90° to the airflow by fifteen 0.3 mm in diameter fuel injection holes, and the hydrogen was injected by ten 1.0 mm in diameter fuel injection holes, that of the throttling air were twenty 3 mm in diameter injection holes. The sampling frequency of pressure transducer was 1 kHz, which was used for measuring the wall pressure. Schlieren images were captured by a CCD camera, and the exposure time was 1μs and the frame rate was 10,0 0 0 fps. The chemiluminescence of CH∗ was used to mark the flame zones in the combustor. The luminosity from CH∗ was imaged by a CCD camera with ± 5 nm bandwidth interference filters centered at 430 nm and the exposure time was 1/20 0 0 s. The test sequence of the studying case was shown in Fig. 3 and Table 1, when hydrogen entered into the facility heater at t = 1.80 s, the cold flow then generated. The running time of the facility was about 600 ms (1.95s–2.57 s), when the supply pressure of hydrogen was kept as constant. The kerosene was injected into the combustor from t = 1.95 s to t = 2.57 s, and the supply pressure of kerosene was increased to 2.0 MPa (equivalence ratio: 0.3) from t = 1.95 s to 2.30 s. The spark in the combustor cavity was working all the test time, so the pilot hydrogen was ignited at once when it was injected into the combustor. The air throttling was started to be injected into the combustor at t = 2.35 s, just 0.04 s before pilot hydrogen was off. The location of air throttling was 0.575 m from the combustor entrance, and the mass flux of air throttling was about 27.2% inflow mass flux. The equivalence ratio (ER) of kerosene was 0.3, and the ER of pilot hydrogen was 0.08. 2.2. Numerical methods In this study, the inhouse CFD code AHL3D [19,23] software which had been introduced in reference [18,19,23] was used for computation. A fully coupled form of species conservation
Fig. 2. Schematic illustration of the combustor.
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Fig. 3. Test sequence of different parameters.
Fig. 4. Wall pressures of pressure monitor, kerosene supply and pilot hydrogen. Table 1 Operation sequence of the test. Operation sequence
Cold flow starts
Kerosene injected on
Hydrogen injected on
Air throttling on
Hydrogen injected off
Kerosene and Air throttling off
Time/s
1.80
1.95
2.05
2.35
2.39
2.57
equations and Reynolds averaged Navier–Stokes equations were used as a governing equation set for a chemically reacting supersonic viscous flow. The governing equations expressed in conservative vector form using the Cartesian coordinate system are:
∂ Q ∂ F ∂ G ∂ E ∂ Fv ∂ Gv ∂ Ev + + + = + + +S ∂t ∂x ∂y ∂z ∂x ∂y ∂z
where, Q = (ρ ,ρ u,ρ v,ρ w,ρ Et ,ρ Ci )T , Ci was the mass concentration for species i. Et was the total energy, including kinetic energy and internal energy. E,F,G were the inviscid fluxes, Fv ,Gv ,Ev were the viscous fluxes. S was the source term, u,v,w were the velocity components in Cartesian coordinate(x,y,z). ρ was the density. Cell-averaged finite volume techniques were used to solve the conservative form governing equations. LU-SGS method was used
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Fig. 5. Wall pressures of different combustion processes.
Fig. 6. Wall pressures at different times.
in time-marching. In space terms difference, third order MUSCL interpolation method and AUSMPW+ scheme were used in inviscid fluxes construction, central difference method was used in viscous fluxes. Kok’s modified k-ω TNT two-equation turbulence mode [22] was used in turbulence simulations. The kerosene reaction mechanism modified version of CARDC’s chemistry mechanism [8], involving 12 elementary reaction steps and 10 reaction species was used. A 2D structured grid with a size of 10 0,0 0 0 grid points was used in this simulation. Considering the calculating costs, the
kerosene atomization process [19,23] would not be considered and two-dimensional numerical simulation was used in the present calculation. 3. Results and discussion In this section, the combustion process of the combustor with air throttling is going to be presented. Time evolution of the pressure at a monitoring station, spatial distribution along the
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Fig. 7. The high-speed schlieren images of part-a and part-b combustion process.
streamwise coordinate of the wall-mounted average pressure over the different stages of the combustion process, schlieren and CH∗ luminosity images of experiments, also non-reacting and reacting simulation data are discussed. Non-reacting and reacting cases are tested and will be compared to assess the relative pressure drop in the combustor as the flame is found to nearly extinguish before air throttle was enabled for stabilization. 3.1. The whole combustion process The monitor pressure with the pressure of kerosene and pilot hydrogen is shown in Fig. 4. During the whole test time (1.95s– 2.60 s), the whole combustion process could be divided into four parts according to the monitor pressure changing. The pressure inside the combustor was monitored at a single location immediately downstream of the cavity flameholder. part-a indicated that the ignition started at t = 2.07 s, 0.02 s after the pilot hydrogen was injected into the combustor. And the flame was nearly blown off at t = 2.22 s, when the kerosene supply pressure was about 0.25 MPa. The kerosene flame was blown off and the pilot flame only existed in the cavity ramp from t = 2.22 s to t = 2.35 s. But the kerosene burnt intensively again when the throttling air was injected into the combustor at t = 2.35 s at the beginning
of part-c, the kerosene kept burning intensively during the whole part-c. The monitor pressure was around 0.28 MPa from t = 2.35 s to t = 2.39 s when the pilot hydrogen was removed (beginning of part-d and ending of part-c). But the pressure went down slightly from t = 2.39 s to t = 2.45 s and then kept steady around 0.25 MPa until t = 2.57 s when the throttling air was removed. The wall pressures of the four parts and non-reacting flow are shown in Fig. 5. The wall pressure of part-a was higher than that of non-reacting flow, especially in the cavity part, which meant the combustion mainly occurred in the cavity. The wall pressure of part-b was almost equal to the non-reacting flow except for the pressure near the cavity ramp, which meant the flame was nearly blow out and only existed near the cavity ramp where the pressure and temperature was higher and the velocity was lower. The wall pressures of part-c and part-d were higher obviously than that of the non-reacting flow and combustion-generated high backpressure had spread into the isolator, but the isolator entrance had not been disturbed. The disturbing distance of part-c was 0.10 m (0.20 m from the isolator entrance) and that was 0.05 m (0.25 m from the isolator entrance) of part-d. The precombustion shock train of part-c extended farther upstream in the isolator than that of part-d, this was because the pilot hydrogen had not been removed, and more heat released in part-c.
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Fig. 8. The CH∗ luminosity images of part-a and part-b combustion process.
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Wall pressures at different time in the whole combustion process are shown in Fig. 6, Wall pressures in part-a (t = 2.10 s, t = 2.16 s, t = 2.20 s) were higher than that of the non-reacting case, which meant combustion existed in the combustor. But wall pressures in part-b (t = 2.25 s, t = 2.30 s, t = 2.33 s) were almost equal to that of non-reacting case, which meant the flame was nearly blown off in the combustor. Wall pressures in part-c (t = 2.38 s and t = 2.39 s) were higher obviously than that of the other parts. Wall pressures in part-d (t = 2.50s– t = 2.58 s) were almost the same, which meant the combustion was stable in the combustor with air throttling. 3.2. The combustion process of part-a and part-b The high-speed schlieren images of part-a and part-b combustion process are shown in Fig. 7. The kerosene located upstream the cavity was gradually injected into the combustor from t = 2.0 0 0 0 s, and the shock waves generated by the cavity ramp and the flow separation zone could be firstly seen at t = 2.0310 s. The kerosene had been filled in the cavity at t = 2.0500 s, and the pilot hydrogen had been injected into the cavity at t = 2.0700 s, then the ignition occurred and the flame was in the cavity near the cavity ramp. The flame propagated into the core flow rapidly at t = 2.10 0 0 s, this was because the kerosene had already mixed with the air well before the pilot hydrogen was injected into the combustor. The supersonic core flow went through the narrow zone caused by the combustion heat release, then the shock waves generated. The shock waves moved upstream rapidly from t = 2.10 0 0 s to t = 2.2025 s, this was because more heat had been released as the mass flux of kerosene increased. And almost no shock waves could be seen in the image at t = 2.2025 s, the shock waves had moved upstream from the cavity region into the isolator, because of the higher combustion backpressure caused by the increasing kerosene. But the shock waves could be seen again when t = 2.2035 s, and the shock waves moved downstream from the isolator into the cavity region, also the flame zone was reduced from the whole cavity to a small area near the cavity ramp at t = 2.2200 s. This was because more room temperature kerosene had been injected into the combustor, and then the flame was nearly blown off, only a small portion of the flame was anchored in the rearward of the cavity. So we could see the wall pressure of part-b was almost equal to the non-reacting flow except for the cavity ramp position. CH∗ luminosity images are shown in Fig. 8, which was used to better analysis the flame distribution and to know if the flame was nearly blown off. The CH∗ signal was only present in the figure from t = 2.110 s to t = 2.220 s during part-a and part-b, and the CH∗ signal distribution area was rather small. When the kerosene supply pressure was increased to 0.25 MPa at t = 2.2220 s, no CH∗ signal could be seen in the figure. Which meant the kerosene flame was blown off at this time.
Fig. 9. The high-speed schlieren images of part-c and part-d combustion process.
meant the flame stabilization was achieved by the throttling air after the pilot hydrogen was removed. CH∗ luminosity images of part-c and part-d are shown in Fig. 10. The CH∗ signal was present in the figure from t = 2.360 s to t = 2.550 s during the whole part-c and part-d, which distributed in the cavity and downstream the cavity near the top wall. From the CH∗ signal configuration, the mixture flame was stable and mainly distributed in the cavity shear layer.
3.3. The combustion process of part-c and part-d 3.4. The effect of air throttling The flame was nearly blown off by the room temperature kerosene after t = 2.2200 s, which can also be seen at t = 2.3500 s in Fig. 9. But the shock wave had been generated by the throttling air at t = 2.3500 s. And the shock waves kept moving upstream from the cavity region into the isolator, and then the flame near the cavity ramp propagated upstream along the cavity wall. The flame was filled in the cavity at t = 2.3600 s, and almost all the shock waves had moved into the isolator at t = 2.40 0 0 s. But the shock waves moved downstream from the isolator at t = 2.4500 s, this was because the combustion heat release was less than before after the pilot hydrogen was removed. Finally the shock waves were stable under the cavity after t = 2.4500 s, which
Based on the above discussion, we found air throttling was an effective method to achieve flame stabilization after pilot hydrogen was removed. The wall pressures of the non-reacting flow with and without air throttling are shown in Fig. 11, and the mass averaged temperature and Mach number of the non-reacting flow are shown in Fig. 12. From the figures, the effect of air throttling increased in the temperature and pressure of the flow in the combustor, along with the decrease in the flow velocity, which would lead to smooth and reliable ignition. In the shock train region, the mass averaged Mach number changed from 1.8 to 0.9, the average static temperature changed from 600 K
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Fig. 10. The CH∗ luminosity images of part-c and part-d combustion process.
to 770 K, the average static pressure changed from 103 kPa to 285 kPa. The shock waves interacted with boundary layer, which would cause the wall boundary layers to separate, so the vorticity could spread into the core flow (seen in Fig. 13), which would improve the fuel/air mixing in the core flow (seen in Fig. 14) as a result of enhanced flow distortion and increased residence time. The reacting simulations of the combustion field in the combustor with air throttling (part-c and part-d) are shown in Figs. 15–19, which were calculated by solving steady Navier–Stokes equations. This was because the combustion flow in part-c and part-d was stable, which could also be explained by the monitor pressure in Fig. 4. The numerical results of wall pressure was used
to compare with the experimental results to show the validity of the simulations in Fig. 15. From the figure, we found the simulated results better matched the experimental results, especially on the disturbing distance of combustion induced backpressure in the isolator. The disturbing distances of part-c and part-d are 0.1 m (x = 0.2 m) and 0.05 m (x = 0.25 m) in Fig. 16, which was in accordance with the information in Fig. 5. The separation zone has been propagated into the isolator from the cavity, but it did not cause the inlet unstart. The core flow of part-d was supersonic, but that of part-c was subsonic. This was because the pilot hydrogen was still injected into the combustor, and more heat released from the mixture. The mass averaged Mach numbers of part-c and part-d were
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Fig. 11. Wall pressures of the non-reacting flow with and without air throttling (Exp.: experimental results, Cal.: calculative results, 27.2%: results with air throttling, 0.0%: results without air throttling).
Fig. 12. Mass averaged Mach number and temperature of the non-reacting flow with and without air throttling.
shown in Fig. 17. The mass averaged Mach number was lower than 1.0 from x = 0.25 m to x = 0.58 m, the combustion mode of pat-c was subsonic combustion, and that of part-d was supersonic combustion. The static pressures of part-c and part-d in the cavity region were shown in Fig. 18, and shock waves generated due to the combustion back pressure. Mass fraction of carbon dioxide of partc and part-d was shown in Fig. 19, the carbon dioxide of part-c had propagated into the isolator along the separation zone near the top wall.
4. Conclusions The combustion process of a kerosene fueled combustor with pilot hydrogen and air throttling was investigated by experiments and numerical simulations. Time evolution of the pressure at a monitoring station, spatial distribution along the streamwise coordinate of the wall-mounted average pressure over the different stages of the combustion process, schlieren and CH∗ luminosity
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Fig. 13. Vorticity contours in the non-reacting flow with and without air throttling.
Fig. 14. The mass fraction of kerosene in thenon-reacting flow with and without air throttling.
Fig. 15. Numerical and experimental wall pressure results of part-c and part-d. (EXP.: experimental results; CAL.: numerical results).
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Fig. 16. Mach number contours in the reacting flow of part-c and part-d.
images of experiments, also non-reacting and reacting simulation data were discussed. We found the combustion process could be divided into four parts according to the monitor pressure changing. The kerosene was ignited by the pilot hydrogen successfully, and the flame was stable during part-a. As the kerosene supply pressure was increasing, the flame was blown off by the room temperature kerosene in part-b. Successful ignition and flame stabilization had been achieved with the aid of air throttling in part-c, and the flame was stable even after the pilot hydrogen was removed in part-d. Compared with previous works, the flame was either stable or blown off in their results, the interesting phenomenon of the flame was from stable and blown off to stable again was found out in this paper, which strengthened the effect of air throttling on flame stabilization.The wall pressure was often used in the previous works to show the flame stable or blown off, but we used pressure monitor, schlieren and CH∗ luminosity images to show the flame development and flow structure, which was easier to understand the combustion process. Finally we discussed the different combustion modes in the combustion process of part-c and part-d, the combustion mode was subsonic combustion when the pilot hydrogen existed (part-c), but it was supersonic combustion when the pilot hydrogen was removed (part-d).
Fig. 18. Static pressure contours in the reacting flow of part-c and part-d.
Fig. 19. Mass fraction of carbon dioxide contours in the reacting flow of part-c and part-d.
Conflicts of interest The authors declare there are no conflicts of interest regarding the publication of this paper.
Fig. 17. Mass averaged Mach number of the reacting flow of part-c and part-d.
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Acknowledgments The project supported by Feng Lei Youthx Innovation Fond of CARDC 20150013 and National Natural Science Foundation of China (Nos. 51376194, 51376193, 51406222 and 91441204). References [1] A. Ben-Yakar, R.K. Hanson, Cavity flame-holders for ignition and flame stabilization in scramjets: an overview, J. Propuls. Power 17 (4) (2001) 869–877. [2] B.J. Tatman, R.D. Rockwell, C.P. Goyne, et al., Experimental study of vitiation effects on flameholding in a cavity flameholder, J. Propuls. Power 29 (2) (2013) 417–423. [3] M.R. Gruber, J.M. Donbar, C.D. Carter, K.Y. Hsu, Mixing and combustion studies using cavity-based flameholders in a supersonic flow, J. Propuls. Power 20 (No.5) (2004) 769–778. [4] A. Ben-Yakar, R.K. Hanson, Cavity flame-holders for ignition and flame stabilization in scramjets-Review and experimental study, AIAA/asme/sae/asee Joint Propulsion Conference & Exhibit 68 (3) (2013) 210–222. [5] T. Mathur, M. Gruber, K. Jackson, J. Donbar, W. Donaldson, T. Jackson, F. Billig, Supersonic combustion experiments with a cavity-based fuel injector, J. Propuls. Power 17 (6) (2001) 1305–1312. [6] T. Mitani, N. Chinzei, T. Kanda, Reaction and mixing controlled combustion in scramjet Engines, J. Propuls. Power 17 (No.2) (2001) 308–314. [7] Wen Bao, Jichao Hu, Youhai Zong, Qingchun Yang, Meng Wu, Juntao Chang and Daren Yu. “Combustion characteristic using O2-pilot strut in a liquid-kerosenefueled strut-based dual-mode scramjet”. Proc. IMechE PartG: J Aerosp. Eng. 227(12) 1870–1880. [8] Jichao Hu, Jiang Qin, Juntao Chang, Wen Bao, Youhai Zong, Qingchun Yang, Combustion stabilization based on a center flame strut in a liquid kerosene fueled supersonic combustor, J. Therm. Sci. 22 (5) (2013) 497–504. [9] Jichao Hu, Juntao Chang, Wen Bao, Qingchun Yang, John Wen. “Experimental study of a flush wall scramjet combustor equipped with strut/wall fuel injection”. Acta Astronaut., 104(2014)84–90. [10] Mohieldin, T.O., Tiwari, S.N., Olynciw M.J., “Asymmetric flow-structures in dual mode scramjet combustor with significant upstream interaction”, AIAA Paper, 2001-3296.
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[11] Bao, W., Hu, J., Zong, Y., et al “Ignition characteristic of a liquid kerosene fueled scramjet by air throttling combined with a gas generator” J. Aerosp. Eng. doi: 10.1061/(ASCE)AS.1943-5525.0 0 0 0329 34. No. 12. [12] Y. Tian, S.H. Yang, J.L. Le, Study on the effect of air throttling on flame stabilization of an ethylene fueled scramjet combustor, Int. J. Aerosp. Eng. 2015 (2015) 10 Article ID 504684, doi:10.1155/2015/504684. [13] Y Vigor, J. Li, Y.C. Jeong, et al. “Ignition transient in an ethylene fueled scramjet engine with air throttling. Part 1: non-reacting flow development and mixing”. AIAA 2010-409 [14] Y. Vigor, J. Li, Y.C. Jeong. et al. “Ignition transient in an ethylene fueled scramjet engine with air throttling. Part 2: ignition and flame development”. AIAA 2010410 [15] Ye Tian, Jialing Le, Shunhua Yang, et al., Numerical study on air throttling influence of ignition transient in the scramjet combustor, J. Aerosp. Power 28 (7) (2013) 1495–1502 (in Chinese). [16] Ye Tian, Jialing Le, Shunhua Yang, et al., Study on the flow structure of an ethylene-fueled scramjet combustor with air throttling, J. Propuls. Technol. 34 (6) (2013) 795–801 (in Chinese). [17] Ye Tian, Jialing Le, Shunhua Yang, et al., Study on the effects of air throttling on combustion performance of a kerosene-fueled scramjet combustor, J. Astronaut. 36 (12) (2015) 1421–1427 (in Chinese). [18] Ye Tian, Jialing Le, Shunhua Yang, Study on the effect of air throttling on flame stabilization of an ethylene fueled scramjet combustor, Int. J. Aerosp. Eng., 2015 (2015) 10 Article ID 504684, doi:10.1155/2015/504684. [19] Ye Tian, Jialing Le, Shunhua Yang, Numerical study on effect of air throttling on combustion mode formation and transition in a dual-mode scramjet combustor, Aerosp. Sci. Technol. 52 (5) (2016) 173–180. [20] Mathur, T., Lin, K.C., Kennedy, P., Gruber, M., Donbar, J., Jackson, T., and Billig, F., “Liquid JP-7 combustion in a scramjet combustor,” AIAA Paper 20 0 0-3581, July 20 0 0. [21] Donbar, J., Powell, O., Gruber, M., Jackson, T., Eklund, D., and Mathur, T., “Posttest analysis of flush-wall fuel injection experiments in a scramjet combustor,” AIAA Paper 2001-3197, July 2001. [22] J.C. Kok. “Resolving the dependence on free-stream values for the K-omega turbulence model”, NLR-TP-99295. [23] Y. Tian, B.G. Xiao, S.P. Zhang, et al., Experimental and computational study on combustion performance of a kerosene fueled dual-mode scramjet engine, Aerosp. Sci. Technol. 46 (2015) 451–458. http://dx.doi.org/10.1016/j.ast.2015.09. 002.
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Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
Investigation of combustion process of a kerosene fueled combustor with air throttling Ye Tian∗, Shunhua Yang, Jialing Le, Fuyu Zhong, Xiaoqiang Tian Science and Technology on Scramjet Laboratory of Hypervelocity Aerodynamics Institute, CARDC, Mianyang 621000, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Revised 14 October 2016 Accepted 24 January 2017
Keywords: Combustion process Flame stabilization Air throttling Kerosene Ignition Blown off
a b s t r a c t An experimental and numerical study was carried out to investigate the combustion process of a kerosene fueled combustor with air throttling. The results were obtained with the inflow conditions of Mach number of 2.0, total temperature of 953 K and total pressure of 0.82 MPa, respectively. The air throttling was located 0.575 m downstream the combustor entrance, and the mass flux of air throttling was 27.2% inflow mass flux. The pilot flame was blown off by the room temperature kerosene when the kerosene supply pressure was 0.25 MPa, but the kerosene was ignited successfully when the throttling air was injected into the combustor, and the flame stabilization was achieved even when the pilot hydrogen was removed. The combustion process could be divided into four parts based on changes in the pressure monitored near the cavity: kerosene was ignited successfully by the pilot flame, and the mixture flame was stable during part-a. As the kerosene supply pressure was increasing, the flame was blown off by the room temperature kerosene in part-b. Successful ignition and flame stabilization had been achieved with the aid of air throttling in part-c, and the combustion mode was subsonic combustion. The flame was stable even after the pilot hydrogen was removed in part-d, but the combustion mode was supersonic combustion. © 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Achieving flame stabilization is a difficult problem in scramjet combustor, because in supersonic combustor, the time available for fuel injected, vaporized, mixed with air, and combustion is very short, of the order of milliseconds [1]. This problem applies especially to hydrocarbon fuels such as kerosene that are often used in the scramjet, Which consists of long chains of hydrogen and carbon molecules with longer reaction times than smaller molecules (such as hydrogen and ethylene) and thus has long ignition delay times, often exceeding a millisecond [2]. So the flameholders should be used in the scramjet combustor in order to achieve flame stabilization. Some different kinds of flame holders, such as cavity [3–5], strut [6–9], step [10] and air throttling [11–14], have been investigated by many researchers. Ignition transients in a scramjet engine with air throttling were investigated by Li and coworkers [13,14]. In their paper, a pre-combustion shock train was generated in the isolator due to the increased back pressure by the throttling air. The resultant increase in the temperature and pressure of the airstream in the combustor, along with the decrease in the flow velocity, lead to smooth and reliable ignition. The incidentally formed separated
∗
Corresponding author. E-mail addresses: [email protected] (Y. Tian), [email protected] (J. Le).
flows adjacent to the combustor sidewall improved fuel/air mixing as a result of enhanced flow distortion and increased residence time. Successful ignition could only be achieved with the aid of air throttling under the present flow conditions. Chemical reactions were intensified and produced sufficient heat release to maintain a flow environment conducive to flame stabilization. A self-sustaining mechanism was thus established between the flow and flame development. Stable flames were achieved even after the deactivation of air throttling. Our previous work studied the effect of air throttling on flow structure, fuel/air mixing, ignition transients and flame stabilization in the scramjet combustor [15–19]. Mathur et al. [20] conducted an experiment using air throttling to initiate combustion in a scramjet combustor. Their results showed that once the air throttling was removed after the flame establishment, the shock train was retained leading to sustained combustion if heat release was sufficient. Conversely, insufficient heat release might result in an unstable shock train and caused flame blowout. Donbar et al. [21] tested the operation sequence of ignition in an ethylene fueled scramjet combustor. Air throttling was used after a stable fuel condition was reached. Once ignition occurred by activating spark igniters, the air throttling was removed, after which sustained combustion proceeded. From the above discussion, we found the effect of air throttling on flame stabilization had been investigated by several researchers, but most of them focused on gas (ethylene or hydrogen) fueled
http://dx.doi.org/10.1016/j.combustflame.2017.01.021 0010-2180/© 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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Fig. 1. Photo of the supersonic combustion facility in CARDC.
Nomenclatures Ma ER D L P t x T
Mach number equivalence ratio depth of cavity length of cavity wall pressure of combustor time distance from combustor entrance mass averaged temperature
combustor. Few published papers investigated the effect of air throttling on room temperature liquid (kerosene) fueled combustor, which was our main purpose of the present paper. We used a small scaled cavity as a flameholder in the combustor in order to reduce the cavity drag, also the pilot hydrogen was used to ignite the room temperature kerosene, and air throttling was used to achieve flame stabilization after pilot hydrogen was removed. 2. Experimental and numerical simulation methods 2.1. Facility and combustor configuration Experimental investigations were conducted on a directconnected supersonic combustion facility (Fig. 1) in China Aerodynamics Research and Development Centre (CARDC). Hydrogen fueled heater was used to heat the air up to 10 0 0 K and additional oxygen was added to maintain a 21% O2 mole fraction in the
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vitiated air, and the mole fraction of H2 O and N2 were 12%, and 67%, respectively. A pulse Mach 2.0 airflow was supplied via a two-dimensional nozzle which was connected to the upstream of the combustor. The total temperature and total pressure of the inflow were 953 K and 0.82 MPa, respectively. The lab-scale combustor [19] was made of stainless steel and divided into two sections, which was shown in Fig. 2, the first section was a rectangular isolator with the length of 430 mm (350 mm straight section and 80 mm expansion section with upwall divergent angle being 1.4°) and the cross section area was 30 × 150mm² . The second section was the combustor which included a cavity (D: 11 mm, L/D: 11) and a four- part expansion section (range: 551mm–1070 mm). There were two fuel injected positions shown in Fig. 2, the first injector was designed for injecting room temperature kerosene and the second injector was designed for introducing pilot hydrogen, and the locations of the two injectors were 410 mm and 440 mm from the isolator entrance, respectively. The kerosene was injected at sonic speed at an angle of 90° to the airflow by fifteen 0.3 mm in diameter fuel injection holes, and the hydrogen was injected by ten 1.0 mm in diameter fuel injection holes, that of the throttling air were twenty 3 mm in diameter injection holes. The sampling frequency of pressure transducer was 1 kHz, which was used for measuring the wall pressure. Schlieren images were captured by a CCD camera, and the exposure time was 1μs and the frame rate was 10,0 0 0 fps. The chemiluminescence of CH∗ was used to mark the flame zones in the combustor. The luminosity from CH∗ was imaged by a CCD camera with ± 5 nm bandwidth interference filters centered at 430 nm and the exposure time was 1/20 0 0 s. The test sequence of the studying case was shown in Fig. 3 and Table 1, when hydrogen entered into the facility heater at t = 1.80 s, the cold flow then generated. The running time of the facility was about 600 ms (1.95s–2.57 s), when the supply pressure of hydrogen was kept as constant. The kerosene was injected into the combustor from t = 1.95 s to t = 2.57 s, and the supply pressure of kerosene was increased to 2.0 MPa (equivalence ratio: 0.3) from t = 1.95 s to 2.30 s. The spark in the combustor cavity was working all the test time, so the pilot hydrogen was ignited at once when it was injected into the combustor. The air throttling was started to be injected into the combustor at t = 2.35 s, just 0.04 s before pilot hydrogen was off. The location of air throttling was 0.575 m from the combustor entrance, and the mass flux of air throttling was about 27.2% inflow mass flux. The equivalence ratio (ER) of kerosene was 0.3, and the ER of pilot hydrogen was 0.08. 2.2. Numerical methods In this study, the inhouse CFD code AHL3D [19,23] software which had been introduced in reference [18,19,23] was used for computation. A fully coupled form of species conservation
Fig. 2. Schematic illustration of the combustor.
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Fig. 3. Test sequence of different parameters.
Fig. 4. Wall pressures of pressure monitor, kerosene supply and pilot hydrogen. Table 1 Operation sequence of the test. Operation sequence
Cold flow starts
Kerosene injected on
Hydrogen injected on
Air throttling on
Hydrogen injected off
Kerosene and Air throttling off
Time/s
1.80
1.95
2.05
2.35
2.39
2.57
equations and Reynolds averaged Navier–Stokes equations were used as a governing equation set for a chemically reacting supersonic viscous flow. The governing equations expressed in conservative vector form using the Cartesian coordinate system are:
∂ Q ∂ F ∂ G ∂ E ∂ Fv ∂ Gv ∂ Ev + + + = + + +S ∂t ∂x ∂y ∂z ∂x ∂y ∂z
where, Q = (ρ ,ρ u,ρ v,ρ w,ρ Et ,ρ Ci )T , Ci was the mass concentration for species i. Et was the total energy, including kinetic energy and internal energy. E,F,G were the inviscid fluxes, Fv ,Gv ,Ev were the viscous fluxes. S was the source term, u,v,w were the velocity components in Cartesian coordinate(x,y,z). ρ was the density. Cell-averaged finite volume techniques were used to solve the conservative form governing equations. LU-SGS method was used
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Fig. 5. Wall pressures of different combustion processes.
Fig. 6. Wall pressures at different times.
in time-marching. In space terms difference, third order MUSCL interpolation method and AUSMPW+ scheme were used in inviscid fluxes construction, central difference method was used in viscous fluxes. Kok’s modified k-ω TNT two-equation turbulence mode [22] was used in turbulence simulations. The kerosene reaction mechanism modified version of CARDC’s chemistry mechanism [8], involving 12 elementary reaction steps and 10 reaction species was used. A 2D structured grid with a size of 10 0,0 0 0 grid points was used in this simulation. Considering the calculating costs, the
kerosene atomization process [19,23] would not be considered and two-dimensional numerical simulation was used in the present calculation. 3. Results and discussion In this section, the combustion process of the combustor with air throttling is going to be presented. Time evolution of the pressure at a monitoring station, spatial distribution along the
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Fig. 7. The high-speed schlieren images of part-a and part-b combustion process.
streamwise coordinate of the wall-mounted average pressure over the different stages of the combustion process, schlieren and CH∗ luminosity images of experiments, also non-reacting and reacting simulation data are discussed. Non-reacting and reacting cases are tested and will be compared to assess the relative pressure drop in the combustor as the flame is found to nearly extinguish before air throttle was enabled for stabilization. 3.1. The whole combustion process The monitor pressure with the pressure of kerosene and pilot hydrogen is shown in Fig. 4. During the whole test time (1.95s– 2.60 s), the whole combustion process could be divided into four parts according to the monitor pressure changing. The pressure inside the combustor was monitored at a single location immediately downstream of the cavity flameholder. part-a indicated that the ignition started at t = 2.07 s, 0.02 s after the pilot hydrogen was injected into the combustor. And the flame was nearly blown off at t = 2.22 s, when the kerosene supply pressure was about 0.25 MPa. The kerosene flame was blown off and the pilot flame only existed in the cavity ramp from t = 2.22 s to t = 2.35 s. But the kerosene burnt intensively again when the throttling air was injected into the combustor at t = 2.35 s at the beginning
of part-c, the kerosene kept burning intensively during the whole part-c. The monitor pressure was around 0.28 MPa from t = 2.35 s to t = 2.39 s when the pilot hydrogen was removed (beginning of part-d and ending of part-c). But the pressure went down slightly from t = 2.39 s to t = 2.45 s and then kept steady around 0.25 MPa until t = 2.57 s when the throttling air was removed. The wall pressures of the four parts and non-reacting flow are shown in Fig. 5. The wall pressure of part-a was higher than that of non-reacting flow, especially in the cavity part, which meant the combustion mainly occurred in the cavity. The wall pressure of part-b was almost equal to the non-reacting flow except for the pressure near the cavity ramp, which meant the flame was nearly blow out and only existed near the cavity ramp where the pressure and temperature was higher and the velocity was lower. The wall pressures of part-c and part-d were higher obviously than that of the non-reacting flow and combustion-generated high backpressure had spread into the isolator, but the isolator entrance had not been disturbed. The disturbing distance of part-c was 0.10 m (0.20 m from the isolator entrance) and that was 0.05 m (0.25 m from the isolator entrance) of part-d. The precombustion shock train of part-c extended farther upstream in the isolator than that of part-d, this was because the pilot hydrogen had not been removed, and more heat released in part-c.
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Fig. 8. The CH∗ luminosity images of part-a and part-b combustion process.
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Wall pressures at different time in the whole combustion process are shown in Fig. 6, Wall pressures in part-a (t = 2.10 s, t = 2.16 s, t = 2.20 s) were higher than that of the non-reacting case, which meant combustion existed in the combustor. But wall pressures in part-b (t = 2.25 s, t = 2.30 s, t = 2.33 s) were almost equal to that of non-reacting case, which meant the flame was nearly blown off in the combustor. Wall pressures in part-c (t = 2.38 s and t = 2.39 s) were higher obviously than that of the other parts. Wall pressures in part-d (t = 2.50s– t = 2.58 s) were almost the same, which meant the combustion was stable in the combustor with air throttling. 3.2. The combustion process of part-a and part-b The high-speed schlieren images of part-a and part-b combustion process are shown in Fig. 7. The kerosene located upstream the cavity was gradually injected into the combustor from t = 2.0 0 0 0 s, and the shock waves generated by the cavity ramp and the flow separation zone could be firstly seen at t = 2.0310 s. The kerosene had been filled in the cavity at t = 2.0500 s, and the pilot hydrogen had been injected into the cavity at t = 2.0700 s, then the ignition occurred and the flame was in the cavity near the cavity ramp. The flame propagated into the core flow rapidly at t = 2.10 0 0 s, this was because the kerosene had already mixed with the air well before the pilot hydrogen was injected into the combustor. The supersonic core flow went through the narrow zone caused by the combustion heat release, then the shock waves generated. The shock waves moved upstream rapidly from t = 2.10 0 0 s to t = 2.2025 s, this was because more heat had been released as the mass flux of kerosene increased. And almost no shock waves could be seen in the image at t = 2.2025 s, the shock waves had moved upstream from the cavity region into the isolator, because of the higher combustion backpressure caused by the increasing kerosene. But the shock waves could be seen again when t = 2.2035 s, and the shock waves moved downstream from the isolator into the cavity region, also the flame zone was reduced from the whole cavity to a small area near the cavity ramp at t = 2.2200 s. This was because more room temperature kerosene had been injected into the combustor, and then the flame was nearly blown off, only a small portion of the flame was anchored in the rearward of the cavity. So we could see the wall pressure of part-b was almost equal to the non-reacting flow except for the cavity ramp position. CH∗ luminosity images are shown in Fig. 8, which was used to better analysis the flame distribution and to know if the flame was nearly blown off. The CH∗ signal was only present in the figure from t = 2.110 s to t = 2.220 s during part-a and part-b, and the CH∗ signal distribution area was rather small. When the kerosene supply pressure was increased to 0.25 MPa at t = 2.2220 s, no CH∗ signal could be seen in the figure. Which meant the kerosene flame was blown off at this time.
Fig. 9. The high-speed schlieren images of part-c and part-d combustion process.
meant the flame stabilization was achieved by the throttling air after the pilot hydrogen was removed. CH∗ luminosity images of part-c and part-d are shown in Fig. 10. The CH∗ signal was present in the figure from t = 2.360 s to t = 2.550 s during the whole part-c and part-d, which distributed in the cavity and downstream the cavity near the top wall. From the CH∗ signal configuration, the mixture flame was stable and mainly distributed in the cavity shear layer.
3.3. The combustion process of part-c and part-d 3.4. The effect of air throttling The flame was nearly blown off by the room temperature kerosene after t = 2.2200 s, which can also be seen at t = 2.3500 s in Fig. 9. But the shock wave had been generated by the throttling air at t = 2.3500 s. And the shock waves kept moving upstream from the cavity region into the isolator, and then the flame near the cavity ramp propagated upstream along the cavity wall. The flame was filled in the cavity at t = 2.3600 s, and almost all the shock waves had moved into the isolator at t = 2.40 0 0 s. But the shock waves moved downstream from the isolator at t = 2.4500 s, this was because the combustion heat release was less than before after the pilot hydrogen was removed. Finally the shock waves were stable under the cavity after t = 2.4500 s, which
Based on the above discussion, we found air throttling was an effective method to achieve flame stabilization after pilot hydrogen was removed. The wall pressures of the non-reacting flow with and without air throttling are shown in Fig. 11, and the mass averaged temperature and Mach number of the non-reacting flow are shown in Fig. 12. From the figures, the effect of air throttling increased in the temperature and pressure of the flow in the combustor, along with the decrease in the flow velocity, which would lead to smooth and reliable ignition. In the shock train region, the mass averaged Mach number changed from 1.8 to 0.9, the average static temperature changed from 600 K
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Fig. 10. The CH∗ luminosity images of part-c and part-d combustion process.
to 770 K, the average static pressure changed from 103 kPa to 285 kPa. The shock waves interacted with boundary layer, which would cause the wall boundary layers to separate, so the vorticity could spread into the core flow (seen in Fig. 13), which would improve the fuel/air mixing in the core flow (seen in Fig. 14) as a result of enhanced flow distortion and increased residence time. The reacting simulations of the combustion field in the combustor with air throttling (part-c and part-d) are shown in Figs. 15–19, which were calculated by solving steady Navier–Stokes equations. This was because the combustion flow in part-c and part-d was stable, which could also be explained by the monitor pressure in Fig. 4. The numerical results of wall pressure was used
to compare with the experimental results to show the validity of the simulations in Fig. 15. From the figure, we found the simulated results better matched the experimental results, especially on the disturbing distance of combustion induced backpressure in the isolator. The disturbing distances of part-c and part-d are 0.1 m (x = 0.2 m) and 0.05 m (x = 0.25 m) in Fig. 16, which was in accordance with the information in Fig. 5. The separation zone has been propagated into the isolator from the cavity, but it did not cause the inlet unstart. The core flow of part-d was supersonic, but that of part-c was subsonic. This was because the pilot hydrogen was still injected into the combustor, and more heat released from the mixture. The mass averaged Mach numbers of part-c and part-d were
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Fig. 11. Wall pressures of the non-reacting flow with and without air throttling (Exp.: experimental results, Cal.: calculative results, 27.2%: results with air throttling, 0.0%: results without air throttling).
Fig. 12. Mass averaged Mach number and temperature of the non-reacting flow with and without air throttling.
shown in Fig. 17. The mass averaged Mach number was lower than 1.0 from x = 0.25 m to x = 0.58 m, the combustion mode of pat-c was subsonic combustion, and that of part-d was supersonic combustion. The static pressures of part-c and part-d in the cavity region were shown in Fig. 18, and shock waves generated due to the combustion back pressure. Mass fraction of carbon dioxide of partc and part-d was shown in Fig. 19, the carbon dioxide of part-c had propagated into the isolator along the separation zone near the top wall.
4. Conclusions The combustion process of a kerosene fueled combustor with pilot hydrogen and air throttling was investigated by experiments and numerical simulations. Time evolution of the pressure at a monitoring station, spatial distribution along the streamwise coordinate of the wall-mounted average pressure over the different stages of the combustion process, schlieren and CH∗ luminosity
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Fig. 13. Vorticity contours in the non-reacting flow with and without air throttling.
Fig. 14. The mass fraction of kerosene in thenon-reacting flow with and without air throttling.
Fig. 15. Numerical and experimental wall pressure results of part-c and part-d. (EXP.: experimental results; CAL.: numerical results).
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Fig. 16. Mach number contours in the reacting flow of part-c and part-d.
images of experiments, also non-reacting and reacting simulation data were discussed. We found the combustion process could be divided into four parts according to the monitor pressure changing. The kerosene was ignited by the pilot hydrogen successfully, and the flame was stable during part-a. As the kerosene supply pressure was increasing, the flame was blown off by the room temperature kerosene in part-b. Successful ignition and flame stabilization had been achieved with the aid of air throttling in part-c, and the flame was stable even after the pilot hydrogen was removed in part-d. Compared with previous works, the flame was either stable or blown off in their results, the interesting phenomenon of the flame was from stable and blown off to stable again was found out in this paper, which strengthened the effect of air throttling on flame stabilization.The wall pressure was often used in the previous works to show the flame stable or blown off, but we used pressure monitor, schlieren and CH∗ luminosity images to show the flame development and flow structure, which was easier to understand the combustion process. Finally we discussed the different combustion modes in the combustion process of part-c and part-d, the combustion mode was subsonic combustion when the pilot hydrogen existed (part-c), but it was supersonic combustion when the pilot hydrogen was removed (part-d).
Fig. 18. Static pressure contours in the reacting flow of part-c and part-d.
Fig. 19. Mass fraction of carbon dioxide contours in the reacting flow of part-c and part-d.
Conflicts of interest The authors declare there are no conflicts of interest regarding the publication of this paper.
Fig. 17. Mass averaged Mach number of the reacting flow of part-c and part-d.
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