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Investigation of ignition characteristics in a kerosene fueled supersonic combustor
Acta Astronautica xxx (xxxx) xxx–xxx
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Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Investigation of ignition characteristics in a kerosene fueled supersonic combustor Ye Tian∗, Shunhua Yang, Baoguo Xiao, Fuyu Zhong, Jialing Le Airbreathing Hypersonics Technology Research Center of CARDC, Mianyang, 621000, China
ABSTRACT
Ignition characteristics of a kerosene fueled supersonic combustor has been numerically and experimentally investigated in the present paper. Flame luminosity images and wall pressure measurements are used for better understanding the ignition and combustion characteristics, air throttling is used to enhance ignition in the combustor. The results are obtained under the inflow condition of Ma number 2.0, total pressure 1.0 MPa and total temperature 1100 K which corresponds to Ma4.5 flight condition. When the ER (Equivalence Ratio) of kerosene is 0.19, the kerosene cannot be ignited at all only by the spark plug. When the flux ratio of air throttling (the ratio of mass flux of air throttling to mass flux of the inflow air) is 9.1%, a small part of kerosene can be ignited, but the flame is blown off soon. When the throttling air flux ratio is increased to 13.7% or 24.5%, the kerosene can be ignited successfully, and the combustion is stable and intense. Cold room temperature liquid kerosene can be ignited successfully by the spark plug with the aid of the throttling air.
1. Introduction Scramjet engines are promising candidates for future airbreathing systems [1–5]. Air entering the scramjet combustor must be supersonic to avoid excessive dissociation of both nitrogen and oxygen gases when the flight Mach number is larger than 6. Consequently, time available for fuel injection, vaporization, fuel/air mixing, and combustion is very short, of the order of milliseconds [6–11]. So, achieving successful ignition in the scramjet combustor is an important factor which should be addressed in the scramjet design. Auto-ignition characteristics in an ethylene/hydrogen fueled supersonic combustor was simulated by Liu et al. by using skeletal mechanism [12], their results showed that the three mechanisms were not appropriate for combustion of ethylene–hydrogen/air mixture, A good agreement was found between the skeletal mechanism and the detailed mechanism in terms of the ignition delay time and the laminar flame speed over a wide range of parameters. Combustion characteristics were investigated in a hydrogen fueled dual-mode scramjet combustor by Wang et al. [13], they found under the stagnation temperature of 1600 K, strong auto-ignition induced the ignition process, auto-ignition contributed to the flame re-stabilization in the jet wake. The spark ignition of kerosene was investigated in a scramjet combustor by Bao et al. [14], the results showed that the injection pressure and injection location had a distinct effect on spark ignition, the injection pressure had both upper and lower limit for local ignition. Dual-pulse laser-induced plasma ignition of kerosene in cavity at model scramjet engine was studied by Li et al. [15,16], they found the
∗
entire ignition process of kerosene could be divided into five stages, which were referred as turbulent dissipation stage, quasi-stable state, combustion enhancement stage, reverting stage and combustion stabilization stage. The middle part of cavity was the most suitable location for ignition as a result of a favorable local equivalent ratio. Ignition schemes of the partially covered cavity in a scramjet combustor were investigated by Cai et al. [17], it revealed that the ignition scheme of the partially covered cavity had a great impact on the ignition and flame stabilization process. The real ways existed an optimized global equivalence ratio of a fixed ignition scheme, and the optimized global equivalence ratio of ignition in the partially covered cavity was lower than that of the uncovered cavity. The ignition and flame stability in a high-density polyethylene solid fueled ramjet were investigated experimentally and numerically by Omer et al. [18], the results showed that using swirl flow decreased the ignition time delay and improved the combustion efficiency and stability. Chang et al. [19–22] experimentally and numerically investigated the ignition and combustion characteristics in a strut/cavity scramjet combustor, results showed that combustion zone distributed in the shear layer behind the strut at a low equivalence ratio. Experimental results showed the combustion characteristics varied significantly with the strut/wall fuel feeding ratio, especially when this ratio was close to its lowest and highest limits. Based on the above discussions, many factors influenced the ignition characteristics in the scramjet combustor. But few published papers focused on the effects of air throttling on ignition in the combustor only with a small energy spark plug, which was our main purpose of the
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Tian).
https://doi.org/10.1016/j.actaastro.2019.03.024 Received 5 December 2018; Received in revised form 17 February 2019; Accepted 8 March 2019 0094-5765/ © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Ye Tian, et al., Acta Astronautica, https://doi.org/10.1016/j.actaastro.2019.03.024
Acta Astronautica xxx (xxxx) xxx–xxx
Y. Tian, et al.
Nomenclatures ER H L Ma P t W x
Equivalence Ratio Depth of Cavity Step Length of Cavity Mach Number Wall Pressure of Combustor Time Width of Cavity Distance from Combustor Entrance
present paper. Wall pressure measurements and high-speed color flame emission images were used for understanding the ignition characteristics and flame development. Fig. 2. a Schematic illustration of the combustor. b Length description of the combustor.
2. Experimental and numerical methods 2.1. Experimental methods and combustor configuration
orifices of 0.3 mm in diameter are used for cold room temperature kerosene injection (Injector K1, 285.0 mm from the isolator entrance or 15.0 mm upstream the cavity step). The cavity flame-holder along with a capacitive-discharge spark plug are mounted on the top wall, which has an expansion angle of 1.4°. The plug is located in the middle of the cavity bottom wall. The impulse excitation energy of the plug is about 6.0 J and the excitation frequency is 50 Hz.
All the experiments in the present study were conducted on a supersonic combustion facility (Fig. 1), which had already been introduced in our previous works [23]. Hydrogen and oxygen are burned in the heater, producing the high enthalpy inflow. Then the high enthalpy air flow is accelerated to Ma = 2.0 through a Laval nozzle, additional oxygen is added to maintain a 21% O2 mole fraction in the vitiated air, and the mole fraction of H2O and N2 are 12%, and 67%, respectively. The facility supplies a total temperature of 1100 K and a total pressure of 1.0 MPa from burnt air. High speed color images are captured by a CCD camera, and the exposure time is 0.5 ms and the frame rate is 2000 fps. The sampling frequency of pressure transducer is 1 kHz, which is used for measuring the wall pressure. There is a pressure monitor near the cavity ramp (500.0 mm from the isolator entrance), which is used to monitor the pressure changing during different tests. Fig. 2 shows the supersonic model scramjet combustor, which has a total length of 1100 mm and consists of a 300-mm-long section of nearly constant cross-sectional area, an L/H = 11.0 cavity (L: length of the cavity; H: depth of the cavity, H = 16.0 mm) and four divergent sections of 58.0, 144.0, 150.0, and 272.0 mm in length and 1.4, 2.0, 8.0, and 15.0 deg in expansion angle, respectively. The cross section of the combustor at its entrance is 30.0 mm in height by 150.0 mm in width. “0″indicates the beginning of the constant cross section and also represents the starting point for the static pressure measurement. Fifteen
2.2. Numerical methods and test sequence In this study, the inhouse CFD code AHL3D software which has been introduced in reference was used for computation [23–25]. A fully coupled form of species conservation equations and Reynolds averaged Navier-Stokes equations are used as a governing equation set for a chemically reacting supersonic viscous flow. Cell-averaged finite volume techniques are used to solve the conservative form governing equations. LU-SGS method is used in timemarching. In space terms difference, third order MUSCL interpolation method and AUSMPW+ scheme are used in inviscid fluxes construction, central difference method is used in viscous fluxes. Kok's modified k-ω TNT two-equation turbulence mode is used in turbulence simulations. The kerosene reaction mechanism modified version of CARDC's chemistry mechanism, involving 12 elementary reaction steps and 10 reaction species is used [23–25]. A 2D structured grid with a size of 100,000 grid points is used in this simulation, the simulation time for each case is about 4 h. The test sequence of the studying cases is shown in Table 1, when hydrogen enters the facility heater at t = 0.90 s, the cold flow then generates. The kerosene and the throttling air are both started to be injected into the combustor at t = 0.97 s, also the spark begins to work at the same time. The air throttling and the spark are switched off at t = 1.27 s, but the kerosene is injected off at t = 1.35 s. The location of air throttling is 625 mm from the combustor entrance. 3. Results and discussions Eight studying cases were used to investigate the ignition characteristics in a kerosene fueled scramjet combustor with air throttling, which were shown in Table 2. The front four cases (case 1∼ case 4) were used to study the effects of air throttling (Flux ratio: ratio of the throttling air flux and inflow air flux) on non-reacting flow structure, the other cases were used to investigate the effects of air throttling on
Fig. 1. Photo of the supersonic combustion facility in CARDC.
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Table 1 Operation sequence of the test. Operation sequence
Cold flow starts
Kerosene injected, spark and air throttling on
Spark and air throttling off
Kerosene injected off
Time/s
0.90
0.97
1.27
1.35
were shown in Fig. 4 and Fig. 5. The flame of case 6 firstly appeared in the front of the cavity at t = 1.214 s, and then spread downstream the combustor. As time went on, the flame zone became smaller and smaller, the flame was nearly blowout, only a small portion of the flame was anchored in the rearward of the cavity and in the boundary layer of the top wall at t = 1.242 s. Finally, the flame was blown off entirely which could not be seen in the photo at t = 1.260 s. The flame of case 8 could be seen at t = 0.978 s, almost at the same time when the kerosene was injected into the combustor. The flame burnt intensively and stably after t = 1.110 s, the kerosene was ignited successfully, and the flame burnt stably with the aid of air throttling. Now the reasons why the air throttling was helpful to the ignition in the supersonic combustor were analyzed in the following part. The numerical and experimental results (case 1, case 2, case 3 and case 4) of wall pressure distribution were shown in Fig. 6, the numerical results well matched the experimental results. The wall pressures of the cases with air throttling were higher obviously than that of the case without air throttling. The numerical results of mass averaged velocity and static pressure of case 1, case 7 and case 8 were shown in Fig. 7, the static temperature of the three cases were shown in Fig. 8. Shock waves generated due to the increased back pressure by the throttling air, which decelerated the high-speed main stream, and increased the static temperature and pressure in the combustor. The shock waves interacted with the cavity shear layer and wall boundary layer, then the cavity shear layer was lifted into the core flow, which would enhance the fuel/ air mixing efficiency (Fig. 9) and ignition. So, the cold room temperature liquid kerosene could be ignited successfully with the aid of air throttling, but the flux ratio of the throttling air should be large enough.
Table 2 Ignition results of the studying cases. case
Kerosene (ER)
Flux ratio
Ignition results
1 2 3 4 5 6 7 8
0.0 0.0 0.0 0.0 0.19 0.19 0.19 0.19
0.0 9.1% 13.7% 24.5% 0.0 9.1% 13.7% 24.5%
– – – – Fail Fail Success Success
4. Conclusions Ignition characteristics of a kerosene fueled combustor with air throttling was investigated by experimental and CFD method in the present paper. A small scaled cavity was used as flame holder and a capacitive-discharge spark plug was used to ignite the kerosene and air mixture. In the experimental results, when the ER of kerosene was 0.19, the kerosene could not be ignited by the spark plug at all without air throttling. When the flux ratio of air throttling was 9.1%, a small part of kerosene could be ignited but the flame was blown off soon. When the throttling was increased to 13.7% or 24.5%, the kerosene could be ignited successfully, and the combustion was stable and intense. In the numerical simulation results, the shock waves were generated due to the increased back pressure by the throttling air, which decelerated the high-speed main stream, and increased the static temperature and pressure in the combustor. The shock waves interacted with the cavity shear layer and wall boundary layer, then the cavity shear layer was lifted into the core flow, which would enhance the fuel/ air mixing efficiency and ignition.
Fig. 3. Wall pressures of pressure monitor, kerosene supply and air throttling.
ignition characteristics. “Fail” in the table meant the kerosene could not be ignited by the spark and “Success” meant the kerosene could be ignited by the spark. From the “Ignition results” in the table, when the flux ratio was 0.0 or 9.1%, the cold room temperature liquid kerosene could not be ignited by the spark. When the flux ratio was 13.7% or 24.5%, the kerosene was ignited successfully. The monitor pressures of case 5, case 6 and case 8 were shown in Fig. 3, the signs of kerosene and air throttling were also added into the figure. The kerosene supply pressure was kept at a constant pressure (4.4 MPa), when the kerosene was injected into the combustor, the monitor pressure of case 5 was still kept as constant (130 kPa), which meant the kerosene could not be ignited by the spark at all. The monitor pressure of case 6 increased at t = 1.214 s, but it went down deeply at t = 1.260 s, which meant a small part of kerosene was ignited successfully and the flame was blown off soon. The monitor pressure of case 8 increased sharply and the wall pressure was about 550 kPa, which was nearly four times larger than that of case 5. The kerosene was ignited successfully, and the combustion was stable, when the throttling air was removed at t = 1.27 s, the flame was blow off. The flame luminosity images of case 6 and case 8 at different times
Conflicts of interest The authors declare there are no conflicts of interest regarding the publication of this paper.
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Fig. 4. Flame luminosity images of case 6 at different times.
Fig. 5. Flame luminosity images of case 8 at different times.
Fig. 7. Mass averaged velocity and static pressure of case 1, case 7 and case 8.
Fig. 6. Numerical and experimental wall pressures of case 1, case 2, case 3 and case 4.
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[6] W. Huang, W. Liu, S. Li, Z. Xia, J. Liu, Z. Wang, Influences of the turbulence model and the slot width on the transverse slot injection flow field in supersonic flows, Acta Astronaut. 73 (2012) 1–9. [7] H. Wang, Z. Wang, M. Sun, N. Qin, Combustion characteristics in a supersonic combustor with hydrogen injection upstream of cavity flameholder, Proc. Combust. Inst. 34 (2013) 2073–2082. [8] Z. Wang, M. Sun, H. Wang, J. Yu, J. Liang, F. Zhuang, Mixing-related low frequency oscillation of combustion in an ethylene-fueled supersonic combustor, Proc. Combust. Inst. 35 (2015) 2137–2144. [9] H. Wang, Z. Wang, M. Sun, H. Wu, Combustion modes of hydrogen jet combustion in a cavity-based supersonic combustor, Int. J. Hydrog. Energy 38 (27) (2013) 12078–12089. [10] M. Sun, Z. Wang, J. Liang, H. Geng, Flame characteristics in supersonic combustor with hydrogen injection upstream of cavity flameholder, J. Propuls. Power 24 (4) (2008) 688–696. [11] H. Wang, Z. Wang, M. Sun, N. Qin, Large eddy simulation of a hydrogen-fueled scramjet combustor with dual cavity, Acta Astronaut. 108 (2015) 119–128. [12] B. Liu, G. He, F. Qin, Simulation of supersonic Ethylene–Hydrogen and air autoignition flame using skeletal mechanism, Acta Astronaut. 152 (2018) 521–533. [13] Y. Wang, Z. Wang, M. Sun, H. Wang, Effects of auto-ignition on combustion characteristics in a hydrogen-fueled dual-mode scramjet combustor, Acta Astronaut. 153 (2018) 154–158. [14] H. Bao, J. Zhou, Y. Pan, The effect of kerosene injection on ignition probability of local ignition in a scramjet combustor, Acta Astronaut. 132 (2017) 54–58. [15] X. Li, W. Liu, Y. Pan, L. Yang, B. An, J. Zhu, Characterization of ignition transient processes in kerosene-fueled model scramjet engine by dual-pulse laser-induced plasma, Acta Astronaut. 144 (2018) 23–29. [16] X. Li, W. Liu, Y. Pan, L. Yang, B. An, J. Zhu, Characterization of kerosene distribution around the ignition cavity in a scramjet combustor, Acta Astronaut. 134 (2017) 11–16. [17] Z. Cai, M. Sun, H. Wang, Z. Wang, Experimental investigation on ignition schemes of partially covered cavities in a supersonic flow, Acta Astronaut. 121 (2016) 88–94. [18] Omer M. Omer, X. Chen, C. Zhou, Experimental and numerical investigation on the ignition and combustion stability in solid fuel ramjet with swirling flow, Acta Astronaut. 137 (2017) 157–167. [19] C. Zhang, J. Chang, Y. Zhang, Y. Wang, W. Bao, Flow field characteristics analysis and combustion modes classification for a strut/cavity dual-mode combustor, Acta Astronaut. 137 (2017) 44–51. [20] J. Hu, J. Chang, W. Bao, Q. Yang, J. Wen, Experimental study of a flush wall scramjet combustor equipped with strut/wall fuel injection, Acta Astronaut. 104 (2014) 84–90. [21] C. Zhang, Q. Yang, J. Chang, J. Tang, W. Bao, Nonlinear characteristics and detection of combustion modes for a hydrocarbon fueled scramjet, Acta Astronaut. 110 (2015) 89–98. [22] W. Shi, J. Chang, J. Zhang, Y. Wang, W. Hou, W. Bao, Numerical investigation on behaviors of shock train in a hypersonic inlet with translating cowl, Acta Astronaut. 152 (2018) 682–691. [23] J. Le, S. Yang, W. Liu, Massively parallel simulations of kerosene-fueled scramjet, AIAA (2005) 2005–3318. [24] Z. Zheng, J. Le, Massively parallel computation of three-dimensional scramjet combustor, Proceedings of the 24th International Symposium on Shock Waves, Beijing, China, July 11–16, 2004, pp. 897–902 Shock Waves; 2005. [25] J. Le, S. Yang, X. Wang, Numerical Investigations of Unsteady Spray Combustion in a Liquid Kerosene Fueled Scramjet, ISABE, 2011, p. 1524.
Fig. 8. Static temperature contours in the main field of case 1, case 7 and case 8.
Fig. 9. Mass fraction distribution of kerosene of case 1, case 7 and case 8.
Acknowledgments The project supported by the National Natural Science Foundation of China (No. 51706237) and China Postdoctoral Science Foundation.. References [1] J. Tishkoff, J. Drummond, T. Edwards, A. Nejad, J. Tishkoff, Future Direction of Supersonic Combustion Research: Air Force/NASA Workshop on Supersonic Combustion, (1997) AIAA Paper 97-1017. [2] P.J. Waltrup, Liquid-fueled supersonic combustion ramjets – a research perspective, J. Propuls. Power 3 (6) (1987) 515–524. [3] M. Sun, C. Gong, S. Zhang, J. Liang, W. Liu, Z. Wang, Spark ignition process in a scramjet combustor fueled by hydrogen and equipped with multi-cavities at Mach 4 flight condition, Exp. Therm. Fluid Sci. 43 (2012) 90–96. [4] W. Huang, Z. Du, L. Yan, R. Moradi, Flame propagation and stabilization in dualmode scramjet combustors: a survey, Prog. Aero. Sci. 101 (2018) 13–30. [5] H. Wang, M. Sun, N. Qin, H. Wu, Z. Wang, Characteristics of oscillations in supersonic open cavity flows, Flow, Turbul. Combust. 90 (1) (2013) 121–142.
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Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Investigation of ignition characteristics in a kerosene fueled supersonic combustor Ye Tian∗, Shunhua Yang, Baoguo Xiao, Fuyu Zhong, Jialing Le Airbreathing Hypersonics Technology Research Center of CARDC, Mianyang, 621000, China
ABSTRACT
Ignition characteristics of a kerosene fueled supersonic combustor has been numerically and experimentally investigated in the present paper. Flame luminosity images and wall pressure measurements are used for better understanding the ignition and combustion characteristics, air throttling is used to enhance ignition in the combustor. The results are obtained under the inflow condition of Ma number 2.0, total pressure 1.0 MPa and total temperature 1100 K which corresponds to Ma4.5 flight condition. When the ER (Equivalence Ratio) of kerosene is 0.19, the kerosene cannot be ignited at all only by the spark plug. When the flux ratio of air throttling (the ratio of mass flux of air throttling to mass flux of the inflow air) is 9.1%, a small part of kerosene can be ignited, but the flame is blown off soon. When the throttling air flux ratio is increased to 13.7% or 24.5%, the kerosene can be ignited successfully, and the combustion is stable and intense. Cold room temperature liquid kerosene can be ignited successfully by the spark plug with the aid of the throttling air.
1. Introduction Scramjet engines are promising candidates for future airbreathing systems [1–5]. Air entering the scramjet combustor must be supersonic to avoid excessive dissociation of both nitrogen and oxygen gases when the flight Mach number is larger than 6. Consequently, time available for fuel injection, vaporization, fuel/air mixing, and combustion is very short, of the order of milliseconds [6–11]. So, achieving successful ignition in the scramjet combustor is an important factor which should be addressed in the scramjet design. Auto-ignition characteristics in an ethylene/hydrogen fueled supersonic combustor was simulated by Liu et al. by using skeletal mechanism [12], their results showed that the three mechanisms were not appropriate for combustion of ethylene–hydrogen/air mixture, A good agreement was found between the skeletal mechanism and the detailed mechanism in terms of the ignition delay time and the laminar flame speed over a wide range of parameters. Combustion characteristics were investigated in a hydrogen fueled dual-mode scramjet combustor by Wang et al. [13], they found under the stagnation temperature of 1600 K, strong auto-ignition induced the ignition process, auto-ignition contributed to the flame re-stabilization in the jet wake. The spark ignition of kerosene was investigated in a scramjet combustor by Bao et al. [14], the results showed that the injection pressure and injection location had a distinct effect on spark ignition, the injection pressure had both upper and lower limit for local ignition. Dual-pulse laser-induced plasma ignition of kerosene in cavity at model scramjet engine was studied by Li et al. [15,16], they found the
∗
entire ignition process of kerosene could be divided into five stages, which were referred as turbulent dissipation stage, quasi-stable state, combustion enhancement stage, reverting stage and combustion stabilization stage. The middle part of cavity was the most suitable location for ignition as a result of a favorable local equivalent ratio. Ignition schemes of the partially covered cavity in a scramjet combustor were investigated by Cai et al. [17], it revealed that the ignition scheme of the partially covered cavity had a great impact on the ignition and flame stabilization process. The real ways existed an optimized global equivalence ratio of a fixed ignition scheme, and the optimized global equivalence ratio of ignition in the partially covered cavity was lower than that of the uncovered cavity. The ignition and flame stability in a high-density polyethylene solid fueled ramjet were investigated experimentally and numerically by Omer et al. [18], the results showed that using swirl flow decreased the ignition time delay and improved the combustion efficiency and stability. Chang et al. [19–22] experimentally and numerically investigated the ignition and combustion characteristics in a strut/cavity scramjet combustor, results showed that combustion zone distributed in the shear layer behind the strut at a low equivalence ratio. Experimental results showed the combustion characteristics varied significantly with the strut/wall fuel feeding ratio, especially when this ratio was close to its lowest and highest limits. Based on the above discussions, many factors influenced the ignition characteristics in the scramjet combustor. But few published papers focused on the effects of air throttling on ignition in the combustor only with a small energy spark plug, which was our main purpose of the
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Tian).
https://doi.org/10.1016/j.actaastro.2019.03.024 Received 5 December 2018; Received in revised form 17 February 2019; Accepted 8 March 2019 0094-5765/ © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Ye Tian, et al., Acta Astronautica, https://doi.org/10.1016/j.actaastro.2019.03.024
Acta Astronautica xxx (xxxx) xxx–xxx
Y. Tian, et al.
Nomenclatures ER H L Ma P t W x
Equivalence Ratio Depth of Cavity Step Length of Cavity Mach Number Wall Pressure of Combustor Time Width of Cavity Distance from Combustor Entrance
present paper. Wall pressure measurements and high-speed color flame emission images were used for understanding the ignition characteristics and flame development. Fig. 2. a Schematic illustration of the combustor. b Length description of the combustor.
2. Experimental and numerical methods 2.1. Experimental methods and combustor configuration
orifices of 0.3 mm in diameter are used for cold room temperature kerosene injection (Injector K1, 285.0 mm from the isolator entrance or 15.0 mm upstream the cavity step). The cavity flame-holder along with a capacitive-discharge spark plug are mounted on the top wall, which has an expansion angle of 1.4°. The plug is located in the middle of the cavity bottom wall. The impulse excitation energy of the plug is about 6.0 J and the excitation frequency is 50 Hz.
All the experiments in the present study were conducted on a supersonic combustion facility (Fig. 1), which had already been introduced in our previous works [23]. Hydrogen and oxygen are burned in the heater, producing the high enthalpy inflow. Then the high enthalpy air flow is accelerated to Ma = 2.0 through a Laval nozzle, additional oxygen is added to maintain a 21% O2 mole fraction in the vitiated air, and the mole fraction of H2O and N2 are 12%, and 67%, respectively. The facility supplies a total temperature of 1100 K and a total pressure of 1.0 MPa from burnt air. High speed color images are captured by a CCD camera, and the exposure time is 0.5 ms and the frame rate is 2000 fps. The sampling frequency of pressure transducer is 1 kHz, which is used for measuring the wall pressure. There is a pressure monitor near the cavity ramp (500.0 mm from the isolator entrance), which is used to monitor the pressure changing during different tests. Fig. 2 shows the supersonic model scramjet combustor, which has a total length of 1100 mm and consists of a 300-mm-long section of nearly constant cross-sectional area, an L/H = 11.0 cavity (L: length of the cavity; H: depth of the cavity, H = 16.0 mm) and four divergent sections of 58.0, 144.0, 150.0, and 272.0 mm in length and 1.4, 2.0, 8.0, and 15.0 deg in expansion angle, respectively. The cross section of the combustor at its entrance is 30.0 mm in height by 150.0 mm in width. “0″indicates the beginning of the constant cross section and also represents the starting point for the static pressure measurement. Fifteen
2.2. Numerical methods and test sequence In this study, the inhouse CFD code AHL3D software which has been introduced in reference was used for computation [23–25]. A fully coupled form of species conservation equations and Reynolds averaged Navier-Stokes equations are used as a governing equation set for a chemically reacting supersonic viscous flow. Cell-averaged finite volume techniques are used to solve the conservative form governing equations. LU-SGS method is used in timemarching. In space terms difference, third order MUSCL interpolation method and AUSMPW+ scheme are used in inviscid fluxes construction, central difference method is used in viscous fluxes. Kok's modified k-ω TNT two-equation turbulence mode is used in turbulence simulations. The kerosene reaction mechanism modified version of CARDC's chemistry mechanism, involving 12 elementary reaction steps and 10 reaction species is used [23–25]. A 2D structured grid with a size of 100,000 grid points is used in this simulation, the simulation time for each case is about 4 h. The test sequence of the studying cases is shown in Table 1, when hydrogen enters the facility heater at t = 0.90 s, the cold flow then generates. The kerosene and the throttling air are both started to be injected into the combustor at t = 0.97 s, also the spark begins to work at the same time. The air throttling and the spark are switched off at t = 1.27 s, but the kerosene is injected off at t = 1.35 s. The location of air throttling is 625 mm from the combustor entrance. 3. Results and discussions Eight studying cases were used to investigate the ignition characteristics in a kerosene fueled scramjet combustor with air throttling, which were shown in Table 2. The front four cases (case 1∼ case 4) were used to study the effects of air throttling (Flux ratio: ratio of the throttling air flux and inflow air flux) on non-reacting flow structure, the other cases were used to investigate the effects of air throttling on
Fig. 1. Photo of the supersonic combustion facility in CARDC.
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Table 1 Operation sequence of the test. Operation sequence
Cold flow starts
Kerosene injected, spark and air throttling on
Spark and air throttling off
Kerosene injected off
Time/s
0.90
0.97
1.27
1.35
were shown in Fig. 4 and Fig. 5. The flame of case 6 firstly appeared in the front of the cavity at t = 1.214 s, and then spread downstream the combustor. As time went on, the flame zone became smaller and smaller, the flame was nearly blowout, only a small portion of the flame was anchored in the rearward of the cavity and in the boundary layer of the top wall at t = 1.242 s. Finally, the flame was blown off entirely which could not be seen in the photo at t = 1.260 s. The flame of case 8 could be seen at t = 0.978 s, almost at the same time when the kerosene was injected into the combustor. The flame burnt intensively and stably after t = 1.110 s, the kerosene was ignited successfully, and the flame burnt stably with the aid of air throttling. Now the reasons why the air throttling was helpful to the ignition in the supersonic combustor were analyzed in the following part. The numerical and experimental results (case 1, case 2, case 3 and case 4) of wall pressure distribution were shown in Fig. 6, the numerical results well matched the experimental results. The wall pressures of the cases with air throttling were higher obviously than that of the case without air throttling. The numerical results of mass averaged velocity and static pressure of case 1, case 7 and case 8 were shown in Fig. 7, the static temperature of the three cases were shown in Fig. 8. Shock waves generated due to the increased back pressure by the throttling air, which decelerated the high-speed main stream, and increased the static temperature and pressure in the combustor. The shock waves interacted with the cavity shear layer and wall boundary layer, then the cavity shear layer was lifted into the core flow, which would enhance the fuel/ air mixing efficiency (Fig. 9) and ignition. So, the cold room temperature liquid kerosene could be ignited successfully with the aid of air throttling, but the flux ratio of the throttling air should be large enough.
Table 2 Ignition results of the studying cases. case
Kerosene (ER)
Flux ratio
Ignition results
1 2 3 4 5 6 7 8
0.0 0.0 0.0 0.0 0.19 0.19 0.19 0.19
0.0 9.1% 13.7% 24.5% 0.0 9.1% 13.7% 24.5%
– – – – Fail Fail Success Success
4. Conclusions Ignition characteristics of a kerosene fueled combustor with air throttling was investigated by experimental and CFD method in the present paper. A small scaled cavity was used as flame holder and a capacitive-discharge spark plug was used to ignite the kerosene and air mixture. In the experimental results, when the ER of kerosene was 0.19, the kerosene could not be ignited by the spark plug at all without air throttling. When the flux ratio of air throttling was 9.1%, a small part of kerosene could be ignited but the flame was blown off soon. When the throttling was increased to 13.7% or 24.5%, the kerosene could be ignited successfully, and the combustion was stable and intense. In the numerical simulation results, the shock waves were generated due to the increased back pressure by the throttling air, which decelerated the high-speed main stream, and increased the static temperature and pressure in the combustor. The shock waves interacted with the cavity shear layer and wall boundary layer, then the cavity shear layer was lifted into the core flow, which would enhance the fuel/ air mixing efficiency and ignition.
Fig. 3. Wall pressures of pressure monitor, kerosene supply and air throttling.
ignition characteristics. “Fail” in the table meant the kerosene could not be ignited by the spark and “Success” meant the kerosene could be ignited by the spark. From the “Ignition results” in the table, when the flux ratio was 0.0 or 9.1%, the cold room temperature liquid kerosene could not be ignited by the spark. When the flux ratio was 13.7% or 24.5%, the kerosene was ignited successfully. The monitor pressures of case 5, case 6 and case 8 were shown in Fig. 3, the signs of kerosene and air throttling were also added into the figure. The kerosene supply pressure was kept at a constant pressure (4.4 MPa), when the kerosene was injected into the combustor, the monitor pressure of case 5 was still kept as constant (130 kPa), which meant the kerosene could not be ignited by the spark at all. The monitor pressure of case 6 increased at t = 1.214 s, but it went down deeply at t = 1.260 s, which meant a small part of kerosene was ignited successfully and the flame was blown off soon. The monitor pressure of case 8 increased sharply and the wall pressure was about 550 kPa, which was nearly four times larger than that of case 5. The kerosene was ignited successfully, and the combustion was stable, when the throttling air was removed at t = 1.27 s, the flame was blow off. The flame luminosity images of case 6 and case 8 at different times
Conflicts of interest The authors declare there are no conflicts of interest regarding the publication of this paper.
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Fig. 4. Flame luminosity images of case 6 at different times.
Fig. 5. Flame luminosity images of case 8 at different times.
Fig. 7. Mass averaged velocity and static pressure of case 1, case 7 and case 8.
Fig. 6. Numerical and experimental wall pressures of case 1, case 2, case 3 and case 4.
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Fig. 8. Static temperature contours in the main field of case 1, case 7 and case 8.
Fig. 9. Mass fraction distribution of kerosene of case 1, case 7 and case 8.
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