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Study of low-temperature ignition characteristics in a supersonic combustor
Energy 195 (2020) 117060
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Study of low-temperature ignition characteristics in a supersonic combustor Jianping Li*, Jindong Li, Kai Wang, Guiqian Jiao, Zilong Liao School of Power and Energy, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2019 Received in revised form 24 December 2019 Accepted 28 January 2020 Available online 29 January 2020
The Ignition combustion experiment in a supersonic combustor was carried out under the conditions with the incoming flow Ma ¼ 2.0, the total temperature at 700 K and the total pressure at 520 KPa(corresponding to the flight Mach number at 3.5). Spray blocks without struts and with struts (with the blockage ratios as 20%, 10%, 7.3% and 5% respectively) were used in the supersonic combustor. The experimental result showed that when there was no strut in the combustor, kerosene could be ignited but was unable to burn stably alone after hydrogen was removed. The addition of struts achieved the independent and stable combustion of kerosene. However, due to the addition of struts, the combustion during the strut test using the blockage ratios at 20%, 10% and 7.3% induced the back pressure to be spread towards the upstream to the entrance of the isolator; When the blockage ratio of the strut was 5%, kerosene could burn stably alone, and the back pressure induced by combustion failed to be spread to the entrance of the combustor. In this paper, the stable flame mode using the cavity and struts was used to achieve stable ignition under the condition with Ma ¼ 2.0, total temperature at 700 K and total pressure at 520 KPa. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Supersonic combustor Low total temperature Ignition characteristics Blockage ratio Induced back pressure
1. Introduction With the continuous development of hypersonic technology, the scramjet engine, which is as the power of the hypersonic vehicle, has become the focus of various technological and military countries in the world [1]. Turbine Based Combined Cycle engine (TBCC) is based on the combination of a turbine engine and a scramjet engine, so it is of advantages such as a wide range of fight Mach good specific impulse performance, horizontal takeoff/landing, and reusability. However, the maximum operating speed upper limit of a turbine engine is generally Mach 2.2 only, while the scramjet engine can start work at Mach 4.0. Whether a turbine engine and a scramjet can achieve efficient connection of the speed and the thrust is the primary program for TBCC combined engines. This paper studied the ignition characteristics of supersonic combustor at the low Mach number and the low total temperature by reducing the limit of working Mach number of the scramjet, which provides technical support for the demonstration of the TBCC combined engines.
* Corresponding author. E-mail address: [email protected] (J. Li). https://doi.org/10.1016/j.energy.2020.117060 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
In recent years, a great number of tests and studies were carried out on the ignition of kerosene fueled supersonic combustors. Tian et al. [2] studied the effect of the thermal throat on the flame stability of a kerosene fueled supersonic combustor under the incoming flow condition of Ma number at 2.0, total temperature at 1100 K and total pressure at 1.0 MPa. The test results showed that due to the existence of the thermal throat, shock trains were generated in the isolator which reduced the high-speed incoming flow, improved the temperature and pressure, and enhanced the mixing efficiency of fuels in combustors, thus having achieved the flame stability. Mansfield A B [3] systematically investigated the ignition properties of simulated syngas mixtures at high-pressure low-temperature conditions relevant to gas turbine combustor operation using the University of Michigan Rapid Compression Facility. The pressures and temperatures after compression ranged from 3 to 15 atm and 870e1150 K respectively. Bao et al. [4] conducted the liquid kerosene spark ignition test for the equipment models of double-cavity scramjet combustors, with the analog flight Mach number at 4.5 and the total temperature at 1032 K. They found that the cavities with a smaller length-depth ratio were suitable for maintaining the local flame in the cavity with direct injection and upstream injection; the larger cavities could be better ignited and expand the local flame, so the length-depth ratio of the
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cavities affected the ignition process of spark plugs. Lu Z et al. [5] used Navier-Stokes (URANS) and large eddy current simulation (LES) to study the effects of spray and turbulence models on the mixing and combustion characteristics of n-heptane spray flames at constant temperature. It was found that the droplet movement near the nozzle had a significant effect on the distribution of fuel vapor, while the penetration length of the liquid was controlled by the evaporation process and was not sensitive to the gas injection model. The calculation results of Ma S et al. [6] showed that the fuel mixture of hydrogen and hydrocarbon had a larger delay time and a smaller flame velocity, which helped to weaken the flame area and reduce the combustion efficiency. Xi Wenxiong, Wang Zhenguo et al. [7,8] conducted a comparative test of ignition scheme on the dual-mode scramjet engine under the condition of the Mach number at 2 and the total temperature at 840 K. Their results showed that the heat jet flame and the spray in the downstream after mixing combustion thought the flame reversed spread to form a cavity-resident flame, the pressure in the combustor was greatly affected by the unsteady characteristics of the heat jet supply and the spray action. Trebs A [9] and others experimentally studied the combustion field downstream of a 10-degree compression ramp injector. The study found that with the increase of the incident boundary layer, the shape of the flame changed, the durability of the vortex core decreased, and the combustion efficiency improved. LIU Wei [10] investigated the ignition and flameholding characteristics of liquid single component hydrocarbon fuels in a model supersonic combustor under the condition of Mach number 2.03 and ranging from 1040 K to 1100 K. Theoretical analysis indicated that the evaporation characteristics of methylcyclohexane was better than n-dodecane’s within the same incoming flow parameters. Lan Z [11] and others found that the single-step reaction mechanism made the fuel easy to ignite and the combustion efficiency was higher, but the obtained temperature was higher; the multi-step reaction mechanism was closer to the actual combustion experiment, but the fuel might be more difficult to ignite, the flame might be easily destroyed during the simulation, and the simulation time might be relatively long. Gao Z [12,13] and others introduced a representative interactive small flame (RIF) model and a small flame/progress variable (FPV) model to develop two flame types for supersonic combustion flow. The numerical results showed that compared with the stable small flame (SF) model, the temperature and main species concentration changes caused by the RIF model were very limited, which meant that the species concentration in the small flame pool was not sensitive to local high Mach number effects. Goldfeld M A [14] measured the kerosene concentration distribution in the combustion chamber attached to the hot wind tunnel, and the Mach number of the combustion chamber entrance was 2.89. The authors showed that the lack of intense combustion in the model burner was caused by insufficient local equivalence ratio. Evans M J [15] analysis of the same unpremixed flow in a laminar diffusion flame showed that for a dilute ethylene fuel with a high-temperature air oxidant, ignition at the same equivalent ratio could better predict the ignition delay. Through the study and survey of the ignition test of the scramjet engine, it is found that there is no ignition test on the supersonic combustor under the condition that the total temperature is lower than 800 K, and the lowest total temperature condition is about 820 K. In this paper, the igniting combustion experiment of a kerosene fueled supersonic combustor was carried out under the condition of the incoming flow at the total temperature of 700 K. The low-temperature ignition characteristics of the kerosene fueled supersonic combustor were studied, and the lower limit of the working Mach number of the supersonic combustor was expanded.
2. Test plans 2.1. Diagram of the test system and the model The test in this paper was based on the direct connection test system with a resistance heater of Northwestern Polytechnical University as shown in Fig. 1. The system bears the total temperature range at 500e1000 K, the total pressure range at 0.2e2.5 MPa, and a mainstream flow range at 0.4e1.5 kg/s, and can simulate the incoming flow conditions of the supersonic combustor with a flight Mach number at 2.5e4.5. The test model used in this paper is a supersonic combustor test model with a single cavity flame holder. Fig. 2 showed the geometry of the test model. Fig. 3 showed the structure of the singlecavity flame holder in the combustor experiment model. It can be seen from the figure that the hydrogen nozzle was located inside the single cavity on the upper wall surface, and the axial distance from the leading edge was x/h ¼ 1.85, which was composed of two nozzle holes having a diameter of 1 mm, and the jetting method was vertical to the wall. The kerosene nozzle was arranged on the kerosene spay block, which was installed at the front edge of the cavity on the upper wall of the cavity. In this paper, the corresponding blockage ratio of the spray block with the strut was 20%, 10%, 7.3% and 5% respectively. The non-strut spray block and the spray block of each blockage ratio used in the test were shown in Fig. 4. Fig. 4 (a) was the non-strut spray block. The structure of the block with the blockage ratio of 20% was shown in Fig. 4(b), the spray block had two struts arranged in front of the nozzle holes on both sides, with the cross-sectional dimension at 8 15 mm2. The size of the block with the blockage ratio of 10% was shown in Fig. 4(c), the strut was arranged in front of the nozzle hole, with the cross-sectional size as 8 15mm2. The structure of the block with the blockage ratio of 7.3% was shown in Fig. 4(d), the strut was arranged in front of the nozzle hole, with the cross-sectional size as 8 11mm2. The blockage ratio of the 5% of the structure of the spray block was as shown in Fig. 4(e), and a strut was provided in front of the nozzle hole, with the crosssectional size as 4 15 mm2. 2.2. Experimental plan Fig. 5 showed the schematic diagram of pioneer hydrogenguided kerosene ignition control timing, including the control timing of the spark plug switch, hydrogen control valve and kerosene control valve. As shown in Fig. 5, the spark plug switch had a working time of 1s~8s, the hydrogen continuous working time was 2s~7s, and the kerosene continuous working time was 5se10s so that the hydrogen burning time was 3s, the hydrogen and kerosene co-burning time was 2s, and the kerosene burning time was 3s. According to the fuel injection situation, the combustion test process can be divided into four stages: no combustion stage, pioneer flame hydrogen combustion (H2) stage, hydrogen and kerosene cocombustion (H2þKerosene) stage, kerosene alone combustion (Kerosene) stage. Under the condition of incoming flow Ma ¼ 2.0, total temperature 700 K and total pressure 520 kPa, the spray block shown in Fig. 4 was selected for the test. The test scheme was shown in Table 1. 2.3. One-dimensional analysis method According to the experimental data acquisition system, the wall pressure value is obtained. The one-dimensional analysis method is used to calculate the other aerodynamic parameters of the combustion chamber, including Mach number, static temperature and
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Fig. 1. Resistance heating direct connection test system.
Fig. 2. Geometrical configuration of the combustor test model.
Fig. 3. Single-cavity flame holder.
total temperature distribution. One-dimensional stationary flow equation based on variable cross-section, chemical reaction, mass addition, and wall friction is shown as below:
1 þ gM 2 j dTt dM j dA gM2 j 4Cf dx ¼ þ þ 2 M D Tt 1M A 2 1 M2 2 1 M2 (1)
dPs gM2 dA gM2 j dTt gM ¼ Ps 1 M 2 A 1 M2 Tt
2
h i 1 þ ðg 1ÞM 2 4Cf dx D 2 1 M2 (2)
2 where: j ¼ 1 þ ðg1Þ 2 M , Ps is static pressure, Tt is total temperature, A is the cross-sectional area, Cf is the friction coefficient, D is the hydraulic diameter, g is the specific heat ratio, and M is the Mach number. By combining the above two equations, we can get:
Cf dM 1 M dA 1 þ gM 2 1 dp ¼ M dx 2 A dx 2gM p dx D h i 2 2 1 þ ð 1 M T g 1ÞM T t t 4C dTt Tt 1 dA 1 dp f ¼ 2j dx j A dx D gM2 j p dx
(3)
(4)
According to the pressure parameters of each measuring point of the combustor wall, we can get static pressure distribution. On this basis, one-dimensional data processing method is utilized to work out other airflow parameters distribution in the combustor. The friction coefficient Cf is set to 0.004, and the static pressure Ps on the wall is known. We first figure out the distribution of airflow M number and total temperature Tt from the above equation. After that, other airflow parameters (like static temperature Ts , total pressure Pt ) could be obtained from the gas-dynamics formula.
3. Test results and analysis The ignition results of the supersonic combustor under the conditions of incoming flow Ma ¼ 2.0, total temperature at 700 K, and total pressure at 520 kPa were shown in Table 2. “Fail” in the table indicated that the fuel can not be successfully ignited, and “Success” indicated that the fuel can be ignited. “Yes” meant that
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Fig. 4. Blocks with different blockage ratios.
stage there was stable combustion. When the test was carried out without the strut, the kerosene can stably burn separately; the method of adding strut in front of the nozzle hole can help kerosene to burn stably and separately, but the combustion induced back pressure easily propagated the upstream to the entrance of the isolator. When the blockage ratio is 20%, 10%, and 7.3% respectively, the induced back pressure was spread to the inlet of the combustor; when the blockage ratio was 5%, not only the kerosene can burn stably and separately, but also the induced back pressure was not propagated to inlet of combustor.
3.1. Analysis of ignition performance of combustor under different blockage ratio conditions Fig. 5. Pioneer hydrogen guided kerosene control timing.
Table 1 Test incoming flow state and equivalence oil and gas ratio. Ma
Total temperature(K)
Total pressure(kPa)
strut
Kerosene(ER)
2.0 2.0 2.0 2.0 2.0
700 700 700 700 700
520 520 520 520 520
e 20% 10% 7.3% 5%
0.10e0.25 0.10e0.25 0.10e0.25 0.10e0.25 0.10e0.25
the induced back pressure during steady combustion of the combustor had spread to the inlet of the combustor, destroying the incoming flow condition; “No” meant that the induced back pressure during combustion had not spread to the inlet of the combustor, and the flow conditions were not destroyed. The test results showed that in all the hydrogen combustion stages of the test and the hydrogen and kerosene co-combustion
Fig. 6 showed the wall pressure distribution and Mach number distribution at kerosene alone combustion stage under different blockage ratios, including no struts and struts (with the blockage ratios at 20%, 10%, 7.3% and 5% respectively). As shown in the figure, when the non-strut spray block was used, the wall pressure distribution was restored to the cold flow level after the pioneer hydrogen was removed, indicating that the kerosene can not burn stably and separately. When the spray block with plate was adapted, the kerosene can burn stably and separately after the pioneer hydrogen was removed. When the blockage ratio was 20%, 10% and 7.3% and the Mach number was 0.68, 0.5, and 0.81 respectively, the wall pressure distribution increased obviously, and the combustion induced back pressure propagated upstream to the entrance of the isolator, which seriously destroyed the incoming flow conditions. When the blockage ratio was 5% and Mach numbers was 1.98, the wall pressure increased and the kerosene can burn stably and separately, induced back pressure did not propagate to the entrance of the isolator, and the flow conditions were not destroyed. Fig. 7 showed the static temperature distribution and total temperature distribution of the combustor
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Table 2 Ignition results of the studying cases. Case
1 2 3 4 5 6 7
strut
e 20% 10% 7.3% 7.3% 5% 5%
H2þKerosene(ER)
0.065 0.087 0.054 0.087 0.087 0.086 0.092
þ þ þ þ þ þ þ
0.18 0.25 0.18 0.16 0.19 0.13 0.19
Ignition results
The induced back pressure disturb the inlet
H2
H2þKerosene
Kerosene
Success Success Success Success Success Success Success
Success Success Success Success Success Success Success
Fail Success Success Success Success Success Success
e Yes Yes Yes Yes No No
Fig. 6. Wall pressure distribution and Mach number distribution at kerosene alone combustion stage under different blockage ratio conditions.
Fig. 7. Static temperature distribution and total temperature distribution at kerosene alone combustion stage under different blockage ratio conditions.
calculated by the wall pressure distribution wall under different blockage ratios. As shown in the figure, the distribution of static temperature and total temperature in the combustor restored to the level of cold flow in the test with non-strut spray block, which indicated that kerosene could not burn stably and separately; when the blockage ratio was 20%, 10% and 7.3% respectively at the kerosene alone combustion stage, the induced back pressure propagated to the inlet of the combustor, the incoming flow conditions were
destroyed, and the static temperature increased. When the blockage ratio was 5%, the induced back pressure did not propagate to the inlet of the combustor, and the static temperature of the entrance was the same as that of the cold flow. The static temperature and total temperature distribution diagrams showed that in the test of spray block with non-strut, spray block with blockage ratio of 20%, spay block with blockage ratio of 10%, spray block with blockage ratio of 7.3%, and spray block with blockage ratio of 5%, the outlet static temperatures of the
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combustor was 656 K, 939 K, 1271 K, 941 K and 996 K respectively, and the total outlet temperatures was 885 K, 1101 K, 1424 K, 1044 K and 1152 K respectively. The results showed that the method of adding plates in front of the nozzle hole helped the kerosene to burn stably and separately. When the blockage ratio was 20%, 10% and 7.3% respectively, the induced back pressure propagated to the inlet of combustor, which seriously destroyed the incoming flow condition. When the blockage ratio was 5%, not only the kerosene can burn stably and separately, but also the induced back pressure did not propagate to the inlet of combustor, and the incoming flow condition was not destroyed. 3.2. Ignition performance analysis with 7.3% blockage under different equivalence ratios Fig. 8 showed the wall pressure distribution and Mach number distribution under different equivalence ratios with the blockage ratio of 7.3%, including the cold flow stage, the pioneer flame hydrogen combustion stage, and the hydrogen and kerosene cocombustion stage and kerosene standalone combustion stage. The figure showed: (1) Pioneer flame hydrogen standalone combustion stage: Compared with the cold flow stage, the wall pressure distribution increased, the combustor airflow became the subsonic mode at X ¼ 290 mm, and the thermodynamic throat existed at X ¼ 560 mm. (2) Hydrogen and kerosene co-combustion stage: Compared with the total equivalence ratio ER ¼ 0.247 (hydrogen ER ¼ 0.087þkerosene ER ¼ 0.16), the wall pressure was higher when the total equivalence ratio was ER ¼ 0.277 (hydrogen ER ¼ 0.087þkerosene ER ¼ 0.19), the corresponding Mach number distribution of the combustor was lower, but the induced back pressure at both two equivalence ratio states propagated to the inlet of the combustor, which seriously destroyed the incoming flow condition, and the combustor airflow was in the subsonic mode. At this time, the inlet Mach numbers were all 0.72. (3) Kerosene standalone combustion stage: Compared with the equivalence ratio (ER ¼ 0.16), the wall pressure was higher when the equivalence ratio was 0.19, and the corresponding Mach number distribution of the combustor was lower. The
wall pressures of the two equivalence ratio tests were slightly lower than the wall pressures at hydrogen and kerosene co-combustion stage, and the Mach number distributions were slightly higher. However, the induced back pressure at both equivalence ratios propagated to the inlet of the combustor, which seriously destroyed the incoming flow conditions, and the inlet Mach number was 0.83. The results showed that under the condition of the total temperature of incoming flow 700 K and the total pressure 520 kPa and blockage ratio 7.3%, the kerosene can burn stably and separately after the pioneer hydrogen was removed, but the induced back pressures at the other stages propagated to the entrance of the isolator except at the hydrogen combustion standalone stage. 3.3. Ignition performance analysis with blockage ratio of 5% under different equivalence ratios Fig. 9 showed the wall pressure distribution and Mach number distribution under different equivalence ratios with the blockage ratio of 5%, including the cold flow stage, the pioneer flame hydrogen combustion stage, and the hydrogen and kerosene cocombustion stage and kerosene standalone combustion stage. The figure showed: (1) Pioneer flame hydrogen standalone combustion stage: Compared with the cold flow stage, the wall pressure distribution increased, and the wall pressure of the equivalence ratio ER ¼ 0.092 was slightly higher than the wall pressure of the equivalence ratio ER ¼ 0.086. When the equivalence ratio was ER ¼ 0.086, the combustor airflow entered the subsonic mode at X ¼ 360 mm, and the thermal throat existed at X ¼ 540 mm. When the equivalence ratio was ER ¼ 0.092, the combustor airflow entered the subsonic mode at X ¼ 300 mm, and there was a thermal throat at X ¼ 560 mm, and the induced back pressure did not propagate to the inlet of combustor. At this time, the inlet Mach numbers were all 1.97. (2) Hydrogen and kerosene Co-combustion stage: Compared with the total equivalence ratio ER ¼ 0.216 (hydrogen ER ¼ 0.086þkerosene ER ¼ 0.13), the wall pressure was higher when the total equivalence ratio was ER ¼ 0.282 (hydrogen 0.092þkerosene 0.19). The corresponding Mach
Fig. 8. Wall pressure distribution and Mach number distribution of the 7.3% blockage test.
J. Li et al. / Energy 195 (2020) 117060
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Fig. 9. Wall pressure distribution and Mach number distribution with blockage ratio of 5%.
number distribution of the combustor was lower, but the induced back pressure at both equivalence ratio stages propagated to the inlet of the combustor, which seriously destroyed the incoming flow condition, and the combustor airflow was in the subsonic mode. The inlet Mach numbers at this time were 0.75 and 0.77, respectively. (3) Kerosene standalone combustion stage: Compared with the equivalence ratio ER ¼ 0.13, the wall pressure was higher when the equivalence ratio was ER ¼ 0.19, and the corresponding Mach number distribution of the combustor was lower. When the equivalence ratio was ER ¼ 0.13, the airflow of combustor became the subsonic mode at X ¼ 360 mm, and there was a thermal throat at X ¼ 570 mm. When the equivalence ratio was ER ¼ 0.19 the airflow of combustor became the subsonic mode at X ¼ 240 mm, and there was thermodynamic throat at X ¼ 760 mm. The induced back pressure at the two equivalence ratios did not propagate to the combustor inlet, and the flow conditions were not destroyed. At this time, the Mach numbers were all 1.98. The results show that kerosene can burn stably and separately after the pioneer hydrogen was removed when the blockage ratio was 5% under the condition of total inlet temperature 700 K and total pressure 520 kPa, and the induced back pressure did not propagate to the inlet of combustor, and the incoming flow condition remained unchanged.
3.4. Numerical simulation analysis of the combustor Different mesh generation methods or the number of meshes may affect the simulation results. In order to ensure the accuracy of the numerical simulation calculation results, the grid independence test was performed on the model grid to obtain the best and most accurate numerical simulation results. In this paper, three structured grids were constructed with 10,000, 50,000, and 100,000 grids; two unstructured grids were constructed with 50,000 and 100,000 grids. Under the same inflow conditions and fuel supply strategies, the numerical simulation calculations on five grids were performed and verified with test results. Fig. 10 showed the calculation results. As shown in the figure, the overall calculation effect and change trend of the five grids were almost the same and the calculation results were not much different and could be ignored. This paper
Fig. 10. Grid independence test result.
chose a structured grid of 50,000 grids for numerical simulation calculation. On the whole, the numerical calculation results in the burning state were basically consistent with the distribution trend of the experimental measurement results. Therefore, the numerical calculation method used in this paper had certain reliability and
Fig. 11. Flow field cloud in the cold flow state of the combustor (non-strut).
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rationality, which could be used to analyze the flow field of the combustor. Fig. 11(a) and Fig. 10(b) showed the incoming flow Mach number as 2.0, total temperature at 700 K, total pressure at 520 kPa at the entrance of combustor model as well as static pressure and Mach number distribution cloud diagram under the condition of nonstrut spray block test and in the cold flow state. As shown in the figure, the velocity of the inlet of the combustor model was high, and the gas flow in the combustor was supersonic. Due to the sudden increase in the flow area from the leading edge of the cavity, an expansion wave was formed at the leading edge of the cavity. The airflow accelerated after the expansion wave, but the static temperature and the static pressure decreased. At the trailing edge of the cavity and the deflection of the shear layer of the cavity, as the flow area suddenly decreased, shock waves formed at the peak point of deviation of the cavity shear layer and on the lower wall surface of the combustor, the air flow decelerated and the pressure increased after the shock wave. The trailing edge of the cavity was an expansion section, and the area was suddenly expanded, so that an expansion wave formed at the position of the trailing edge of the cavity. Due to the interaction between the shock wave and the shock wave, and the interaction between the shock wave and the expansion wave, the pressure in the combustor first increased and then decreased (as shown in Fig. 6). Fig. 12(a) and Fig. 12(b) showed the static pressure and Mach number distributions of the central section of the combustor under the cold flow condition, when Mach number was 2.0, total temperature was 700 K, total pressure was 520 kPa and blockage ratio was 5%. As can be seen from the figure, there was a low-speed recirculation zone in the cavity, due to its characteristic of low speed and high temperature, it could increase the residence time of the kerosene fueled in the combustor and increase the temperature of mixing gas, which contributed to the ignition of the supersonic combustor and the flame stability. However, it can be seen from the comparison with Fig. 11 that, the size of the recirculation zone in the cavity of the combustor flow channel with the strut was smaller than that with the non-strut, and the cavity shear layer was more deflected toward the bottom of the cavity. As the strut was arranged in front of the nozzle hole, the airflow flow was more complicated due to the complicated wave system in the downstream part of the strut affected by it, and the supersonic flow was blocked by the strut, and a small part of the low speed zone existed behind the strut, which helped to increase the penetration depth of liquid kerosene and made it more fully mixed with air. The trailing edge of the cavity was an expansion section, and there was an area expansion. Therefore, an expansion wave was formed at the position of the trailing edge of the cavity, and then the expansion wave was reflected from the wall surface, and the airflow was accelerated
Fig. 12. Flow field cloud image of the central section in the cold flow state of the combustor (with blockage ratio at 5%).
and decompressed by the expansion wave, so the pressure distribution in the expansion section was gradually decreased. Fig. 13 and Fig. 14 showed the static pressure and Mach number distribution of the center section of the combustor under the conditions of Mach number at 2.0, total temperature at 700 K, total pressure at 520 kPa, blockage ratio at 5% and kerosene equivalence ratio ER ¼ 0.13 and 0.19, respectively. As shown in Fig. 13, compared with the cold flow stage, the wave system of the flow flied in the combustor was more complex. The induced back pressure rose sharply and propagated upstream to the isolator. When the kerosene equivalence ratio was ER ¼ 0.13, the pressure disturbed to about X/L ¼ 0.08 position. When the kerosene equivalence ratio was ER ¼ 0.19, the pressure disturbed to about X/L ¼ 0.04 position. With the increase in kerosene equivalence ratio, the induced back pressure moved upward gradually. From Fig. 14, it can be seen that the induced back pressure rise caused the wall boundary layer to separate and form a shock train. The interaction between the shock train and the boundary layer resulted in a larger boundary layer separation zone near the upper wall. The high-speed incoming flow decelerated and supercharged after passing through the shock train, and the combustor was in a subsonic combustion mode. Fig. 15 and Fig. 16 showed the static temperature distributions of the central section of the combustor and the three X-axis cross sections in the combustion state, respectively. From the static temperature distribution, it can be seen that with the increase in the kerosene equivalence ratio, the combustion area, that was the heat release concentration area, gradually diffused from the vicinity of the cavity to the whole cavity. From the cross section of X-axis, it can be seen that the kerosene was injected through a single hole and there was a supporting plate in front of the hole, the combustion area at the leading edge of the cavity was located near the upper wall of the combustor axis, and then gradually diffused into the cavity until it filled almost the whole cavity. As can be seen from the figure, when kerosene equivalence ratio was ER ¼ 0.13 and 0.19, the combustion can be stable and products can be stable generated near the cavity. 4. Conclusion In this paper, the ignition and combustion characteristics of kerosene fueled supersonic combustor at low temperature were studied using hydrogen-guided flame ignition and no-support palate spark block and spark block with support block under the incoming flow condition of Ma ¼ 2.0, total temperature 700 K and total pressure 520 kPa. The following conclusions were drawn as follow: 1. The mode of the cavity with the strut stabilizing flame was used in this paper. When the blockage ratio was 20%, 10% and 7.3% respectively, kerosene can burn separately and stably after hydrogen was removed, but the combustion induced back pressure had been propagated upstream to the entrance of the isolator, which destroyed the incoming flow condition of the
Fig. 13. Static pressure distribution of the central section of the combustor(burning).
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Fig. 14. Mach number distribution cloud of the center section of the combustor.
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flow state, the recirculation zone was smaller than that in the non-strut when the blockage ratio was 5%, but the flow field wave system in the combustor was more complex, which helped to mix kerosene and airflow and increase the mixing temperature. In the combustion state, the flow field wave system in the combustor was more complex. The interaction between shock train and boundary layer made the upper wall have a larger boundary layer separation zone nearby, the combustion can be stable and products can be stably generated near the cavity, and with the increase in kerosene equivalence ratio, the combustion induced back pressure moved upward gradually.
Declaration of competing interest In the research process of the paper, Li Jianping was in charge of experiment and thesis writing; Li Jindong was in charge of literature collection; Wang Kai was in charge of experiment and experimental data processing, Jiao Guiqian was in charge of experimental data acquisition and Liao Zilong was in charge of experimental operation.
References Fig. 15. Cloud section of the combustor center and cross-section static temperature distribution (ER ¼ 0.13).
Fig. 16. Cloud cross section of the combustor and cross-section static temperature distribution (ER ¼ 0.19).
combustor. With the decrease in blockage ratio of strut, the combustion induced back pressure was more hardly to propagate to upstream. When the blockage ratio of the strut was 5%, kerosene can burn stably after pioneer hydrogen was removed, and the combustion induced back pressure did not propagate to the entrance of the isolator. This indicated that the combustor configuration adopted in this paper could work normally under the flight Mach number of about 3.5. This study extended the lower limit of the working Mach number of the ramjet. 2. According to the results of the ignition and combustion test, the flow path configuration of the supersonic combustor without the strut and the flow path configuration with the strut (with blockage ratio at 5%) were numerically simulated: in the cold
[1] Zhang Lijing, Liu Dongsheng, Yu Cungui, et al. Hypersonic vehicles. Aviation Weapons 2010;2:13e6. [2] Tian Y, Zeng X, Yang S, et al. Study on the effects of thermal throat on flame stabilization in a kerosene fueled supersonic combustor. Energy Convers Manag 2018;166:98e105. [3] Mansfield AB, Wooldridge MS. High-pressure low-temperature ignition behavior of syngas mixtures. Combust Flame 2014;161(9):2242e51. [4] Bao H, Zhou J, Pan Y. Effect of cavity configuration on kerosene spark ignition in a scramjet combustor at Ma 4.5 flight condition. Acta Astronaut 2015;117(1):368e75. [5] Lu Z, Zhou L, Ren Z, Lu T, Law CK. Effects of spray and turbulence modelling on the mixing and combustion characteristics of an n-heptane spray flame simulated with dynamic adaptive chemistry. Flow, Turbul Combust 2016;97(2):609e29. [6] Ma S, Zhong F, Zhang X. Numerical study on supersonic combustion of hydrogen and its mixture with ethylene and methane with strut injection. Int J Hydrogen Energy 2018;43(15):7591e9. [7] XI Wenxiong, Wang Zhenguo, Liu Weidong, et al. Experimental comparison on the scheme of ignition in dual-mode scramjet. J Propuls Technol 2013;34(3):383e9. [8] Wenxiong XI. Experiment and factors affecting ethylene ignition in supersonic flow-field. J Aero Power 2018;33(12):18e28. [9] Trebs A, Roa M, Heister S, Anderson W, Lucht R. Ramp injector scale effects on supersonic combustion. J Propul Power 2014;30(2):426e37. [10] Liu Wei, Yu-zhen Lin, Zhang Yan, Wang Jian-chen. Ignition and flameholding characteristics of n-dodecane and methylcyclohexane in the supersonic combustor. J Propuls Technol 2015;36(2):261e8. [11] Lan Z, Deng T, Zhong J, Sun Z. Multi-step reduced reaction mechanism simulation of hydrogen supersonic combustion process. Beijing Daxue Xuebao Ziran Kexue Ban/Acta Scientiarum Naturalium Universitatis Pekinensis 2017;53(3):397e404. [12] Gao Z, Jiang C, Lee CH. Representative interactive flamelet model and flamelet/ progress variable model for supersonic combustion flows. In: Proceedings of the combustion institute, vol. 36; 2017. p. 2937e46. 2. [13] Zhao GY, Sun MB, Wu JS, Wang HB. A flamelet model for supersonic nonpremixed combustion with pressure variation. Mod Phys Lett B 2015;29(21):440e8. [14] Goldfeld MA, Starov AV. Determination of the fuel concentration distribution in a supersonic combustion chamber. Combust Explos Shock Waves 2019;55(3):274e81. [15] Evans MJ, Medwell PR, Tian ZF, Frassoldati A, Stagni A. Ignition characteristics in spatially zero-, one- and two-dimensional laminar ethylene flames. AIAA J 2016;54(10):1e10.
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Energy journal homepage: www.elsevier.com/locate/energy
Study of low-temperature ignition characteristics in a supersonic combustor Jianping Li*, Jindong Li, Kai Wang, Guiqian Jiao, Zilong Liao School of Power and Energy, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2019 Received in revised form 24 December 2019 Accepted 28 January 2020 Available online 29 January 2020
The Ignition combustion experiment in a supersonic combustor was carried out under the conditions with the incoming flow Ma ¼ 2.0, the total temperature at 700 K and the total pressure at 520 KPa(corresponding to the flight Mach number at 3.5). Spray blocks without struts and with struts (with the blockage ratios as 20%, 10%, 7.3% and 5% respectively) were used in the supersonic combustor. The experimental result showed that when there was no strut in the combustor, kerosene could be ignited but was unable to burn stably alone after hydrogen was removed. The addition of struts achieved the independent and stable combustion of kerosene. However, due to the addition of struts, the combustion during the strut test using the blockage ratios at 20%, 10% and 7.3% induced the back pressure to be spread towards the upstream to the entrance of the isolator; When the blockage ratio of the strut was 5%, kerosene could burn stably alone, and the back pressure induced by combustion failed to be spread to the entrance of the combustor. In this paper, the stable flame mode using the cavity and struts was used to achieve stable ignition under the condition with Ma ¼ 2.0, total temperature at 700 K and total pressure at 520 KPa. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Supersonic combustor Low total temperature Ignition characteristics Blockage ratio Induced back pressure
1. Introduction With the continuous development of hypersonic technology, the scramjet engine, which is as the power of the hypersonic vehicle, has become the focus of various technological and military countries in the world [1]. Turbine Based Combined Cycle engine (TBCC) is based on the combination of a turbine engine and a scramjet engine, so it is of advantages such as a wide range of fight Mach good specific impulse performance, horizontal takeoff/landing, and reusability. However, the maximum operating speed upper limit of a turbine engine is generally Mach 2.2 only, while the scramjet engine can start work at Mach 4.0. Whether a turbine engine and a scramjet can achieve efficient connection of the speed and the thrust is the primary program for TBCC combined engines. This paper studied the ignition characteristics of supersonic combustor at the low Mach number and the low total temperature by reducing the limit of working Mach number of the scramjet, which provides technical support for the demonstration of the TBCC combined engines.
* Corresponding author. E-mail address: [email protected] (J. Li). https://doi.org/10.1016/j.energy.2020.117060 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
In recent years, a great number of tests and studies were carried out on the ignition of kerosene fueled supersonic combustors. Tian et al. [2] studied the effect of the thermal throat on the flame stability of a kerosene fueled supersonic combustor under the incoming flow condition of Ma number at 2.0, total temperature at 1100 K and total pressure at 1.0 MPa. The test results showed that due to the existence of the thermal throat, shock trains were generated in the isolator which reduced the high-speed incoming flow, improved the temperature and pressure, and enhanced the mixing efficiency of fuels in combustors, thus having achieved the flame stability. Mansfield A B [3] systematically investigated the ignition properties of simulated syngas mixtures at high-pressure low-temperature conditions relevant to gas turbine combustor operation using the University of Michigan Rapid Compression Facility. The pressures and temperatures after compression ranged from 3 to 15 atm and 870e1150 K respectively. Bao et al. [4] conducted the liquid kerosene spark ignition test for the equipment models of double-cavity scramjet combustors, with the analog flight Mach number at 4.5 and the total temperature at 1032 K. They found that the cavities with a smaller length-depth ratio were suitable for maintaining the local flame in the cavity with direct injection and upstream injection; the larger cavities could be better ignited and expand the local flame, so the length-depth ratio of the
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cavities affected the ignition process of spark plugs. Lu Z et al. [5] used Navier-Stokes (URANS) and large eddy current simulation (LES) to study the effects of spray and turbulence models on the mixing and combustion characteristics of n-heptane spray flames at constant temperature. It was found that the droplet movement near the nozzle had a significant effect on the distribution of fuel vapor, while the penetration length of the liquid was controlled by the evaporation process and was not sensitive to the gas injection model. The calculation results of Ma S et al. [6] showed that the fuel mixture of hydrogen and hydrocarbon had a larger delay time and a smaller flame velocity, which helped to weaken the flame area and reduce the combustion efficiency. Xi Wenxiong, Wang Zhenguo et al. [7,8] conducted a comparative test of ignition scheme on the dual-mode scramjet engine under the condition of the Mach number at 2 and the total temperature at 840 K. Their results showed that the heat jet flame and the spray in the downstream after mixing combustion thought the flame reversed spread to form a cavity-resident flame, the pressure in the combustor was greatly affected by the unsteady characteristics of the heat jet supply and the spray action. Trebs A [9] and others experimentally studied the combustion field downstream of a 10-degree compression ramp injector. The study found that with the increase of the incident boundary layer, the shape of the flame changed, the durability of the vortex core decreased, and the combustion efficiency improved. LIU Wei [10] investigated the ignition and flameholding characteristics of liquid single component hydrocarbon fuels in a model supersonic combustor under the condition of Mach number 2.03 and ranging from 1040 K to 1100 K. Theoretical analysis indicated that the evaporation characteristics of methylcyclohexane was better than n-dodecane’s within the same incoming flow parameters. Lan Z [11] and others found that the single-step reaction mechanism made the fuel easy to ignite and the combustion efficiency was higher, but the obtained temperature was higher; the multi-step reaction mechanism was closer to the actual combustion experiment, but the fuel might be more difficult to ignite, the flame might be easily destroyed during the simulation, and the simulation time might be relatively long. Gao Z [12,13] and others introduced a representative interactive small flame (RIF) model and a small flame/progress variable (FPV) model to develop two flame types for supersonic combustion flow. The numerical results showed that compared with the stable small flame (SF) model, the temperature and main species concentration changes caused by the RIF model were very limited, which meant that the species concentration in the small flame pool was not sensitive to local high Mach number effects. Goldfeld M A [14] measured the kerosene concentration distribution in the combustion chamber attached to the hot wind tunnel, and the Mach number of the combustion chamber entrance was 2.89. The authors showed that the lack of intense combustion in the model burner was caused by insufficient local equivalence ratio. Evans M J [15] analysis of the same unpremixed flow in a laminar diffusion flame showed that for a dilute ethylene fuel with a high-temperature air oxidant, ignition at the same equivalent ratio could better predict the ignition delay. Through the study and survey of the ignition test of the scramjet engine, it is found that there is no ignition test on the supersonic combustor under the condition that the total temperature is lower than 800 K, and the lowest total temperature condition is about 820 K. In this paper, the igniting combustion experiment of a kerosene fueled supersonic combustor was carried out under the condition of the incoming flow at the total temperature of 700 K. The low-temperature ignition characteristics of the kerosene fueled supersonic combustor were studied, and the lower limit of the working Mach number of the supersonic combustor was expanded.
2. Test plans 2.1. Diagram of the test system and the model The test in this paper was based on the direct connection test system with a resistance heater of Northwestern Polytechnical University as shown in Fig. 1. The system bears the total temperature range at 500e1000 K, the total pressure range at 0.2e2.5 MPa, and a mainstream flow range at 0.4e1.5 kg/s, and can simulate the incoming flow conditions of the supersonic combustor with a flight Mach number at 2.5e4.5. The test model used in this paper is a supersonic combustor test model with a single cavity flame holder. Fig. 2 showed the geometry of the test model. Fig. 3 showed the structure of the singlecavity flame holder in the combustor experiment model. It can be seen from the figure that the hydrogen nozzle was located inside the single cavity on the upper wall surface, and the axial distance from the leading edge was x/h ¼ 1.85, which was composed of two nozzle holes having a diameter of 1 mm, and the jetting method was vertical to the wall. The kerosene nozzle was arranged on the kerosene spay block, which was installed at the front edge of the cavity on the upper wall of the cavity. In this paper, the corresponding blockage ratio of the spray block with the strut was 20%, 10%, 7.3% and 5% respectively. The non-strut spray block and the spray block of each blockage ratio used in the test were shown in Fig. 4. Fig. 4 (a) was the non-strut spray block. The structure of the block with the blockage ratio of 20% was shown in Fig. 4(b), the spray block had two struts arranged in front of the nozzle holes on both sides, with the cross-sectional dimension at 8 15 mm2. The size of the block with the blockage ratio of 10% was shown in Fig. 4(c), the strut was arranged in front of the nozzle hole, with the cross-sectional size as 8 15mm2. The structure of the block with the blockage ratio of 7.3% was shown in Fig. 4(d), the strut was arranged in front of the nozzle hole, with the cross-sectional size as 8 11mm2. The blockage ratio of the 5% of the structure of the spray block was as shown in Fig. 4(e), and a strut was provided in front of the nozzle hole, with the crosssectional size as 4 15 mm2. 2.2. Experimental plan Fig. 5 showed the schematic diagram of pioneer hydrogenguided kerosene ignition control timing, including the control timing of the spark plug switch, hydrogen control valve and kerosene control valve. As shown in Fig. 5, the spark plug switch had a working time of 1s~8s, the hydrogen continuous working time was 2s~7s, and the kerosene continuous working time was 5se10s so that the hydrogen burning time was 3s, the hydrogen and kerosene co-burning time was 2s, and the kerosene burning time was 3s. According to the fuel injection situation, the combustion test process can be divided into four stages: no combustion stage, pioneer flame hydrogen combustion (H2) stage, hydrogen and kerosene cocombustion (H2þKerosene) stage, kerosene alone combustion (Kerosene) stage. Under the condition of incoming flow Ma ¼ 2.0, total temperature 700 K and total pressure 520 kPa, the spray block shown in Fig. 4 was selected for the test. The test scheme was shown in Table 1. 2.3. One-dimensional analysis method According to the experimental data acquisition system, the wall pressure value is obtained. The one-dimensional analysis method is used to calculate the other aerodynamic parameters of the combustion chamber, including Mach number, static temperature and
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Fig. 1. Resistance heating direct connection test system.
Fig. 2. Geometrical configuration of the combustor test model.
Fig. 3. Single-cavity flame holder.
total temperature distribution. One-dimensional stationary flow equation based on variable cross-section, chemical reaction, mass addition, and wall friction is shown as below:
1 þ gM 2 j dTt dM j dA gM2 j 4Cf dx ¼ þ þ 2 M D Tt 1M A 2 1 M2 2 1 M2 (1)
dPs gM2 dA gM2 j dTt gM ¼ Ps 1 M 2 A 1 M2 Tt
2
h i 1 þ ðg 1ÞM 2 4Cf dx D 2 1 M2 (2)
2 where: j ¼ 1 þ ðg1Þ 2 M , Ps is static pressure, Tt is total temperature, A is the cross-sectional area, Cf is the friction coefficient, D is the hydraulic diameter, g is the specific heat ratio, and M is the Mach number. By combining the above two equations, we can get:
Cf dM 1 M dA 1 þ gM 2 1 dp ¼ M dx 2 A dx 2gM p dx D h i 2 2 1 þ ð 1 M T g 1ÞM T t t 4C dTt Tt 1 dA 1 dp f ¼ 2j dx j A dx D gM2 j p dx
(3)
(4)
According to the pressure parameters of each measuring point of the combustor wall, we can get static pressure distribution. On this basis, one-dimensional data processing method is utilized to work out other airflow parameters distribution in the combustor. The friction coefficient Cf is set to 0.004, and the static pressure Ps on the wall is known. We first figure out the distribution of airflow M number and total temperature Tt from the above equation. After that, other airflow parameters (like static temperature Ts , total pressure Pt ) could be obtained from the gas-dynamics formula.
3. Test results and analysis The ignition results of the supersonic combustor under the conditions of incoming flow Ma ¼ 2.0, total temperature at 700 K, and total pressure at 520 kPa were shown in Table 2. “Fail” in the table indicated that the fuel can not be successfully ignited, and “Success” indicated that the fuel can be ignited. “Yes” meant that
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Fig. 4. Blocks with different blockage ratios.
stage there was stable combustion. When the test was carried out without the strut, the kerosene can stably burn separately; the method of adding strut in front of the nozzle hole can help kerosene to burn stably and separately, but the combustion induced back pressure easily propagated the upstream to the entrance of the isolator. When the blockage ratio is 20%, 10%, and 7.3% respectively, the induced back pressure was spread to the inlet of the combustor; when the blockage ratio was 5%, not only the kerosene can burn stably and separately, but also the induced back pressure was not propagated to inlet of combustor.
3.1. Analysis of ignition performance of combustor under different blockage ratio conditions Fig. 5. Pioneer hydrogen guided kerosene control timing.
Table 1 Test incoming flow state and equivalence oil and gas ratio. Ma
Total temperature(K)
Total pressure(kPa)
strut
Kerosene(ER)
2.0 2.0 2.0 2.0 2.0
700 700 700 700 700
520 520 520 520 520
e 20% 10% 7.3% 5%
0.10e0.25 0.10e0.25 0.10e0.25 0.10e0.25 0.10e0.25
the induced back pressure during steady combustion of the combustor had spread to the inlet of the combustor, destroying the incoming flow condition; “No” meant that the induced back pressure during combustion had not spread to the inlet of the combustor, and the flow conditions were not destroyed. The test results showed that in all the hydrogen combustion stages of the test and the hydrogen and kerosene co-combustion
Fig. 6 showed the wall pressure distribution and Mach number distribution at kerosene alone combustion stage under different blockage ratios, including no struts and struts (with the blockage ratios at 20%, 10%, 7.3% and 5% respectively). As shown in the figure, when the non-strut spray block was used, the wall pressure distribution was restored to the cold flow level after the pioneer hydrogen was removed, indicating that the kerosene can not burn stably and separately. When the spray block with plate was adapted, the kerosene can burn stably and separately after the pioneer hydrogen was removed. When the blockage ratio was 20%, 10% and 7.3% and the Mach number was 0.68, 0.5, and 0.81 respectively, the wall pressure distribution increased obviously, and the combustion induced back pressure propagated upstream to the entrance of the isolator, which seriously destroyed the incoming flow conditions. When the blockage ratio was 5% and Mach numbers was 1.98, the wall pressure increased and the kerosene can burn stably and separately, induced back pressure did not propagate to the entrance of the isolator, and the flow conditions were not destroyed. Fig. 7 showed the static temperature distribution and total temperature distribution of the combustor
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Table 2 Ignition results of the studying cases. Case
1 2 3 4 5 6 7
strut
e 20% 10% 7.3% 7.3% 5% 5%
H2þKerosene(ER)
0.065 0.087 0.054 0.087 0.087 0.086 0.092
þ þ þ þ þ þ þ
0.18 0.25 0.18 0.16 0.19 0.13 0.19
Ignition results
The induced back pressure disturb the inlet
H2
H2þKerosene
Kerosene
Success Success Success Success Success Success Success
Success Success Success Success Success Success Success
Fail Success Success Success Success Success Success
e Yes Yes Yes Yes No No
Fig. 6. Wall pressure distribution and Mach number distribution at kerosene alone combustion stage under different blockage ratio conditions.
Fig. 7. Static temperature distribution and total temperature distribution at kerosene alone combustion stage under different blockage ratio conditions.
calculated by the wall pressure distribution wall under different blockage ratios. As shown in the figure, the distribution of static temperature and total temperature in the combustor restored to the level of cold flow in the test with non-strut spray block, which indicated that kerosene could not burn stably and separately; when the blockage ratio was 20%, 10% and 7.3% respectively at the kerosene alone combustion stage, the induced back pressure propagated to the inlet of the combustor, the incoming flow conditions were
destroyed, and the static temperature increased. When the blockage ratio was 5%, the induced back pressure did not propagate to the inlet of the combustor, and the static temperature of the entrance was the same as that of the cold flow. The static temperature and total temperature distribution diagrams showed that in the test of spray block with non-strut, spray block with blockage ratio of 20%, spay block with blockage ratio of 10%, spray block with blockage ratio of 7.3%, and spray block with blockage ratio of 5%, the outlet static temperatures of the
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combustor was 656 K, 939 K, 1271 K, 941 K and 996 K respectively, and the total outlet temperatures was 885 K, 1101 K, 1424 K, 1044 K and 1152 K respectively. The results showed that the method of adding plates in front of the nozzle hole helped the kerosene to burn stably and separately. When the blockage ratio was 20%, 10% and 7.3% respectively, the induced back pressure propagated to the inlet of combustor, which seriously destroyed the incoming flow condition. When the blockage ratio was 5%, not only the kerosene can burn stably and separately, but also the induced back pressure did not propagate to the inlet of combustor, and the incoming flow condition was not destroyed. 3.2. Ignition performance analysis with 7.3% blockage under different equivalence ratios Fig. 8 showed the wall pressure distribution and Mach number distribution under different equivalence ratios with the blockage ratio of 7.3%, including the cold flow stage, the pioneer flame hydrogen combustion stage, and the hydrogen and kerosene cocombustion stage and kerosene standalone combustion stage. The figure showed: (1) Pioneer flame hydrogen standalone combustion stage: Compared with the cold flow stage, the wall pressure distribution increased, the combustor airflow became the subsonic mode at X ¼ 290 mm, and the thermodynamic throat existed at X ¼ 560 mm. (2) Hydrogen and kerosene co-combustion stage: Compared with the total equivalence ratio ER ¼ 0.247 (hydrogen ER ¼ 0.087þkerosene ER ¼ 0.16), the wall pressure was higher when the total equivalence ratio was ER ¼ 0.277 (hydrogen ER ¼ 0.087þkerosene ER ¼ 0.19), the corresponding Mach number distribution of the combustor was lower, but the induced back pressure at both two equivalence ratio states propagated to the inlet of the combustor, which seriously destroyed the incoming flow condition, and the combustor airflow was in the subsonic mode. At this time, the inlet Mach numbers were all 0.72. (3) Kerosene standalone combustion stage: Compared with the equivalence ratio (ER ¼ 0.16), the wall pressure was higher when the equivalence ratio was 0.19, and the corresponding Mach number distribution of the combustor was lower. The
wall pressures of the two equivalence ratio tests were slightly lower than the wall pressures at hydrogen and kerosene co-combustion stage, and the Mach number distributions were slightly higher. However, the induced back pressure at both equivalence ratios propagated to the inlet of the combustor, which seriously destroyed the incoming flow conditions, and the inlet Mach number was 0.83. The results showed that under the condition of the total temperature of incoming flow 700 K and the total pressure 520 kPa and blockage ratio 7.3%, the kerosene can burn stably and separately after the pioneer hydrogen was removed, but the induced back pressures at the other stages propagated to the entrance of the isolator except at the hydrogen combustion standalone stage. 3.3. Ignition performance analysis with blockage ratio of 5% under different equivalence ratios Fig. 9 showed the wall pressure distribution and Mach number distribution under different equivalence ratios with the blockage ratio of 5%, including the cold flow stage, the pioneer flame hydrogen combustion stage, and the hydrogen and kerosene cocombustion stage and kerosene standalone combustion stage. The figure showed: (1) Pioneer flame hydrogen standalone combustion stage: Compared with the cold flow stage, the wall pressure distribution increased, and the wall pressure of the equivalence ratio ER ¼ 0.092 was slightly higher than the wall pressure of the equivalence ratio ER ¼ 0.086. When the equivalence ratio was ER ¼ 0.086, the combustor airflow entered the subsonic mode at X ¼ 360 mm, and the thermal throat existed at X ¼ 540 mm. When the equivalence ratio was ER ¼ 0.092, the combustor airflow entered the subsonic mode at X ¼ 300 mm, and there was a thermal throat at X ¼ 560 mm, and the induced back pressure did not propagate to the inlet of combustor. At this time, the inlet Mach numbers were all 1.97. (2) Hydrogen and kerosene Co-combustion stage: Compared with the total equivalence ratio ER ¼ 0.216 (hydrogen ER ¼ 0.086þkerosene ER ¼ 0.13), the wall pressure was higher when the total equivalence ratio was ER ¼ 0.282 (hydrogen 0.092þkerosene 0.19). The corresponding Mach
Fig. 8. Wall pressure distribution and Mach number distribution of the 7.3% blockage test.
J. Li et al. / Energy 195 (2020) 117060
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Fig. 9. Wall pressure distribution and Mach number distribution with blockage ratio of 5%.
number distribution of the combustor was lower, but the induced back pressure at both equivalence ratio stages propagated to the inlet of the combustor, which seriously destroyed the incoming flow condition, and the combustor airflow was in the subsonic mode. The inlet Mach numbers at this time were 0.75 and 0.77, respectively. (3) Kerosene standalone combustion stage: Compared with the equivalence ratio ER ¼ 0.13, the wall pressure was higher when the equivalence ratio was ER ¼ 0.19, and the corresponding Mach number distribution of the combustor was lower. When the equivalence ratio was ER ¼ 0.13, the airflow of combustor became the subsonic mode at X ¼ 360 mm, and there was a thermal throat at X ¼ 570 mm. When the equivalence ratio was ER ¼ 0.19 the airflow of combustor became the subsonic mode at X ¼ 240 mm, and there was thermodynamic throat at X ¼ 760 mm. The induced back pressure at the two equivalence ratios did not propagate to the combustor inlet, and the flow conditions were not destroyed. At this time, the Mach numbers were all 1.98. The results show that kerosene can burn stably and separately after the pioneer hydrogen was removed when the blockage ratio was 5% under the condition of total inlet temperature 700 K and total pressure 520 kPa, and the induced back pressure did not propagate to the inlet of combustor, and the incoming flow condition remained unchanged.
3.4. Numerical simulation analysis of the combustor Different mesh generation methods or the number of meshes may affect the simulation results. In order to ensure the accuracy of the numerical simulation calculation results, the grid independence test was performed on the model grid to obtain the best and most accurate numerical simulation results. In this paper, three structured grids were constructed with 10,000, 50,000, and 100,000 grids; two unstructured grids were constructed with 50,000 and 100,000 grids. Under the same inflow conditions and fuel supply strategies, the numerical simulation calculations on five grids were performed and verified with test results. Fig. 10 showed the calculation results. As shown in the figure, the overall calculation effect and change trend of the five grids were almost the same and the calculation results were not much different and could be ignored. This paper
Fig. 10. Grid independence test result.
chose a structured grid of 50,000 grids for numerical simulation calculation. On the whole, the numerical calculation results in the burning state were basically consistent with the distribution trend of the experimental measurement results. Therefore, the numerical calculation method used in this paper had certain reliability and
Fig. 11. Flow field cloud in the cold flow state of the combustor (non-strut).
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rationality, which could be used to analyze the flow field of the combustor. Fig. 11(a) and Fig. 10(b) showed the incoming flow Mach number as 2.0, total temperature at 700 K, total pressure at 520 kPa at the entrance of combustor model as well as static pressure and Mach number distribution cloud diagram under the condition of nonstrut spray block test and in the cold flow state. As shown in the figure, the velocity of the inlet of the combustor model was high, and the gas flow in the combustor was supersonic. Due to the sudden increase in the flow area from the leading edge of the cavity, an expansion wave was formed at the leading edge of the cavity. The airflow accelerated after the expansion wave, but the static temperature and the static pressure decreased. At the trailing edge of the cavity and the deflection of the shear layer of the cavity, as the flow area suddenly decreased, shock waves formed at the peak point of deviation of the cavity shear layer and on the lower wall surface of the combustor, the air flow decelerated and the pressure increased after the shock wave. The trailing edge of the cavity was an expansion section, and the area was suddenly expanded, so that an expansion wave formed at the position of the trailing edge of the cavity. Due to the interaction between the shock wave and the shock wave, and the interaction between the shock wave and the expansion wave, the pressure in the combustor first increased and then decreased (as shown in Fig. 6). Fig. 12(a) and Fig. 12(b) showed the static pressure and Mach number distributions of the central section of the combustor under the cold flow condition, when Mach number was 2.0, total temperature was 700 K, total pressure was 520 kPa and blockage ratio was 5%. As can be seen from the figure, there was a low-speed recirculation zone in the cavity, due to its characteristic of low speed and high temperature, it could increase the residence time of the kerosene fueled in the combustor and increase the temperature of mixing gas, which contributed to the ignition of the supersonic combustor and the flame stability. However, it can be seen from the comparison with Fig. 11 that, the size of the recirculation zone in the cavity of the combustor flow channel with the strut was smaller than that with the non-strut, and the cavity shear layer was more deflected toward the bottom of the cavity. As the strut was arranged in front of the nozzle hole, the airflow flow was more complicated due to the complicated wave system in the downstream part of the strut affected by it, and the supersonic flow was blocked by the strut, and a small part of the low speed zone existed behind the strut, which helped to increase the penetration depth of liquid kerosene and made it more fully mixed with air. The trailing edge of the cavity was an expansion section, and there was an area expansion. Therefore, an expansion wave was formed at the position of the trailing edge of the cavity, and then the expansion wave was reflected from the wall surface, and the airflow was accelerated
Fig. 12. Flow field cloud image of the central section in the cold flow state of the combustor (with blockage ratio at 5%).
and decompressed by the expansion wave, so the pressure distribution in the expansion section was gradually decreased. Fig. 13 and Fig. 14 showed the static pressure and Mach number distribution of the center section of the combustor under the conditions of Mach number at 2.0, total temperature at 700 K, total pressure at 520 kPa, blockage ratio at 5% and kerosene equivalence ratio ER ¼ 0.13 and 0.19, respectively. As shown in Fig. 13, compared with the cold flow stage, the wave system of the flow flied in the combustor was more complex. The induced back pressure rose sharply and propagated upstream to the isolator. When the kerosene equivalence ratio was ER ¼ 0.13, the pressure disturbed to about X/L ¼ 0.08 position. When the kerosene equivalence ratio was ER ¼ 0.19, the pressure disturbed to about X/L ¼ 0.04 position. With the increase in kerosene equivalence ratio, the induced back pressure moved upward gradually. From Fig. 14, it can be seen that the induced back pressure rise caused the wall boundary layer to separate and form a shock train. The interaction between the shock train and the boundary layer resulted in a larger boundary layer separation zone near the upper wall. The high-speed incoming flow decelerated and supercharged after passing through the shock train, and the combustor was in a subsonic combustion mode. Fig. 15 and Fig. 16 showed the static temperature distributions of the central section of the combustor and the three X-axis cross sections in the combustion state, respectively. From the static temperature distribution, it can be seen that with the increase in the kerosene equivalence ratio, the combustion area, that was the heat release concentration area, gradually diffused from the vicinity of the cavity to the whole cavity. From the cross section of X-axis, it can be seen that the kerosene was injected through a single hole and there was a supporting plate in front of the hole, the combustion area at the leading edge of the cavity was located near the upper wall of the combustor axis, and then gradually diffused into the cavity until it filled almost the whole cavity. As can be seen from the figure, when kerosene equivalence ratio was ER ¼ 0.13 and 0.19, the combustion can be stable and products can be stable generated near the cavity. 4. Conclusion In this paper, the ignition and combustion characteristics of kerosene fueled supersonic combustor at low temperature were studied using hydrogen-guided flame ignition and no-support palate spark block and spark block with support block under the incoming flow condition of Ma ¼ 2.0, total temperature 700 K and total pressure 520 kPa. The following conclusions were drawn as follow: 1. The mode of the cavity with the strut stabilizing flame was used in this paper. When the blockage ratio was 20%, 10% and 7.3% respectively, kerosene can burn separately and stably after hydrogen was removed, but the combustion induced back pressure had been propagated upstream to the entrance of the isolator, which destroyed the incoming flow condition of the
Fig. 13. Static pressure distribution of the central section of the combustor(burning).
J. Li et al. / Energy 195 (2020) 117060
Fig. 14. Mach number distribution cloud of the center section of the combustor.
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flow state, the recirculation zone was smaller than that in the non-strut when the blockage ratio was 5%, but the flow field wave system in the combustor was more complex, which helped to mix kerosene and airflow and increase the mixing temperature. In the combustion state, the flow field wave system in the combustor was more complex. The interaction between shock train and boundary layer made the upper wall have a larger boundary layer separation zone nearby, the combustion can be stable and products can be stably generated near the cavity, and with the increase in kerosene equivalence ratio, the combustion induced back pressure moved upward gradually.
Declaration of competing interest In the research process of the paper, Li Jianping was in charge of experiment and thesis writing; Li Jindong was in charge of literature collection; Wang Kai was in charge of experiment and experimental data processing, Jiao Guiqian was in charge of experimental data acquisition and Liao Zilong was in charge of experimental operation.
References Fig. 15. Cloud section of the combustor center and cross-section static temperature distribution (ER ¼ 0.13).
Fig. 16. Cloud cross section of the combustor and cross-section static temperature distribution (ER ¼ 0.19).
combustor. With the decrease in blockage ratio of strut, the combustion induced back pressure was more hardly to propagate to upstream. When the blockage ratio of the strut was 5%, kerosene can burn stably after pioneer hydrogen was removed, and the combustion induced back pressure did not propagate to the entrance of the isolator. This indicated that the combustor configuration adopted in this paper could work normally under the flight Mach number of about 3.5. This study extended the lower limit of the working Mach number of the ramjet. 2. According to the results of the ignition and combustion test, the flow path configuration of the supersonic combustor without the strut and the flow path configuration with the strut (with blockage ratio at 5%) were numerically simulated: in the cold
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