Investigation On Dynamic Process And Performance Of RBCC In Strut-Rocket/Scramjet Mode

Rocket based combined cycle（RBCC） propulsion system combines a rocket of high thrust-to-weight ratio with a ramjet of high sp ecific impulse in an integrated flowpath organically, which can operate in a wide envelope and trajectory with the ability of acceleration, maneuver and cruise. The steady and efficient operation of RBCC engine in scramjet mode plays an important role on realizing the flight in a high altitude and high speed range, which is also the necessary for the RBCC engine operating at various modes.In this dissertation, taking into account a central-strut RBCC engine operating in ejector mode and ramjet mode as the research object, the dynamic process and the performance of RBCC combustor in scramjet mode were studied. The liquid kerosene was used as a secondary fuel. The research contents specifically included: the ignition characteristic of the strut-rocket jet, the dynamic propagation process and diffusion range of the flame, flameholding methods, combustion organization schemes, fuel injection strategies, the change feature of the combustor pressure and the dynamic propagation feature of the shock trains in the isolator during the transition of combustion organization strategy, etc. The design method of the flowpath geometry, the quantitative fuel injection strategies, and the better combustion performance of RBCC combustor in scramjet mode were obtained. The main research contents and conclusions were as follows:（1） The assistant ignition of the strut-rocket jet as pilot flame and the dynamic propagation of the flame were investigated through experiments and numerical simulations. The strut-rocket operated under the relative low flowrate and fuel-rich conditions. The diffusion flame of high temperature formed by the afterburning between strut-rocket jet and airflow enhanced the evaporation and atomization of kerosene, reducing the ignition delay time of kerosene. The low velocity formed by the flame of rocket jet increased the residence time of the kerosene, promoting the mixing between kerosene and airflow. When the distance between fuel-pylon and strut-rocket exit Lpr, the transverse spacing distance between the pylons S1 satisfied the conditions of 0<Lpr/Wr≤7.46, 0<S1/Wr≤1.38（Wr is the flame width of the efficient afterburning between rocket jet and airflow）, the good flame propagation and spread ability of the strut-rocket jet in the transverse and longitudinal direction could ignite the liquid kerosene and realize the stable combustion under different conditions of fuel injection strategies, operation parameters of the strut-rocket, flight Mach number and dynamic pressure of the airflow. The strut-rocket jet played an important role on reliable ignition and flameholding.（2） For expanding the operation stability of RBCC engine further, t he flameholding schemes in the combustor were investigated through experiments and numerical simulations after the strut-rocket turned off, while analyzing the flame structures of different flameholding schemes. After the strut-rocket turned off, the flow characteristics and the flameholding schemes were changed obvious in the combustor. The result showed that the combustion of the fuel could not be sustained just by the wake of the fuel-pylon. The joint flameholding between cavity and fuel-pylon, and the flameholding of the recirculation downstream the base of the strut-rocket could realize the stable combustion of the kerosene. As the cavity approached the base of the fuel-pylon, the countercurrent spread flame formed by the cavity could ignite the fuel injected from the fuel-pylon, with the initial position of the flame moving forward and the zone of flame distribution enlarging. The flame maintained between the base of the fuel-pylon and the cavity, which enhanced the ability of flame stabilization in the combustor. The flame formed in the recirculation downstream the base of the strut-rocket ignited the fuel injected upstream the central-strut reliably, which produced the diffusion flame. The diffusion flame acted as the flame of the strut-rocket jet similarly, which ignited the fuel injected from the fuel-pylon, and the flame spread towards both sides of the flowpath further.（3） The effects of various positions of the fuel-pylon and cavity in the combustor on flameholding and combustion performance were studied through experiments, and the design method of the flowpath geometry of RBCC combustor in scramjet mode was obtained. After the fuel-pylon moved backward along the longitudinal direction（Lpr/Wr was changed from 2.59 to 4.22）, the effect of the flameholding of strut-rocket weakened. When the distance between the leading edge of cavity and the base of the fuel-pylon L_cp/W_pc was 0.68 or 3.93(W_pc is the flame width of the efficient afterburning between fuel and airflow, with the fuel injected from the fuel-pylon), the use of the cavity could promote the mixing and combustion between fuel and airflow obviously, and the strut-rocket jet and the cavity stabilized flame together, by which the effect of the flameholding in the combustor enhanced. As the cavity moved towards the fuel-pylon, the joint flameholding between the rocket jet and cavity enhanced the effect of flame stabilization further. When L_cp/W_pc was 0.68 or 3.93, the thrust of the engine differed by 3 percent maximum, with the fuel-pylon moving backward along the longitudinal direction. With the strut-rocket turning off, when the distance between cavity and fuel-pylon was far(L_cp/W_pc was 7.18), only the cavity stabilized flame, and the cavity and fuel-pylon didn’t stabilize flame together. The combustion generated near the cavity, and the position of the heat release was more after, which caused the combustion performance becoming poor. As the cavity moved upstream(L_cp/W_pc was 0.68 or 3.93), the effect of the joint flameholding between cavity and fuel-pylon enhanced, and the combustion heat release generated between the fuel-pylon and the cavity mainly, thus the comb ustion performance increased notably.（4） The experiments under the simulated flight Mach number of 5.5 and 6 conditions were performed with investigation of combustion organization and operation performance, in which the effects o f the height of fuel-pylon,the transverse spacing distance between the pylons, fuel equivalence ratio, fuel injection positions on the combustion performance were analyzed. The quantitative fuel injection strategies and the better combustion performance of RBCC combustor in scramjet mode were obtained. Comparing with the use of the fuel-pylon of half height, the use of the fuel-pylon of full height could make the degree of fuel-rich state decreasing obviously, thus the combustion efficiency increased, and the thrust of the engine increased by 34 percent. Increasing the transverse spacing distance between the pylons could enhance the mixing between fuel and airflow, thus the thrust of the engine increased by 15 percent. The fuel equivalence ratio of the fuel-pylon should be between 0.57 and 0.74 at flight condition of Ma 5.5. The combination of the central-strut injection（equivalence ratio was between 0.21 and 0.31） with fuel-pylon injection（equivalence ratio was between 0.64 and 0.7） could promote the combustion organization in the combustor, then the combustion heat release increased, which resulted in the realization of distributed heating and the augment of the thrust of the engine. The fuel equivalence ratio of the fuel-pylon should be below 0.88 at flight condition of Ma 6. When the fuel equivalence ratio of the fuel-pylon was 0.74 and the fuel equivalence ratio of the central-strut injection was 0.33, the combustion effect of the engine was much better. As the flight Mach number increased, the combustion origination between fuel and airflow in the combustor was not changed substantially, but the heat release decreased. The thrust and the specific impulse also dropped, with the increment of the total pressure loss at the exit of the combustor.（5） Under the conditions of the variation of fuel equivalence ratio, fuel injection position and the closure of the strut-rocket, the process of the transition of combustion organization strategy was carried out based on experiments. The use of different methods for the transition of combustion organization strategy could realize the successful transition. The transition time of combustion organization strategy was close to the different transition methods. During the transition of combustion organization strategy, the position of the shock trains in the isolator moved as the combustor pressure changed, which could match the variation of the combustor pressure well. The shock trains always stabilized in the isolator, which didn’t have a disturbance on the flow at the entrance of the isolator. The averaged Mach number along the flow path was above 1 after decreasing the fuel equivalence ratio and the closure of the strut-rocket. With the characteristics of the flow structure, the state of combustion in the combustor was still mixed with subsonic combustion and supersonic combustion combined and translated gradually to the supersonic combustion dominated.