Study On Propagation Stability Of Rotating Detonation Wave And Optimization Of Engine Propulsion

Detonation combustion has the advantages of fast energy release rate,small entropy increase and high thermal cycle efficiency compared with deflagration.The rotating detonation engine,which releases the chemical energy in the form of detonation,is considered as one of the most promising aerospace engines with simpler structure and higher thermal cycle efficiency than traditional aerospace engines.By means of theoretical analysis and numerical simulation,the propagation stability of rotating detonation waves and thrust performance of the engine are invested and optimized.A numerical simulation codes suitable for initiation and propagation process computation of rotating detonation wave is developed based on the finite volume method.The Euler equations coupled with detail chemical reaction model are used as the governing equations.The Strang splitting method is used to decouple the chemical reaction and flow field calculation.Convective terms are evaluated with the second order central-upwind scheme of Kurganov,Noelle and Petrova.The chemical reactions are solved byɑ-QSS method and the time integration is performed with third-order Runge-Kutta method.Numerical results are in good agreement with the theoretical and experimental results.The initiation process of rotating detonation wave is numerically simulated.One kind of ejector is designed for speed up the deflagration-to-detonation transition in the pre-detonator.The formation mechanism of hot spots in the flow field was discussed.In addition,a direct initiation method to establish rotating detonation wave was proposed which improve the ignition success rate of the traditional direct initiation method.Simulation results show that the initial filling rate of fuel affects the accumulation of fuel and the propagation stability of detonation wave in the first cycle after ignition.When the injection total pressure is 0.4MPa,initial fuel filling rate range from 13.4%to 26.6%could lead a successful establishment of rotational detonation waves.Increasing the injection total pressure is conducive to increasing the initial fuel filling rate range of successful initiation.In order to study the propagation stability of rotational detonation waves and the formation mechanism of different propagation modes,the initiation processes and spontaneous transition process of different propagation modes are numerically simulated.In the given engine model,the flow-field evolution became unstable when a single detonation wave was released.New detonation waves formed spontaneously,changing the propagation mode from single-wave to four-wave.However,when two or three detonation waves were released,the flow field evolved in a quasi-steady manner.Further study revealed that the newly formed detonation wave resulted from an accelerated chemical reaction on the contact surface between the detonation products and the reactive mixture.To satisfy the stable propagation requirements of detonation waves,we propose a parameter called N_L,which can be compared with the number of detonation waves in the combustor to predict the evolution(quasi-stable or unstable)of the flow field.Finally,we verify the effectiveness of N_L in a redesigned engine.This work may assist the operational mode control in rotating detonation engine experiments.Fuel injection conditions,detonation wave propagation mode and the propulsion performance are coupled to each other.Simulation results showed early burning of the fuel is not conducive to the improvement of thrust performance.When the early burning ratio increases,the specific impulse of the engine decreases.The change of detonation waves propagation mode is accompanied by the drastic fluctuation of engine thrust.The increasing number of detonation waves in combustor can reduce the oscillation of thrust thus improve the propulsive stability.The propagation mode is affected by fuel injection condition.When injection total pressure increases,the number of detonation waves increases,and the thrust and specific impulse increase.When injection total temperature increases,the number of detonation waves increases,and the thrust and specific impulse decrease.When the mole fraction of dilution gas increases,the engine thrust decreases and the specific impulse increases.To maximization the energy released by detonation combustion,stratified injection mode is proposed.The proposed fuel injection mode functions by suppressing the isobaric combustion process occurring on the contact surface between fuel and detonation products and thus the proportion of fuel consumed by detonation wave increased from 67%to 95%,leading to more self-pressure gain and lower entropy generation.A pre-mixed hydrogen-oxygen-nitrogen mixture is used as a reactive mixture.The computational results show that the propulsive performance and the operation stability of the engine with stratified injection were both improved,the temperature of the flow field was notably decreased,and the specific impulse of the engine was improved by 4.4%and the average temperature of the engine with stratified injection was reduced by 19.1%.Also,the fine structure of the rotational detonation flow field is revealed.On the wave surface,there exist complex small-scale structures caused by the intrinsic instability of the detonation wave.It is smooth away from the fuel inlet,but triple points and transverse shock waves gradually appear on the lower area.In the area behind the detonation wave,interference phenomenon appears.