Investigations On The High Efficiency Of The Pulse Detonation Rocket Engine

As a thrust device working intermittently,a pulse detonation engine has its advantage of high thermal efficiency and low entropy rise.However,in practical applications,severer wall heat loss,flow momentum loss and under/over expansion loss may inevitably take place due to high temperature and unsteady flow,causing great irreversible entropy rise.This paper mainly focuses on the efficient utilization of energy to conduct theoretical experimental research,trying to demonstrate the energy loss problem in the working process of a PDRE and providing an improvement on performance loss for engineering application.1.Analysis on the complex wave movement in direct detonation initiation were conducted and gas state parameters were calculated,including static filling parameter,VN peak parameter,CJ detonation parameter,gas parameter of the Taylor expansion wave from the closed end,pressure plateau parameter of the Taylor expansion wave,parameters of the Taylor expansion wave infected by the expansion wave from the open end,gas parameters after exhaust expansion wave.By integrating the thrust wall pressure with operating time,the thrust and specific impulse of a detonation cycle can be calculated.Main performance loss in a real combustion process was also analyzed theoretically,including the relationship between the length to diameter ratio of a tube and wall heat loss,as well as total pressure loss in the detonation initiation process.Heat loss is proportional to the length to diameter ratio.Compared to the full expansion purging process situation,the specific impulse loss of a tube with no nozzle can be up to 19.6%.2.Heat loss measurement and its effect on the performance loss based on the RP3/oxygen two-phase detonation were studied numerically.When the operating frequency was 10-20 Hz,the heat power of one detonation cycle was 25-30 k W/m²,which can be used to estimate the maximum heat flux.Because of the fast detonation speed,the heat exchange time to the wall is very short.Thus the transferred heat of a detonation wave is 20-60%lower than a deflagration wave,indicating that detonation can effectively transforming chemical energy to thrust power,rather than releasing heat to the environment.Also,the detonation wave speed and pressure peak cannot be affected by heat loss within such short time.However,as the time passes,the thrust wall pressure will decay more greatly compared to the adiabatic condition,thus the thermal dissipation will reduce the engine specific impulse performance.In the experiments,the heat loss of the two phase detonation combustion on the wall was 11.1%～12.8%of the fuel calorific value.According to the linear relationship of the heat loss and the specific impulse loss,the estimated specific impulse loss was 5.6%～6.4%,with the error less than 6.7%.3.Based on the unsteady characteristic of detonation wave,a method of determining the total thermal resistance value of a regenerative cooling system after solving the equivalent gas temperature according to the active cooling data was proposed.The regenerative cooling system designed with this method successfully achieved detonation under the thermal equilibrium condition of regenerative cooling.When the temperature of oil and wall both rise,the wave speed loss decreases,the combustion stability increases,the two phase detonation peak pressure increases,the detonation initiation distance reduces,and the detonation wave speed rises.The initial purging total pressure at the exit and the purging time both increase and thus the engine can obtain greater thrust.However,if the wall temperature is too high during the regenerative cooling process,the reactants will expand and dissipate.Therefore an improvement on the detonation cycle was provided to make sure the regenerative cooling system may be put into engineering use.4.A hot jet obstacle technique was proposed based on the current method of using fluid jet to increase turbulence and shorten the DDT distance.This technique can work self-adaptively with single ignition source and adding no supplying systems.With same blockage ratios,cold fluidic obstacles have the same triggering detonation effect as the solid obstacles,but the hot fluidic obstacles have the best effect because it has no cold mixing effect to the flow.There are three aspects of the detonation triggering mechanism of hot fluidic obstacles.Firstly,the leading shock before the flame front in the jet cavity disturbing the flow in the tube;secondly,the burned gas in the jet cavity compressing the flow in the tube and causing the flame front to be wrinkled and enhancing the chemical reactivity,thirdly,the strong shock diffraction near the jet hole after detonation occurs in the jet cavity and then triggering direct initiation of detonation or re-imitation in the main tube.5 Based on the numerical and theoretical analyses,the nozzle design method for the unsteady exhausting process of detonation was studied.There are four stages of unsteady exhausting process,isobaric filling,rapid pressure decrease,pressure plateau and low-pressure purging.In the C₂H₄/O₂ detonation,the purging pressure is within 90.6 p₀～1.72 p₀,if unsteady nozzles are used,the expansion ratio will be 13～1,but if the Fixed surface nozzles are used,the shock wave loss should be avoided under the over-expansion condition.The nozzle design parameters can be calculated using the pressure plateau parameters of the purging process whose thrust coefficient can be achieved 95%.The design point parameter of the fixed surface nozzle corresponds to the burned gas parameter of the last Taylor expansion wave exiting the open end of the tube.Using the classical theoretical formula,fast design of the detonation nozzle can be achieved.