Mechanisms And Regulation Characteristics Of A High-Frequency Pulse Detonation Rocket Engine

The pulse detonation rocket engine(PDRE)is a new concept propulsion system based on detonation mode,with potential advantages of simple structure and high thermal cycle efficiency.Many challenging problems need to be solved before the PDRE can be used for practical aircraft and spacecraft.One of the key issues is how to improve the operating frequency,stability and controllability of the PDRE.Theoretical,experimental and numerical investigations were carried out on this issue.(1)A novel method was developed to generate high-frequency detonation waves.A detonation tube was designed,which is closed at one end and open at the other end.Reactants were injected toward the closed end in the middle of the tube and ignited near the open end.Detonation waves propagating to the closed end can be obtained under the appropriate conditions.Experimental investigations on this method were carried out.Detonation waves at 200 Hz were successfully obtained in the 14 mm inner diameter tube.Shadowgraphs of the exhaust process were captured by a high speed camera.A numerical model of a single cycle operation was designed to study the exhaust process and the propulsive performance.The results indicated that the specific impulse based on the mixtures in the tube was nearly to that in the traditional straight tubes.However,the waste of reactants was observed in this method.It means that this method is not suitable for propulsion,but can be used to generate high frequency detonation waves.(2)The mechanism of the high-frequency valveless PDRE was introduced.The operating cycle can be divided to four parts,including filling of propellants,ignition and propagation of detonation wave,propagation of expansion wave from the outlet to the closed end,decrease of pressure at the closed end.The characteristic times of four processes during one cycle were analyzed based on the one dimensional flow model in the detonation tube.The results indicated that filling of propellants injection occupied more than 80% of the time during one cycle,which limited the maximum frequency of the engine.(3)Experiments were carried out to find the reliable conditions for the stable operation of the valveless PDRE.The effects of igniton frequency,oxygen concentration and filling pressure of the oxidizer were considered.Four types of operating state of the PDRE were observed in the experiment,including 1)stable operation,2)half-frequency operation,3)continuous combustion,4)unstable operation.The continuous combustion,which was caused by the failure of isolation and was extremely harmful to the engine performance,was investigated in detail.The developing process of the continuous combustion was captured by a high speed camera.The main effects responsible for this state were analyzed based on the thermal ignition model.The model indicated that the continuous combustion can be avoided by decreasing the temperature of the wall and the combustion products,decreasing the oxygen concentration in the oxidizer and decreasing the filling pressure of the oxidizer.(4)In order to improve the isolation effects and avoid continuous combustion,a valveless mode with the purge gas based on liquid fuel for PDRE was developed.The fuel,oxidizer and purge gas were filled into the detonation tube at valveless mode.The application of independent purge gas can improve the isolation effects significantly.The PDRE can work stably up to 160 Hz in this mode.Fully-developed detonation waves were proved to be obtained successfully at a maximum frequency of 130 Hz.The propulsive performance of a practical PDRE at a wide operating frequency range was also tested.The thrust increased with operating frequency until 140 Hz and then decreased.The increase was linear below 80 Hz and nonlinear from 90 Hz to 140 Hz.(5)Numerical investigations were carried out on the backflow and the re-filling of propellants in the valveless PDRE.Several factors were studied including filling pressure,inner diameter of the inlet,ignition location,distribution length of obstacles,and detonation tube length.The results indicated that the backflow and re-filling process were obviously different when change the filling pressure or the inner diameter of the inlet.It is feasible to shorten the cycle time by improving the filling pressure if the isolation is reliable.Besides,the flow field at different time during the filling process can be regarded as the initial flow field at different frequencies.The average thrust at different frequencies was calculated.The change of thrust with frequency was similar to the experimental results,which can be explained by the filling state at different frequencies.(6)A frequency modulation valveless PDRE was studied.A frequency conversion ignition system was employed to control the operating frequency of a valveless PDRE.Steady operation was obtained at a wide range of ignition frequency.Conversion between different frequencies was also realized.Fully-developed detonation waves can be obtained in lower frequencies.The thrust at different frequencies are also tested.It was proved that controlling the operating frequency and thrust by the ignition frequency is practicable.