Research On Changing Operation Combustion Characteristics And Combustion Stabilities Of Hydrogen/oxygen Engine

To change operation mode of rocket engine, the mass flow rate of propellants in the feeding pipeline is modulated, which will cause fuel-oxidizer velocity ratio and mixture ratio change, and hence the combustion flow field, combustion efficiency and the wall heat fluxes will be changed. The combustion characteristics and stabilities of a hydrogen(H2)/oxygen(O2) rocket engine on changing operation mode are investigated by numerical simulation and experiments in the presented thesis.Firstly, in order to obtain reliable and effective numerical simulation model, the combustion status of the engine was investigated with different turbulence model, turbulence-chemistry interaction model, and combustion mechanism. These models were verified by comparing the computational results with the experimental results acquired by testing the wall heat fluxes. The results indicated that the combustion flame estimated by the standard k ?? model is shorter than that calculated by the k ?? ?sst model, and the wall heat fluxes predicted by the latter provides better match to the experiments data. In addition, the laminar finite-rate model predicts higher temperature in specific local region than that predicted by the eddy-dissipation model.Secondly, after the reliable and effective models were gained, the combustion characteristics on changing operation mode, including the effect of velocity ratio and mixture ratio on combustion flow field and wall heat fluxes, and the impact of velocity ratio on combustion efficiency, were investigated. The results suggested that increasing the fuel-oxidizer velocity ratio will reduce the flame length, low temperature region and the fuel concentration. The plateau of maximum heat flux is insensitive to both the velocity ratio and mixture ratio, and the varying of the velocity ratio and mixture ratio will not change the profile of the wall heat fluxes. Higher combustion efficiency presents for the larger velocity ratio on the same cross section.Finally, based on the understanding of combustion characteristics on changing operation mode, the investigation of combustion stabilities in changing operation mode was conducted. It was found that heat release rate fluctuations was mainly due to the forming, merging and colliding process of the large-scale vortex in gas/gas combustion. The computational results demonstrated a dramatic increase in the amplitude of dominant mode oscillations for the lower fuel-oxidizer velocity ratio due to combustion oscillations. Entirely, in order to reduce the combustion oscillations, it is beneficial to increase the fuel-oxidizer velocity ratio. If the forcing frequency differs the combustor resonant frequency, the response of combustor pressure to the varying operation is transient, and when pressure levels off, the combustor will only present resonant behaviors.