Research On The High Frequency Combustion Instability In The Model Of LOX/Kerosene Engine

High frequency combustion instability happens frequently in the combustionchamber of liquid rocket engine, which often devastates the normal work of engine.However, the triggering and evolutionary mechanism has not yet been fully understood.Therefore, investigations of the mechanism of self-excited high-frequency combustioninstability in liquid rocket engine have very important academic significance andapplication value.Three dimensional transient turbulent two-phase reacting flow in the chamber ofLOX/kerosene bipropellant liquid rocket engine model is numerically investigated byusing RANS method in this paper. The predicted pressure and mean axial velocity arequalitatively consistent with the experimental measurements of other investigators.Self-excited pressure oscillations are obtained without any disturbance introducedthrough the initial and boundary conditions, and the corresponding frequency is in goodagreement with test data, which validates the results of self-excited high frequencypressure oscillations predicated by the present numerical simulation method.The phenomenon of “pressure peak” is observed in the time history of pressureoscillations. Corresponding analysis shows that the local combustible premixtures thatgenerated immediately by the instantaneous gasification of fuel droplets reaching theircritical state will go through the “quasi-constant volume combustion” and result in thebombing effects distinguished by the high amplitude increase of pressure andtemperature. It is the “quasi-constant volume combustion” that stimulates the “pressurepeak”. The temporal and spatial distribution of “pressure peak” indicates that it occursfrequently and stochastically in the region near the injector panel of combustionchamber, and its propagation and reflection in the chamber will ultimately stimulate theself-excited high frequency combustion instability.A third Damk hler number is defined as the ratio of the characteristic time of achemical reaction to the characteristic time of a pressure wave expansion, whichcorresponds to the “pressure peak” and further indicates the mechanism of“quasi-constant volume combustion” which stimulates the “pressure peak”. The thirdDamk hler number can well classify the combustion process in liquid rocket engine into quasi-constant volume combustion, constant pressure combustion and combustionwith partial expansion and pressure increase.The effects of chamber pressure, initial droplet diameter and temperature on theself-excited pressure oscillations are investigated. If the average chamber pressure islower than the critical pressure of propellants with weak volatility, the combustionchamber will have better stability because of no appropriate conditions of forming“pressure peak”. It is shown that the intensity of self-excited pressure oscillations willbe augmented first and then weakend with the increase of droplet diameter. For thedroplet with smaller or larger diameter, there is no suitable conditions for forming“pressure peak” or the “pressure peak” occurs far away from the injector panel whichwill result in little coupling effect with the severe chemical reaction there, so highamplitude pressure oscillations can not be stimulated or sustained; For the droplet withmediate diameter, the propagation of “pressure peak” will couple with the severechemical reaction in the region of injector panel, thus the high frequency and amplitudepressure oscillations will be stimulated and sustained. When the initial temperature ofpropellant increases, the amplitude of pressure oscillation will be augmented, thecorresponding analysis is consistent with that of the droplet diameter.The damping mechanism of baffles on the self-triggered high-frequencycombustion is investigated, which indicates that baffles can effectively restrain thepressure oscillations in the combustion chamber. However, the “oscillating source”which stimulates the “pressure peak” can not be eliminated completely. The effects ofbaffle can be interpreted as blocking wave propagation process induced by the“oscillating source” and weakening the coupling between different regions where thereare “oscillating sources”. Furthermore, an optimal length interval of baffle is obtained,which indicates that the damping effect of baffle will be reduced if baffle length isbeyond this interval, because self-excited pressure oscillation will have a long timeresidence in the cavity of baffles which will enhance the coupling effect betweenpressure oscillation and acoustic characteristic of the chamber.