Study On Influencing Factors And Suppression Methods Of Combustion Instability Of Solid Rocket Motor

With the continuous updating of missile weapon and equipment,the required standards for weapon system have been continuously raised and the requirements of the power system index have repeatedly broken new heights.Large aspect ratio,high-loading,high-energy composite propellant engines are widely used.However,this type of engine has repeatedly experienced the problem of unstable combustion at the end of the work.In order to reveal the causes of unstable combustion in the final stage,theoretical analysis,numerical calculations,and experimental measurements were combined to investigate the main gains and damping factors of solid rocket motors.The main work includes the following aspects:The data analysis of the unstable combustion phenomenon of the solid rocket motor during the process of the experimental test was performed to obtain its frequency spectrum characteristics.At the same time,the inherent frequencies were calculated using the finite element method.In order to diagnose and find out the type of the unstable combustion phenomenon of the engine,comparing the inherent acoustic vibration frequency of the engine with the experimental measurement oscillation frequency,it was found out that the first-order axial theoretical calculation value was similar to the first-order experimental one.And it was concluded that the type of the unstable combustion is the acoustic unstable combustion,which pointed out the direction of solving the unstable combustion problem.Using the single phase large eddy simulation method,the influence of thermal baffle parameter design on the pressure oscillation caused by the coupling between vortex shedding and acoustic field was studied and the regular patterns of the length and bending angle were obtained,which was based on the typical VKI model.And then,the numerical simulation work was used to study the flow field characteristics,pressure oscillation characteristics,and vorticity characteristics of the engine with complex grain structure at three typical operating times.The change regulation of the pressure oscillation amplitude caused by the complex structure design of the grain during engine working process was analyzed.It was found that the pressure oscillation amplitude decreased gradually with the retrogression of combustion surface.The amplitude of pressure oscillation caused by the complex structure design of the grain at the end of the working period was about two orders of magnitude less than that of the previous period.Based on the theory of short nozzle,the nozzle damping mechanism and damping coefficient expression were derived.Numerical simulation method was used to study the influence of engine design parameters on the nozzle damping coefficient,such as the engine characteristic length and the ratio of the nozzle throat area to the combustion chamber aeration area.It was found that the numerical simulation calculated values were similar to the theoretical ones,which proved the rationality of the numerical calculation method.Then,the factors that were unable to be predicted and judged by the theoretical formula were carried out,such as the nozzle convergence half angle and the convergence surface.It was found out that the nozzle convergent half angle played a vital role in nozzle damping coefficient.Small convergent half angle was beneficial to the nozzle damping.Using a convex profile was also beneficial to improve the damping characteristics of the nozzle.At last,the effects of the working pressure and gas temperature on the damping coefficient were studied and the influence mechanisms were revealed.In addition,the conversion relations between the cold flow test and the heat engine test were obtained.It was also found that the change of working pressure had almost no effect on nozzle damping coefficient with the fixed working conditions and constant nozzle throat radius.The particle relaxation theory was derived on one dimensional level.And then the theoretical calculation formula of the particle damping coefficient and the mechanism of the loss of acoustic energy caused by the particles were obtained.The particle damping test was carried out on the basis of the double burning surface secondary attenuation method using the T burner with new charge scheme designed.The propellants used in the first two groups were identical and the third group of propellants had a lower percentage content of aluminum.The working pressure of the three experiments was about 11.5MPa.It was found that the particle damping coefficient values obtained from the first two experiments were similar,which proved the repeatability of the experimental method.The particle damping coefficient measured in the third group was obviously lower than that in the first two groups,which indicated that the percentage of aluminum in propellant had a very important influence on the particle damping.And then,the conclusion was verified and the relation between the percentage of the particle mass content and the particle damping coefficient was obtained by using the numerical simulation method.Aiming at the acoustic unstable combustion appearing at the end of solid rocket motor working process,the change of main gains and damping factors with the burning surface retrogression was analyzed.It was found that the combustion response gain remained unchanged at the end of the engine operation and the gain caused from the coupling between vortex shedding and acoustic field was so small as to be negligible.And it was known that the acoustic unstable combustion was mainly induced by the reduction of the value of the total damping coefficient.However,the particle damping coefficient decreased first and then increased during working process.The coefficient of the wall damping was also so small as to be negligible.Therefore,it can be concluded that the reduction of the damping coefficient of the nozzle was the main inductive factor of unstable combustion at the end of the working period.In order to solve the problem of acoustic unstable combustion,the overall structural design of the engine was optimized in order to increase the loss of acoustic energy.And then,the optimized design was obtain and validated by the numerical calculation method,which provided a viable solution to solving the problem of acoustic unstable combustion.