Analysis On Trigger Factor Of Combustion Instability In Unconstrained Solid Rocket Motor

Combustion instability in solid rocket motors is an intractable problem that often appears in motor developing process.It is mainly manifested as the periodic or nearly periodic changes of flow parameters such as combustion chamber pressure and propellant burning rate.In recent years,some solid rocket motors both at home and abroad experienced combustion instability in their flight tests,although there was no instability detected in their ground tests.This phenomenon cannot be explained using the traditional combustion instability theory.It can be resorted to two possible reasons.The first is that the fixed constraint adopted by the motor ground test changes the intrinsic vibration mode of the motor shell,so that it does not couple with the acoustic vibration of the internal flow field.The second is that there is no external load of the motor under flight condition in ground test,thus no effect of triggering factors caused by it.Therefore,studying the influence of trigger factors on motor combustion instability in free state is of great significance for reproducing combustion instability phenomena in ground tests and solving this inconsistency problem between the flight and ground test.The focus of the present study is on the influence of different trigger factors on the combustion instability of solid rocket motors in free flight.A combination of mode analysis and bidirectional fluid-solid interaction coupled calculation is used to obtain the combustion chamber pressure oscillation and shell vibration.Finite element method is used to calculate the solid vibration mode and the acoustic mode of the flow field in the combustion chamber to obtain the inherent vibration and acoustic properties of the motor;user-defined function(UDF)is applied to write source items to simulate the grain loading,and the combustion stability under the triggering action is studied numerically.The main research contents are as follows:(1)A physical model is established based on the American Ram missile,according to which,vibration mode analysis is carried out concentrating on three typical working moments of the motor,viz.,the early stage(10% grain regression),middle stage(50% grain regression),and final stage(90% grain regression).It is found that with the grain regression,the frequency of the first-order bending mode increases gradually,and becomes more and more closer comparing with the first-order longitudinal acoustic frequency.(2)The influences of the pulse intensity,exerting stage,and duration at the head of the combustion chamber on the internal flow field pressure oscillation and the motor shell vibration are studied.It is found that the attenuation constants under different pulse intensities are very close,but there exists obvious first-order acoustic frequency vibration at the head of the motor shell.At the early working stage of a large throat-to-pass ratio motor,pulse triggering can hardly cause pressure oscillation with first-order acoustic frequency due to large nozzle damping.(3)A fluid-structure interaction numerical simulation is carried out,aiming at revealing the effects of vibration and external impact on the inflight solid rocket motor performance.The simulations show that the pressure oscillation amplitude in the combustion chamber increases as the shell vibration frequency gets closer to the combustion chamber acoustic frequency.When the shell vibration frequency is the same as the combustion chamber acoustic frequency,a frequency coupling will occur between the two.In this case,the acoustic pressure will increase with time and finally reach and stay at the limit amplitude.As the grain regression,throat to port ratio becomes smaller,and the pressure oscillation excited by the shell vibration becomes more and more intense.