| Pressure oscillation phenomenon is widely found in the combustion chamber of spacecraft power system.However,strong pressure oscillation will lead to catastrophic consequences such as abnormal internal ballistic,deviation of external ballistic and structural damage of spacecraft.Therefore,this phenomenon has attracted worldwide attention.However,the problem of pressure oscillation in combustion chamber has never really been solved due to the complex generation mechanism and numerous influencing factors.It has always been one of the most challenging problems in spacecraft power system.Recently,in the development of tactical missile SRMs(solid rocket motors)in China,the problem of pressure oscillations has arisen repeatedly.Therefore,the study of pressure oscillation in combustion chamber has both the important academic value and the practical significance.In this paper,based on the oscillating flow-field in an SRM,the influence factors and suppression methods of pressure oscillation in combustion chamber are studied by means of experimental verification,theoretical derivation and numerical simulation.It is concluded that the frequency of external disturbance and the characteristic structure of flow-field in combustor have significant influence on pressure oscillation.In addition,a feedback control method for solid rocket motor is proposed through theoretical derivation.Finally,the control method is validated by numerical technology.The purpose of this paper is to find a way to solve the problem of strong pressure oscillation in the combustion chamber of SRMs.Firstly,the influence of external structural vibration on the pressure oscillation in the cavity is confirmed by experimental method.Based on this,the oscillation characteristics of the VKI(Von Karman Institute of Fluid Dynamics)model with inhibitor removed are simulated by using acoustic modal analysis,acoustic harmonic response analysis and large eddy simulation.The mass-flow-inlet boundary condition with periodic disturbance is realized by UDF(user-defined-function),which represents the superposition of periodic injection and extraction of mass and average mass injection.The simulation results show that the disturbance frequency has a significant effect on the amplitude-frequency characteristics of pressure oscillation in the combustion chamber.In present work,the pressure amplitude at the head-end of the combustion chamber can change more than 80 times under the same amplitude and different frequency disturbance.When the disturbance frequency is close to the acoustic frequency of combustion chamber,the pressure amplitude is large;on the contrary,the pressure amplitude is small.Under the inlet disturbances at specific frequencies,the pressure oscillations are excited at the frequency of the disturbance and acoustic modes that are close in value to the disturbance frequency.However,under the inlet disturbances at frequencies close to the acoustic frequencies,the pressure oscillations are excited only at a frequency between the disturbance and acoustic frequencies and its multiplications.When the disturbance frequency is close to the acoustic frequency,the combustion chamber shows the standing wave acoustic field characteristics,while far away from the acoustic frequency,the traveling wave acoustic field characteristics.Secondly,the acoustic finite element analysis(FEA)and renormalization group(RNG)k-ε method,together with the experimental thought of pulse attenuation,are used for a comparative study on the stability of finocyl and axil combustion chamber with the same main size.The results show that the characteristic structure of the combustion chamber has a significant influence on the acoustic response and pulse attenuation characteristics of the combustor.In present work,the combustion chamber with finocyl structure has strong response to the first three order acoustic frequency,and the axil combustion chamber has strong response to the third to fifth order acoustic frequency.For the head-end of the chamber,the weighted average value of acoustic pressure in axil chamber is less than 55%of that in finocyl chamber at the first order acoustic frequency in working time.Especially in the middle of working time,the acoustic pressure of axil chamber at the first acoustic frequency is less than 35%of that of finocyl chamber.Acoustic response characteristics of the two type combustion chambers are also reflected in pulse attenuation simulation.Under the same pulse,the relaxation time of pressure in the axil chamber is shorter than that in the finocyl chamber.The RMS value of pressure oscillation in frequency domain is significantly smaller than that in the finocyl chamber.Moreover,compared with the finocyl chamber,the oscillation energy of the axil chamber is more concentrated at higher acoustic frequencies.Various results show that the stability of a axil chamber which has not been used in China is better than that of a finocyl chamber.Finally,by theoretical deduction and summarizing the existing control methods,a feedback control method of pressure oscillation using pressure-coupled response time delay is proposed based on the principle of anti-phase noise elimination for SRMs.When the time lag of pressure-coupled response is between one quarter and three quarters of the oscillation period,the pressure-coupled response on the propellant surface damps the pressure oscillation.If the time lag is half of the oscillation period,the oscillation suppression effect is the most significant.By compiling UDF,the establishment of standing wave acoustic field in a combustion chamber and the disturbance inlet condition for expressing the real part and time lag information of pressure coupled response function are realized.Based on the idea of steady-state wave attenuation experiment,the feedback control method of pressure oscillation for T-burner is validated by standard k-ε method.The feedback control method for internal flow-field oscillation of an SRM can be self-validated and obeys Rayleigh criterion.At the same time,the influence of pressure-coupled response function value on pressure oscillation under the same time delay is considered and discussed by numerical technique.When there is no time delay in pressure-coupled response,the increase rate of amplitude provided by combustion surface increases with the increase of pressure-coupled response function value.When the time lag of pressure-coupled response is half period of pressure oscillation,the ability of combustion surface to attenuate amplitude increases with the increase of pressure-coupled response value.The pressure oscillation control method proposed in this paper may found an uncharted field of feedback control for SRMs,and the pressure-coupled response delay proposed in this paper may become an important parameter to measure the stability of propellant. |
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