| The phenomenon of pressure oscillation widely exists in all kinds of combustion chambers,it has been restricting the research and development of power system,so this phenomenon has been widely concerned by scholars all over the world.The phenomenon of pressure oscillation in combustion chamber is also called unstable combustion.At present,scholars are more concerned about the combustion instability caused by thermo acoustic coupling,but few scholars have studied the instability combustion phenomenon caused by the coupling of structural vibration and combustion chamber pressure oscillation.In recent years,many types of solid rocket motors with large aspect ratios have successively appeared instability combustion phenomena.At the same time,the structure of the entire missile vibrates at the acoustic fundamental frequency or multiplier frequency of the combustion chamber.It can be considered that the coupling phenomenon of structural vibration and combustion chamber pressure oscillation appears in this type of instability combustion.In this paper,based on the mechanical problem of the interaction between the pressure oscillation and the structural vibration of combustion chamber,the experimental methods and numerical simulation methods are used to analyze the possibility of the coupling between the acoustic cavity oscillation and the structural vibration in the combustion chamber,and the characteristics of the pressure oscillation in the combustion chamber with the structural excitation.It is concluded that there will be a significant effect on the pressure oscillation excited by the pulse excitation when the structure breathing mode frequency is close to the combustion chamber acoustic frequency,and the pressure amplitude can reach the maximum value when the boundary disturbance frequency is close to the acoustic frequency.The purpose of this paper is to further deepen the understanding of the generation mechanism of the instability combustion phenomenon,provide theoretical support for designers to select the means to suppress instability combustion,and enrich the research on the interaction between fluid oscillation and structural vibration.Firstly,a set of models suitable for calculating the oscillation characteristics of the internal flow field of an axisymmetric combustion burner with the structural boundary disturbances are proposed,and the rationality of the calculation method is verified by experimental means.At the same time,according to the experimental platform,the vibration transmission characteristics between the acoustic pressure in the combustion chamber and the shell structure with different propellant thickness are also studied.According to the experimental results,when the whole system is in a vibrating state,the acoustic cavity and the structure should be treated as a whole.The vibration of the acoustic cavity is not only related to the inner surface of the rubber,but also coupled with the structure and affects each other.At the same time,according to the experiment,when the thickness of rubber changes,the system vibration characteristics are obviously different,which makes it clear that it is necessary to study the vibration characteristics with the interaction between combustion chamber structure and flow field at different working times(i.e.different propellant thickness).Secondly,based on the finocyl and axil configuration combustion chambers,the acoustic modal frequencies and acoustic response characteristics of the two combustion chambers at different working times were analyzed by acoustic finite element,and the acoustic pressure response amplitude characteristics of the monitoring points at the combustion chamber head and end were analyzed by the combined time-frequency analysis method.The structural modal change characteristics of the two motors during the whole working process were analyzed by using the structural finite element method.According to the research,for the combustion chambers with the same axial length and different geometrical characteristics,the acoustic frequency value and the characteristics of the acoustic frequency varied with the working time are obviously different,but this difference is more obvious in the high-order acoustic mode.There is no significant difference in first acoustic mode.According to the calculation of the structural mode,it can be seen that different changing laws of the geometrical characteristics of the combustion chamber during the working process will cause the changing laws of the centroid positions to be different.Finally,it can affect the changing laws of the breathing mode frequency.According to the analysis of acoustic response,the conclusion can be obtained that different characteristic structures can lead to large differences in acoustic response characteristics for the combustion chambers with the same axial length.The acoustic response capability of the axil combustion chamber is obviously weaker than finocyl during the whole working process,which indicates that the acoustic stability of axil configuration combustion chamber is significantly better than the finocyl.Then,according to the acoustic harmonic response analysis,the models of the two kinds of the combustion chambers at three typical working time were used,and the pressure oscillation characteristics of the combustion chambers excited by structural disturbances of different frequencies were studied by computational fluid dynamics.In the calculation,it is assumed that the structural vibration causes a certain intensity of pressure oscillation near the burning surface,and the propellant burning rate oscillates with the pressure due to the pressure coupling response characteristics of the propellant,which makes the inlet boundary of the combustion chamber oscillate,and finally leads to the pressure oscillation in the whole flow field.The simulation results show that when the disturbance frequency is close to the acoustic frequency of the combustion chamber,the pressure oscillation amplitude will be significantly increased.If the disturbance frequency is far away from the acoustic frequency,the pressure oscillation amplitude will be reduced.The pressure oscillation caused by the disturbance of the first-order acoustic frequency of the combustion chamber is significantly greater than the second-order acoustic frequency.The results show that the pressure oscillation in the first-order acoustic frequency plays a decisive role in the pressure oscillation of the combustion chamber.At the same time,some special geometric structures in the combustion chamber,such as annular groove and sudden expansion cavity,can significantly inhibit the pressure oscillation.Finally,using the calculation model constructed in this paper,the pressure oscillation characteristics of the combustion chamber excited by the impulse structural acceleration load of finocyl and axil combustion chamber at three typical times are studied.With the excitation of the impulse structural acceleration load,the pressure oscillation of the first-order acoustic frequency is dominant,and the distribution of the pressure amplitude in the axial position is similar to the acoustic mode shape.And the structural vibration of the motor also has the similar characteristics.The closeness of the structure breathing modal frequency to the acoustic frequency of the combustion chamber can lead to a resonance phenomenon that greatly increases the amplitude of the pressure and structural vibrations.When the frequencies are far away,the phenomenon of coupling will be significantly weakened.Therefore,it is possible to find means to suppress the occurrence of combustion instability in the combustion chamber from avoiding the closeness between the acoustic frequency and the structural breathing mode frequency.In addition,the special geometry of the combustion chamber can significantly dampen pressure oscillations.At the same time,various calculation results show that the use of the annular slotted structure instead of the wing structure can significantly reduce the pressure oscillation intensity of the combustion chamber excited by the structural pulse,and improve the stability of the combustion chamber. |
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