Investigation On Aluminum Agglomeration On The Burning Surface Of Solid Propellants By Optical Observation Technology

Aluminum particles in solid propellants accumulate,melt and agglomerate on the burning surface,thereby form a large particle,and finally leave the burning surface.The agglomerates on the burning surface are the initial process of aluminum combustion in the solid rocket motor.The size distribution of the agglomerates affects the combustion process of aluminum in the combustion chamber,and it has a great influence on the size of the final combustion products.Also,the large agglomerates can cause two-phase flow loss in the flow combustion process.However,the agglomerate size distribution on burning surface is related to the formulation of the solid propellant.Therefore,the research of the aluminum agglomeration on burning surface can be helpful to understand the mechanism,the key factors and rules of the agglomeration.It is critical important to establish the agglomeration model to predict the particle size distribution of agglomeration.In this thesis,the experimental system to study the agglomeration process under high pressure is established,in which the optical camera method with a high-speed camera and a long distance microscope are adopted.By image processing technology,the diameter of the initial agglomerations is extracted from the image.Based on the experimental system and examination methods,burning rate,pressure,initial aluminum average particle diameter and RDX,the factors that influence the aluminum agglomeration process,are studied and analyzed.The mechanism of the agglomeration and the coalesce process of the agglomerates on burning surface are obtained as well.Finally,the agglomeration model is established to predict the diameter distribution of the agglomerates,and the simulation results are compared to the results of the experiments.The main research work and results of this thesis are as follows:?1?An optical method is developed,which is suitable for investigating the agglomeration on the burning surface under 5MPa.The use of a suitable transmittance of the neutral density filter successfully solves the problem of excessive exposure of the image caused by the burning of the propellant.And a calculation method of the transmittance of the neutral density filter is presented.This method can be used for approximately estimation and selection in the experiment.?2?The effect of burning rate on the aluminum agglomeration is studied under the same pressure.The agglomerate diameter range of low-burning-rate is less than that of high-burning-rate.With the increase of burning rate,the average mass diameter of the agglomerates(D_agg)decreases.The increase of burning rate also shortens the residence time of the agglomerate on the burning surface.It decreases the agglomeration on the burning surface.?3?The effect of the pressure on agglomeration are studied while the burning rate and the propellant formulations are kept constant.Experimental results show that the change of pressure does not lead to the obvious change of the agglomerate diameter and Dagg.Therefore,pressure has no direct effect on the agglomeration on burning surface.?4?The initial aluminum particle size(D⁰_Al)in the propellant formulations is 16?m,29?m and 36?m.The experiments are carried out under the same pressure and burning rate.It is found that the agglomeration degree and D_agg of D⁰_Al=29?m propellant are the largest.Since the residence time of the agglomerates does not change with the initial aluminum particle average size,the agglomerate diameter distribution on the burning surface highlights the importance of the distribution of aluminum in the packing array of the propellant and the combustion environment above the burning surface.The initial aluminum size does not determine the agglomerate diameter distribution directly.?5?With the same pressure and burning rate,the effect of adding RDX in the propellants for the aluminum agglomeration is studied.The experimental results show that the agglomerate diameter obviously increases as RDX increases.?6?The agglomeration on burning surface can be divided into two types.The main reason that leads to the two different types is the degree of aluminum particles density distribution in the solid propellant.The coalesce process on the burning surface of two or more agglomerates results in the formation of larger agglomerates?larger than 350?m?.?7?Based on the idea of the pocket model,a comprehensive agglomeration model is proposed.Through scanning electron microscopy for the solid phase of the propellants,the pocket size of the region formed by adjacent coarse AP particles are calculated by the image processing technology.Involving the effects of the burning rate and the coalesce process on the agglomeration,the diameter of every agglomerate from the pocket region is able to obtained.Therefore,the agglomeration model can predict the agglomerate diameter distribution on the burning surface.The calculated results agree well with the experiments.