Numerical Study On Mesoscopic Structure Model Of Ablative Material And Its Heat Transfer Characteristics

The internal thermal insulation material plays an important role in the thermal protection system(TPS)of solid rocket motor.So far,the thermal insulation material in most ablation models was described as homogenous material.Actually,the thermal insulation material is consisted of particle filler,short fiber and rubber matrix,which is a kind of heterogeneous material.Obviously,the ablation models based on the heterogeneous mesoscopic structure,in which filler and short fiber are dispersed in matrix,should be developed to understand the thermo-chemical mechanism in detail and evaluate the ablation rate of the thermal insulation material precisely.Furthermore,the evaluation methods should also be established to evaluate the feasibility of the ablation models based on the heterogeneous mesoscopic structure.On the basis of the random sequential adsorption(RSA)method,a series of MATLAB programs to generate the heterogeneous mesoscopic structure model of insulation material were developed.In the model,the particle filler and short fiber were non-overlapped and distributed evenly and randomly.Meanwhile,the size,orientation and volume fraction of the filler or short fiber in the model were adjustable.The methods to evaluate the feasibility of the models based on the heterogeneous mesoscopic structure were developed.The coordinates of centroid and the moments of inertia were used to evaluate the uniformity of filler in the model.The radial distribution function was used to evaluate the uniformity of short fiber in the model.The second order orientation tensor was used to evaluate the orientation characteristics of short fiber in the model.A heterogeneous mesoscopic structure model of silicon rubber based ablative material was calculated based on the formulation of the material.The size of the model was 1mm×1mm×1mm.The volume percentage of the particle filler was 14.6vol% and that of short fiber was 6.7vol%.The size distribution of particles in the model followed the Gaussian distributions,in which the average diameter of particles is 0.005 mm,and standard deviation of the diameter was 0.001 mm.The length of short fibers in the model was 1mm,and its diameter was 0.01 mm.The calculated results of the model showed that the formalities of the particle filler and short fiber were good enough to meet the requirement and the orientation distribution of the fibers was acceptable.The results showed that the model was reasonable.Due to the complex structure,the model is hard to divide the grids by the conventional methods.In this paper,a background lattice method was introduced to divide the grids for the mesoscopic heterogeneous structure model of the ablative material.Firstly,the regular hexahedron grids of the model were generated.Secondly,the material property of the grids was judged one by one.Finally,the finite element analysis(FEA)model was established.Based on the heat transfer theory and finite element analysis method,the thermal conductivities of the insulation material at room temperature were calculated and the meso-heat transfer characteristics were analyzed with the finite element software of ANSYS.According to the calculated results,several conclusions can be made:(1)When the thermal conductivity of filler was greater than that of matrix,the thermal conductivity of insulation material would increase with the higher volume fraction of filler.(2)When the volume fraction of filler was lower than 0.2,the thermal conductivity of insulation material would almost keep the same with the change of the filler’s particle size in the range of 10~100 μm,but the value of temperature field of the material with smaller particle size of filler would become more uniform.(3)The addition the filler with lower thermal conductivity would decrease the thermal conductivity of the material remarkably,and the opposite is also true.(4)The greater the number of the short fiber which orientation of fibers was parallel with the direction of heat flux,the greater the thermal conductivity of the insulation material would be.(5)When the ratio of the fiber’s length and its diameter increased in the range of 10~50,the thermal conductivity of insulation material would increase.