Numerical Simulation On Ablation Behavior And Thermal Response Of Thermal Protection Materials

Thermal protection materials are very important to design and develop advanced thermal protection system.They play an irreplaceable role in new power system,re-entry vehicle,space exploration vehicle,near space vehicle,reusable carrier and other aircrafts.Furthermore,both the reliability and safety of the relevant power system and aircraft are directly affected by the reliability of such materials.As typical thermal protection materials,carbon/carbon composite,carbon/phenolic composite and ultra-high temperature ceramic have been the research hotspots in terms of thermal protection system.Therefore,it is necessary to investigate the ablation and thermal response behavior of such thermal protection materials,especially to accurately predict the distribution and evolution of temperature field,which can provide effective reference and basis for the design and optimization of thermal protection system and thermal protective material.In this paper,three kinds of materials were regarded as research objects,including a carbon/carbon composite,a three-dimensional braided carbon/phenolic composite and an ultra-high temperature ceramic material.Based on the calculation models for thermal response of different thermal protective materials,numerical calculation method was adopted.In addition,the ablation and thermal response behavior under high temperature service condition was calculated.The main work is listed as follows:Firstly,on the basis of the difference between the fiber and the matrix,the heat transfer model has been applied.Moreover,the surface thermochemical ablation model for the carbon/carbon composite was established by means of the surface thermochemical ablation mechanism.The recession rate of the ablation surface was captured by the dynamic mesh method of the finite element.The thermochemical surface ablation behavior of the carbon/carbon composite was calculated in high temperature environment.The surface temperature,ablation surface recession,line ablation rate of the fiber and the matrix,and the volume and mass loss were predicted.Secondly,according to the performance of the three-dimensional braided carbon/phenolic composite under high temperature service condition,the volume ablation model of the thermal protection material was established from the perspective of composition based on the energy and mass conservation.In previous calculation models,the difference between the matrix and the fiber was generally ignored.As a result,the larger analysis errors were produced in the calculation of ablation properties.For more accurate calculations,the matrix density and the material thermal properties at high temperature were fully considered in our volume ablation model,and the thermal decomposition process of the matrix was calculated by using Arrhenius equation.Considering the thermal blocking effect of pyrolysis gas on the ablation boundary,the volume ablation behavior of the three-dimensional braided carbon/phenolic composite was calculated.The transient temperature field distribution,density change,thermal properties,pyrolysis degree,mass loss rate and char layer thickness of the thermal protection material were predicted.At high temperatures,as the volume ablation of the three-dimensional braided carbon/phenolic composite proceeds,the material inevitably undergoes thermal deformation with a resultant deformation displacement.On one hand,both the deformation form and deformation displacement of the thermal protection material are directly related to the stability and the safety of the thermal protection system.On the other hand,the accurate prediction of deformation is the prerequisite to design and optimize the thermal protection system.Therefore,based on the temperature field,the high temperature deformation of the three-dimensional braided carbon/phenolic composite was calculated.In this calculation process,the nonlinear variation of the thermal expansion coefficient of the matrix caused by the phenolic resin pyrolysis was considered.The deformation displacement distribution and the thermal expansion coefficient of the matrix were predicted.Finally,based on the original calculation method,the thermal response behavior of the ultra-high temperature ceramic material on the aircraft surface in the supersonic environment was calculated by the fluid-solid-thermal coupling method.In this process,the Spalart-Allmaras turbulence model and the pressure far-field boundary condition were selected for the flow field,and the deformation displacement of the structure was captured by using the dynamic mesh method.Hereafter,the calculation method was used to predict the flow state of supersonic flow field,the aerodynamic heat flux,the aerodynamic pressure,the temperature distribution and deformation displacement of the ultra-high temperature ceramic.1.The results show that the ablation surface of the carbon/carbon composite has obvious recession,and the recession distance of the matrix is larger than that of the fiber.Additionally,the fiber and the matrix have different volume and mass loss,and their linear ablation rates are also different,indicating that they have different ablative properties during ablation.2.The three-dimensional braided carbon/phenolic composite has an uneven temperature field distribution during ablation,and there is a large temperature difference inside the material.With the increase of temperature,the matrix density of the heating surface and its vicinity firstly decreases,and the decreasing area extends to the thickness direction of the material structure.Besides,there is a trough in the decomposition rate curve of phenolic matrix at different time,which represents the most serious position of material decomposition.As the ablation process progresses,the most severely decomposed position continuously moves toward deeper regions,and the absolute value of the decomposition rate gradually decreases.The generation of the surface char layer requires a transition period,and the average char layer thickness is almost zero during the transition time.3.In the initial stage,the thermal expansion coefficient of the phenolic matrix in the top region first decreases,and a region of decreasing thermal expansion coefficient is formed near the top.The thermal expansion coefficient of the material is stratified from top to bottom.Moreover,the deformation displacement of the carbon/phenolic composite increases with time.Compared with the deformation displacement of the fibers,the whole deformation is mainly concentrated in the matrix,and the deformation is most serious near the top surface of the composite.4.There are different aerodynamic heat flux at different positions of the ultra-high temperature ceramic material on the aircraft surface.At the stagnation point of the aircraft leading edge,the aerodynamic heat flux rapidly rises to a very high level in a short period,and then decreases gradually with the increase of time.Furthermore,the deformation of the thermal protection material at the leading edge is the largest,and the deformation displacement is mainly concentrated in the x direction.