Study On Ablation Mechanism And Thermal Conductivity Inversion Of Charring Material

When the hypersonic vehicle reenters the earth’s atmosphere,it will be subjected to severe aerodynamic heating.In order to protect the structure,the aerodynamic surface and the payload,some thermal protection measures should be taken.Relatively speaking,ablation thermal protection holds many practical advantages.It is efficient,adaptable,simple and reliable.It is the most successful means of thermal protection at present.According to the organic matrices,ablative composites can be divided into two groups:charring materials and non-charring materials.Charring materials decompose into volatile monomers at high temperature,leaving solid residues.The carbonaceous residue on the one hand can operate as a binder,on the other hand can work as an additional heat sink to absorb heat through sublimation.So the charring material has a broader range of applications.In this paper,two typical applications of charring materials,namely,reentry warhead and recoverable satellite or spacecraft reentry capsule,are studied,and appropriate ablative analysis models are built respectively.The thermal environment characteristics of reentry warhead are high pressure,high enthalpy,high heat flux and short time.The research focus on surface ablation.Silicon-based material is the main thermal protection material for the first generation reentry warhead.Based on the comprehensive consideration of carbon-silica reaction,the oxidation of carbon,the loss of fused silica,a revised liquid layer model with higher accuracy is established.The thermal environment characteristics of recoverable satellite and spacecraft reentry capsule are low pressure,high enthalpy,low heat flux and long time.The research focus on the internal thermal response of materials and the convection heat transfer of pyrolysis gas.Carbon phenolic materials are often used.This paper analyses the gas boundary layer,the compatibility relations of ablation surface and the internal heat transfer.Based on the established ablation model,the endothermic mechanism of charring materials under two typical ablative environment is analyzed,and the proportion of each endothermic mechanism is calculated.To better compare different ablative materials,the ablative performance evaluation index is established.For non-ablative materials,the effective heat capacity,which is an evaluation index of thermal insulation performance,is defined.Then optimization method is studied to obtain the optimal combination of material parameters.By comparing with the existing material parameters,the improvement direction is pointed out.Finally,based on the measured temperature distribution,a thermal conductivity inversion method of charring materials is proposed,which does not require any prior information on the functional form.The temperature-dependent thermal conductivity is parameterized by discrete data points with cubic spline interpolation between them,which makes the curve continuous and smooth and improve numeric stability.Because the density of charring material significantly decreased during ablation,the thermal conductivity is also affected by the density.So a multivariable function inversion method is proposed.The three-dimensional surface of the thermal conductivity of charring materials varying with temperature and density coordinates is obtained.