Study On Three-dimensional Finite Element Thermal Response Calculation Method Applied To Ablative Thermal Protection System

For nonreusable re-entry vehicles,thermal protection systems(TPS)that use ablative insulators are commonly required to protect structural components from extreme aerothermochemical environments resulting from high entry velocities.To minimize the weight of ablative TPS,in-depth temperature distribution of ablators must be accurately predicted.Since arc jet test is expensive and can not perfectly simulate the environments that re-entry vehicles experience,it is necessary to develope numerical ablative thermal response analysis tools.Researchers from America and Europe have done lots of research on ablative thermal response problem.They have developed a series of calculation tools,ranging from one-dimensional to three-dimensional.In contrast,there are rarely practicable three-dimensional analysis tools in our country.Therefore,this dissertation focuses on numerical simulation of ablative thermal response,aims at establishing a high fidelity analysis tool for our TPS designers.Mechanism analysis and physical modeling are prerequisite for accurate simulation of the phenomena associated with ablation heat transfer.Based on previous research,this dissertation has built a physical model with comprehensive consideration of surface ablation,in-depth material pyrolysis and pyrolysis gas transmission.Governing differential equations are derived due to fundamental conservation laws.A computer program is developed to numerically solve the governing differential equations.There are totally five chapters in this dissertation.The first chapter is Introduction.In this chapter,background and research status are summarized.The second chapter is an overview of research method.In-depth conservation and surface energy balance equations are built based on mechanism analysis.All the equations are transformed to the moving coordinate to account for the material's motion due to ablation.Finite element method is also introduced briefly.In the third chapter,One-dimensional ablative thermal response calculations are researched primarily,as preparation for solving more complicated three-dimensional problems.The solution of moving boundary heat transfer due to surface ablation is realized using “front-fixing” method,and the governing differential equations are solved by the finite element method in combination with implicit Newton-GMRES iteration.Verification results from exact solutions are presented to show the spatial and temporal orders of accuracy.The results of present code are also compared to classical program and test data,as a code validation.The forth chapter expands the numerical method to three-dimensions.A three-dimensional “front-fixing” based moving grid methodology is introduced for the purpose of surface recession simulation.Darcy law is also used to describe multi-dimensional pyrolysis gas flow.Discretization of the three-dimensional governing differential equations is performed by finite element method,and the computer code is built using MOOSE framework,similar to former one-dimensional case.Analytical solution and code comparisons are used as verifications of the numerical implementation.Analysis of cross-section heat transfer and pyrolysis gas flow are also demonstrated to show the multi-dimensional effects.The capability of the code to deal with complex geometry is examined by an asymmetrical ablation case.Finally,in the fifth chapter,the total work of this dissertation is summarized,and the future research is forecasted.