Numerical Investigation On Transpiration Cooling With Liquid Water Phase Change

"Thermal barrier" is one of the key factors restricting the development of hypersonic aircraft.At present,the thermal protection design is to use thermal protection materials with high temperature resistance and anti-oxidation of the matrix.Facing the demand for greater range,higher speed and longer time in the future,the thermal environment will become more severe,and the current performance of thermal insulation materials has been used to the limit.An effective thermal protection technology is urgently needed to solve this problem.Transpiration cooling technology with a high cooling efficiency by fluid phase change dissipation,can effectively reduce the heat inside the aircraft,it is considered one of the most potential to address the future challenges of hypersonic vehicle thermal protection technology.In the early research of transpiration cooling,more attention is paid to the simple case of one-dimensional,steady-state and no phase change.There are few numerical studies on the flow and phase change of liquid water in porous media.To solve this problem,in this paper,the COMSOL finite element software is used to realize the numerical simulation of the transpiration cooling of liquid water with phase change,and the flow and heat transfer laws of one-dimensional and two-dimensional under unsteady conditions are carried out.The main work is as follows:Firstly,the theory of flow and heat transfer in transpiration cooling of porous media is summarized and analyzed.The characteristic parameters of micro structure in porous media are listed,and the influence of these parameters on the flow and heat transfer in porous media is evaluated.The mathematical models of flow and heat transfer in transpiration cooling theory are reviewed,and the applicable conditions of each model are discussed.Compared with the existing phase transformation model,the defects in the model are proposed,and the phase transformation model is improved by combining with the COMSOL finite element software.Secondly,the one-dimensional transient transpiration cooling process is studied.Based on the local thermal equilibrium model,the transient characteristics of liquid water transpiration cooling are discussed,the evolution characteristics of the initial stage of liquid water transpiration cooling are obtained,and the effects of thermal diffusion coefficient,heat flow and mass flow rate on the flow and heat transfer of liquid water transpiration cooling are analyzed.Based on the local thermal non-equilibrium model,the influence of convective heat transfer coefficient on the local heat non-equilibrium effect is analyzed,and the sweating cooling characteristics of different magnitude convective heat transfer coefficient are obtained,and compared with the results of the local heat balance model,the results show that the local heat non-equilibrium effect can not be ignored under the low convective heat transfer coefficient.Finally,a two-dimensional plate model is used to study the transpiration cooling process of liquid water under the non-uniform heat flux and pressure distribution on the hot end surface.The effects of the non-uniformity of the external environment on the temperature response,pressure distribution,phase transformation and transport characteristics of the two-dimensional plate are analyzed.The results show that the nonuniformity of the environmental conditions will lead to uneven distribution of the coolant in the plate,local high temperature and "heat transfer deterioration" phenomenon.