Theoretical And Experimental Investigations On Basic Problems Of Transpiration Cooling

Aerospace technology has developed rapidly and influenced human life greatly since twentieth century, and now becomes the key project of all the developed countries for its significant effects on military strategy, national economy and social life. However, the further development of aerospace technology has been limited by existing thermal protection system (TPS), therefore more efficient enhanced cooling technology is the focus of aerospace industry at present. Transpiration cooling, as a potential thermal protection approach, has attracted more and more attention from the researchers due to its excellent cooling efficiency. However there are still many problems in the investigations of experimental methods, theoretical models and numerical approaches, such as more accurate models and boundary conditions to describe the fluid flow, heat and mass transfer and phase change in porous media, the optimal design of transpiration cooling system, the evaluation of coolant consumption under a certain thermal circumstance and so on.Surrounding the above problems and based on the theoretical analysis, this dissertation will present experimental and numerical investigations on the basic problems of transpiration cooling. Main works include:(1) The present mathematical models of single-phase and multi-phase flows within porous media which during transpiration cooling process are summarized and analyzed. Firstly, the momentum equation, empirical formulas summarized by experimental data and constitutive relations of single-phase flow within porous media are particularized and compared, and the subject area, physical model and flow condition in which those flow models are applicable are discussed. Then based on the mass, momentum and energy conservation equations of single-phase flow, the interactive multi-phase flow with phase change or chemical reaction are further analyzed. By investigating the mathematical models which describe the performances of fluid flow, heat transfer and phase change within porous media, the deficiencies of the previous models are pointed out, which is helpful for the further improvement of the theoretical model of transpiration cooling.(2) Based on the traditional model, the momentum equation of fluid within porous media at ambient temperature is modified in this work, so that it can be applicable in the circumstance with high temperature or great temperature gradient. Both the natural and forced convection of fluid within porous matrix, and the variation of fluid properties with temperature and pressure are considered in the modified model, and then transpiration cooling experiment using air as coolant is conducted to validate the model. Theoretical analysis and experimental results indicate that the effect of environmental condition on fluid flow is significant when transpiration cooling system works in high temperature and pressure environment, and varied fluid properties should be used. Finally, an expression of apparent permeability is obtained, which can embody the temperature, pressure and inertia effect caused by high speed flow of fluid, and ensure Darcy’s law keeps its concise linear form at the same time.(3) A series of new conservation equations for mass, momentum and energy are presented in this work, to describe the performances of fluid flow, heat absorption and phase change in porous media. The differences from the previous models include, firstly, considering the compressibility of vapor in the momentum and energy equation; secondly, adding a term of momentum transfer caused by liquid phase change into the momentum equations of vapor and liquid phases in two-phase region; finally, in the energy equation of two-phase region, taking the variations of temperature and pressure into account, eliminating the assumptions that the enthalpy is only dependent on temperature, and saturation temperature is constant. Transpiration cooling experiment using liquid water as coolant is conducted to validate the model. The distributions of temperature, pressure and velocity of one-dimension steady-state problem are analyzed by using the verified model and numerical approach, and the effects of coolant volume and external heat flux are discussed. The numerical simulations show that:the temperature of liquid and vapor phases in two-phase region is not constant, but rises in coolant flow direction; the momentum moving from liquid to vapor caused by phase change has a significant effect on the distributions of pressure and velocity in two-phase region. So the new model and numerical approach developed in this work can provide more accurate description and solutions to the phase change process involved in transpiration cooing using liquid as coolant, and improves the present mathematical models.(4) The states of fluid flow and heat transfer probably occurring during the phase change procedure are numerically investigated, and the optimized design of transpiration cooling system is discussed. The optimization includes two parts:1) The variation of the thickness and location of two-phase region, capillary pressure and driving force with heat flux and coolant volume are analyzed, and a desired case of transpiration cooling is determined. From the relationships between the external heat flux and coolant mass flow rate, an approach is given to estimate the maximal heat flux afforded and the minimal coolant consumption required by the desired case of transpiration cooling. Thus the pressure and coolant consumption required in a certain thermal circumstance can be determined.2) The effects of thermal protection material, coolant and working environment on the transpiration cooling with liquid phase change are numerically investigated. The optimizations from the choice in coolant, and the thermal protection material and structure such as thermal conductivity, porosity and particle diameter, are discussed with the ultimate targets of high cooling efficiency and low driving force. The estimation of coolant consumption and driving force are important in the practical application of transpiration cooling, and the analysis of thermal protection material and structure can provide valuable experience to the optimized design of cooling system in the future.