| With the fast development of hypersonic vehicle technology,one of the most severe challenges faced is increasingly harsh thermal environment,especially in the leading edge region of the vehicles.Therefore,developing more effective thermal protection systems has attracted more and more attention in this field.Due to the huge specific surface area of porous media,transpiration cooling has been proven to have excellent cooling capacity.However,there are still many deficiencies in the investigations on the mechanism of transpiration cooling,especially the transpiration cooling with liquid phase change.This thesis exhibits an in-depth invstigation on the mechanism of transpiration cooling by numerical simulations.The characteristics of the cooling capacity of a gaseous transpiration cooling system,i.e.,the maximum heat flux a gaseous transpiration cooling system can withstand under a certain coolant consumption,is numerically investigated,and it is discovered that the cooling capacity of a transpiration cooling system is joinly restricted by coolant heat absorption capacity and fluid-solid heat exchange capacity.On this basis,the effects of solid thermal conductivity,particle diameter,thickness of porous media on the cooling capacity under different coolant consumption are analyzed.Compared to the gaseous transpiration cooling,liquid transpiration cooling with phase change,due to the huge phase change latent heat and boiling heat transfer coefficient,has much higher cooling capacity.However,its mathematical model is so complicated that the mechanism investigations on the liquid transpiration cooling are very lacking.Through a mathematical equivalent transformation,traditional local thermal non-equilibrium two-phase mixture model is improved and a simpler while equivalent mathematical model is obtained.On this basis,a multi-region numerical strategy is proposed to realize the coupling calculation between the porous region and mainstream region.By numerically simulating liquid transpiration cooling processes within a wedge-shaped porous cone,the characteristics of coolant flow and heat transfer are analyzed.The simulation results reveal a dramatic differences in the coolant flow characteristics between the cases where coolant is vaporized completely and vaporized incompletely in the porous cone.When coolant cannot vaporized completely in the porous cone,a large amount of coolant will flow along the liquid and two-phase regions to the end of the porous cone and then flow out,resulting in a very uneven mass flux distribution on the hot side of the cone,and a huge waste of coolant phase change latent heat The relationship between coolant mass flow rate and injection pressure is "N-shaped",i.e.,when the mass flow rate is very small or very large,they are positively associated,but when the mass flow rate is in the middle section,they are negative associated.Both of the two interesting phenomena are caused by the huge change of coolant kinematic viscosity during phase transition.In order to provide a reference for the design and optimization of liquid transpiration cooling systems,the effects of solid thermal conductivity,particle diameter and porous media thickness on the liquid transpiration cooling performance are numerically investigated.It is indicated that there are significant differences between the effects of these parameters on the liquid and gaseous transpiration cooling,and this is mainly caused by the existence of a very thin two-phase layer,which undertakes the most of fluid-solid heat exchange,in the liquid transpiration cooling.It is difficult to utilize the phase change latent heat fully when liquid coolant is directly injected into a large area.To achieve an effective thermal protection for the large area of a hypersonic vehicle and reduce the coolant consumption as much as possible,a new combined cooling method,which consists of liquid transpiration cooling,gaseous transpiration cooling,convection cooling and film cooling,is proposed. |
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