| With the improvement on speed and cruising time of the hypersonic vehicles,the thermal environment of the leading edge and other key parts is more and more severe.Therefore,the development of efficient and lightweight active thermal protection technology has become a hot topic.Transpiration cooling using porous material has a great potential in thermal protection of reusable hypersonic vehicles,because it can achieve a good cooling effect with less coolant consumption.Through some experimental and numerical investigations,the feasibility of transpiration cooling has been verified,and the law of coolant flow and heat transfer in the porous matrix has been preliminarily mastered.However,in order to achieve its mature application in engineering,there are still some thorny problems to be studied.For example,the mechanism of instability phenomenon during the transpiration cooling with phase change is unclear,the design of coolant internal channel for transpiration cooling is insufficient,and the structure of transpiration cooling on the leading edge needs to be further optimized.In this dissertation,to solve above problems,a series of wind tunnel experiments and full-filed coupled numerical simulations of transpiration cooling and related cooling technologies are carried out,and the main work is summarized as follows:(1)In order to study the instability phenomenon during the transpiration cooling with phase change,under constant mass flow rate of liquid water,transient experiments by using a porous flat plate are carried out in a subsonic wind tunnel.Through collecting the transient variations of surface temperature and coolant injection pressure,the synchronous periodic fluctuation on temperature and injection pressure are captured,the mechanism of instability phenomenon such as fluctuations is deeply analyzed,and then the influences of mainstream temperature and coolant mass flow rate on cooling response time and fluctuation characteristics are investigated.In addition,a simplified prediction method is proposed to estimate the transient maximum increase of coolant injection pressure,which provides a reference for the pressure design of coolant injection system.(2)In order to eliminate the instability phenomenon during the transpiration cooling with phase change,a novel open natural circulation system is proposed,which can be used as the internal cooling channel of transpiration cooling.In this system,by loading non-uniform heat flux on the pipe wall,the coolant can be driven to circular flow in the loop and the generated vapor can be released with the circulation.Therefore,the loop maximum temperature can be maintained at the coolant boiling point,the fluctuation phenomenon caused by coolant phase change in the porous matrix can be avoided,and the coolant driving and control devices can be reduced to improve the effective loads of vehicles.Through the transient numerical simulations of the natural circulation system starting state,the characteristics of fluid flow and heat transfer are intuitively obtained,the cooling law of the system is discussed,and the influences of heat flux distribution and loop shape on the starting state and starting mode of the circulating flow are revealed.(3)In order to improve the cooling effect near the stagnation region of transpiration cooling on nose cone,a new combined cooling method,transpiration cooling with forward-facing cavity is proposed,which can effectively improve the cooling effect of the whole structure and significantly reduce the coolant injection pressure.By full-field coupled simulations of transpiration cooling with forward-facing cavity on the nose cone,the feasibility and efficiency of the structure are verified,and the influences of mainstream Mach number and coolant injection pressure on the coolant flow characteristics and cooling effect are investigated.Finally,an optimization method of lip passivation is proposed,to further reduce the temperature gradient of the structure. |
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