Research On The Mechanism Of Platelet Heat-pipe-cooled Leading Edge Of Hypersonic Vehicle

When flying at a hypersonic speed,the leading edge of the vehicle will encounter severe aerodynamic heating environment,which needs thermal protection system to guarantee its safety of flight.Conventional thermal protection systems usually use the ablative coating method,which absorbs heat by endothermic transformation,namely phase or chemical change to the polymeric coating,and then the heat is dissipated as the single-use coating eventually vaporizes,which also bring change of aerodynamic shape of the leading edge.The heat-pipe-cooled thermal protection system,which is based on the working fluid's evaporation,flow and condensation,can absorb the aerodynamic heating from the stagnation area and transport it through the flow of the vapor in the pipe caused by the pressure difference to the back area,which is then released through the condensation of the vapor,conducted through the wick and wall of heat pipe,and eventually radiated to the air.For the heat-pipe-cooled thermal protection system don't change the aerodynamic shape of the leading edge,it can be used for long periods of time.The work of this thesis is about the mechanism of platelet heat-pipe-cooled leading edge.The numerical calculation module of the aerodynamic heating of the hypersonic vehicle leading edge is firstly established.Comparing with the wall heat flux of open experiment,the numerical calculation model is proved to be good accuracy,but the adopting of the wall as a cold temperature condition also induces the overestimation of the aerodynamic heating of the leading edge.In order to well solve this problem,based on the principle of the thermal balance,the engineering calculation method is then proposed,with an isothermal high temperature wall as calculation condition but not a cold wall temperature,which brings more reasonable heat flux results for the further mechanism analysis of the heat pipe.To investigate the flow and heat transfer of the heat-pipe-cooled thermal protection system,a numerical model based on the principles of working fluid's flow and phase change is proposed and the calculation result is compared with an open experiment to make sure the numerical method's accuracy.Using the numerical method,the temperature of the solid/liquid/vapor fields,the pressure and the flow of the fluid are acquired.Comparing with the non-cooling designed leading edge,the heat-pipe-cooled one shows the dredging of the aerodynamic heating from the head stagnation small area to the back flat large area,resulting the decrease of the temperature of the stagnation area and then achieve the thermal protection of the leading edge.According the heat condition,materials and fluid's properties,the heat transfer limits of the platelet heat-pipe are also studied for the different geometry factor designs.Besides the high temperature induced by the aerodynamic heating,the materials of the leading edge must can endure the high thermal stress leaded by the high temperature grades and the mechanical stress leaded by the high vapor pressure inside of the pipes.In order to study these problems,the platelet heat-pipe structure without operation on the ground condition is firstly investigated numerically.Then using the effective conductivity coefficient of the vapor core,a seven heat-pipes constructed leading edge with different wall materials and working fluids is investigated numerically,and its temperature and stress fields show its better thermal-structural characters than the wedge-shaped heat-pipe-cooled leading edge.An engineering quick calculation method is also proposed here to compute the thermal-structural properties of platelet heat-pipe structure,and its comparison with the numerical results shows its approximately accuracy.In order to investigate the thermal-structural character furthermore,the fail case and burn out of the outer wall case of a single heat pipe is studied respectively,and the effect of different materials and working fluids is discussed.And then a dual level platelet heat-pipe cooled leading edge structure is proposed,and the numerical calculation show its good thermal protection character for a single heat pipe fail and burn out case.To study the capillary character of wick,the vapor-liquid interface shape and its capillary of rectangular grooved wick and sphere wick are computed respectively using a numerical method,and its comparison with open experiments shows its good accuracy.Using this numerical method,the effect of different geometry designs and different liquid filling levels are discussed for the two sorts of wicks.The wicking rate is also investigated using a theoretical model,and the effect of geometry designs for the wicks is also studied.To study the evaporation heat transfer character of the vapor-liquid interface of the two sort of micro wick structures,a numerical model is proposed and its feasibility is verified by comparing with an open theoretical results.The evaporation characters are then discussed under different super heat,saturation temperature,solid/liquid contact angle,liquid filling level,materials,working fluids and other geometry design parameters.The further study show the rectangular grooved wick's better phase change heat transfer character than the sphere wick under the same porosity and filling level for the high temperature alkali metal heat pipe.To verify the accuracy of the numerical simuliation using the effective conductivity coefficient of the vapor core,and the effection of the heat-pipe cooled leading edge,platelet flat heat pipe and leading edge structure are designed respectively.The plat heat pipe,which is heated using local uniform heat source,shows nearly isothermal operating temperatures at the steady state and good agreement with the numical results.The heat pipe cooled leading edge,heated by a halogen quartz lamp,present temperature inflection points during the experiments,which means the end of the start procedure.And the leading edge experiments also show the good isothermal operating temperatures,with a sharp temperaure fall at the tip region and a substantial rise at the back area.Besides,an impingement cooling is used at the condensation region of the heat pipe cooled leading strcture,and the results shows a further temperature drop of the whole surface.