Investigation On Overall Thermal Protection Characteristics And Structure Optimization On Laminated Cooling Configuration With Thermal Barrier Coating

With the development of aircraft engines,a single thermal protection technology is no longer sufficient to cope with the harsh high-temperature environment of turbine hot-end components.The next generation of thermal protection systems will require the coordinated development of passive insulation and advanced active cooling technologies.Hence the combination of laminated cooling configuration and thermal barrier coating(TBC)will be a key technology for extending the service life of turbine hot components.Unfortunately,the lack of understanding of the interaction mechanisms between these two thermal protection methods makes it difficult to achieve a "1+1>2" overall thermal protection effect.Therefore,it is necessary to carefully discuss the efficient coupling methods of TBC passive insulation and active laminated cooling technology,clarify the matching criteria between laminated structure parameters and TBC design parameters,and provide necessary theoretical support for the efficient integrated design of laminate/TBC coupling structure.Firstly,based on the similarity criterion of hot-side material Biot number,this work establishes a coupled heat transfer experimental platform with TBC passive insulation and active airfoil cooling,which verifies the efficient thermal protection performance of laminated/TBC coupling structure and reveals the design optimization direction through sensitivity analysis.Secondly,in response to the problem of high TBC surface temperature in the coupled structure,this work proposes a design approach of using TBC spraying to form trench and crater near the film hole,and uses a comprehensive research method combining measurement experiments of heat transfer coefficient and adiabatic film effectiveness with numerical simulation to analyze the complex interactions between key structural parameters of trenched-holes and crater-holes.Finally,the multi-objective optimization algorithm is used to provide the optimal geometric parameters of trenched-holes and crater-holes based on laminate/TBC coupling structure under twelve typical working conditions.The specific work and conclusions are as follows:(1)The overall thermal protection characteristics of the laminate/TBC coupling structure under different wall curvature are studied by coupling heat transfer experiments and simulations in the present work.Besides,the sensitivity of the overall cooling performance of the coupled structure to the internal layout of the laminated cooling configuration,TBC thickness,cooling air amount and the wall curvature were analyzed in detail.The results show that the combination of laminated cooling technology and TBC can significantly reduce the metal wall temperature,however the dominated internal intensive convection cooling was isolated to cause an increase of exterior surface temperature of entire cooling scheme,resulting in a higher temperature on outer TBC surface.Relative to the coolant amount and TBC thickness,the sensitivities of cooling effectiveness with internal layout were non-significant.In addition,wall curvature mainly affects the external film cooling of the laminate/TBC coupled structure and has almost no effect on internal heat transfer.Among flat,convex,and concave models,the concave model has the worst overall cooling performance.(2)In view of the problems that the high external surface temperature of laminate/TBC coupled structure,the further design of the external film configuration in such coupled cooling structure is of great necessity.Forming trenched-hole by spraying TBC is one of the strategies to solve the aforementioned problem.Given that the thin film plate and small length-to-diameter of film hole for laminated configuration,the film cooling effectiveness and heat transfer coefficient of trenched-hole with varied length-to-diameter are investigated experimentally and numerically.The combined influences of hole-length,trench-depth,compound angle and cooling air amount on the film effectiveness and heat transfer features of trenched-holes are discussed deeply.The results reveal the various jet-mechanisms and heat transfer characteristics for various trenched-hole and point out that the combined influences of various parameters could result in inadequate thermal protection for short hole jets under the constraint of thin walls.To effectively improve film coverage and reduce the heat flux from hot mainstream to the solid,the short-holes are properly embedded in the deeper trench,and the sensitivity analysis of trench-depth to discharge coefficient showed the negligible variation below 5%.Besides,the application of compound angle can improve the film effectiveness of the short trenched-holes and also reduce the heat flux in case of the shallowest trench.(3)Forming the locally crater-hole by TBC spraying is another way to reduce the external surface temperature of the laminate/TBC coupled structure.Through fluidsolid coupling heat transfer simulation,the influence of crater configuration on overall thermal protection characteristics of laminate/TBC coupled structure is investigated.The results show that the lateral expansion of crater can significantly improve the overall cooling effectiveness and reduce the wall heat flux.As the width of crater width increases,the overall cooling performance of the coupled structure usually first increases and then decreases,indicating the existence of an optimal width for crater.Moreover,the optimal crater width will increase with TBC thickness increase.In addition,a larger width of crater width can lead to a decrease in TBC surface temperature with the increase of TBC thickness.(4)The method of multi-objective optimization algorithm combined with the fluid-solid coupling heat transfer simulation is used to realize the optimization design of the laminate/TBC coupled structure.Based on the plat and concave laminate/TBC coupled structure,the multi-objective optimization of the crater width and compound angle of film hole were conducted respectively under the design of the crater-hole and trenched-hole.The results intuitively provide the optimal geometric parameters under twelve typical operating conditions.Compared with the original model,the optimized model can improve the overall cooling effectiveness of both metal and TBC surfaces simultaneously,the maximum increments are reached 12%and 32%respectively.