Numerical Investigations On Cooling Characteristics Of Turbine Blade Endwall And Leading Edge

With the continuous development of aero-engine technology,modern gas turbines are usually operating at the high temperatures exceeding the melting point of turbine blade materials.Therefore,it is important and necessary to develop effective cooling methods,especially for the turbine blade endwall and leading edge,because they are directly exposed to the extreme environment with heavy heat load and aerodynamic force.According to whether the coolant enters into the high-temperature mainstream,the present cooling strategies can be divided into external cooling and internal cooling.The external cooling strategy mainly includes film cooling and laminated cooling,while the internal cooling strategy includes ribbed-channel cooling,impingement cooling and vortex cooling.In recent years,there have been a large number of studies concerned on the heat transfer mechanism,influence factors and structural optimization of the cooling strategies of blade endwall and leading edge.However,there is still a certain gap between the current theoretical turbine cooling investigations and practical applications.For example,in most investigations,the simplified geometrical models or boundary conditions were usually used to substitute the real complicated turbine structures or running conditions,respectively,which could cause non-ignorable errors on the predictions of cooling effectiveness.In addition,it is still a major problem to get a high cooling performance without increasing the coolant consumption.In this dissertation,validated numerical calculations by conjugate heat transfer algorithm are used to obtain the heat transfer and flow characteristics of the realistic cooling structures in blade endwall and leading edge.The main work is summarized as follows:(1)The influence of flat endwall simplification on film cooling effectiveness is discussed.Through the comparison of film cooling effectiveness between a real annular turbine endwall and simplified flat endwall,it can be concluded that the influences of flat endwall simplification on film cooling effectiveness are different under different conditions.Under the real running conditions with high mainstream Reynolds number,the "Matthew effect" is exhibited on the annular endwall,i.e.,better overall cooling performance appears in good film-covered region,but worse overall cooling performance appears in poor film-covered region.Therefore,the temperature gradient in annular endwall is much higher.With other words,using flat endwall simplification may cause a high risk of endwall lifespan.(2)The feasibility and accuracy of the mass transfer analogy experiments in turbine endwall film cooling are analyzed.Through theoretical analyses,the turbulent heat and mass diffusive coefficients are compared.Then the heat and mass transfer simulations on the film effectiveness of turbine endwall are conducted,using air and CO2 as coolant,respectively.It can be concluded that the feasibility and reliability of the analogy experiments are closely related to the local turbulent characteristics.Only when the values of turbulent heat and mass diffusive coefficients are close,and the turbulent transportation dominates in the whole field of heat or mass transfer,the experimental results obtained by the analogy are reliable.For a real turbine endwall,the difference of film effectiveness between the heat and mass simulations is large in the upstream laminar region,but small in the mid-passage with turbulence transportation.In the downstream of the endwall,the film effectiveness by mass transfer data is lower than that by real heat transfer.(3)A novel conception of multistage vortex cooling configuration is introduced,aiming to improve the cooling performances of the leading edges without increasing the coolant consumption.The numerical simulations are carried out to study the flow and heat transfer characteristics of the novel cooling configuration,and the results show that compared to the traditional single stage model,the multistage cooling strategy can provide a better cooling performance,a more uniform heat transfer rate.Although the total pressure loss becomes higher,the thermal performance of the multistage model is better.(4)The flow and heat transfer characteristics in the double wall cooling structure of blade leading edge are investigated.Three different total pressure ratio of coolant to mainstream are included in the numerical simulations,and from the results,a unfavorable phenomenon is observed,i.e.,high-temperature-mainstream reversely flows back to the film holes.In addition,the reasons of this phenomenon are revealed,a series of improvements are proposed,and the cooling effectiveness on the target wall is successfully improved(5)Under the real conditions of non-uniform mainstream inlet temperature(hot streak),the performances of film cooling and double wall cooling in the blade leading edge are numerically investigated.The results show that,a local high temperature region can be observed in the leading edge surface caused by hot streak.Compared to the film cooling,double wall cooling structure shows a better cooling ability and a lower thermal stress on the vane surface,but the problem of the extremely high thermal stress within the pin-fins should be carefully considered.