| The temperature at the turbine inlet is rising to improve the engine thrust and the cycle efficiency. Such temperature has exceeded the thermal yield limit of the blade material, thus an effective cooling system is needed to maintain the engine operation. Accurately predicting the blade thermal load becomes rather essential to improve the cooling efficency of cooling system and to extend the blade operating life. Nowadays coupled heat transfer (CHT) technique has been widely applied to predict the blade thermal field. Otherwise there still are few investigations of numerical methods influencing on the CHT results. The purpose of this study is to improve the accuracy and reliability of CHT simulatioins, hence the effect of several numerical methods, including coolant diffusion, computational grids, coupling method, laminar-to-turbulent transition, closure of time-averaged energy equation and so on, on the blade thermal load prediction are investigated. And the server is HIT-3D.Firstly the influences of coolant diffusion on the aerodynamic and thermal parameter distribution are investigated. Coolant mixing exists in the passages of film cooled turbines. Since the components of the cooling air and the gas are different, the coolant mixing would result in the variation of gas component in the passage, and then the thermal property of gas. A two-stage film cooled turbine is selected as the test case, and the aerodynamic simulations for gas with variation components and constant component are carried out. The comparision between the numerical results reveals that the simulation for the gas with variation components predicts different flow structure and shock wave postion compared with that for the constant component gas. Hence the component variation induced by coolant mixing should be taken account of in simulations of the film cooled turbines.Secondly the influences of computational grids on the CHT simulation results are investigated. Physically the CHT simulation, relating to the data transimission between fluid and solid domains, is a non-linear process. Furthermore, such non-linarility is strengthened by the coupling of velocity and thermal boundary layers near the fluid/solid interface in the fluid domain. And the iteration could be affected by the orthogority of computational grids. The H-type and H-O-H-type grids are employed to discretize the turbine passage domain, and the No. 5411 case of NASA-MARKâ…¡vane is served as the test case. The CHT results show that the simulation with O-type grids near the fluid/solid interface in the fluid domain normally converges, while that with H-Type grids diverges.Thirdly the influences of coupling methods on the stability and convergence of CHT simulations are studied. There are two kinds of the methods, the indirect coupling method and the direct coupling method. With the same test case as that mentioned above, CHT simulations with different coupling methods are carried out. And the numerical results show that the simulation by the indirect coupling method is with much instability, but that by the direct coupling mthod is rather stable and it converges quickly.Fourthly the effects of laminar-to-turbulent transition on the thermal prediction of CHT simulations are investigated. Boundary layer transition is rather common in the boundary layer flows on the low-pressure turbine surface. With three different operating conditions of NASA-MARKâ…¡vane as test cases, CHT simulations by several full turbulence models and transition models are carried out. The numerical results are compared with the measured ones. It shows that the heat transfer is strongly affected by the laminar-to-turbulent transition in the boundary layer flow, and that the transition flow also exists on the cooling air channel walls when the cooling air mass flow is low. The transition predition is quite an essential factor that affects the accuracy of CHT simulations. The full turbulence models are not able to predict the transition onset and the length of transition zone, thus the simulations with such models over predict the vane thermal load. The transition models are able to predict the transition pocess, thus the simulations with such models predict thermal load much close to the measured ones. The algebraic transition model negelets the intermittency transportation along the normal direction to the wall, and it leads to lower temperature than the measured one at several nodes. The intermittency transportation equation, able to predict intermittency distribution in the complex and 3-D flow field, needs much finer grids than the algebraic one, and it costs more computational resourses. The comparision between the results by HIT-3D and those by CFX10 with Gama-Theta transition model proves the ability of HIT-3D in CHT simulations. The developed solver HIT-3D would predict results closer to the measured ones than CFX10 in the blade surface except the shock-induced transition zone.Fifthly the effect of time-averaged energy equation closure on the CHT simulation is studied. The Reynolds analogy is a widely applied method for closing the time-averaged energy equation in simulations of inner flows. With such method, the constant turbulent Pradtl number is always employed. Otherwise the thermal boundary layer, similar to the velocity boundary layer, could be divided into several layers. Thus the turbulent Pradtl number is not constant. An algebraic correlation is utilized to coupute the turbulent Prandtl number in the thermal boundary layer. The CHT simulation with such algebraic correlation shows that the turbulent Prandle number vanishes along the outer normal direction to the wall. The predicted profile temperature is compared with that by the simulation with constant turbulent Pradtle number, but there is slignt difference between the predicted profile temperature distributions.Finally the CHT simulatons of a low-pressure turbine in pratical operating conditions are carried out. And the effects of numerical methods on the thermal prediction of CHT simulations are discussed. Compared with the adiabatic results, the vane profile temperature with single cooling air channel is largely reduced, and the highest temperature in the vane surface exists at the leading and trailing edges. For the vane with two air cooling channels and a slot at the trailing edge, the traling edge is effectively cooled, but the cooling efficiency is affected by the flow in the cooling air channel. There is few even none cooling air at the upstream of vane trailing edge, and most of the cooling air flow towards the downstream of the vane. Taking account of the coolant diffusion effect, the gas constant changes slightly during the whole passage, but the constant-pressure specific heat is lower than that of consant component gas in the regions with more cooling air. The comparision between the predicted profile pressure of the CHT simulations for the gases with variable components and constant component is carried out, and little difference is found out. There is also comparision between the predicted profile temperature distributions, and the largest difference exists near the trailing edge in the pressure side.
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