Thermal-flow-elastic Coupling Numerical Simulations Based On Unstructured Mesh In Air-cooled Turbine

The development of aviation industry requires better performance of turbine engines, mainly including two important parameters: efficiency and thrust-weight ratio. Since the inlet temperature plays a critical role in turbine efficiency, the temperature needs to be increased in order to elevate turbine efficiency. However, the increase of inlet temperature requires effective thermal protection method. Also, the accurate prediction of temperature of air-cooled turbine blade becomes more important to the optimization of cooling structure and prediction of safety and stability of blade. Along with the progress of computer science, the conjugate heat transfer simulation (CHT) has become an important tool to predict blade temperature. Due to the contact between high temperature gas and coolant, great temperature gradient would exist in blade. The uneven temperature distribution and thermal expansion under constraint will produce large thermal stress and deformation, which will lower the life span of blade, even damage blade. Thus, to ensure safety and stability of blade, it is necessary to take into account of thermal-elastic coupling simulation. The main task of this paper includes the development of a full three-dimension (3D) thermal-flow-elastic coupling self-programming code based on unstructured mesh, the application of this code in heat transfer and strength analysis in air-cooled turbine, and the investigation on high order accuracy discrete method and multiphysical fields coupling method.First, the finite volume method (FVM) discrete format of3D Navier-Stokes (NS) equations is derived. The MUSUL method is introduced. The least-squares method is used to compute gradients. Venkatakrishnan limiter is used to guarantee stability. For the negative value of linear weight of high order WENO on unstructured meshes, a technique of solving optimal linear weight is presented; and detailed mathematical model is established. Through front step flow and double Mach reflection cases, the code’s stability and sensibility of processing discontinuities was verified. The detailed progress of preconditioning and AUSM+format for solving convective terms were presented. The detailed implicit LU-SGS method based on preconditioning was derived. Through cavity flow, inviscid bump flow and plate flow, the code’s accuracy of calculating convective flux and viscid flux was verified. T3A、T3A-and S&K cases were used to verify transition model. For negative value of high-order WENO linear weight on two-dimension (2D) unstructured mesh, a solving method of optimal linear weight was proposed; and detailed solving mathematical model was established. Through front-ward step flow and double Mach reflection cases, the sensitiveness and stability of the code to solve discontinuity problem were verified.Second, solving method of solid field was investigated; and FVM was used to solve heat conduction equation. The full implicit solving method was used, which has relatively high calculation efficiency. The weighted least-squares method was used to construct a high order accuracy solving method of gradient. Analytic solutions were used to verify that the effective improvement of the heat transfer calculation. High order accuracy FEM was used to solve thermal-elastic problems. The detailed process and solving method of establishing FEM discrete equation were introduced. The code was verified by analytic solutions of thermal stress produced by thermal expansion of a finite length cylinder. The result shows that the general accuracy of the FEM code is very high. Under given accurate temperature solution, the calculated displacement agrees well with the analytic solution; and the error is around1%. The calculated axial, circumferential, and radial stress are in good agreement with analytic solutions. And a low-pressure turbine guide vane was used as a case to compare the calculated temperature, thermal deformation and thermal stress with the result of a commercial program ACE. The accuracy and adaptation of complex model of this code was verified.Finally, a thermal-flow-elastic coupling simulation code, which is used to solve multiphysical fields, was developed by different coupling methods. The CHT part is bidirectional coupling. The thermal-elastic part is undirectional coupling. The area-weighted interpolation method was used on the data transfer at interface. Through comparison with given accurate solutions, the accuracy of the interpolation code was verified. CHT simulation was conducted on C3X4521case; and the result was compared with heat transfer experimental data. The CHT simulation accuracy was verified; and the calculated heat transfer coefficient agrees well with experimental data. From MARKⅡ4311and5411operating cases, the effect of transition on heat transfer was investigated. From the comparison, it can be noted that the transition model can improve the heat transfer simulation accuracy in laminar region and transition region. The effect of transition on pressure is slight. For the region where shock interacts with boundary layer, the heat transfer simulation is still not accurate. Under the basis of an existing code HIT-3D, the effect of CHT and transition model on temperature calculation was verified. The result shows the wall temperature of CHT is30%lower than that of adiabatic wall. The error of temperature in transition region between BL and experimental data is10%. The error between calculated temperature of q-ω model、BL+AGS transition model and SST-Gama model and experimental data is5%. The calculation accuracy is relatively high. The thermal-elastic analysis of heat transfer results of MARKⅡ vane and a low-pressure turbine vane was conducted. It proves that transition model can help verify the safety and stability of turbine blade more accurately. Last, thermal-flow-elastic multiphysical coupling simulation of a low-pressure turbine vane was conducted by using the thermal-flow-elastic coupling platform. The transition flow and heat transfer characteristics were studied; and the high temperature partial zone and largest stress concentration position were analyzed. These give help of remodeling cooling structure. The accuracy of the result of the code was verified by comparing with commercial code. Although the supernormal stress phenomenon induced by over constraints exists, the stress distribution trend can still be used to help remodel and optimize blade profile.