Numerical Research On Ablation Problem For Gas Turbine Cascade Of Aero-engine

With the rapid development of modern aero engines, the turbine inlet temperature will be further increased in order to improve the thrust-weight ratio and cycle efficiency. Working in the long-lasting high temperature environment, a certain range of ablation phenomenon would more or less takes place on the turbine cascades, seriously affecting their life and reliability. Considering the difficulty of experiment, it is hard to assess the influence of ablation on the engine operating conditions, thus it is critical to apply numerical simulation tool to study the turbine cascade ablation problem for the sake of research on the ablation characteristics of blade so as to effectively prevent the engine failure caused by the blade ablation, which is of great significance to realize the reliable and safe operation of aircraft engine and gas turbine.Centering on the core issue of how to accurately predict the ablation problem of turbine cascades and other hot sections, meshfree smoothed particle hydrodynamic method is applied to perform numerical simulation in this paper.The existing meshfree SPH program framework of our research group is developed further where turbulence, transition, gas-solid two phase flow and ablation modules are added, moreover, the flow/solid/heat couple and boundary condition modules are also improved based on the meshfree and material cloud theory together with object-oriented programming under the guidance of Professor Zhi-bo Ma. With respect to the complex flow, heat transfer and ablation phenomenon within the turbine of aero engine, an aero-thermodynamic ablation physical model of turbine blade applicable to internal flow is proposed on the basis of ablation model for the reentry of aircraft by introducing boundary layer thickness to substitute the thermal boundary layer boundary condition of turbine blade for the free stream boundary condition. For the improvement aspect of meshfree smoothed particle hydrodynamic method, the following three parts are proposed based on the latest international research work:(1).The selection guideline for the optimal initial smoothing length of kernel function is investigated based on wavelet analysis and multi-scale reproducing kernel function theory, moreover, the adaptive rule of smoothing length during calculation is also discussed. (2).Based on approximate Riemann solver, adaptive artificial viscosity according to local flow field without artificial parameter is constructed as an improvement for the traditional Monaghan artificial viscosity, on this basis, by analogy, dissipative term in the discrete form of NS thermal energy equation is also constructed to avoid wall heating problem. (3).Based on the research work in literature about the discrete SPH conservative scheme of NS momentum equation, discrete schemes of diffusion term in mass and heat transfer equations are unified which are analogous to the viscous term in NS momentum equation.On the basis of this program, numerical research is performed on a more complex high pressure and inner-cooling turbine blade.Simulation results show that:(1).Small scale ablation mainly occurs in the suction and pressure side of blade, resulting in the small loss of material, which is negligible considering its influence on the shape change of blade, and only increases surface roughness compared with the original turbine blade; Large scale ablation emerges around the leading and trailing edge of the blade, where considerable loss of material and even block missing phenomenon occurs, thus leading to significant change of the turbine blade shape.(2).Due to centrifugal force and Coriolis force, compared with stator, certain difference exists as to the heat transfer and ablation characteristics of rotor blade surface.Then the research object is extended further to the turbine cascade environment, the ablation characteristics of turbine blades under rotor stator interaction are studied. Results show that:(1).Under different conditions of the rotor rotating speed, the circumferential pressure difference of stator passage varies so as to change the propagation of horseshoe vortex of the pressure surface branch, which affects the heat transfer and ablation characteristics of the pressure surface of stator. (2). In the case of medium rotating speed, the leading edge of the rotor blade endures less ablation; In the case of high rotating speed, the flow field changes intensively due to the relative motion of rotor and stator, thus, the passage vortex and horseshoe vortex before the leading edge of the rotor blade grows by absorbing more energy from the large deformative flow field, strengthening heat transfer of the leading edge of the rotor and consequently increasing the ablation degree; In the case of low rotating speed, the continuous interaction of the trailing edge shock from stator also results in relatively serious ablation on the leading edge of the rotor blade.Finally, the ablation characteristics of the turbine blade under the conditions of different main flow parameters are investigated, also the following research to be carried out about the ablation problem is pointed out.