A Safety Design Evaluation on Transient Thermal Stress in Turbine Disk

Abstract

In the safe mode design stage of aero-engine, the whole engine safety can be improved by enhancing the safety of engine life-limited parts (ELLP). As one of ELLP, turbine disk works in thermal environments characterized by complexity and transient. This paper presents the analytical solutions of the transient temperature, thermally induced stress and equivalent stress distributions in a hollow disk rotating at a speed of 4000 r/min and subjected to complicated thermal boundary conditions. The outer surface of the disk is kept at a constant temperature, and the windward side is exposed to a forced convention to the cooling air. The inner surface is insulated until t = t_Qi after which the inner surface is uniformly heated by extra thermal loading Qi. The theoretical derivation of the governing energy equation with nonhomogeneous boundary and initial conditions is obtained by using the method of separation of variable (in other words, the eigenfunction method) for space variables and the method of Laplace transforms for the time variable. The transient calculations are performed individually for various start times of the thermal loading imposed on the inner surface, t_Qi = 0, 40, 100 and 120 s, as well as various values of the thermal loading, Qi = 60, 100 and 140W, until the system attains thermal equilibrium state. From the perspective of the safety design, the influences of the start time and the value of the thermal loading Qi on the safety margin of a disk are evaluated respectively. The results of this study clearly demonstrate that the reverse temperature difference at the hub of a disk rises, the value of maximum equivalent stress decreases and the shifting time of the position of maximum equivalent stress delays with advance of the start time t_Qi as well as increasing of the thermal loading on the inner surface Qi. Thus, the safety margin of a turbine disk can be enhanced by using the thermal loading imposed on the inner surface.