Investigation of Tip Clearance Flow Effects on an Open 3D Steam Turbine Flutter Test Case

Abstract

Blade failure caused by flutter is a major problem in the last stage of modern steam turbines. It is because rotor at this stage always has a large scale in spanwise, which provides low structural frequency as well as supersonic tip speeds. Since most of the unsteady aerodynamic work is done in the tip region, transonic tip-leakage flow that influences the tip region flow could have a remarkable effect on the aerodynamic stability of rotor blades. However, few research had been done on the tip-leakage flow influence on flutter characteristic based on full-scale steam turbine numerical models. In this paper, an open 3D steam turbine stage model designed by Durham University was applied, which was widely analyzed and representative for the last stage of modern industrial steam turbines. The average Mach number at the rotor outlet is 1.1. URANS simulation carried by both numerical software CFX and LUFT code is applied, and the two solvers show an agreement on steady and unsteady results. The numerical results indicate that the influence of tip leakage flow on blade stability is based on two types of flow mechanisms. Both mechanisms act on the suction side of near tip region. The first type of mechanism is produced by the reduction of passage shock near the leading edge, and the other type of mechanism at the rear of blade is caused by the interaction between tip leakage vortex and trailing edge shock of the neighbor blade. In conclusion, tip leakage flow has a significant influence on steam turbine flutter boundary prediction and requires further analysis in the future.