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.

Influence of the Tip Clearance on the Aeroelastic Characteristics of a Last Stage Steam Turbine

“…Compared with the no tip gap model, an 8.4-mm tip gap increased the mass flow rate by 0.6% and decreased the total to static isentropic efficiency by 1.9% at the studied working point. According to previous research [8], the tip clearance flow can have a more dominant impact on the flow field near the blade tip region. The influence of tip clearance flow on both blade loading and the aeroelastic stability is presented in the following sections, and the underneath mechanism of those variations will be presented in Section 5.…”

“…The energy method uses a one-way fluid-structure interaction calculation, in which the blade is vibrating at the selected traveling wave mode and the aerodynamic work done on the blade surface W aero by fluid perturbations is integrated. Validation of the energy method and the 3D computational fluid dynamics (CFD) simulation setups applied in this paper have been presented in [8,11,12] by comparison with the result from the influence coefficient method and other CFD codes.…”

“…However, only one layer of mesh was located inside the tip gap, which may not be able to resolve the tip clearance vortex structure. Previous study on the KTH Steam Turbine Flutter Test Case [8] showed that a tip gap equal to 1.25% of chord length increased the blade aeroelastic stability at the least stable IBPA. The stabilization effects were mainly produced by the interaction between the tip clearance vortex and the shock generated on the trailing edge of the neighbor blade.…”

“…The blade material was assumed to be 17PH4 high strength steel in the modal analysis, the properties of which are shown in Table 1. The first blade-dominated bending mode of the KTH Steam Turbine Flutter Test Case was applied in the flutter analysis, as shown in Figure 2, which has been verified to be aeroelastically unstable at this working point [8]. The modal frequency of the first flap mode was 92.953 Hz, and therefore, the corresponded reduced frequency was about 0.2.…”

“…The combination of these effects can lead to the non-monotonic relationship between the tip gap height and the aerodynamic damping at the least stable IBPA. However, although the impact on flutter characteristics due to shock oscillation can be significant in the blade with transonic tip speeds [6,8], the sensitivity of tip gap height on the aeroelastic stability of realistic-scale steam turbines has not been widely studied. To reveal the impact of tip clearance on steam turbine flutter characteristics and the physics behind the differences observed, the KTH Steam Turbine Flutter Test Case with tip gap heights varied from 0-5% chord length is analyzed in this paper.…”