ICOPE-15-C143 Unsteady Study on the Performance in a Low-pressure Turbine Stages with Tip Clearance under Design and Off-design Load Conditions

Abstract

Previous research has shown that tip clearance in a low-pressure turbine stage is of practical significance in evaluating the overall loss and off-design condition always occur due to the complex operating conditions. Both of the vortices and operation under low load condition generate strong unsteadiness and oscillating, which may give rise to unreliability of steam turbine. This research mainly focus on investigating the unsteady performance of last two stages with tip clearance in a 100MW industrial steam turbine under design and off-design load conditions. An unsteady numerical simulation was conducted to predict the transient flow properties in the flow field, in which the K-w SST turbulence model has been used. This paper gives the characteristics curve, then the design point and low volume flow conditions were respectively focused on. The scrolled tip leakage vortex and its interaction with the mainstream were analyzed. Correspondingly, the aerodynamic losses caused by the flow near the wall were also shown. Comparison of the transient results under different volume flow conditions was made. The results show that the variation of the leakage vortex varies with time and different volume flow conditions. The structure, intensity and trajectory of the leakage vortexes and other vortexes were captured. The extending of tip leakage vortex stretches and shrinks with a regular periodic process for the leakage flow at tip clearance. Under design condition, the interaction of tip leakage flow with the mainstream was strong. And under low volume flow condition, a reverse flow region was shown and the unsteadiness was stronger because of the presence of many vortexes. The vortexes influence the flow patterns in stages, and the flow patterns give rise to the corresponding aerodynamic characteristics, since the blade load may show the influence of the tip clearance. The conclusions drawn by this paper may provide a reference for the further low-pressure stage blade design of 100MW industrial steam turbine. By numerical study in this paper, the mechanism and consequence of flow with leakage flows were understood more deeply under design and off-design load conditions.