Unsteady flow mechanisms in the last stage of a transonic low pressure steam turbine—multistage effects and tip leakage flows

Ilias Papagiannis,Altug Basol,Anestis Kalfas,Asad Raheem,Shigeki Senoo,Hisataka Fukushima,Reza Abhari

doi:10.22261/f4iw8s

Abstract

AbstractIn this paper, an unsteady investigation of the last two stages of a low-pressure steam turbine with supersonic airfoils near the tip of the last stage’s rotor blade is presented. Goal is the investigation of multistage effects and tip leakage flow in the last stage of the turbine and to provide insight on the stator-rotor flow interaction in the presence of a bow-shock wave. This study is unique in a sense of combining experimental data for code validation and comparison with a numerical simulation of the last two stages of a real steam turbine, including tip-cavity paths and seals, steam modelling and experimental data used as inlet and outlet boundary conditions. Analysis of results shows high unsteadiness close to the tip of the last stage, due to the presence of a bow-shock wave upstream of the rotor blade leading edge and its interaction with the upstream stator blades, but no boundary layer separation on stator is detected at any instant in time. The intensity of the shock wave is weakest, when the axial distance of the rotor leading edge from the upstream stator trailing edge is largest, since it has more space available to weaken. However, a phase shift between the maximum values of static pressure along the suction side of the stator blade is identified, due to the shock wave moving with the rotor blades. Additionally, the bow-shock wave interacts with the blade shroud and the tip leakage flow. Despite the interaction with the incoming flow, the total tip leakage mass flow ingested in the tip-cavity shows a steady behaviour with extremely low fluctuations in time. Finally, traces of upstream stage’s leakage flow have been identified in the last stage, contributing to entropy generation in inlet and outlet of last stage’s stator blade, highlighting the importance of performing multistage simulations.

Highlights

Steam turbines currently hold the largest share in the energy market, which highlights the importance of having robust and efficient machines
Recent experimental measurements conducted in a low-pressure test facility of Mitsubishi Hitachi Power Systems, Ltd (MHPS) (Bosdas et al, 2016), confirm the presence of high unsteadiness in flow due to interaction of the travelling bow shock wave with the upstream stator but found no clear evidence of a boundary layer separation
An understanding is required of the unsteady interaction of the bow shock wave with its surroundings, i.e., the upstream stator blade row as well as the tip-cavity path of L-0 rotor and the leakage mass flow ingested in it

Summary

Introduction

Steam turbines currently hold the largest share in the energy market, which highlights the importance of having robust and efficient machines. The design is quite challenging due to the fact that inlet flow conditions close to tip are supersonic in relative frame of reference This leads to a generation of a bow shock wave upstream of the rotor’s leading edge (Senoo, 2012) that, if not given sufficient space to decay, will interact with the upstream stator blade row, causing high flow unsteadiness or even potential boundary layer separation on the stator and increased kinetic energy losses (Senoo and Ono, 2013). The goal of the current work is to investigate multistage effects and tip leakage flow in the last stage of the steam turbine Before analysing these topics, an understanding is required of the unsteady interaction of the bow shock wave with its surroundings, i.e., the upstream stator blade row as well as the tip-cavity path of L-0 rotor and the leakage mass flow ingested in it

Objectives

Methods

Results