Influence of the Radial and Axial Gap of the Shroud Cavities on the Flowfield in a 2-Stage Turbine

Abstract

An important goal in the development of turbine bladings is to improve their efficiency for an optimized usage of energy resources. This requires a detailed insight into the complex 3D-flow phenomena in multi-stage turbines. In order to investigate the flow characteristics of modern highly loaded turbine profiles a test rig with a two stage axial turbine has been set up at the Institute of Steam and Gas Turbines, RWTH Aachen University. The test rig is especially designed to investigate the influence of different cavity sizes. In order to analyze the influence of the cavity size on the secondary flow and to discuss the effects of the blade loading, the 3D flow through the 2-stage turbine with shrouded blades is investigated numerically, using the steady Navier-Stokes inhouse computer code, CHT-Flow. The turbine blading is designed to concentrate the mass flow in the middle of the passage in order to keep the main flow away from the secondary flow regions at the endwalls of the blade. The simulations include a comparison of a configuration without cavities (design case) and two configurations, where the axial gap between the shroud and the endwalls is about 5 mm and the radial gap between the shroud and the endwall is varied between 0.8 mm (open radial gap) and radial gaps “near zero” (closed radial gap). The investigations are done with focus on the secondary flow phenomena in the second guide vane. For a detailed analysis of the blade load the design point and an off-design point are simulated for each blading. The flow conditions are taken from experimental investigations performed at the Institute of Steam and Gas Turbines, Aachen University. In the experimental setup, the turbine is operated at a low pressure ratio of 1.4 with an inlet pressure of 3.2·105 Pa. The numerical results will also be compared to the corresponding experimental data at the outlet of the second stage.