Using Profiled Endwalls, Blade Lean and Leading Edge Extensions to Minimize Secondary Flow

Abstract

Methods to reduce secondary flows and losses in turbines have been studied for many years. This paper presents the latest results from a series of projects using profiled end walls, blade lean and modifying the blade profile near the end wall. In addition a review is made of the series of designs and experiments with the aim of improving our understanding of these methods. Three distinct mechanisms are identified, the first reducing the cross passage pressure gradient in the early part of the blade passage by the wall profiling. The second uses reversed compound lean which reduces mid-span losses and forces the secondary flow region closer to the end wall by inducing spanwise pressure gradients. The third uses a leading edge extension, resulting in local sweep at the end wall which reduces the blade loading at the leading edge, and hence the early cross passage pressure gradient. The results are shown not to be additive, and although large reductions in secondary flow can be achieved, these do not always result in reduced losses. Computational based design techniques, backed up by experiment are essential to produce optimal designs.