Reynolds stress measurements in a turbine tip leakage vortex

Abstract

An experimental study of the structure of turbulence in a tip leakage vortex was conducted in a large scale, linear turbine cascade. The study was performed in a cross sectional plane 0.96 of the axial chord length downstream of the blade leading edge. The cascade consisted of five blades with an aspect ratio of one and an axial chord of 235.2 mm. The tip clearance gap was nominally 2.1% of the blade span.

A specially designed rotatable x-wire hot wire probe with high spatial resolution was used to measure both the mean velocity distribution and all six components of the Reynolds stress tensor. An automated probe positioning and digital signal processing system was developed to facilitate quick, accurate measurements at a large number of data points. A geared stepper-motor drive mechanism was used to rotate the endflow hot-wire probe about its axis. Performance of the probe and the data analysis routines was verified by measurements in a fully developed pipe flow.

The contribution of turbulence to the over a1l 1osses was investigated by studying both the general level of turbulence and the work done on the fluid by the turbulence stresses. This allowed the study of the physical mechanisms of loss production; spanwise mixing processes were found to be major contributors. The high level of turbulence in the tip leakage vortex was found to amount to about 36% of the total pressure loss generated by the plane near the blade trailing edge.