Tip Leakage Losses in Subsonic and Transonic Blade-Rows

In this paper the effect of blade-exit Mach number on unshrouded turbine tip-leakage flows is investigated. Previously published experimental data of a high-pressure turbine blade are used to validate a CFD code, which is then used to study the tip-leakage flow at blade-exit Mach numbers from 0.6 to 1.4. Three-dimensional calculations are performed of a flat-tip and a cavity-tip blade. Two-dimensional calculations are also performed to show the effect of various squealer-tip geometries on an idealized tip-flow. The results show that as the blade-exit Mach number is increased the tip leakage flow becomes choked. Therefore the tip-leakage flow becomes independent of the pressure difference across the tip and hence the blade-loading. Thus the effect of the tip-leakage flow on overall blade loss reduces at blade-exit Mach numbers greater than 1.0. The results suggest that for transonic blade-rows it should be possible to raise blade loading within the tip region without increasing tip-leakage loss.