Loss mechanisms in turbine tip clearance flows

Abstract

Numerical simulations of tip clearance ow have been carried out to dene the loss generation mechanisms associated with tip leakage in unshrouded axial turbines. Mix- ing loss between the leakage, which takes the form of a strong embedded streamwise vortex (u=ux 1 in the vortex core), and the mainstream ow is found to be the main source of loss. Vortex line contraction, and consequent vortex core expansion, and also vortex breakdown, are identied as the two important mechanisms that determine mixing loss. Because of these vortex dynamic features, the behavior is dierent from the conventional view of the effect of pressure level on mixing of non- uniform flows. More specifically, it is shown, through control volume arguments and axisymmetric computations, that as a strongly swirling ow passes through a pres- sure rise, the mixed-out loss can either decrease or increase, the latter occurring if the deceleration becomes large enough to initiate vortex breakdown. It is further shown that tip vortices in turbines experience pressure rises large enough to cause vortex breakdown. The effect of pressure distribution on tip leakage losses is illustrated through examination of two turbine blades, one designed with a forward loaded tip and one with an aft loaded tip. The computations show a 16% difference in tip clearance loss between the two, due to the lower pressure rise encountered by the clearance vortex, and hence lower vortex breakdown losses, with the forward loaded blade. Other computational experiments, on the effects of blade loading, incidence, and solidity, are also shown to be consistent with the ideas developed about blade pressure distribution effects on vortex breakdown and hence clearance mixing loss.