Numerical simulations have been carried out to define the loss generation mechanisms associated with tip leakage in un-shrouded axial turbines. Tip clearance vortex dynamics are a dominant feature of two mechanisms important in determining this loss: (i) decreased swirl velocity due to vortex line contraction in regions of decreasing axial velocity, i.e., adverse pressure gradient and (ii) vortex breakdown and reverse flow in the vortex core. The mixing losses behave differently from the conventional view of flow exiting a turbine tip clearance. More specifically, it is shown, through both control volume arguments and computations, that as a swirling leakage flow passes through a pressure rise, such as in the aft portion of the suction side of a turbine blade, the mixed-out loss can either decrease or increase. For turbines the latter typically occurs if the deceleration is large enough to initiate vortex breakdown, and it is demonstrated that this is the case in modern turbines. The effect of blade pressure distribution on clearance losses is illustrated through computational examination of two turbine blades, one with forward loading at the tip and one with aft loading. A 15% difference in leakage loss is found between the two, due to lower clearance vortex deceleration (lower core static pressure rise) with forward loading, and hence lower vortex breakdown loss. Additional computational experiments, carried out to define the effects of blade loading, incidence, and solidity, are found to be consistent with the proposed ideas linking blade pressure distribution, vortex breakdown and turbine tip leakage loss.

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