Effects of Rotor Tip Blade Loading Variation on Compressor Stage Performance

Abstract

Changes in loss generation associated with altering the rotor tip blade loading of an embedded rotor-stator compressor stage are assessed with unsteady three-dimensional computations, complemented by control volume analyses. Tip-fore-loaded and tip-aft-loaded rotor blades are designed and assessed to provide variation in rotor tip blade loading distributions for determining if aft-loading rotor tip would yield a stage performance benefit in terms of a reduction in loss generation. Aft-loading rotor blade tip delays the formation of tip leakage flow resulting in a relatively less mixed-out tip leakage flow at the rotor outlet and a reduction in overall tip leakage mass flow, hence a lower loss generation; however, the attendant changes in tip flow angle distribution are such that there is an overall increase in the flow angle mismatch between tip flow and main flow leading to higher loss generation. The latter outweighs the former so that rotor passage loss from aft-loading rotor tip is marginally higher unless a constraint is imposed on tip flow angle distribution so that associated induced loss is negligible; a potential strategy for achieving this is proposed. Tip leakage flow, which is not mixed-out at the rotor outlet, enters the downstream stator, where it can be recovered. The tip leakage flow recovery process yields a higher benefit for a relatively less mixed-out tip leakage flow from aft-loading a rotor blade tip. These characterizing parameters together determine the attendant loss associated with rotor tip leakage flow in a compressor stage environment. A revised design hypothesis is thus as follows: rotor should be tip-aft-loaded and hub-fore-loaded while stator should be hub-aft-loaded and tip-fore-loaded with tip/hub leakage flow angle distribution such that it results in no additional loss. For the compressor stage being assessed here, an estimated 0.15 points enhancement in stage efficiency is possible from aft-loading rotor tip only. In the course of assessing the benefit from unsteady tip leakage flow recovery in the downstream stator, it was determined that tip clearance flow is inherently unsteady with a time-scale distinctly different from the blade passing time. The disparity between the two timescales: (i) defines the periodicity of the unsteady rotor-stator flow, which is an integral multiple of blade passing time; and (ii) causes tip leakage vortex to enter the downstream stator at specific pitchwise locations for different blade passing cycles, a tip leakage flow phasing effect. Despite the inherent unsteadiness from tip leakage flow, the recovery process is demonstrated to be beneficial on a time-averaged basis.