Effects of rotor tip blade loading variation on compressor stage performance

Abstract

Changes in loss generation associated with altering the rotor tip loading of an embedded compressor stage is assessed. Steady and unsteady three-dimensional computations, complemented by control volume analyses, for varying rotor tip loading distributions provided results 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 yields a relatively less-mixed-out tip leakage flow at the rotor exit 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. 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, which is a tip leakage flow phasing effect. Because of an inadequate grid resolution defining the unsteady interaction of tip flow with downstream stator, the benefit from unsteady tip flow recovery is the lower bound of its actual benefit. 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% enhancement in stage efficiency is possible from aft-loading rotor tip only.