Impact of hub gap leakage on stator endwall flow in an axial compressor stage with casing treatment

Abstract

Abstract In an axial compressor stage equipped with slot-type casing treatment, simulation without hub gap between rotors and stators achieves much lower stall margin enhancement than experiment, which results from the over-predicted corner stall in stators. Aiming at clarifying the combined effect of hub leakage flow and casing treatment on operating range and the flow mechanism of leakage flow interacting with stator flow, a detailed numerical research is performed in this compressor stage. The effects of hub leakage mass flow are elaborated in terms of compressor performance and flow fields. Unsteady simulations with appropriate leakage mass flow condition are able to predict the stall margin improvement and total pressure characteristics preferably. Casing treatment slots tend to re-distribute the loading of downstream stator by decreasing loading above 70% span and increasing that below 70% span, which result in the exaggerated corner stall in stators. Hub leakage flow from cavity resists the tendency brought by casing treatment and succeeds in relieving the heavy loading near the hub. The flow mechanism for the effect of cavity leakage flow on stall margin is elucidated. Cavity hub leakage gives rise to a thickened boundary layer upstream stator inlet by producing a separation bubble and aggravating the boundary layer skewing. The thickened boundary layer produces a horseshoe vortex in the stator passage, which induces main flow to rolling up as the pressure-side branch and avoids the emergence of corner stall.