THE ROLE OF TIP LEAKAGE FLOW IN AXIAL COMPRESSOR SPIKE-TYPE ROTATING STALL INCEPTION

The operating range of axial compressors is frequently limited, at low flow rates, by spike-type stall inception. Previous work has identified spikes as radial vortex tubes, created by leading edge separation. In this thesis, a series of multi-passage unsteady computations, verified by experiments, have been used to determine the role of tip leakage flow in spike formation. The calculations have predominately used the E3 Rotor B, a design representative of the subsonic intermediate/high pressure stages of gas turbine axial compressors. It is shown that there are two routes to the high incidence required for leading edge separation and hence spike formation: 1. Spillage of the tip leakage jet in front of the adjacent leading edge creates a propagating disturbance. This disturbance impinges on the leading edge, and triggers a leading edge separation. Geometries that follow this route to incidence are designated jet critical. 2. Blockage from casing corner separation increases the incidence onto the adjacent leading edge and triggers a leading edge separation. Geometries that follow this route to incidence are designated separation critical. Once leading edge separation has occurred, the development of the spike is qualitatively independent of the route to incidence. Tip leakage flow determines which of the two routes to incidence occurs. At zero clearance the route to incidence is blockage from casing corner separation. Opening the tip clearance suppresses the casing corner separation, but increases the strength of the tip leakage jet. Stall margin is maximised (at 0.26 design point flow coefficient for the E3 Rotor B) at the tip clearance (0.5% chord) at which the casing corner separation and spillage of the tip leakage jet occur at the same flow coefficient. At clearances above this optimum, the route to incidence is spillage of the tip leakage jet. At large clearances (>1.6% chord for the E3) stall margin is independent (0.11+/-0.02) of clearance. The tip leakage flow is characterised by the chord-wise distribution of axial momentum. Shifting the tip leakage flow axial momentum distribution, at a fixed magnitude, towards the trailing edge increases stall margin of jet critical geometries (an improvement of 43% is demonstrated for the E3). Shifting the tip leakage flow axial momentum distribution, at a fixed magnitude, towards the leading edge increases stall margin of separation critical geometries. It is shown that, for a given blade, flow coefficient at stall can be mapped to tip leakage flow total axial momentum and the centroid of the tip leakage flow axial momentum distribution, at the design point. Based upon the unsteady stall point calculations of a subset of geometries, a correlation of stalling flow coefficient, as a function of these two parameters, can be used to produce a tip leakage flow characterisation map. This permits a design process where steady calculations at the design point are used as a preliminary indication of stall margin. At a fixed, jet critical, tip clearance (1.8% chord) this design process is applied to the E3 Rotor B: forward sweep, negative dihedral and positive near-tip leading edge recamber improve stall margin (by up to 53%, 37% and 26% respectively) by reducing the magnitude of tip leakage flow axial momentum and, for dihedral and recamber, shifting the tip leakage flow axial momentum distribution towards the trailing edge. Leading edge shape has minimal effect on stall margin. Combining forward sweep, negative dihedral and positive leading edge recamber, with blade restagger to recover lost pressure rise, a stall margin improvement of 90% is calculated