Effects of Bleed Rate and Endwall Location on the Aerodynamic Behaviour of a Circular Hole Bleed Off-Take

In a jet engine bleed off-takes on the hub and casing endwalls, part-way through the compressor, supply high-pressure air for cooling, sealing, de-icing and aircraft cabin air applications; bleed also assists compressor operation at part-speed conditions. Two separate issues are of interest: the bleed off-take air pressure and the interaction of the bleed off-take with the primary flow through the blade passage. In this paper, the aerodynamic behaviour is presented for a circular-hole bleed off-take at three endwall locations within a stationary cascade blade passage: at mid-passage; near the blade pressure-surface; and near the blade suction-surface. Results from low-speed cascade experiments are complemented by three-dimensional numerical calculations using an unstructured mesh-based solver, in which the blade passage and bleed off-take geometry are fully modelled. The bleed off-take location and the magnitude of bleed rate influence the off-take air pressure and the interaction with the primary passage flow. For optimum design at zero and low bleed rates, off-takes near the blade pressure-surface give the highest bleed air pressures and minimum loss in the blade passage. For minimum blade passage loss at higher bleed rates, however, it is necessary to take bleed near the blade suction-surface. The paper discusses the causes for this pattern of behaviour.