The Influence of Axial Throughflow Swirl on Buoyancy-Induced Flow in a Compressor Cavity

Next-generation aero-engine compressors will operate with overall pressure ratios exceeding 70:1. This will require shorter compressor blades, presenting a challenge to the designer when predicting tip clearance and efficiency. Buoyancy-induced flow within co-rotating compressor discs drives the heat transfer that determines rotor expansion and the resulting blade-tip clearance. This inherently unstable flow is influenced by the radial temperature distribution of the discs, rotational speed, as well as enthalpy and momentum exchange with an axial throughflow of cooled air at low radius. Due to the rotation of the engine compressor, this throughflow may become swirled, altering the temperature, mass exchange and swirl within the rotating cavity. The University of Bath Compressor Cavity Rig has been adapted to introduce pre-swirl into the axial throughflow by passing it through rotating holes. The effects of inlet swirl have been characterised in terms of Rossby and Reynolds numbers. Measurements of disc temperature, shroud heat flux and unsteady pressure in the rotating frame of reference are used to quantify the effects of ingestion (entrainment) of fluid into the cavity. The unsteady dynamics and rotation of the core relative to the disc have been measured in both the stationary and rotating frames of reference with consistent results. A single correlation between shroud Nusselt and Grashof numbers has been established, effectively capturing the impact of swirl, Rossby number and free convection.