Entrainment of Swirled Axial Throughflow in a Rotating Compressor Cavity under Centrifugal Buoyancy-Driven Convection

Abstract

Future jet engine compressors are expected to feature overall pressure ratios (OPR) that approach 70:1. The core of the compressor will downsize with higher OPR, increasing the importance of managing blade-tip clearance. This clearance is influenced by compressor rotor expansion, which is dictated by the flow structure and heat transfer inside the cavities between co-rotating discs. This unstable flow structure is induced by buoyancy under centrifugal acceleration at high Grashof numbers. Further, the flow is destabilised by the enthalpy and momentum exchange with an axial throughflow of cooler air at low radius. The throughflow forms part of the secondary air system and inherently features swirl from rotating components within the compressor. In some cases, the swirl relative to the rotating discs an exceed unity (over-swirl). This paper presents an experimental study of the influence of entrained fluid into the rotating compressor cavity over a range of Rossby numbers and axial throughflow swirl. The University of Bath Compressor Cavity Rig was used to measure the radial distribution of air temperature in the throughflow, the radial distribution of temperature on the discs inside the cavity, the shroud heat flux, and the resultant flow structure from unsteady pressure. The data produces universal Grashof-number correlations for shroud Nusselt number and the radial mass flow within the buoyancy-induced structures that form in the cavity. Swirl is shown to be a fundamental governing non-dimensional parameter determining the slip of the structures and the number of vortex pairs in the cavity. A theoretical model based on first principles is coupled to the data and supports the practical thermal-mechanical design of compressor rotors in industry.