Ingress wave model with purge-mainstream density ratio

Abstract

Aeroengines operate with a cooling flow (purge) at a significant purge-mainstream density ratio (DR), which is principally created by the differences in temperatures of these two streams. This paper will show there is a profound influence of DR on ingress, purge flow rates, and sealing effectiveness - all crucial to the superordinate aim of achieving a high thermodynamic efficiency for the engine. A new theoretical (low-order) model is introduced to enable the engine designer to flexibly predict the required purge to prevent ingress over a range of typical operating conditions. The Ingress Wave Model is based on the physical principle that unsteadiness, in the form of large-scale rotating instabilities, forms a circumferential pressure gradient driving fluidic motion against the Coriolis force. The shear created by the difference in tangential momentum between adjacent flow streams is assumed to be the primary mechanism in the process. This allows a set of equations to be derived from dimensional analysis and the assumption that flow entrainment is a function of the relative egress momentum and ingress density. The model is validated against data collected at both DR = 1 and 1.52, with good quantitative agreement across a range of purge and annulus flow conditions. Typical engine design practice exploits information captured in experimental rigs operating in benign conditions at low technology readiness level (TRL) and DR = 1. The new model is used to scale such data collected from six experimental facilities to the density ratios expected in current state-of-the-art (DR = 1.5) and future (DR = 2) engines. The result is a requirement for significantly reduced purge, with profound practical implications for the engine designer, in particular for future engines which operate at higher purge-mainstream density ratios.