Synchronisation of the Unsteady Pressure Field: an Explanation for Amplified Ingress

Abstract The efficiency of aero-engines is linked to increased turbine entry temperature and a secondary air system that protects vulnerable components under high thermal stresses and metal temperatures. Purge (or sealing) air from the compressor is used to limit the ingress of hot mainstream annulus gases into rotorstator cavities in the high-pressure turbine. Accurately predicting ingress, and understanding conditions under which it is amplified, is a significant challenge for the engine designer. Experimental data gathered from a 1.5-stage turbine facility and a mathematical, physics-informed model are used to link the rotation of large-scale structures (instabilities) near the rim seal with amplified ingress. The Ingress Wave Model identifies the swirl of cyclonic-anticyclonic vortex pairs (instabilities) in the cavity as the transport mechanism for ingress. The intensity of these unsteady rotating structures is maximised if the circumferential pressure field in the cavity is synchronised (hence superposition) to that in the annulus. Cross-correlation of unsteady pressure measurements in the cavity forward of the rotor revealed this synchronisation was to the pressure field caused by downstream rotating blades. In the aft cavity, this synchronisation was in the stationary frame of reference and associated with the downstream vanes. The effects of amplified ingress are shown to be significant and exist in turbine rigs featuring a wide range of blade and vane counts. In terms of new knowledge and originality, the synchronisation to the pressure field provides the first explanation of this important physical mechanism. A criterion for the engine designer to avoid this phenomenon is proposed.