The Adverse Effect of Pre-Swirl on Ingestion in a Downstream Cavity

Gas turbine designers demand accurate predictions of metal temperature to ensure acceptable operating life of components experiencing high thermal stress. Rotor-stator cavities ingest hot mainstream gas through rim seals when inadequately purged with relatively cool air bled off the compressor. Superfluous use of purge, and any associated windage increase, creates a parasitic loss in overall efficiency. Shear interaction caused by the difference in swirl between the purge and mainstream flow is a principal driver for ingestion; pre-swirled purge flow has the potential to alter the swirl gradient. This paper presents the first assessment of purge conditioning in a downstream cavity. An experimental campaign was conducted in a new aeroengine representative 1.5-stage test facility designed to facilitate expedient changeover of modular components in the downstream stator assembly. Purge flow in the downstream cavity was supplied through a series of angled injectors contained in a single component at mid-radius. Three co-swirled injection angles were tested. Measurements of CO2 gas concentration, static pressure and swirl were taken in the cavity to examine the relationship between purge-mainstream swirl gradient and ingress downstream of a rotor blade. The aero-engine designer must balance caution when employing pre-swirl to reduce disc windage; co-swirled purge increased the purge-mainstream swirl gradient and subsequently increased shear-driven ingestion.