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; preswirled purge flow has the potential to alter the swirl gradient. This article presents the first assessment of purge conditioning in a downstream cavity. An experimental campaign was conducted in a new aero-engine 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 coswirled 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 preswirl to reduce disc windage; coswirled purge increased the purge-mainstream swirl gradient and subsequently increased shear-driven ingestion.