The Effect of Purge-Mainstream Density Ratio on Rim Seal Fluid Mechanics

Significant density ratios arise in a gas turbine due to severe temperature gradients between the hot mainstream gases leaving the combustor and superposed purge flow injected from the secondary air system. Engineers seek to minimise the ingestion of hot annulus gas through the rim-seal at the periphery of the turbine wheel-space to maximise component life, while continuing to increase turbine entry temperature in pursuit of optimised thermodynamic cycle efficiency. The majority of experimental ingestion facilities assess sealing performance at a near-unity purge-mainstream density ratio which negates the impact of this significant contributor to ingestion. This study investigates the impact of density ratio on the fluid mechanics across the rim-seal of a single-stage turbine facility. The results demonstrate that the purge-mainstream density ratio is a crucial consideration when designing the rim-seal architecture, particularly with the transition to alternative fuels which have the potential to augment the temperature gradient. A density affected region at intermediate superposed purge flows was identified where the non-unity density ratio has the greatest impact on outer cavity swirl and sealing effectiveness. Furthermore, unsteady pressure spectra in this region exhibited a suppression of the low frequency spectral band as density ratio was increased, highlighting a causal link between unsteadiness and ingress.