Theoretical modelling of hot gas ingestion through turbine rim seals

The rim seals of gas turbines are used to prevent or reduce the ingestion of hot mainstream gas into the wheel-space between the turbine rotor and its adjacent stationary casing. The ingestion is caused by local pressure differences between the mainstream and the wheel-space; ingress usually occurs where the mainstream pressure is higher than that in the wheel-space and egress occurs where it is lower. Sealing air, which is supplied to the wheel-space, flows through the seal clearance and joins the mainstream flow. Too much sealing air is inefficient; too little can lead to disastrous consequences.The nozzle guide vanes create three-dimensional (3D) variations in the distribution of pressure in the mainstream annulus and the turbine blades create unsteady effects. Computational fluid dynamics (CFD) is both time-consuming and expensive for these 3D unsteady flows, and engine designers tend to use correlations or simple models to predict ingress. This paper describes the application of simple ‘orifice models’, the analytical solutions of which can be used to calculate the sealing effectiveness of turbine rim seals. The solutions agree well with available data for externally-induced ingress, where the effects of rotation are negligible, for rotationally-induced ingress, where the effects of the external flow are small, and for combined ingress, where the effects of both external flow and rotation are significant.