2012 Volume 4 Issue 1 Pages 10-18
Ingress occurs when hot mainstream gas from the turbine annulus is ingested through a rim seal into the wheel-space between a rotating turbine disc (the rotor) and an adjacent stationary casing (the stator). The stationary vanes and rotating blades in the annulus create circumferential variations of pressure that drive hot gas inwards into the wheel-space in regions of high external pressure. This is called externally-induced ingress. (The rotating flow in the wheel-space creates a radial pressure gradient that separately promotes radially inward flow into the wheel-space. This is known as rotationally-induced ingress). The high temperature of the ingested mainstream gas in an engine can lead to fatigue and damage to important components. This paper describes simplified computational studies of externally-induced ingress into a rotor-stator system with an axial clearance rim seal. Axisymmetric steady-state computations are carried out using the commercial computational fluid dynamics code CFX. The SST model of turbulence is used. The model geometry and boundary conditions are based on an experimental rig designed and built at the University of Bath to study fundamental features of ingestion and rim seal effectiveness, in a related project having substantial industrial involvement. It is known that simplified steady flow models can significantly underestimate ingestion from the mainstream annulus into the wheel-space. In the present work, ingestion is prescribed in the steady model through the use of boundary conditions at the axial clearance seal. The computation of the flow and heat transfer in the wheel-space is validated by comparison with previously-published experimental measurements for a simple rotor-stator system without ingestion. The computations are carried out for values of rotational Reynolds number up to around 1.25 x 106 as typically used in experimental studies, and using sealing air flow-rates corresponding to non-dimensional values relevant to engine applications. The computed results show that the flow structure and heat transfer in the wheel-space at high sealing flow rates agree well with measured values for rotor-stator systems, the effects of differences between geometries being mostly small. Due to the recirculating secondary flow in the wheel-space, ingested fluid is drawn toward the surface of the stator. At lower sealing flow rates, the higher swirl of the ingested mainstream flow causes changes in the flow structure in the wheel-space. The ingested mainstream flow can come into contact with the rotor, and this could have serious consequences in practice.