Prediction of ingestion through turbine rim seals part 2: externally-induced and combined ingress

Ingress of hot gas through the rim seals of gas turbines can be modelled theoretically using the so-called orifice equations. In Part 1 (ASME GT 2009-59121) of this two-part paper, the orifice equations were derived for compressible and incompressible swirling flow, and the incompressible equations were solved for axisymmetric rotationally-induced (RI) ingress. In Part 2, the incompressible equations are solved for non-axisymmetric externally-induced (EI) ingress and for combined EI and RI ingress. The solutions show how the nondimensional ingress and egress flow rates vary with Theta(0), the ratio of the flow rate of sealing air to the flow rate necessary to prevent ingress.

For EI ingress, a 'saw-tooth model' is used for the circumferential variation of pressure in the external annulus, and it is shown that epsilon, the sealing effectiveness, depends principally on Theta(0); the theoretical variation of epsilon with Theta(0) is similar to that found in Part 1 for RI ingress. For combined ingress, the solution of the orifice equations shows the transition from RI to El ingress as the amplitude of the circumferential variation of pressure increases. The predicted values of e for El ingress are in good agreement with available experimental data, but there are insufficient published data to validate the theory for combined ingress.