Experimental measurements of ingestion through turbine rim seals. Part 4: Off-design conditions

This paper describes results obtained from an experimental facility which models ingress through the rim seal into the upstream wheel-space of an axial-turbine stage. The experimental rig included 32 nozzle guide vanes and 41 symmetrical turbine blades, and the paper presents measurements of ε (the sealing effectiveness) for single- and double-clearance seals for both over-speed (where the blades rotate faster than at the design point) and under-speed conditions. The design flow coefficient was CF = 0.538, and tests were conducted for 0 < CF < 0.9, which is larger than the range experienced in engines. The measured values of ε were correlated by the ‘effectiveness equations’ for rotationally-induced (RI) and externally-induced (EI) ingress. The correlated effectiveness curves were used to determine Φmin′ (the value of the sealing flow parameter when ε = 0.95), and the variation of Φmin′ with CF was in mainly good agreement with the theoretical curve for CI (combined ingress), which covered the transition from RI to EI ingress. Departure of the measured values of Φmin′ from the CI curve occurred at very low values of CF for all the seals tested; this was attributed to the effects of separation of the mainstream flow over the turbine blades at large ‘deviation angles’ between the flow and the blades. The measurements are expected to be qualitatively similar to but quantitatively different from those experienced in engines.