Modulation and Radial Migration of Turbine Hub Cavity Modes by the Rim Seal Purge Flow

Abstract

In the present paper, the results of an experimental and numerical investigation of the hub cavity modes and their migration into the main annulus flow are presented. A one-and-a-half stage, unshrouded and highly loaded axial turbine configuration with three-dimensionally shaped blades and cylindrical end walls has been tested in an axial turbine facility. Both the blade design and the rim seal purge flow path are representative to modern high-pressure gas turbines. The unsteady flow field at the hub cavity exit region has been measured with the fast-response aerodynamic probe (FRAP) for two different rim seal purge flow rates. Furthermore, fast-response wall-mounted pressure transducers have been installed inside the cavity. Unsteady full-annular computational fluid dynamics (CFD) simulations have been employed in order to complement the experimental work. The time-resolved pressure measurements inside the hub cavity reveal clear cavity modes, which show a strong dependency on the injected amount of rim seal purge flow. The numerical predictions provide information on the origin of these modes and relate them to pronounced ingestion spots around the circumference. The unsteady probe measurements at the rim seal interface show that the signature of the hub cavity induced modes migrates into the main annulus flow up to 30% blade span. Based on that, an aerodynamic loss mechanism has been found, showing that the benefit in loss reduction by decreasing the rim seal purge flow rate is weakened by the presence of turbine hub cavity modes.

Flow mechanisms in axial turbine rim sealing

“…Schädler et al [35] reported unsteady flow measurements in a stage and a half turbine with a more geometrically complex axial rim seal than those considered by Cao et al and Jakoby et al "Hub cavity modes" in the frequency range ~7Ω to 19Ω were detected and found to propagate up to 30% of span in the main flow annulus. These modes were suppressed at higher purge flow rates and their effect on aerodynamic pressure losses was highlighted, lessening the expected reduction in losses as purge flow rate was reduced.…”

“…[33], Savov et al [34], Schädler et al [35], Jakoby et al [24] and Town et.al. [36] have all indicated unsteady rim seal cavity flow features which are unrelated to blade passing.…”

“…As indicated in figure 1 and table 1, axial clearance geometries have been studied experimentally by Cao et al [11], Jakoby et al [24] and Schädler et al [35]. The seal and cavity geometries were quite different in these three cases, but all these cases were tested within turbine stages with a high swirling annulus flow.…”

“…Unsteady flow features were captured with low frequency from 13.3Ω to 20Ω for both uniform and varying seal clearances. Schädler et al [35] complemented their experimental studies with a full 360 ∘ annulus URANS model including 36 vanes and 54 blades. Low frequency unsteadiness was captured but showed a broader frequency band than the measurements.…”

Flow analysis and performance improvement of a radial inflow turbine with back cavity under variable operation condition of compressed air energy storage

“…Khilnani and Bhavnani 21 revealed sealing characteristics of a typical rotor-stator cavity mainstream flow experimentally. Schädler 22 conducted an experimental and numerical investigation for the cavity leakage flow that migrating into the main annulus flow. A hub cavity mode that weakens the benefit in loss reduction was also proposed.…”