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In order to model the flow between an air-cooled gas turbine rotor and its stationary casing, a simple isothermal plane rotating disk and stator are used. In tests reported earlier, the cavity between the rotor and stator was sealed by a stationary cylindrical shroud, and the dimensionless minimum amount of “coolant,” Cw, min, necessary to prevent a radial inflow (or ingress) of “hot gas” through the axial clearance between the shroud and the rotor, was determined. In the current tests, a number of seals with a radial clearance between the cylindrical shroud and the rotor are tested. Unlike their axial-clearance counterparts, radial-clearance seals can exhibit a pressure-inversion effect, where the pressure inside the cavity increases, rather than decreases, with increasing rotational speed. Using pressure measurements and flow visualization, correlations showing the variation of Cw, min with clearance ratio and rotational Reynolds number are presented, and it is shown that — under equivalent conditions — a seal with a radial clearance can be much more effective than one with an axial clearance.

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