Measurements have been made for tip clearances up to 15% of chord in a turbine cascade which has been used previously to study the tip-leakage flow at smaller clearances. The work is part of a study of the aerodynamics of turbines which have experienced in-service damage and is related to Engine Health Monitoring. Detailed flow field measurements have been made upstream and downstream of the cascade and static pressure distributions have been measured on the endwall and at the midspan of the blade. The most unexpected result was that the blade end losses (that is, the sum of the secondary and tip-leakage losses) reached a maximum for a clearance of about 6% of chord. At the largest clearance of 15%, the end losses were substantially reduced and were in fact comparable with the secondary losses measured for zero clearance. The paper discusses in some detail the physics of the flow and the reasons for the observed reduction in losses. The pressure difference across the gap was found to be considerably attenuated at the largest clearance. This would have the effect of reducing the mass flow rate per unit area through the gap as well as the kinetic energy of the gap flow. These effects appear to account for a large part of the reduction in losses. As might be expected, the turning of the flow near the endwall was considerably reduced at the large clearances.

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