To gain further insights into the details of the tip-gap flow in axial turbines, a test section has been constructed with a single, idealized, large-scale tip gap. The single “blade” forms a circular arc with 90 degrees of turning and has a constant thickness of 78 mm. For a plain, flat tip four clearances have been examined, varying from 0.292 to 0.667 of the blade thickness (corresponding to physical gap heights of 22.8 to 52.1 mm). The large proportions made it possible to obtain very detailed measurements inside the gap.

The paper discusses the structure of the gap flow in some detail. One new feature, involving multiple vortices on the tip, probably helps to explain the “burnout” which sometimes occurs on turbine tips near the pressure side. Quantitative results are presented for the static pressures, total pressures and velocity vectors through the gap. In addition, contraction coefficients for the flow at the separation bubble, discharge coefficients for the gap, and the gap losses have been extracted for comparison with the assumptions made in recent gap-flow models.

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