MEASUREMENTS OF THE FLOW IN AN IDEALIZED TURBINE TIP GAP

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 are with 90 deg of turning and has a constant thickness of 78 mm. For a plain, flat tip Sour clearances have been examined varying from 0.299 to 0.66 7 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 d etailed measurements inside the gap. The paper discusses the structure of the gap flow in some detail. One new feature, involving multiple v ortices on the tip, probably helps to explain the ''burnout'' that som etimes occurs on turbine tips near the pressure side. Quantitative res ults are presented for the static pressures, total pressures, and velo city vectors through the gap. In addition, contraction coefficients fo r the flow at the separation bubble, discharge coefficients for the ga p, and the gap losses have been extracted for comparison with the assu mptions made in recent gap-flow models.