Experimental Analysis of Tip Clearance Effects on Turbine Performance and Flow Structures Considering Gap Height Variations and Dusting Holes

Abstract

This study presents an experimental assessment of tip clearance effects for a squealer tip rotor in a 2½-stage high pressure Rotating Turbine Rig (RTR). Three tip clearance heights were tested at the aerodynamic design point in the Next Generation Turbine facility (NG-Turb). The facility configuration and the applied measurement technologies, including standard probes and time-resolved tip clearance measurements, are summarized. The study investigates two rotor blade configurations: one uncooled and one cooled design featuring a dust hole at the squealer cavity. Turbine performance is assessed using integral data from rakes and probe measurements and compared with corresponding tip clearance data. Stationary 2D contour plots upstream and downstream of the blades, obtained with 6-hole pressure probes and miniaturized Pitot probes, quantify the effects of tip clearance variation on total pressure and flow angle along with the unsteady data from the tip clearance sensors. Smaller clearances reduce tip leakage flows, modify the secondary flow field, and increase blade loading and gap-sealing effectiveness. The combined uncooled and cooled datasets offer a consistent basis for numerical model validation. Comparisons with established tip clearance loss and deviation correlations demonstrate their limited predictive capability for squealer tip configurations particularly under the influence of cooling flows.