Effects of Tip Clearance on Blade Loading in a Planar Cascade of Turbine Blades

The paper examines in detail the structure of the tip leakage flow and its effect on the blade loading in a large-scale planar cascade of turbine blades. The tip clearance was varied from 0.0 to 2.86 percent of the blade chord. One of the blades is instrumented with 14 rows of 73 static taps which allowed a very detailed picture of the loading near the tip to be obtained. In addition to the measurements, extensive flow visualization was conducted using both smoke and surface oil flow. A new feature found in the present experiment was the formation of multiple, discrete tip-leakage vortices as the clearance was increased. Their presence is clearly evident from the surface oil flow and they account for the multiple suction peaks found in the blade pressure distributions. Integration of the pressure distributions showed that for larger values of the clearance the blade loading increases as the tip is approached and only begins to decline very near the tip. The increase was found to occur primarily in the axial component of the force.