Tip-clearance-affected flow fields in a turbine blade row

For effective validation of computational fluid dynamics (CFD) codes for design, numerical simulations must be compared and contrasted with experimental data sets which are well posed and measured. This paper presents, with the emphasis placed primarily upon the description and explanation of the experimental results, a test case designed for CFD validation concerning the flow field generated by the presence of tip clearance in an annular cascade turbine blade row. The test case is based upon a moderately loaded prismatic blade profile, operating with axial inflow and a nominal mid-span exit Mach number of 0.5. In addition to the case with no clearance, three gap heights, representing 1, 3 and 5 per cent of chord length have been selected. These tip clearances were chosen because they represent three conditions where the tip clearance is below, approximately equal to and substantially larger than the inlet boundary layer displacement thickness. At the leading edge this produces different behaviours, with the smallest gap producing a stagnation point, equivalent to the case without tip clearance, while the largest clearance allows throughflow over the leading edge into the tip gap. In the presence of larger tip clearances, the tip leakage how induces a stagnating flow in the trailing-edge region, which results in a large area of blocked flow in the blade passage downstream of the trailing edge.