Interaction Between Inlet Boundary Layer, Tip-Leakage and Secondary Flows in a Low-Speed Turbine Cascade

An experiment has been conducted in a large-scale linear turbine cascade to examine the interaction between the inlet endwall boundary layer, tip-leakage and secondary flows. Detailed flow field measurements have been made upstream and downstream of the blade row for two values of inlet boundary layer thickness (δ*/c of about 0.015 and 0.04) together with three values of tip clearance (gap heights of 0.0, 1.5 and 5.5 percent of blade chord). In the downstream plane, the total pressure deficits associated with the tip-leakage and secondary flows were discriminated by examining the sign of the streamwise vorticity. For this case, the streamwise vorticity of the two flows have opposite signs and this proved an effective criterion for separating the flows despite their close proximity in space. It was found that with clearance the loss associated with the secondary flow was substantially reduced from the zero clearance value, in contradiction to the assumption made in most loss prediction schemes. Further work is needed, notably to clarify the influence of relative tip-wall motion which in turbines reduces the tip-leakage flow while enhancing the secondary flow.