Influence of Chord Length and Inlet Boundary Layer on the Secondary Losses of Turbine Blades

In the present paper results concerning the influence of chord length and inlet boundary layer thickness on the endwall losses are discussed. The investigations were performed in a low speed cascade tunnel using the turbine profile T40. The deflection of 90 and 70 deg, the velocity ratio in the cascade from 1.0 to 3.5 as well as the chord length of 100,200 and 300 mm were predetermined. In a measurement distance behind the cascade of s2 /l = 1, an approximate proportionality of endwall losses and chord length was observed in a wide range of velocity ratios. At small measurement distances (e.g. s2 /l = 0.4), this proportionality does not exist. If aside from the flow path behind the cascade the flow path in the cascade is approximately incorporated, a proportionality to the chord length at small measurement distances can be obtained, too. Then to a large extent, the magnitude of the endwall losses is dependent on the length in main flow direction. At velocity ratios near 1.0, the influence of the chord length decreases rapidly, while at a velocity ratio of 1.0, the endwall losses are independent of chord length. By varying the inlet boundary layer thickness no correlation of displacement thickness and endwall losses was achieved. With a calculation method according to the modified integral equation by van Driest, the velocity gradient on the wall, the wall shear stress and the local friction coefficient were determined and their influence on the endwall losses analyzed.Copyright © 2010 by ASME