Investigation of the Effectiveness of Various Types of Boundary Layer Transition Elements of Low Reynolds Number Turbine Bladings

The paper describes an experimental investigation of the use of different types of boundary layer transition elements for the control of boundary layer separation at low Reynolds numbers. The tests are carried out in a low speed cascade tunnel for Reynolds numbers between 30000 and 200000. For convenience the author used an existing HP turbine guide vane with ∼63 degree turning. To obtain representative adverse pressure gradients as those existing on the rear suction side of highly loaded LP blades the tests are run at a pitch-to-chord ratio of 1. The transition elements include tripwires, single and double rows of spherical roughness elements, balloon type transition elements and a metal sheet actuated by shape memory alloy springs. The optimum position and height of the transition elements are obtained with systematic tests with the trip wire. All other elements are placed at the same position and have approximately the same height. As expected, the transition elements are very beneficial at low Re numbers but deteriorate the performance at high Re numbers. The advantages and drawbacks of the various configurations are discussed and suggestions for real turbine applications are made.