The Development of Unsteady Boundary Layers on the Rotor of an Axial-Flow Turbine,
CAMBRIDGE UNIV (ENGLAND) DEPT OF ENGINEERING
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Aerodynamic efficiency of an axial-flow turbine is significantly less than that predicted by measurements made on equivalent cascades which operate with steady inflow. This difference is strongly dependent upon the rotor-stator axial spacing. An experimental investigation was conducted into the effects of rotor-stator interactions upon the efficiency of a large scale, low speed turbine. Comparison is made between performance of the mid-span section of the rotor blades and of a rectilinear cascade of identical geometry. Both wind tunnels were operated at a Reynolds number of 315,000. This paper presents measurements of blade surface boundary layers and their associated profile losses. The profile loss of the turbine rotor at mid-span ws approximately 50 higher than that of the rectinlinear cascade. The mean velocity profiles of the boundary layers indicate that this difference is mainly due to higher growth rate of the suction surface boundary layer on the turbine rotor. Time-resolved boundary layer measurements indicate that the increased growth rate is caused by interaction of the wakes of the upstream nozzle row with the blade surface boundary layers. This results in a localized transition of the otherwise laminar boundary layers. Boundary layers thus alternate between characteristically laminar and turbulent states during one wake passing cycle.