Secondary flow and losses in a compressor cascade

Abstract

Three ways in which secondary flow can be generated in a straight compressor cascade have been investigated. 1. Wakeflow. The inlet flow is characterized by a constant inlet angle and a varying stagnation pressure over the span. 2. Skewed flow. The inlet flow is characterized by a constant stagnation pressure and a varying inlet angle over the span. 3. Skewed wakeflow. The inlet flow is characterized by a variation of both stagnation pressure and inlet angle over the span. For a certain combination of inlet angle and stagnation pressure distribution (presented in a formula) in the skewed wakeflow case no secondary flow is generated behind the cascade. The kinetic energy of the secondary flow was found to be very small in all three cases. The secondary flow itself did not create any losses but the blades stalled in the skewed flow layer and in the skewed wakeflow layer causing great losses. The stream pressure and the tangential blade force were lower in the disturbed flow region. A small perturbation theory (for a non-viscous, incompressible fluid) had been developed to describe analytically the secondary flow. Formulae readily adaptable for numerical calculations of flow deviation angles, kinetic energy of the secondary flow, and tangential blade force are presented. Good and satisfactory correlation of theory and experiment was found. Applied to compressor design the results imply: The secondary flow is very small and may occur as an overturning or an underturning of the flow at the casings depending on the actual design. The kinetic energy of the secondary flow may be neglected when considering the losses. The ends of the blades stall in the skewed boundary layer at the casings causing losses. This could be reduced by twisting the blade ends to account for the increased incidence angle. The stream pressure varies in spanwise direction through the boundary layer on the casings.