A study of the effect of boundary layer control on an axial flow compressor stage

The problem of increasing the pressure ratio per stage of an axial flow compressor is studied in part by considering the effect of boundary layer control by area suction on the three dimensional turbulent boundary layer flow on the casing in one stage of a compressor. The effect of this boundary layer control, on the prevention of separation is investigated. Crossflow and crosswise variation of crossflow in the boundary layer are factors contributing to the tendency toward separation and the effect of boundary layer control on this phenomenon is determined. This analysis is limited to the boundary layer on the casing and does not take into consideration the possible separation taking place at the blades. The boundary layer equations are used to obtain the momentum integral equations with suction applied at the surface. The method of small perturbations is applied to the equations in order to simplify them so that a solution may be reached with relatively little effort. Expressions are assumed for the boundary layer profiles, flow path outside the boundary layer, and shear stresses. These are in a form which closely resembles the actual conditions. In order to show what effect boundary layer control exerts, a simplified single stage compressor is studied. The results show that crossflow can be reduced by applying suction to the boundary layer with a resultant decreased tendency toward separation of the flow, but this is reached only after a certain minimum suction velocity is attained, If the suction velocity is less than this minimum, in spite of the decrease in boundary layer thickness the crossflow will be increased over that of a solid wall and likewise the tendency toward separation will increase.