Secondary flow and losses in a compressor cascade

Abstract

The shear flow on the annular walls of axial-flow turbo-machines is generally the primary cause of the difficulties encountered in the improvement of the performance and reliability of these machines. It creates a flow of lower momentum along the walls on which the pressure gradients developed in the main stream are impressed. Within a blade row, as a turning in the boundary layer equal to the turning of the free stream would not be sufficient to obtain the balance between pressure gradients and the centrifugal forces, the low-momentum fluid is turned more than the main fluid. This deviation in flow from the main stream is called the secondary flow. The secondary flow results in the accumulation of the low-momentum flow on the suction side of the blades, and the axial symmetry is destroyed. Furthermore, in the above-mentioned accumulation area, a high loss core is often discernible. The experimental work described in this report shows that this loss core in the corner may not only be due to the secondary flow, as previously thought, but to the separation of the boundary layer in the corner formed by the junction of the wall and the blade where an important adverse gradient of pressure exists. These losses due to a stall in the corner will reduce appreciably the performance of the turbo-machines. It is also reported that this kind of stall is associated with appreciable velocity fluctuations which could in certain cases initiate vibrations of blades such as flutter. Means of eliminating this sort of stall are discussed, and further study of the development of boundary layer in a straight corner formed by two semi-infinite planes at right angles is proposed, particularly in the case of an adverse gradient of pressure.