An experimental and numerical investigation of the performance of compressor cascades with stalled flow
This investigation was conducted to determine how design variables affect the basic flow characteristics and performance of compressor cascades with stalled flow. The performance of stalled cascades is required for analyzing with computer models stall and post stall behavior of axial flow compressors. In this investigation, the unsteadiness of the stalled flow and the stalled cascade performance as indicated by the blade normal force and total pressure loss were evaluated.
The investigation consisted of both experimental and numerical phases. The effects of stagger, angle of attack and Reynolds number were investigated experimentally using a two-dimensional cascade facility. Surface flow visualization, smoke flow visualization, velocity measurements and pressure measurements were used to evaluate the flow. The flow was modeled numerically by solving the Navier-Stokes equations for a cascade of flat plates.
All of the results indicate that blade stagger is a key variable in determining the performance of a stalled cascade. The smoke flow visualization revealed that propagating stall occurred for the cascades with staggers of 36.5 and 45 degrees at all angles of attack greater than or equal to the angle of full stall inception. Propagating stall was never observed for the cascade with a 25 degree stagger. The flow in the passages of the 25 degree stagger cascade was characterized by two distinct regions of flow, a potential or inviscid region with no losses and a separated region with high losses. For the two higher stagger cascades, two distinct regions did not exist. The performance data for the cascades were consistent with the qualitative results obtained in the flow visualization. When presented as a function of angle of attack, the performance parameters indicate that the loss curve is steeper, the maximum value of the normal force coefficient is lower, and the maximum normal force occurred at a lower angle of attack for the higher staggered cascades. The numerical study revealed trends in cascade performance similar to those found in the experimental study and showed that the predicted losses continue to rise as the limiting inlet angle is approached.