Stall Margin Improvement in a Transonic Compressor With a Casing Treatment: Flow Mechanism

Abstract

Abstract A compressor casing treatment with circumferential casing grooves (CCGs) is studied in detail. The primary objective of the current paper is to unearth the main driving fluid mechanism that changes the stall margin in a transonic compressor with CCGs. Large Eddy Simulation (LES) is applied to calculate the transonic compressor flow fields with and without CCGs. Detailed comparisons between measured and computed flow fields show that changes in flow structures due to CCGs are very well represented with LES. The measured data show that flow blockage generation in the tip gap area is reduced significantly when CCGs are applied, which delays compressor stall onset. Calculated flow fields with and without CCGs are examined to understand how the blockage generation is reduced with CCGs. Tip leakage flow typically travels upstream, opposite to the incoming main flow. When CCGs are applied on the compressor casing, part of the tip leakage flow goes into the grooves and is later reinjected into the main flow passage. Therefore, CCGs modify the collision between the incoming main passage flow and the tip leakage flow. Two main aspects of the change of tip leakage flow due to CCGs are examined in the present study. The first aspect is whether the total mass flow through the tip gap changes substantially with CCGs. The second aspect is whether changes in the collision and mixing of the incoming main passage flow and the tip leakage flow due to CCGs substantially affect blockage generation. The present investigation shows that CCGs reduce the mass flow through the tip gap by about 16 % near the stall-condition. Calculated flow fields show that most of this reduction of tip leakage flow occurs near the CCGs. Reinjected flow from the CCGs pushes the tip leakage flow radially inward below the casing and changes how the tip leakage flow collides with the incoming main passage flow. However, detailed examination of the calculated flow in the tip region shows that the reinjected flow does not contribute to the reduction of the overall blockage generation. The primary driver reducing blockage generation with CCGs is the reduction of overall mass flow rate through the tip gap. It has been thought that circumferential grooves always incur small additional losses near the design condition due to mixing between the flow exiting the grooves and the main flow, in addition to an increased wetted area. In the present investigation, measurements show such a decrease in efficiency with CCGs, although the difference in efficiency is within the measurement uncertainty. Results from the LES simulation at the design condition with CCGs show that the tip leakage vortex (TLV) is pulled toward the blade suction side and double leakage flow is eliminated. The result is that the simulated efficiencies with and without CCGs are almost the same.