An Experimental Study of Stall Suppression and Associated Changes to the Flow Structures in the Tip Region of an Axial Low Speed Fan Rotor by Axial Casing Grooves

The effects of axial casing grooves (ACGs) on the performance and flow structures in the tip region of an axial low speed fan rotor are studied experimentally in the JHU refractive index-matched liquid facility. The four-per-passage semicircular grooves are skewed by 45 deg, overlapping partially with the blade leading edge (LE) and extending upstream. They reduce the stall flow rate by 40% compared to the same machine with a smooth endwall. Stereo-particle image velocimetry (SPIV) measurements show that the inflow into the downstream side of the grooves and the outflow from their upstream side vary periodically, peaking when the inlet is aligned with the blade pressure side (PS). This periodic suction has three effects: first, substantial fractions of the leakage flow and the tip leakage vortex (TLV) are entrained into the groove, causing a reduction in TLV strength starting from midchord. Second, the grooves prevent the formation of large-scale backflow vortices (BFVs), which are associated with the TLV, propagate from one blade passage to the next, and play a key role in the onset of rotating stall in the untreated fan. Third, the flow exiting from the grooves causes periodic variations of the relative flow angle around the blade LE, presumably affecting the blade loading. The distributions of turbulent kinetic energy (TKE) provide statistical evidence that in contrast to the untreated casing, very little turbulence originating from the TLV and BFV of one blade propagates across the tip gap to the next passage.