OPTIMIZED CASING TREATMENT TO IMPROVE STALL MARGIN UNDER CIRCUMFERENTIAL PRESSURE DISTORTION IN A HIGH-SPEED AXIAL FLOW FAN

Aviation fans often suffer from the negative effect of distorted inflow on the stall margin. An effective way to solve the aforementioned problem is to apply casing treatment for extending the stall margin. However, previously, it was observed that casing treatments often increase the stability but with reduced efficiency. Therefore, an optimized axial slot casing treatment (OPCT) was designed that improves the stall margin by 7.31% with 1.97% efficiency gain at a design speed of 100% in a high-speed axial flow fan. The stability-enhancing effect of the OPCT under distorted inflow was further evaluated. Based on the airworthiness standard, the distorted inflow with 9% of composite distortion index was tested with OPCT. At 100% design speed, the OPCT can still improve the stall margin by 6.19% with an improvement in efficiency by 1.63%. The stall margin improvement due to the OPCT at 80% design speed can be up to 13.31% with an improvement in efficiency by 0.87%. The unsteady measurement results under distorted inflow indicate that the location of the shock wave obviously moves forward towards the blade tip leading edge as the rotor blade rotates out of the distorted region. This in turn leads to an earlier stall of the fan compared to that at uniform inflow. When the OPCT is applied, although the fluctuation of the tip leakage vortex in the slot region is stronger than that in smooth casing, the strength of the interaction between the shock wave and tip leakage flow can be suppressed, and the location of the shock wave at the distorted region is pushed back towards the blade passage by the OPCT. Consequently, the transition from the disturbance downstream of the distortion region to the stall inception is postponed. It was experimentally discovered that the casing treatment does not alter the instability mode of the fan, and the fan eventually surges, which is consistent with B parameter value of 2.942. However, under the distorted inflow, B parameter can be reduced from 2.942 to 0.863, and thereby, the instability mode of the fan is changed from surge under uniform inflow to stall under distorted inflow. The aforementioned phenomena can guide the instability prediction when the fan encounters distortion or casing treatment is applied in future studies.