Investigation of Recirculating Casing Treatment for Low-Pressure Ratio Mixed-Flow Micro-Compressor

The mixed-flow micro-compressors are used as actuators in Co-flow Jet (CFJ) Active Flow Control (AFC) airfoils. The micro-compressor is required to have a wide operating range to meet the broad flight envelop of aircraft. The recirculating casing treatment (RCT) is an effective technique to extend compressor stall margin. RCT removes tip blockage by extracting flow from blade aft part and injects it to the upstream. It requires certain pressure difference between extraction and injection location. However, due to the energy efficiency consideration of CFJ airfoil, the micro-compressor may be designed with a very low total pressure ratio such as 1.095, which makes the application of RCT challenging.

This paper investigates the recirculating casing treatment of a low total pressure ratio micro-compressor to achieve stall margin enhancement while minimizing the design point efficiency penalty. Various RCT injection and extraction configurations are studied, including three types of RCT geometries, ducts combined with slot, ducts only, and slots only. The numerical approach is validated with a tested micro-compressor using RCT and a very good agreement is achieved between the predicted performance and the measured data. To minimize the design point efficiency, it is found that the optimal location of extraction and injection is where the recirculated flow rate can be minimized at the design point. For extending stall margin, extraction location should favor minimizing the tip blockage. The study shows extracting near the start of the tip separation is desirable.

Overall, a small efficiency penalty of 0.89% at the design point is achieved with the stall margin increased by 6.38%. The highest stall margin enhancement achieved is 10.2% with the duct geometry that has the extraction holes at 53.3% chordwise location, which has a higher design point efficiency penalty of 1.45%. More detailed results and analysis will be presented in the final paper.