A novel experimental control method to suppress instability in a centrifugal compressor with two counter and co-rotating rotors

The use of a counter-rotating rotor in turbomachines can be a solution to improve performances by allowing reduction in size and weight, especially compared to multistage configuration. The present study aims are to demonstrate another advantage of the counter-rotating rotors: their application to expand the operating range of centrifugal compressors toward extremely low flow rates based on an active control method. This method was performed on two different counter-rotating configurations. The principle of the active control method is to adjust independently the rotation speed of each rotor to push back the instability appearance. Experimental results show that reducing the rotation speed of the upstream-rotor while fixing the speed of the downstream rotor can push the instability phenomenon toward a lower mass flow rate while decreasing the pressure rise and efficiency. Co-rotating mode is applied to push further the stable region toward low mass flow rates allowing a significant extension of the operating range. The experimental results reveal that the map width is shifted by about 50% over the studied range toward a lower mass flow rate for both counter-rotating centrifugal compressors when using the control method. This can be achieved when the absolute value of the upstream-rotor speed is within the range of 48%–61% of the rotational speed of the downstream rotor.