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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. [ABSTRACT FROM AUTHOR]

Turbomachinery with double counter-rotating impellers offers more degrees of freedom in the choice of design and control parameters compared to conventional machines. For these innovative machines, the literature review shows that more publications concerning axial type turbomachines are available than centrifugal ones. This work deals with a design and experimental performance analysis, applied to two counter-rotating impellers of a centrifugal compressor "CRCC". CRCC was designed with a specifically developed tool based on mean-line approach coupled with optimization algorithms and a stream-curvature through-flow method to satisfy the design criteria. This paper presents an experimental validation of the CRCC design tool and its performances against the baseline "SR", composed of one centrifugal impeller and a volute for which experimental data are available. CRCC numeric simulations are also validated by experimental data. For a fair comparison between CRCC and SR, the same volute is used for both configurations. The CRCC studied here includes a first conventional impeller with an axial inlet and a radial outlet, while the second impeller is parametrically designed and can be considered a rotating bladed diffuser with a radial inlet and outlet. The obtained results show that CRCC can deliver a pressure rise increase of two compared to SR, along with an increase of isentropic efficiency and also validate the design method of this novel layout. The experimental results also show that the speed ratio of CRCC has a positive effect on the surge and shock margin. [ABSTRACT FROM AUTHOR]