A self-adaptive casing treatment for unshrouded centrifugal compressor was proposed in our previous studies. It is a kind of passive control techniques. The experimental results proved that the stable working range of the compressor was extended greatly with the technique. As for the stability mechanism, there is no convinced explanation. Many researchers believe that the unsteady flow could be one of the key points. In the paper, steady and unsteady numerical simulations were carried out to get the performances of the centrifugal impeller by ANSYS CFX software. The numerical method was validated by comparing with the experimental results. It was found that there were two types of flow pattern in the bleeding-recirculation passages by the numerical simulation with the self-adaptive casing treatment. One was the recirculation flow at the smaller flowrate working conditions and the other was bypass flow at the larger flowrate working conditions. The pressure at the bleeding ports was more than that at the recirculation port at the smaller flowrate. It would result in the recirculation flow in the bleeding-recirculation passages. Otherwise, it would result in the bypass flow in the bleeding-recirculation passages. The numerical results of each bleeding-recirculation passage provided the variation of mass flowrate in it with the pressure difference. The relation of the pressure drop coefficient and Reynolds number based on the bleeding hole was fitted. It was different for the recirculation flow and bypass flow. It is helpful to decide the position of the bleeding ports during the centrifugal compressor design process. Moreover, an unsteady numerical simulation method with the increasing back pressure boundary condition was proposed to investigate the unsteady process approaching to the numerical stall point or unstable flow. The dynamic pressure data in impeller and diffuser were recorded. The amplitudes of the data were picked up to compare the time dependent process. The dynamic pressure at the inlet of diffuser fluctuated more strongly than those at the other positions while the back pressure was increased to the numerical stall point. The experimental data provided the similar phenomena. It suggested that the unstable flow tendency could be caught up by the unsteady simulation process with the increasing back pressure boundary condition. Furthermore, the time dependent flow fields at the blade tip region were compared on the conditions with and without the self-adaptive casing treatment. The effect of the self-adaptive casing treatment was proved by unsteady numerical method with the increasing back pressure boundary condition. The stability mechanism of the self-adaptive casing treatment was explained to some extent.

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