Active control of the rotating stall in a radial vaneless diffuser

An active control method for suppression of the rotating stall in a radial vaneless diffuser is proposed. The activator is driven by the prestall pressure fluctuation detected at the shroud wall of the diffuser. In order to extract the characteristic frequency of the prestall signal, an analogue bandpass filter system as well as a digital signal processor (DSP) system based on an auto-regressive model are adopted and the results obtained using the two systems are compared. The latter realizes a real-time response to the varying frequency of the pressure fluctuation of the prestall. It is confirmed that the critical flow rate is decreased by 12% by the DSP method, yielding a further extension of the operation range of the vaneless diffuser as compared to that of the filter system. The phase angle is found to be an important parameter for the optimum operation of the present control system.