Experimental investigation on unstable flow in multi-stage centrifugal blower with inlet guide vane

This study experimentally investigates the unstable flow in a five-stage centrifugal blower with inlet guide vanes (IGVs) upstream of the first stage. Performance characteristics of the blower were obtained under three IGVs opening conditions. Measurements of the unsteady pressure were performed using six high-response pressure transducers. To distinguish rotating stall from surge, three of these transducers were located thirty degrees apart in the circumferential direction at the vaneless space upstream of the diffuser vane. Three types of pressure fluctuation in the low flowrate region were confirmed. The first one occurred in the positive slope region of the total-system head curve with the largest amplitude and the lowest frequency under the full opening condition of IGVs. The second one was the spike-like fluctuation propagating in the circumferential direction in the vaneless space. The last one appeared under the partial opening condition of IGVs with smaller amplitude and higher frequency than those of the first one. It was confirmed that this pressure fluctuation oscillated at the same phase in the vaneless space and occurred even in the steeply negative slope region of the total-system head curve under the minimum opening condition of IGVs. It was considered that the first one was deep surge and the second one was rotating stall. The last one was considered to be mild surge. Under the minimum opening condition of IGVs, the slope of the head curve of the first stage becomes steeply negative by prewhirl. This negative slope stabilizes the total system of the blower. By contrast, the slope of the head curve of the other stages becomes positive in the low flowrate region. This positive slope destabilizes the total system of the blower. Therefore, even if the total-system head curve keeps a steeply negative slope, the system operating point is considered to oscillate slightly, corresponding to mild surge situation, due to these stability balances.