This study experimentally examines unstable flow phenomena occur in a five-stage centrifugal blower equipped with inlet guide vanes (IGVs) before the first stage.

High response pressure transducers were mounted on a suction nozzle, an U-turn bend between a diffuser and return channel in the second stage, and a discharge nozzle to detect surge. Moreover, at a vaneless space between an impeller and a vaned diffuser in the second stage, three pressure transducers were located thirty degrees apart in the circumferential direction to distinguish rotating stall in the diffuser from surge. Measurements were performed at several flowrates changing the IGVs from fully opening condition to partially opening conditions.

We confirmed three types of pressure fluctuation in low flowrate region. The first is the fluctuation showing the largest amplitude and the lowest frequency (about 7–9 percent of impeller rotation frequency) and observed at all the sensors. This appears in the positive slope region of the total system head curve. The second is the spike-like fluctuation intermittently observed only in the vaneless space. This occurs at the IGVs partially opening conditions and shows observed time lag among the three sensors. The last is the fluctuation with smaller amplitude and higher frequency (about 14–23 percent of impeller rotation frequency) than that of the first and observed at all the sensors. This occurs at the IGVs partially opening conditions. At 4.5 percent IGVs opening, this fluctuation is observed in the steep negative slope region of the total system head curve, where the whole compression system is supposed to be stable. Moreover, this fluctuation is measured at the same phase in the vaneless space.

We concluded that the first-mentioned fluctuation is deep surge and the second-mentioned fluctuation is rotating stall in the diffuser. On the other hand, we have considered the last-mentioned fluctuation to be mild surge and the reason why this occurs in the negative slope region to be as follows. At 4.5 percent IGVs opening, slope of the first stage head curve becomes steeply negative by prewhirl and this negative slope stabilizes the system. By contrast, at the second to the last stage, the slope of the head curve becomes positive in low flowrate region and this positive slope destabilizes the system. Therefore, even if the total system head curve maintains a steep negative slope, the system operating point oscillates slightly (mild surge situation) due to a balance of these stabilization and destabilization effects.

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