SUPPRESSION OF UNSTABLE FLOW IN CENTRIFUGAL BLOWERS BY THREE-DIMENSIONAL SEPARATION CONTROL

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In most low specific speed type centrifugal impellers having backward leaning blade, the unstable flow such as rotating stall occurs in the vaneless diffuser and it is caused by the 3-D boundary layer separation mainly on the hub side wall not on the shroud side wall. On the other hand, in most high specific speed type open shrouded centrifugal impellers with radial blade, the unstable flow is caused by the flow separation in the inducer due to the excessive deceleration of relative velocity. The objective of this study is to suppress the unstable flow in each type of centrifugal blower without deteriorating blower characteristics.By positioning the completely rough wall locally on the hub side diffuser wall alone in the vaneless diffuser, the flow rate of rotating stall inception was decreased by 42% at a small pressure drop less than 1%. This is based on the fact that the local reverse flow occurs firstly in the hub side in most centrifugal blowers with the backswept blade impeller. … More The 3-D boundary layer calculation shows that the increase in wall shear component normal to the main-flow direction decreases markedly the skewed angle of the 3-D boundary layer, and results in suppression of the 3-D separation. It is also clarified theoretically that the diffuser pressure recovery is hardly deteriorated by the rough wall positioned downstream of R=1.2 because the increase in the radial momentum change, resulting from reduction in the skewed angle of the 3-D boundary layer, supports the adverse pressure gradient.A ring groove arrangement was proposed to suppress unstable flow in a centrifugal blower with the radial blade impeller. The ring groove arrangement connects the impeller inlet upstream and the inducer throat tip, and forms an inlet recirculation through a bypass for the two regions. The inlet recirculation at small flow rates results in a decrease in the flow rate of unstab1e flow inception. An up to 8% improvement in surge margin was obtained by the ring groove arrangement at a small expense of the delivery pressure drop. A 3-D turbulent flow calculation was conducted to investigate effect s of the inlet recirculation on the main through-flow and unstable flow suppression in the impeller. The calculated results were compared with the experimental ones. It is found that the critical flow incidence is about 7.4 degrees for separation bubble formation at the blade leading edge, and the surge suppression is mainly due to decrease in flow incidence at the impeller leading based on the inlet recirculation flow. Less