Verification of Unstable Flow Mechanism due to Inducer Tip Leakage Vortex in a Centrifugal Blower

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In order to clarify the mechanism of unstable flow inception in a high specific speed type open shrouded centrifugal impeller with inducer, 3-D turbulent flow simulations and experimental works as well were performed and the following concluding remarks are obtained. (1)At first, it is found that the steady flow simulation was very helpful for understanding unsteady stall phenomena. (2)The incipient unstable flow occurs due to a rolling-up vortex flow resulting from an interaction between the tip leakage flow and the reverse flow accumulated at the pressure side immediately downstream of the inducer tip throat. (3)By forming the inlet recirculation flow, the tip leakage vortex is sucked into the suction ring groove, and the flow incidence is decreased simultaneously. (4)The unstable flow range of the test blower was reduced by about 45% without deteriorating the impeller characteristics by implementing both the ring groove arrangement and the narrowed diffuser width. (5)By the optimized configuration of the front and rear ring grooves, the unstable flow range of the test impeller was reduced by about 50% without deterioration of the impeller characteristic even at the 125% flow rate.In order to obtain a wider operating range, guide vanes were installed circumferentially in the annular bypass of the ring groove arrangement, and the setting angle of the guide vane was changed. (6)The fundamental concept for surge suppression is to achieve the flow incidence less than but close to the critical one. (7)A higher pressure rise in the inducer is obtained at the flow incidence close to the critical one by suppressing the pre-whirl which is induced by the recirculation flow. (8)The unstable flow rate range of the tested impeller was reduced by about 53% almost without deterioration of the impeller efficiency in the whole flow rate range.