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

为了阐明高比转速开式诱导轮离心叶轮内部不稳定流动的形成机理，对叶轮内部进行了三维湍流数值模拟和实验研究，得到了以下结论。(1)At首先，发现定常流模拟对理解非定常失速现象是非常有帮助的。（2）由于叶尖泄漏流与在导流轮叶尖喉部紧下游的压力侧积聚的反向流之间的相互作用而产生的卷起涡流，出现了初期不稳定流。(3)By形成进口回流，叶尖泄漏涡被吸入吸环槽，同时减小了流动冲角。（4）通过采用环形槽布置和窄扩压器宽度，在不影响叶轮性能的情况下，试验风机的不稳定流量范围减小了约45%。(5)By通过对前后环槽的优化配置，使试验叶轮在125%流量下的不稳定流动范围减小了50%左右，而叶轮性能没有恶化;为了获得更宽的工作范围，在环槽布置的环形旁路内沿周向安装了导叶，并改变了导叶的安装角。（6）喘振抑制的基本概念是使气流冲角小于但接近临界冲角。(7)A通过抑制由回流引起的预旋，在接近临界冲角的冲角处获得了诱导轮中较高的压力升高。（8）试验叶轮的不稳定流量范围减少了约53%，在整个流量范围内叶轮效率几乎没有恶化。

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.