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

为了弄清高比速带诱导轮的开闭式离心叶轮内不稳定起动的机理，进行了三维湍流模拟和实验研究，得到了以下结论。(1)首先，定常流动模拟对了解非定常失速现象有很大的帮助。(2)由于叶尖泄漏流和导叶喉部正下游压力侧积累的逆流相互作用，形成卷涡流，从而产生了初始不稳定流。(3)通过形成进口回流，将叶尖泄漏涡吸进吸力环槽内，同时减小了流动攻角。(4)通过采用环槽布置和缩小扩散器宽度，在不影响叶轮性能的情况下，试验风机的不稳定流动范围减小了约45%。(5)通过优化前后环槽的结构，使试验叶轮在125%流量下的不稳定流动范围减小了约50%，同时为了获得更宽的工作范围，在环槽布置的环形旁通处设置了导叶，并改变了导叶的安装角度。(6)喘振抑制的基本概念是使水流入射小于但接近临界点。(7)通过抑制回流引起的预旋流，在接近临界点的流量冲角处，可以获得较高的导流板升压。(8)在整个流量范围内，叶轮的不稳定流量范围减小了约53%，而叶轮的效率几乎没有下降。

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.