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

在大多数低比速型后倾叶片离心叶轮中，旋转失速等不稳定流动发生在无叶扩压器内，主要是由轮毂侧壁而非叶冠侧壁的三维边界层分离引起的。另一方面，在大多数高比速型径向叶片开式冠状离心叶轮中，由于相对速度的过度减速，导致了诱导管内的流动分离，从而导致了不稳定的流动。本研究的目的是在不恶化风机特性的情况下抑制各类型离心式鼓风机的不稳定流量。通过在无叶扩压器中仅在轮毂侧扩压器壁上局部放置完全粗糙的壁面，在小于1%的小压降下，旋转失速发生的流速降低了42%。这是基于大多数后掠叶片叶轮离心式鼓风机的局部逆流首先发生在轮毂侧的事实。三维边界层计算结果表明，增加垂直于主流方向的壁面剪切分量可显著降低三维边界层的偏斜角度，从而抑制三维分离。从理论上也阐明了扩散器压力恢复几乎不会因为R=1.2下游的粗糙壁面而恶化，这是由于三维边界层倾斜角度的减小导致径向动量变化的增加，从而支持了逆压梯度。为了抑制径向叶片叶轮离心鼓风机内的不稳定流动，提出了一种环形槽布置方法。环形槽布置将叶轮进口上游与诱导器喉部尖端连接，并通过两个区域的旁路形成进口再循环。小流量进口再循环使不稳定流起始流量减小。通过环形槽布置，以较小的输送压降为代价，获得了高达8%的喘振裕度改善。通过三维紊流计算，研究了进口再循环对叶轮内主通流和抑制不稳定流动的影响。计算结果与实验结果进行了比较。研究发现，叶片前缘分离泡形成的临界气流攻角约为7.4度，抑制喘振的主要原因是在进口再循环流量的基础上减小了叶轮前缘的气流攻角。少

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