The LDV Measurement Of Axial-flow Pump Impeller Internal3D Flow Field

Internal flow of the axial-flow impeller has important influence on the hydraulic performance, the cavitation characteristics and the operational stability of the pumps. At present, the CFD technique is the major method to analyze internal3D flow field to disclose the internal flow mechanism and the flow phenomena in the axial-flow impeller. But, most of its numerical simulation results are short of effective experimental verification. Therefore, it is necessary to increase the test research on the internal flow field of the axial-flow pump in order to support the development of high performance axial-flow pump.This study measures the internal3D flow field of the axial-flow impeller with3D-LDV. It is4-blade axial-flow impeller. The ratio of hub is0.4. The specific speed is about900. A long straight tube is installed in front of the impeller and used to insure that the inlet flow is a steady uniform current. The flow rates of the five test conditions are respectively0.52Q,0.70Q,0.85Q,1.0Q and1.2Q."Q" is the flow rate of the highest efficiency condition. The phase locking technique is used to test the3D velocity along the12radial test lines inside and outside the impeller. There are36points per line, and at the ends of these lines, there are more points than in the middle parts.360points are arranged along the circumferential direction. At the inlet, one test line is arranged, while at the outlet, two test lines are arranged. What’s more,3D velocity of the5conditions at both the inlet and outlet is measured.9test lines are arranged in the impeller and3D velocity of the conditions of0.52Q,1.0Q and1.2Q is measured.Impeller inlet and outlet experimental results indicate that there is obvious turbulent pulsation in the3D velocity and along radial and circumferential orientation,3D velocity change is also obvious. At impeller inlet (Test Line a), the radial distribution of circumferentially averaged axial velocity is relatively even and the boundary effect is not obvious. On high flow conditions, axial velocity is higher on the hub side. On low flow conditions (Condition One and Condition Two), axial velocity is negative on the blade tip, which means there is backflow, and accordingly, there is obvious circumferential velocity and prewhirl flow. At impeller outlet (Test Line b and c), the radial distribution of circumferentially averaged velocity is different from each other on different conditions. There is obvious boundary effect in the radial distribution of axial velocity and the major flow concentrates in the middle. On low flow conditions, the major flow of axial velocity leans towards blade tip and there is backflow on the hub side. The trend of the radial distribution of circumferential velocity is that it is high at both ends and it is low in the middle. As the flow decreases, circumferential velocity on the blade tips is rising obviously. The circumferential distribution of the phase average velocity at the inlet is typical cascade flow; axial velocity is low near the blades and is high on the middle of the vane pitch. From the blade back to the pressure side of blade, circumferential velocity changes from positive to negative. The circumferential distribution of the phase average velocity at the outlet is approximately cascade flow type, but velocity fluctuates severely, the lower the flow, the severer the velocity.Impeller internal flow experimental results show that the major flow of axial velocity first concentrate in the middle of the groove, and then gradually lean towards blade back as the water flow from the inlet to the outlet; and the circumferential velocity amplifies; inside of the impeller, near the inlet side, there is negative circumferential velocity near the blade back; radial velocity fluctuates at axial section, and it is higher in the middle of the section; on low flow conditions, there is backflow near the inwall of the inlet; water rotates severely inside the impeller and as the flow rate amplifies, circumferential velocity become smaller, therefore, on high flow rate condition, the water almost does not rotates; there is also backflow near the hub on measured condition and the closer to the outlet, the larger the backflow is; the larger the flow rate is, the smaller the backflow becomes.