Rotating Stall Behavior in a Diffuser of Mixed Flow Pump and Its Suppression

Abstract

The relationship between pump characteristic instability and internal flow was investigated on a mixed flow pump with specific speed ωs = 1.72 (dimensionless) or 700 (m3/min, m, min−1) by using a commercial CFD code and a dynamic PIV (DPIV) measurement. As a result, it was clarified that the diffuser rotating stalls causes the positive slope of a head-flow characteristic and the backflow at hub-side of the vaned diffuser plays an important role on the onset of the diffuser rotating stall. The complex behaviors of diffuser rotating stall were visualized by the DPIV measurements and CFD simulation. Moreover, the internal flow was investigated in detail and the inception of characteristic instability was presumed as follows: At the partial flow rate, low energy fluids are accumulated in the corner between the hub surface and the suction surface of the diffuser vane. As the flow rate is further decreased, the low energy fluids region at the corner axi-symmetrically expands along the hub and become unstable due to adverse pressure gradient. Then, strong backflow occurs and impinges against passage flow from the impeller at the inlet of the vaned diffuser. In addition, the backflow blocks the passage flow from impeller and the inlet flow angle at the leading edge of adjacent diffuser vane is reduced. Therefore, flow separation occurs near the inlet of suction surface of the vaned diffuser, and a strong vortex is generated there. After that, the vortex develops and becomes a stall core. Based on above considerations, pump design parameter studies were numerically carried out and diffuser rotating stall was suppressed and pump characteristic instability was controlled by enlarging the inlet diameter of diffuser hub.

Dynamic Characteristics of Rotating Stall in Mixed Flow Pump

“…To suppress the rotating stall in the diffuser, Miyabe et al enlarged the diameter of the hub at the diffuser inlet and placed a swirl stop at the pump entrance. The positive slope of the head-flow performance still occurred at partial flow rate [16]. Figure 10, which is compared with the numerical simulation result of a conventional pump.…”

“…He concluded that a rotating stall is flow instability caused by a positive slope of the head-flow rate performance curve, whereby the rotational frequency of rotating stall depends on the performance and the geometry of the rotor. Miyabe et al visualized the unsteady internal flow in diffuser with Dynamic PIV [15,16]. The authors clarified that the diffuser rotating stall causes the positive slope of a head-flow characteristic, and the backflow (i.e., a stall cell or a vortex) at the hub side of the diffuser plays an important role in the onset of a diffuser rotating stall.…”

Influence of different blade numbers on the performance of “saddle zone” in a mixed flow pump

“…Miyabe et al. 25,26 used dynamic PIV technology to test and measure a mixed-flow pump, and found that the backflow in the guide vane played a crucial role in the occurrence of stall, and stall always occurred along with the decrease of flow rate. Zhu 27 found that the low momentum region near the suction side of the blade grows upstream as the flow rate decreases, resulting in a smaller stall margin for mixed-flow compressors, and that two different forms of tip leakage are the main cause of the low momentum region.…”

Numerical and experimental investigation of the pressure fluctuation in a mixed-flow pump under low flow conditions

“…Energy performance and flow patterns of a mixed-flow pump with different tip clearances were investigated by Hao et al 7 and Liu et al, 8 based on which the influence of blade rotational angle and T-shape tip clearance was also studied. 9,10 DPIV was employed by Miyabe et al [11][12][13][14][15][16] to illustrate the inner flow of the vane. It was found that the periodic backflow at the inlet of guide vane under the low flow condition is one of the reasons for the positive slope of the performance curve.…”