JAPANThis paper treats the flow instabilities in a mixed flow pump with a vaned diffuser. Test pump has a positive slope of a head-flow performance curve at 65% flow rate of BEP (Best Efficiency Point) because of a rotating stall. Dynamic Particle Image Velocimetry (PIV) and pressure fluctuation measurements are used for investigating the propagation mechanism of a rotating stall. It was found that unstable performance was caused by periodical large scale abrupt backflow generated from the vaned diffuser to the outlet of impeller. Further, the relation between the static pressure at the inlet of diffuser vane and the internal flow condition was clarified. From these experimental results, in order to improve the positive slope of a head-flow performance curve, to suppress the growth of strong vortex toward the inlet of diffuser vane was proved to be a key point. K eywords: mixed flow pump, vaned diffuser, flow instabilities, rotating stall, dynamic PIV. CLC number: TK72 Document code: A

The difference of pump characteristics between two kinds of mixed flow pumps with low specific speed of 350 (min -1 , m 3 /min, m), which have the same impeller and the different diffuser vanes, is presented in the present paper. It was confirmed from the previous study that a diffuser rotating stall (DRS) occurs in the original type of mixed flow pump at about 65% flow rate of best efficiency point (BEP) and there is an abrupt drop of the total head characteristic. The relationship between pump characteristic instability and internal flow is investigated in detail by using a dynamic PIV measurement system (DPIV) and a commercial CFD code. As a result, the cause of characteristic instability is supposed for the original type as follows. The flow on the vaned diffuser hub-side becomes unstable due to adverse pressure gradient and strong backflow occurs at partial flow rate. Then it impinges against downstream flow from the impeller and the secondary flow from hub-to casing-sides occurs. This secondary flow blocks the downstream flow from the impeller and the inlet flow angle at the leading edge of adjacent diffuser vane decreases. Therefore, the flow separates on the suction surface of the adjacent diffuser vane inlet and a strong vortex is generated. After that, it develops and becomes a stall core. Next, the modified type of pump, where only diffuser vanes are modified, is tested. As a result, the flow rate, at which characteristic instability occurs, is shifted to lower one and the pump operating range becomes widened. It is clarified upon above considerations that the secondary flow has been restricted and diffuser performance has been improved in comparison with the original type.

The flow instability in a low specific speed mixed-flow pump, having a positive slope of head-flow characteristics was investigated. Based on the static pressure measurements, it was found that a rotating stall in the vaned diffuser occurs at about 65% flow rate of best efficiency point (BEP). A dynamic Particle Image Velocimetry (DPIV) measurement and the numerical simulations were conducted in order to investigate the flow fields. As a result, the diffuser rotating stall was simulated even by Computational Fluid Dynamics (CFD) and the calculated periodic flow patterns agree well with the measured ones by DPIV. It is clarified that a periodical large scaled backflow, generated at the leading edge of the suction surface of the diffuser vane, causes the instability. Furthermore, the growth of the strong vortex at the leading edge of the diffuser vane induces the strong backflow from the diffuser outlet to the inlet. The scale of one stall cell is covered over four-passages in total thirteen vane-passages.

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.

The purpose of this paper is to present optimization method of an inducer blade shape to improve its suction performance and clarify the relationship between pump performance and design parameters. In order to conduct the optimization process a response surface based optimization framework was established. Baseline was designed in previous research [1]. The inducers were 3Dprinted in ABS plastic and their wetted and cavitating characteristics were measured. It was confirmed that the optimized inducer can maintain its wetted performance at lower cavitation numbers. A response surface is a mathematical model that approximates the relationship between the input parameters and the objective function from a finite number of learning points within the design space. The design space was defined by four parameters: sweep angle, sweep radius, incidence angle and blade solidity at the tip that controlled the blade shape. The performance of each design was evaluated with a CFD simulation established in a commercial solver. The optimization goal was to minimize the critical cavitation number that corresponds to a 5% drop of pressure increase through the pump due to cavitation. A starting point of the optimization was the industrial pump designed by a Japanese company Teral [1]. The results of the numerical optimization show that the critical cavitation number was decreased by 17.6% with respect to the baseline design. In the experimental results, an average improvement of 15.4% was achieved.