Akinori Furukawa

Theoretical Analysis of Transitional and Partial Cavity Instabilities

We describe a new time marching calculation of blade surface cavitation based on a linearized free streamline theory using a singularity method. In this calculation, closed cavity models for partial and super cavities are combined to simulate the transitional cavity oscillation between partial and super cavities. The results for an isolated hydrofoil located in a 2-D channel are presented. Although the re-entrant jet is not taken into account, the transitional cavity oscillation with large amplitude, which is known to occur when the cavity length exceeds 75 percent of the chord length, was simulated fairly well. The partial cavity oscillation with relatively high frequency was simulated as damping oscillations

Steady Analysis of the Thermodynamic Effect of Partial Cavitation Using the Singularity Method

It is well known that the suction performance of turbopumps in cryogenic fluids is much better than that in cold water because of the thermodynamic effect of cavitation. In the present study, an analytical method to simulate partially cavitating flow with the thermodynamic effect in a cascade is proposed; heat transfer between the cavity and the ambient fluid is modeled by a one-dimensional unsteady heat conduction model under the slender body approximation and is coupled with a flow analysis using singularity methods. In this report, the steady analysis is performed and the results are compared with those of experiments to validate the model of the present analysis. This analysis can be easily extended into unsteady stability analysis for cavitation instabilities such as rotating cavitation and cavitation surge.

Pressure Fluctuation in a Vaned Diffuser Downstream from a Centrifugal Pump Impeller

Periodic flows downstream from a centrifugal pump impeller in vaneless and vaned diffusers were measured by using a single hole yawmeter and a phase-locked sampling method. The flows were also calculated by an inviscid flow analysis using the blade-surface singularity method. The periodic variations in calculated static pressure with the impeller rotating quantitatively agree well with the measured ones. The flow behaviors in the vaned diffuser are discussed, citing measured and calculated results. The potential interaction between the impeller and the diffuser blades appears more strongly than the impeller-wake interaction. The appearance of static pressure fluctuations due to the impellers rotating in the fully vaned zone is different from that in the semivaned zone of the diffuser. The existence of the peripheral blade surface of the impeller outlet with an outlet edge of the pressure surface causes violent pressure fluctuations in the vaned diffuser.

Analysis of Thermodynamic Effects on Cavitation Instabilities

The suction performance of turbopumps in cryogenic fluids is basically much better than that in cold water because of the thermodynamic effect of cavitation. However, it is not still clear how the thermodynamic effect works on cavitation instabilities, such as rotating cavitation and cavitation surge. In the present study, the unsteady heat exchange between the cavity and the surrounding liquid is taken into account in a stability analysis using a singularity method. The results are qualitatively compared to existing experiments to clarify the research needs for deeper understanding.

Low Speed Design of Rear Rotor in Contra-Rotating Axial Flow Pump

Abstract The application of contra-rotating rotors for higher specific speed pump has been proposed in our studies, which is in principle effective for reducing the rotational speed and/or the pump size under the same specification of conventional axial flow pump. In the previous experiments of our prototype, the cavitation inception at the tip region of the rear rotor rather than that of the front rotor and the strong potential interaction from the suction surface of the rear rotor blade to the pressure surface of the front one were observed, indicating the possibility to further improve the pump performance by optimizing rotational speed combination between the two rotors. The present research aims at the design of rear rotor with lower rotational speed. Considering the fact that the incoming flow velocity defects at the tip region of the rear rotor, an integrated inflow model of ‘forced vortex’ and ‘free vortex’ is employed. The variation of maximum camber location from hub to tip as well as other related considerations are also taken into account for further performance improvement. The ideas cited above are separately or comprehensively applied in the design of three types of rear rotor, which are subsequently simulated in ANSYS CFX to evaluate the related pump performance and therefore the whole low speed design idea. Finally, the experimental validation is carried out on one type to offer further proofs for the availability of the whole design method.

A Behavior of the Diffuser Rotating Stall in a Low Specific Speed Mixed-Flow Pump

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.

Theoretical analysis of thermodynamic effect of cavitation in cryogenic inducer using singularity method

Vapor production in cavitation extracts the latent heat of evaporation from the surrounding liquid, which decreases the local temperature, and hence the local vapor pressure in the vicinity of cavity. This is called thermodynamic/thermal effect of cavitation and leads to the good suction performance of cryogenic turbopumps. We have already established the simple analysis of partially cavitating flow with the thermodynamic effect, where the latent heat extraction and the heat transfer between the cavity and the ambient fluid are taken into account. In the present study, we carry out the analysis for cavitating inducer and compare it with the experimental data available from literatures using Freon R-114 and liquid nitrogen. It is found that the present analysis can simulate fairly well the thermodynamic effect of cavitation and some modification of the analysis considering the real fluid properties, that is, saturation characteristic, is favorable for more qualitative agreement.

Air/Water Two-Phase Flow Performance of Contra-Rotating Axial Flow Pump and Rotational Speed Control of Rear Rotor

An application of contra-rotating rotors, consisting of front and rear rotors rotating in the opposite direction from each other, has been proposed against a demand for developing a higher specific speed axial flow pump with a more compact structure, higher efficiency and higher cavitation performance. As axial flow pumps are used for standby operations of air-lock and air/water mixing discharge to prevent floods, air/water two-phase flow performance of the contra-rotating pump has to be also investigated. In the present paper, therefore, experimental results on air/water two-phase flow performance of a test pump with contra-rotating rotors are shown and compared with those of a conventional axial flow pump, consisting of a front rotor and a rear stator. Even under two-phase flow conditions head characteristic curve of the contra-rotating type has a more strongly negative slope than that of the conventional type. The contra-rotating type maintains higher head and higher efficiency even in the low flow rate range and vice versa in the high flow rate range. This result will be discussed by considering the change of outlet flow from front rotor due to two-phase flow with the help of observed air behavior in the rotors. Then effects of changes of rear rotor rotational speed different from front rotor speed, which is an advantage of the contra-rotating axial flow pump, on two-phase flow performance are examined. Under the condition of constant ratio of air to water flow rates, the head rise of the rear rotor linearly increases with rear rotor rotational speed. Air/water two-phase flow performance of the contra-rotating axial flow pump can be improved by this control procedure for the rear rotor rotational speed.Copyright

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

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.Copyright

Influences of Impeller Diameter and Diffuser Blades on Air-Water Two-Phase Flow Performance of Centrifugal Pump

A tandem arrangement of double rotating cascades and single diffuser cascade, proposed as a centrifugal pump with high performance in air-water two-phase flow condition, yields lower head due to the smallness of the impeller outlet in comparison with a impeller with large outlet diameter and no diffuser. Influences of impeller diameter change and installation of diffuser blades on two-phase flow performance were experimentally investigated under the case of the same volute casing. As the result, the similarity law of the diameter of impeller having the similar blade geometry and the rotational speed is satisfied even in two-phase flow condition. Comparing pump performances between a large impeller without diffuser blades and a small one with diffuser blades, higher two-phase flow performance is obtained by controlling the rotational speed of a small impeller with diffuser blades in the range of small water flow rates, while a large impeller with no diffuser gives high performance in the range of high water flow rate and small air flow rate.Copyright