Hongyuan Xu

Numerical Simulation of Cavity Shedding from a Three-Dimensional Twisted Hydrofoil and Induced Pressure Fluctuation by Large-Eddy Simulation

Simulation of cavity shedding around a three-dimensional twisted hydrofoil has been conducted by large eddy simulation coupling with a mass transfer cavitation model based on the Rayleigh-Plesset equation. From comparison of the numerical results with experimental observations, e.g., cavity shedding evolution, it is validated that the unsteady cavitating flow around a twisted hydrofoil is reasonably simulated by the proposed method. Numerical results clearly reproduce the cavity shedding process, such as cavity development, breaking-off and collapsing in the downstream. Regarding vapor shedding in the cavitating flow around three-dimensional foils, it is primarily attributed to the effect of the re-entrant flow consisting of a re-entrant jet and a pair of side-entrant jets. Formation of the re-entrant jet in the rear part of an attached cavity is affected by collapse of the last shedding vapor. Numerical results also show that the cavity shedding causes the surface pressure fluctuation of the hydrofoil and the force vibration. Accompanying the cavity evolution, the wave of pressure fluctuation propagates in two directions, namely, from the leading edge of the foil to the trailing edge and from the central plane to the side of the hydrofoil along the span. It is seen that the large pressure fluctuation occurs at the central part of the hydrofoil, where the flow incidence is larger.

Unsteady Numerical Simulation of Cavitating Turbulent Flow Around a Highly Skewed Model Marine Propeller

The cavitating flows around a highly skewed model marine propeller in both uniform flow and wake flow have been simulated by applying a mass transfer cavitation model based on Rayleigh‐Plesset equation and k- shear stress transport (SST) turbulence model. From comparison of numerical results with the experiment, it is seen that the thrust and torque coefficients of the propeller are predicted satisfactory. It is also clarified from unsteady simulation of cavitating flow around the propeller in wake flow that the whole process of cavitating-flow evolution can be reasonably reproduced including sheet cavitation and tip vortex cavitation observed in the experiments. Furthermore, to study the effect of pressure fluctuation on the surrounding, pressure fluctuations induced by the cavitation as well as the propeller rotation are predicted at three reference positions above the propeller for comparison with the experimental data: The amplitudes of the dominant components corresponding to the first, second, and third blade passing frequencies were satisfactorily predicted. It is noted that the maximum difference of pressure fluctuation between the calculation and experiment reached 20%, which might be acceptable by usual engineering applications. DOI: 10.1115/1.4003355

Experimental study of the pressure fluctuations in a pump turbine at large partial flow conditions

Frequent shifts of output and operating mode require a pump turbine with excellent stability. Current researches show that large partial flow conditions in pump mode experience positive-slope phenomena with a large head drop. The pressure fluctuation at the positive slope is crucial to the pump turbine unit safety. The operating instabilities at large partial flow conditions for a pump turbine are analyzed. The hydraulic performance of a model pump turbine is tested with the pressure fluctuations measured at unstable operating points near a positive slope in the performance curve. The hydraulic performance tests show that there are two separated positive-slope regions for the pump turbine, with the flow discharge for the first positive slope from 0.85 to 0.91 times that at the maximum efficiency point. The amplitudes of the pressure fluctuations at these unstable large partial flow conditions near the first positive slope are much larger than those at stable operating condtions. A dominant frequency is measured at 0.2 times the impeller rotational frequency in the flow passage near the impeller exit, which is believed to be induced by the rotating stall in the flow passage of the wicket gates. The test results also show hysteresis with pressure fluctuations when the pump turbine is operated near the first positive slope. The hysteresis creates different pressure fluctuations for those operation points even though their flow rates and heads are similar respectively. The pressure fluctuation characteristics at large partial flow conditions obtained by the present study will be helpful for the safe operation of pumped storage units.

Numerical investigation of unsteady cavitating turbulent flow around a full scale marine propeller

This paper treats the unsteady cavitating turbulent flow around a full scale marine propeller operated in non-uniform ship wake. The RANS method combined with k−ω SST turbulence model and the mass transfer cavitation model was applied for the flow simulation. It is noted that both the propeller performance and the unsteady features of cavitating turbulent flow around the propeller predicted by the numerical calculation agreed well with the experimental data. Due to the non-uniform wake inflow and gravity effect, there occurred periodical procedure for cavity development such as cavitation inception, growth, shrinking, etc near the blade tip for the propeller. The study also indicated that there was considerably large pressure fluctuation near the propeller during the operation. The 1st order frequency of pressure fluctuation predicted by numerical simulation equaled the rotating frequency of propeller blades. Both amplitude and frequency agreed with the experimental results fairly well.

Numerical Simulation of the Unsteady Flow in a High-Head Pump Turbine and the Runner Improvement

The unsteady flow in a high-head pump-turbine whose head-discharge curve has the positive slopes at high-partial-load operation condition was investigated. It is noted that the numerical methods is very important for predicting this kind of head-discharge curve with positive slopes, and better agreement between calculation results and experimental data was achieved by using Spalart-Allmaras turbulence model and mesh strategy with y+ controlling for numerical simulation. From the analysis of hydraulic losses at different parts in the pump turbine, it is found that the head loss at the flow passage of the guide vane and stay vane was not small at pump mode. In order to make clear the reason why the positive slopes at head-discharge curve occur, the flow between the impeller exit and the inlet of spiral casing was checked carefully. Much intensive vortex was observed near the impeller shroud, and there was strong rotor stator interaction for those operation conditions with positive slope. It is suspected the instability such as positive slope at head-discharge curve was resulted from the vortex formation near the flow channel wall. Based on the flow analysis, the runner optimization was conducted so as to mitigate the intensive rotor stator interaction. It is noted that the pressure fluctuation as well as the flow pattern was improved by applying the optimized impeller.Copyright

Cavitation in semi-open centrifugal impellers for a miniature pump

Cavitation in miniature pumps was investigated experimentally for two semi-open centrifugal impellers. Although both impellers had the same blade cross-section, one impeller had a two-dimensional blade, while the other had a leaned blade. The flows were also analyzed using a numerical model of the three-dimensional turbulent flow in the pumps near the peak efficiency point using the k-ɛ turbulence model and the VOF cavitation model. The average cavitation performance of each impeller was satisfactorily predicted by the numerical simulations. The results show that the miniature pumps have similar cavitation performances as an ordinary-size pump, with the cavitation performance of the semi-open impeller reduced by increased axial tip clearances. Also, both the hydraulic and cavitation performance of the semi-open impeller were improved by the leaned blade. The results also show that uniform flow upstream of the impeller inlet will improve the cavitation performance of a miniature pump.

NUMERICAL AND EXPERIMENTAL STUDY ON UNSTEADY SHEDDING OF PARTIAL CAVITATION

Periodically unsteady shedding of partial cavity and forming of cavitation cloud have a great influence on hydraulic performances and cavitation erosion for ship propellers and hydro machines. In the present study, the unsteady cavitating flow around a hydrofoil has been calculated by using the single fluid approach with a developed cavitation mass transfer expression based on the vaporization and condensation of the fluid. The numerical simulation depicted the unsteady shedding of partial cavity, such as the process of cavity developing, breaking off and collapsing in the downstream under the steady incoming flow condition. It is noted that good agreement between the numerical results and that of experiment conducted at a cavitation tunnel is achieved. The cavitating flow field indicates that the cavity shedding was mainly caused by the re-entrant jet near cavity trailing edge, which was also clearly recorded by high-speed photographing.

Hysteresis phenomena in hydraulic measurement

Hysteresis phenomena demonstrate the lag between the generation and the removal of some physical phenomena. This paper studies the hysteresis phenomena of the head-drop in a scaled model pump turbine using experiment test and CFD methods. These lag is induced by complicated flow patterns, which influenced the reliability of rotating machine. Keeping the same measurement procedure is concluded for the hydraulic machine measurement.

Investigation on the Flow Field Upstream of a Centrifugal Pump Impeller

The flow upstream of a centrifugal pump impeller has been investigated by both experimental test and numerical simulation. For experimental study, the flow field at four sections in the pump suction is measured by using PIV method. For calculation, the three dimensional turbulent flow for the full flow passage of the pump is simulated based on RANS equations combined with RNG k- turbulence model. From those results, it is noted that at both design lo ad and quarter load condition, the pre-swirl flow whose direction is the same as the impeller rotation exists at all four sections in suction pipe of the pump, and at each section, the pre-swirl velocity becomes obviously larger at higher rotational speed. It is also indicated that at quarter load condition, the low pressure region at suction surface of the vane is large because of the unfavorable flow upstream of the pump impeller.

Design optimization for a shaft-less double suction mini turbo pump

In order to further satisfy the operation needs for social applications, a shaft-less double suction mini turbo pump with outer impeller diameter of 24 mm and specific speed of 188 min-1⋅m3min-1⋅m has been designed. In order to simulate the three dimensional steady turbulent flow in the mini pump so as to improve the pump impeller design, RANS equations and k-ω SST turbulence model are used. Based on the detailed analysis of the internal flow in the pump, six new impellers have been designed to investigate the effects of impeller parameters on the performance of the mini pump. Based on those results, the following conclusions are drawn: (1) For the double-suction shaft-less mini turbo pump, the averaged wall shear stress has very low level and the maximum hydraulic efficiency is larger than 80%. Those favourable features must be related to the symmetric suction design of the mini pump; (2) Large vane angle at the trailing edge is suitable for a mini turbo pump in many applications so as to obtain higher head and smaller impeller size. On the other hand, the impellers with β1 =90° may result in large wall shear stress at the vane leading edge at small flow rate; (3) Because the radial impeller is much convenient for manufacture and creates much larger head, it is preferable for a mini turbo pump if the wall shear stress can be controlled within the acceptable range due to further design optimization.