B.P.M. van Esch

Experimental and theoretical study of the flow in the volute of a low specific-speed pump

The flow in the volute of a low specific-speed pump was studied both experimentally and numerically near its design point. Measurements included time-averaged values of velocity and static pressure at a large number of locations in the volute. The numerical computations were based on the unsteady three-dimensional potential flow model for the core flow. Viscous losses were quantified using additional models that use the potential flow as input. It is shown that near the design point of this pump, the core flow behaves like a potential flow, provided that no boundary layer separation occurs. Explanations are given for the presence of local deviations due to secondary flow. These local deviations do not influence the overall potential flow characteristics significantly.

Hydraulic Performance of a Mixed-Flow Pump: Unsteady Inviscid Computations and Loss Models

The hydraulic performance of an industrial mixed-flow pump is analyzed using a three-dimensional potential flow model to compute the unsteady flow through the entire pump configuration. Subsequently, several additional models that use the potential flow results are employed to assess the losses. Computed head agrees well with experiments in the range 70 percent–130 percent BEP flow rate. Although the boundary layer displacement in the volute is substantial, its effect on global characteristics is negligible. Computations show that a truly unsteady analysis of the complete impeller and volute is necessary to compute even global performance characteristics; an analysis of an isolated impeller channel and volute with an averaging procedure at the interface is inadequate.

Performance of a Novel Rotating Gas-Liquid Separator

A novel gas separation process makes use of a rotating phase separator to separate micron-sized droplets from a gas stream. Based on an industrial scale design, a water/air separator is constructed and tested. The first experiment concerns the drainage of large fractions of separated liquid. During operation, drainage is observed via windows and a descriptive model is formulated. Because of the major influence on overall separation efficiency, liquid drainage is a key issue in the separator design. The second experiment comprises a droplet collection efficiency measurement using micron-sized droplets dispersed within the airstream. The separation efficiency of fine droplet removal is measured. This is an important factor in reducing capital costs. DOI: 10.1115/1.4001008

A superelement-based method for computing unsteady three-dimensional potential flows in hydraulic turbomachines

A numerical method is presented for the computation of unsteady, three-dimensional potential flows in hydraulic pumps and turbines. The superelement method has been extended in order to eliminate slave degrees of freedom not only from the governing Laplace equation, but also from the Kutta conditions. The resulting superelement formulation is invariant under rotation. Therefore the geometrical symmetry of the flow channels in the rotor can be exploited. This makes the method especially suitable to performing fully coupled computations of the unsteady flow phenomena in both rotor and stator, the so-called rotor-stator interaction. The developed numerical method is used to simulate the unsteady flow in an industrial mixed-flow pump. Two types of simulation are considered: one in which unsteady wakes behind the trailing edges of the rotor blades are taken into account and one in which these are neglected. Results are given that show the importance of unsteady flow phenomena. However, the computed head-capacity curve is hardly influenced by whether or not unsteady wakes are taken into account.

Numerical analysis of the unsteady behavior of cloud cavitation around a hydrofoil based on an improved filter-based model

The unsteady cavitation evolution around the Clark-Y hydrofoil is investigated in this paper, by using an improved filter-base model (FBM) with the density correction method (DCM). To improve the prediction accuracy, the filter scale is adjusted based on the grid size. The numerical results show that a small filter scale is crucial for the unsteady simulations of the cavity shedding flow. The hybrid method that combines the FBM and the DCM could help to limit the overprediction of the turbulent viscosity in the cavitation region on the wall of the hydrofoil and in the wake. The large value of the maximum density ratio, ρl/ρv, clip promotes the mass transfer rate between the liquid phase and the vapor phase, which results in a large sheet cavity length and the vapor fraction rise inside the cavity. The cavity patterns predicted by the improved method are verified by the experimental visualizations. The time-average lift, the drag coefficient and the primary oscillating frequency St for the cavitation number σ = 0.8, the angle of attack, α=8α, at a Reynolds number Re = 7×10 are 0.735, 0.115 and 0.183, respectively, and the predicted errors are 3.29%, 3.36% and 8.93%. The typical three stages in one revolution are well-captured, including the initiation of the sheet/attached cavity, the growth toward the trailing edge (TE) with the development of the re-entrant jet flow, and the large scale cloud cavity shedding. It is observed that the cloud cavity shedding flow induces the vortex pairs of the TE vortices in the wake and the shedding vortices. The positive vorticity vortex of the re-entrant jet and the TE vortices interacts and merges with the negative vorticity vortex of the leading edge (LE) cavity to produce the shedding flow.

Quantification of Hydrodynamic Forces due to Torsional and Axial Vibrations in Ship Propellers

CFD is used to compute the hydrodynamic coefficients for torsional and axial vibrations, for one type of the Wageningen B-series of ship propellers in open-water condition. It is shown that the wakes shed from the blades have an influence on the magnitude and the phase of the damping forces. The dependency on reduced frequency of the vibratory motion is explained. This phenomenon can be related to the lift deficiency of trailing vortices in the wake of an oscillating plate, as derived by Theodorsen and Von Karman and Sears, and is frequently overlooked by more recent investigations. Results of the calculations are compared with theoretical and experimental data from literature.Copyright

Numerical analysis of cavitation shedding flow around a three-dimensional hydrofoil using an improved filter-based model

The cavitation shedding flow around a 3-D Clark-Y hydrofoil is simulated by using an improved filter-based model (FBM) and a mass transfer cavitation model with the consideration of the maximum density ratio effect between the liquid and the vapor. The unsteady cloud cavity shedding features around the Clark-Y hydrofoil are accurately captured based on an improved FBM model and a suitable maximum density ratio. Numerical results show that the predicted cavitation patterns and evolutions compare well with the experimental visualizations, and the prediction errors of the time-averaged lift coefficient, drag coefficient and Strouhal number St for the cavitation number σ = 0.8, the angle of attack at α = 8° Reynolds number Re = 7 × 105 are only 3.29%, 2.36% and 9.58%, respectively. It is observed that the cavitation shedding flow patterns are closely associated with the vortex structures identified by the Q-criterion method. The predicted cloud cavitation shedding flow shows clearly three typical stages: (1) Initiation of the attached sheet cavity, the growth toward the trailing edge. (2) The formation and development of the re-entrant jet flow. (3) Large scale cloud cavity sheds downstream. Numerical results also indicate that the non-uniform adverse pressure gradient is the main driving force of the re-entrant jet, which results in the U-shaped cavity and the 3-D bubbly structure during the cloud cavity shedding.

Numerical and Experimental Investigation of Hydrodynamic Forces Due to Non-Uniform Suction Flow to a Mixed-Flow Pump

This paper presents an investigation of the effect of a non-uniform suction flow on forces on the impeller of a waterjet pump. In such a pump, used for ship propulsion, the non-uniformity of the suction flow is caused by the boundary layer under the hull of the ship and the shape of the inlet duct. The paper covers both experimental and numerical studies. A model of a mixed-flow waterjet pump is built into a closed-loop test rig. In order to measure the instantaneous forces and bending moments on the impeller, a newly designed co-rotating dynamometer is used, which is built between the impeller and the shaft of the pump. The design of the dynamometer will be presented. Various entrance flow distributions to the pump are achieved by means of a device situated in the suction pipe. In this manner the axial velocity at the inlet of the pump is varied between uniform and non-uniform distributions. Results of measurements show the influence of suction flow and blade interaction on forces. Results of experiments are compared with CFD calculations of a waterjet pump installation with similar entrance flow conditions. Quasi-steady calculations are performed for the pump which is equipped with a vaned stator bowl. Calculations show a good quantitative agreement with measurements.© 2005 ASME

Axial Thrust Prediction for a Multi-Stage Centrifugal Pump

The prediction of axial thrust for centrifugal pumps has been an important topic for many years. This is especially the case for multi-stage pumps with opposed or inline impellers, as the correct selection of balancing device(s) and bearings depends highly on the accuracy of the calculated thrust. Up till now, many investigations regarding axial thrust have focused on fully analytical or (semi-)empirical relations while others have tried to predict the axial thrust using CFD simulations. Full analytical or empirical methods tend to give poor results or need tuning for each specific pump, while the full CFD methods are costly in both setup time and computer resources. This paper presents a hybrid method to calculate the axial thrust of a multi-stage pump with an inline impeller design. The hybrid method combines analytical methods and CFD to reduce the required setup time and computation costs. The CFD calculation of the main flow is used as a boundary condition for the semi-empirical models for the side chambers and the inter-stage seals, such that these tight regions can be excluded from the CFD calculation. To verify and validate the hybrid method, results are compared with measurements and with full CFD calculations that include the side chambers and seals. These results show that the hybrid method and the full CFD method give comparable results, but there is still some difference with the measurements.

Exit Loss Model for Plain Axial Seals in Multi-Stage Centrifugal Pumps

Plain axial seals are often used in centrifugal pumps as a means to achieve acceptable sealing against leakage flow without the much higher friction losses that are associated with mechanical seals. Examples of their application are the front seals in shrouded radial and mixed-flow pumps and the inter-stage seals in multi-stage pumps. Knowledge about the relation between leakage flow rate and pressure drop over the seal is vital, not only for estimating the volumetric losses, but also for calculating the axial thrust and shaft power of a pump. Investigations up till now have mainly concentrated on the frictional pressure drop in the seal (e.g. Yamada [1], Weber [2]), and hardly on the expansion losses at the exit of the seal. These exit losses are commonly modelled by a kinetic loss coefficient equal to or close to 1, but recent measurements by Storteig [3] have shown that exit loss coefficients can have values well above 1. This paper presents an analytical method to compute the exit loss coefficient of a plain axial seal. It is derived from energy and momentum balances and assumes power-law profiles for the velocity distribution in the seal. The power-law coefficients are computed using CFD and are found to only depend on the Reynolds numbers based on axial flow, Reax, and Couette flow in circumferential direction, ReW. The resulting exit loss coefficients are shown to range between 1 and 2, depending on the ratio of Reax and ReΩ. Results of the analytical model are compared with measurements and CFD calculations. This new analytical model can help improve the prediction of rotor dynamic stability, efficiency and axial thrust of turbomachinery without the need for dedicated CFD calculations in these tight clearances.