Leqin Wang

Numerical Simulation of the Transient Flow in a Centrifugal Pump During Starting Period

Computational fluid dynamics were used to study the three-dimensional unsteady incompressible viscous flows in a centrifugal pump during rapid starting period (≈0.12 s). The rotational speed variation of the field around the impeller was realized by a dynamic slip region method, which combines the dynamic mesh method with nonconformal grid boundaries. In order to avoid introducing errors brought by the externally specified unsteady inlet and outlet boundary conditions, a physical model composed of a pipe system and pump was developed for numerical self-coupling computation. The proposed method makes the computation processes more close to the real conditions. Relations between the instantaneous flow evolutions and the corresponding transient flow-rate, head, efficiency and power were analyzed. Relative velocity comparisons between the transient and the corresponding quasisteady results were discussed. Observations of the formations and evolutions of the primary vortices filled between the startup blades illustrate the features of the transient internal flow. The computational transient performances qualitatively agree with published data, indicating that the present method is capable of solving unsteady flow in a centrifugal pump under transient operations.

Hydraulic Improvement to Eliminate S-Shaped Curve in Pump Turbine

For pump turbines, an S-shaped curve can lead to failures in synchronization. To improve the hydraulic design, the component that is responsible for the formation of the S-shaped curve was identified by a hydraulic loss analysis using previous computational fluid dynamics (CFD) results, which indicates that the formation of the S-shaped curve can be ascribed to the runner. To improve the hydraulic design of the runner, a simple numerical approach for direct problem analysis was proposed, based on the bladeloading distributions of runners with and without an S-shaped curve, and directly analyzed. It was implied from the differences in the blade-loading distributions that, when the meridional passage was broadened, the formation of the S-shaped curve was suppressed. To validate this, two runners with different meridional sections were designed by means of the inverse design method. Through model tests, it was verified that the S-shaped curve was eliminated completely and the performance curve of the modified hydraulic model satisfied the requirements for safe operation in a pumped storage plant. [DOI: 10.1115/1.4023851]

Transient flow around an impulsively started cylinder using a dynamic mesh method

The finite volume method (FVM) with a dynamic mesh method (DMM) to deal with the moving boundary was applied to the simulation of two-dimensional incompressible viscous flow past a circular cylinder that is impulsively started into rotation and translation. The non-dimensional rotating to translating speed ratio α is varied from 0.28 to 2.07, with the Reynolds number being 200 for the range of α. The computation covers a period, during which the cylinder translates seven times its diameter. The current scheme handles the impulsively moving boundary directly by DMM, which is implemented using both mesh deforming and local remeshing. The instantaneous asymmetrical flow configurations for various α are presented and compared with the experimental visualizations. Quantitatively, the velocity distributions with drag and lift coefficients are also compared with the experimental and numerical results. Results show that the flow is strongly influenced by the rotation. Comparisons are found to be satisfactory.

Effects of water compressibility on the pressure fluctuation prediction in pump turbine

The compressible effect of water is a key factor in transient flows. However, it is always neglected in the unsteady simulations for hydraulic machinery. In light of this, the governing equation of the flow is deduced to combine the compressibility of water, and then simulations with compressible and incompressible considerations to the typical unsteady flow phenomenon (Rotor stator interaction) in a pump turbine model are carried out and compared with each other. The results show that water compressibility has great effects on the magnitude and frequency of pressure fluctuation. As the operating condition concerned, the compressibility of water will induce larger pressure fluctuation, which agrees better with measured data. Moreover, the lower frequency component of the pressure signal can only be captured with the combination of water compressibility. It can be concluded that water compressibility is a fatal factor, which cannot be neglected in the unsteady simulations for pump turbines.

MOC-CFD Coupled Approach for the Analysis of the Fluid Dynamic Interaction between Water Hammer and Pump

AbstractA one-dimensional (1D) system and three-dimensional (3D) component cosimulation method based on the method of characteristic (MOC) and computational fluid dynamics (CFD) was proposed to study the interaction between valve-induced water hammer and pump during the rapid closing of the valve. In a long pipeline system, the pump was treated as a 3D CFD model using Fluent code, whereas the remaining system models (e.g.,xa0pipe, valve) were represented by 1D components using MOC. The numerical coupling and integration of both models were realized by a coupled code written using Visual Basic. The coupling theory and procedure were described in detail. MOC-CFD steady coupling in the case of the fully open state of the valve was first conducted. The corresponding results showed that the steady cosimulation can accurately determine the operating conditions. A transient cosimulation for the rapid closing procedure of the valve was conducted to study the interaction. The relative dynamic behavior obtained by MO...

A study on numerical methods for transient rotating flow induced by starting blades

Based on the finite volume method, three methods for rotational region treatment were presented and validated by simulating two-dimensional accelerating rotational flows. Separate transient incompressible flows induced by cross-shaped blades during starting process were simulated using the dynamic mesh, sliding mesh and dynamic reference frame methods. The computing performance and stability of the three methods were evaluated by comparing numerical results, and the transient characteristics of the accelerating rotational flow were analysed numerically. Results showed that the starting process affected the flow structure and transient characteristics of the accelerating rotational flows. The sliding mesh method showed higher computational efficiency and accuracy compared with other methods, and could easily be extended to solve three-dimensional transient flows in hydraulic machineries under transient operations, such as start-up and shutdown.

Investigation of CFD calculation method of a centrifugal pump with unshrouded impeller

Currently, relatively large errors are found in numerical results in some low-specific-speed centrifugal pumps with unshrouded impeller because the effect of clearances and holes are not accurately modeled. Establishing an accurate analytical model to improve performance prediction accuracy is therefore necessary. In this paper, a three-dimensional numerical simulation is conducted to predict the performance of a low-specific-speed centrifugal pump, and the modeling, numerical scheme, and turbulent selection methods are discussed. The pump performance is tested in a model pump test bench, and flow rate, head, power and efficiency of the pump are obtained. The effect of taking into consideration the back-out vane passage, clearance, and balance holes is analyzed by comparing it with experimental results, and the performance prediction methods are validated by experiments. The analysis results show that the pump performance can be accurately predicted by the improved method. Ignoring the back-out vane passage in the calculation model of unshrouded impeller is found to generate better numerical results. Further, the calculation model with the clearances and balance holes can obviously enhance the numerical accuracy. The application of disconnect interface can reduce meshing difficulty but increase the calculation error at the off-design operating point at the same time. Compared with the standard k-ɛ, renormalization group k-ɛ, and Spalart-Allmars models, the Realizable k-ɛ model demonstrates the fastest convergent speed and the highest precision for the unshrouded impeller flow simulation. The proposed modeling and numerical simulation methods can improve the performance prediction accuracy of the low-specific-speed centrifugal pumps, and the modeling method is especially suitable for the centrifugal pump with unshrouded impeller.

Experimental Study on a Medium Consistency Pump

An experimental testing rig is built to study the performance of a centrifugal pump used in medium consistency technology (MC technology). Pump performances are tested in different pulp concentration, degas pressure differential, and tip clearance ratio. The results show that in medium concentration, pump head and efficiency decreases with increase of pulp concentration. There is a critical concentration above which pump performance drops. Gas separation effect is influenced by the pressure differential over the degas system and the air content in the pulp suspension. In high concentration and small tip clearance conditions, friction loss increases a great deal and results in an efficiency reduction. [DOI: 10.1115/1.4024865]

Computation of stress distribution in a mixed flow pump based on fluid-structure interaction analysis

The internal flow evolution of the pump was induced with impeller movement. In various conditions, the peak load on centrifugal blade under the change of rotational speed or flow rate was also changed. It would cause an error when inertia load with a safety coefficient (that was difficult to ascertain) was applied in structure design. In order to accurately analyze the impeller stress under various conditions and improve the reliability of pump, based on a mixed flow pump model, the stress distribution characteristic was analyzed under different flow rates and rotational speeds. Based on a three-dimensional calculation model including impeller, guide blade, inlet and outlet, the three-dimension incompressible turbulence flow in the centrifugal pump was simulated by using the standard k-epsilon turbulence model. Based on the sequentially coupled simulation approach, a three-dimensional finite element model of impeller was established, and the fluid-structure interaction method of the blade load transfer was discussed. The blades pressure from flow simulation, together with inertia force acting on the blade, was used as the blade loading on solid surface. The Finite Element Method (FEM) was used to calculate the stress distribution of the blade respectively under inertia load, or fluid load, or combined load. The results showed that the blade stress changed with flow rate and rotational speed. In all cases, the maximum stress on the blade appeared on the pressure side near the hub, and the maximum static stress increased with the decreasing of the flow rate and the increasing of rotational speed. There was a big difference on the static stress when inertia load, fluid load and combined loads was applied respectively. In order to more accurately calculate the stress distribution, the structure analysis should be conducted due to combined loads. The results could provide basis for the stress analysis and structure optimization of pump.

The research of 1D / 3D coupling simulation on pump and pipe system

The research of performances of hydraulic mechanical depends on static complete characteristic curves, which have great difference compared with the actual work condition and have accidents potential. So we need a new way to compute the dynamic system, which is more reasonable. So the method to couple one dimensional simulation and three dimensional CFD analysis based on Flowmaster and Fluent is explored, and the dynamic characteristics and internal flow of the pumping system are analyzed. First, a pipe system model is created in Flowmaster and a pump model is created in Fluent; then VB code and scheme code are used to realize the automated operation for Flowmaster and Fluent; at last, the exchange of data between these two parts is realized by an interface program. In this paper, the interaction between pumps and pipe system are analyzed by coupling one-dimensional and three-dimensional simulations. This study would be helpful to identify the influences of the rapid adjustment process on stability of system and provide guides for design of pump system.