Experimental study on movement of particles in centrifugal impeller

The slurry pump is the key component of a dredger. Solid particles have strong influence on the performance of a slurry pump. The movement of solid particles in a centrifugal impeller was studied using particle image velocimetry (PIV) measurement. The experiments were conducted in a dredging pump model at Hohai University. Some transparent glass spheres with diameter of 02-04 mm were used as solid particles. The concentration and relative velocities of the particles were analyzed to investigate the particle trajectory. The results show that the concentration of the particles on the pressure surfaces of the blades is higher than on the suction surfaces, and the particles tend to move towards the suction surfaces. Moreover, the particles have faster relative velocities than the liquid phase through the flow channels of the impeller.     

Computational fluid dynamics of left ventricular assist device under unsteady flow

Left ventricular assist device (LVAD) in this study is a mechanical tool that is used to support blood flow in the patient with heart disease. It supports left ventricle by building up the pressure to the pump outlet connected to the aorta. This pump was designed based on the magnetic driven centrifugal pump with a unique small washout hole constructed inside the impeller to generate the washout flow passage to prevent the stagnation at the region underneath and around the rotor. Computational fluid dynamics (CFD) was adopted in this study to assess the performance and optimize the design to avoid recirculation and high shear stress which is the main cause of stagnation and blood damage. Transient simulation was used for this study due to the asymmetric design of the washout hole and the complication of the bottom support of the impeller that have a risk of thrombosis, also, it was used to predict the variation of hydraulic performance caused by the rotation of the impeller and pulsed flow at the pump inlet. The simulation results show no excessive stress and no recirculation observed within the computational domain; in addition, the research result also provided information for further optimization and development to the pump.     

Numerical simulation of cavitationturbulence in Francis turbine runnerwith splitter blades

Cavitation will reduce the turbine performance and even damage the turbine components. To verify the effects of splitter blades on improving the cavitation performance,the cavitation flow inside a Francis turbine runner with splitter blades was numerically simulated by using the Singhal cavitation model and the standard k-εturbulence model. The distributions of static pressure and gas volume fractions on the surface of the runner blades were predicated under different conditions,and the cavitation in the flow field of the runner was analyzed. The results show that the static pressure and gas volume fractions are more uniformly distributed on the short blades than those on the long blades in Francis turbines with splitter blades,and there is almost no cavitation on the short blades; their distributions are more uniform under small flow conditions than those under large flow conditions; and large gas volume fractions are concentrated at the outlet tip near the band on the suction side of the long blade. The installation of splitter blades can improve the cavitation performance of conventional Francis turbines.     

Influence of long and short guide vanes on hydraulic performance of water-jet pump

Different guide vane structures will affect the flow inside the pump,and then affect the transformation of the pressure energy and kinetic energy,and change the velocity distribution of the pump outlet.In order to study the influence of long and short guide vanes on the water-jet pump,on the basis of conventional design,eight schemes of guide vane with different vertical heights were designed in the method of computational fluid dynamics for numerical calculation,the performance curve of water-jet pumps with different long and short guide vanes was obtained,and finally the influence of different guide vanes on hydraulic performance and internal flow was analyzed.The results show that all of schemes reducing the height of blade can improve the head and efficiency.In the schemes reducing the height on the shroud,the guide vanes that the height of the blade is equal to the height difference between hub and shroud in impeller have the highest head and efficiency.In all schemes decreasing the blade height,with the increase of the height difference,the velocity increases gradually and the distribution of turbulence kinetic energy becomes more reasonable in the guide vane outlet.The schemes reducing the height on the hub have more reasonable distribution of velocity and turbulence kinetic energy according to schemes reducing the height on the shroud.The guide vanes of long and short blades can be used to stagger the position of the diffusion flow generated by adjacent blades,which can reduce the effect of the velocity circulation and make the flow of the outlet position more stable.     

Experimental study on inlet vortex in pump sump

In order to study the mechanism of the vortex generation at the bottom of the pump sump below the flare tube, twenty pressure pulsation monitoring points were arranged at the bottom of the pump sump below the flare tube, and the pressure fluctuation experiments were carried out under different flow conditions. The experimental results show that the frequency of pressure fluctuation at the bottom of the pump sump is twice of the rotational frequency of the impeller blade; the vortex below the flare tube is easy to generate under the large flow conditions mainly concentrated at the right front position below the flare tube. The position of the vortex occurring is corresponding to the position of the low-pressure region below flare tube.     

Effects of silt particle on cavitation flow in centrifugal pump

Based on numerical method,effects of silt particle with certain silt mean diameter and silt concentration on the evolution of cavitation in a centrifugal pump were studied. Silt mean diameter 0. 005 mm and silt concentration 1.0% were adopted in numerical simulations. Cavitation flow in a flatnosed cylinder was simulated to validate the designed algorithm. Cavitaton flows of water and silt-laden water were simulated and compared. The results indicate that the silt particles promote the evolution of cavitation. At the outlet pressure of 6.0×10~5 Pa,cavitation bubbles do not exist in the water flow,but a few cavitation bubbles appeare in the silt-laden water flow,demonstrating the silt particles induce the formation of cavitation bubbles. At the outlet pressure of 5.29×10~5 Pa,the vapor volume fraction in the silt-laden water flow is much larger than that in the water flow,indicating that the silt particles enhance the evolution of cavitation. The properties of silt particle,static pressure,flow field structure,turbulent kinetic energy and density difference have a close relationship with the evolution of cavitation.     

Numerical simulation of transient flow performance during different periods in centrifugal pump

The instantaneous variations of the hydraulic characteristics take place in centrifugal pumps during their start up, shutdown and other variable speed operations. In this paper, the variable speed method was proposed to simulate the transient internal flow field and the external performance of the pump during starting and stopping periods. [JP2]The terms of accelerations due to variable speeds in the flow governing equations were analyzed[JP] in a multiple reference of frame (MRF). A transient CFD simulation was performed for a typical centrifugal pump by using ANSYS CFX with the standard k ε turbulence model.[JP2]The entire simulation process was composed of four stages: start up, normal run, shutdown and post shutdown. The function of rotating speed with regard to time was set by CEL language directly into the impeller domain in the pre processor of the software to conduct variable speed simulation. The variations of the flow field in the centrifugal pump were obtained from the transient simulation. The change laws of flow rate, head and other performance parameters over time were also analyzed and summarized.     

Numerical simulation of cavitation around a Clark-Y hydrofoil with different LES models in OpenFOAM

A finite volume, multiphase solver in the framework of OpenFOAM is used to calculate the flow field of the cavitating flow over the Clark-Y hydrofoil. This solver uses Transport Based Equation Model (TEM) to solve the liquid volume fraction, and utilizes volume of fluid (VOF) technique to predict the interface between liquid and vapor phases. The simulation is designed to study the cavitation shedding and different fluid characteristics in the cloud cavitation regime when adopting two different Large Eddy Simulation (LES) models, namely, one equation eddy viscosity (oneEqEddy) model and Smagorinsky model. It is shown that these two models can be used to study the cavitation shedding dynamics and predict the velocity profiles.     

Analysis of flow response to fluctuation ofrotational speed in a radial impeller

By discretizing the convection terms with AUSM+ up scheme in the rotating coordinate system, a finite volume analysis code based on multi block structured grids was developed indepen dently so as to realize the numerical solving of internal flow fields of turbomachineries. Taking an unshrouded radial impeller with the working fluid of water vapour as the research object, the flow response to the fluctuation of rotational speed was calculated. By comparing the surface pressure profiles and velocity contours calculated by the code and commercial software respectively, the accuracy of flow solver was verified. The analysis of flow response data indicates that, as the wor king condition shifts closer towards the surge boundary, the response of flow parameters such as mass flow and aerodynamic torque will be more nonsynchronous with the fluctuation of rotational speed, and also the influence of density variation on mass flow variation will be smaller. Moreover, the transient variation region of working condition performance will deviate farther away from the steady performance curve as the working condition approaches the surge boundary. Compared to the working conditions with small mass flows, the distribution characteristics of pressure difference load on the blade surface vary little under large mass flow conditions. The reduction of fluctuation amplitude of rotational speed exerts no influence on abating the hysteresis of flow response.     

Comparisons between numerical calculations and measurements in vaned diffuser of SHF impeller

The paper presents analysis of the performance and the internal flow behaviour in the vaned diffuser of a radial flow pump using PIV(particle image velocimetry)and pressure probe traverses.PIV measurements have already been performed at middle height inside one diffuser channel passage for a given speed of rotation and various mass flow rates.These results have been already presented in several previous communications.New experiments have been performed using a three-hole pressure probe traverses from hub to shroud diffuser width at different radial locations between the two diffuser geometrical throats.Numerical simulations are also realized with the commercial codes Star CCM+7.02.011 and CFX.Frozen rotor and fully unsteady calculations of the whole pump have been performed.Comparisons between numerical results,previous experimental PIV results and new probe traverses one’s are presented and discussed for one mass flow rate.In this respect,a first attempt to take into account fluid leakages between the rotating and fixed part of the pump has been checked since it may affects the real flow structure inside the diffuser.     

CFD studies of scraper built in SPA clarifying tank based on mixture solid-liquid two-phase flow model

In the industrial process of producing the strong phosphoric acid (SPA), clarification of the solution is essential to the ultimate product. However, the large viscosity of sediment and the induced interface interaction result in difficulties when the SPA is clarified. CFD numerical methodology is applied to simulate internal flow field and performance of the low speed scraper based on Mixture solid liquid two phase flow model. Sediment deposition is generated by loading solid particles at the bottom of clarifying vessel. The moving mesh and RNG k-ε model are used to simulate the rotational turbulent flow in clarifying tank. Variables studied, amongst others, are the scraper rotation speed and the mounting height, which can affect the solid suspension height. Features of flow field and solid volume fraction distribution in computational domain are presented and analyzed. The numerical reports of the scraper torque and velocities of inlet and outlet filed are obtained. It seems the torque value of rotational axis and particle suspending height augment with an increasing rotating speed. Meanwhile, a high revolving speed is good for the deposition discharge. The particle fraction distribution in meridional surface and horizontal surface at fixed rotation speed are analyzed to determine the corresponding optimal installation height. The simulating results reflect the flow field is marginally stirred by the scraper and proper working parameters are obtained, in which case the comprehensive properties of the scraper and the clarifying tank are superior.     

Pressure pulsation in stages ofelectric submersible pump at shut-off under various speeds

Electric submersible pumps were widely used in agricultural fields, petroleum and various other industries. The pressure pulsation caused fatigue failure due to vibration in electric submersible pump and affects the life and performance of its system. The objective of this study was to experimentally investigate the characteristics of pressure pulsation which were generated at various stages of a multistage electric submersible pump during closed valve operation at different speeds. An electric submersible pump with five stages was selected for conducting experiments. A variable frequency drive(VFD)was used to operate the electric submersible pump at five different speed settings from 40 to 60 Hz. Piezoresistive pressure transducers were mounted at each stage of the electric submersible pump to capture the unsteady pressure signals. At each speed setting, the electric submersible pump was operated at the shut-off condition and the signals of unsteady pressure from all the five stages were captured. A fast fourier transformation(FFT)was carried out on the pressure signals to convert into frequency domain. From the spectra of pressure pulsation signals, the characteristics of pressure pulsation are obtained for each stage and for various speed settings which are then used to understand its variation with speed and stages.     

Orthogonal optimization of flow uniformity at exit section of elbow-inlet passages

Elbow-inlet passage is widely used in large drainage pumping stations.Flow uniformity at the exit section directly determines its hydraulic performance.Flow uniformity must be optimized to improve the operational efficiency of the large axial-flow pumping station.Modeling and numerical simulation methods were used to investigate the elbow-inlet passage,and the accuracy of the calculation results was verified.The key geometric parameters affecting the uniformity of the flow were optimized by the orthogonal experiment design.The optimal schemes were obtained and compared with the original scheme.The results show that flow uniformity V u after optimization is 95.41%,which is increased by 1.04%.The pumping station efficiency is increased by 1.89%,thereby confirming the applicability and accuracy of the proposed scheme,especially for the optimization of flow uniformity of the exit section of the elbow-inlet passage.     

Optimal Design of Vortex Pump Using Approximate Model and the Non-dominated Sorting Genetic Algorithm北大核心CSCD

【Objective】The cavity width L, blade width b2 and the number of blades Z are the most important geometric parameters affecting the performance of vortex pump. This paper aims to present a method to optimize the vortex pump. 【Method】The study was based on approximate model and the non-dominated sorting genetic algorithm II (NSGA-II), in which the pump cavity length, blade width and the number of blades were calculated using the central composite design of DoE (design of experiment). The performance of the designed pump was examined using CFD, and the effect of L, b2 and Z on vortex pump efficiency and shear stress on the blade wall was calculated using the Kriging model. The NSGA-II was used to optimize the geometric parameters.【Result】The optimal parameters calculated from the methods were L=25 mm, Z=8, b2=26.45 mm.【Conclusion】We proved that CFD and NSGA-II can be used in a combination to calculate the optimal parameters of the vortex pump, and they can significantly improve efficiency of the pump and reduce the shear stress on the blade. Our results revealed that the optimization can reduce the width of the non-blade cavity by 16.67%, and increase the number of blades of the impeller and the outlet width of the blade by 1 and 25.95% respectively. At the design flow rate, the optimal design increased pump efficiency by 1.06% and reduced the average shear stress on the blade from 274.37 Pa to 204.57 Pa. The optimal design made the shear stress on the blade more uniform, in addition to reducing the shear stress on the outlet of the blade. © 2019 Authors. All rights reserved.     

Effect of leading edge roughness on cavitation inception and development on thin hydrofoil

The cavitation incipience and development of water flow over a thin hydrofoil placed in the test section of high-speed cavitation tunnel were investigated.Hydrofoils with smooth and rough leading edge were tested for different upstream velocities and incidence angles.The observations clearly revealed that cavitation incipience is enhanced by roughness at incidence angle below 2°.This is in line with the former reports,according to whose roughness element decreases the wettability and traps a larger amount of gas.As a result,surface nucleation is enhanced with an increased risk of cavitation.Surprisingly,for higher incidence angles(3°),it was found that cavitation incipience is significantly delayed by roughness while developed cavitation is almost the same for both smooth and rough hydrofoils.This unexpected incipience delay is related to the change in the boundary layer structure due to roughness.It was also reported a significant influence of roughness on the dynamic of developed cavitation and shedding of transient cavities.     

Vibration analysis of the fluid-structure  interaction in water hammer

In consideration of the problem that the effect of conduit structure on water hammer has been ignored in the classical theory, the Poisson coupling between the fluid and the pipeline was stu died and a fourteen equation mathematical model of fluid-structure interaction (FSI) was developed in this paper. Then, the transfer matrix method (TMM) was used to calculate the modal frequency, modal shape and frequency response. The results were compared with that in experiment to verify the correctness of the TMM and the results show that the fluid-structure coupling has a greater impact on the modal frequencies than the modal shape. Finally, the influence on the response spectrum of diffe rent damping ratios was studied and the results showed that the natural frequency under different dam ping ratios has changed little but there is a big difference for the pressure spectrum. With the decrease of damping ratio, the damping of the system on frequency spectrum is more and more significant and the dispersion and dissipation is more and more apparent. So the appropriate damping ratio should be selected to minimize the effects of the vibration of the FSI.     

Irrigation Uniformity Affected by Spatial Arrangement of Sprinklers北大核心CSCD

【Objective】Irrigation uniformity is an important parameter in irrigation design. The purpose of this paper is to experimentally study the impact of spatial arrangement of sprinklers on irrigation uniformity.【Method】The FYRB471 sprinkler nozzles were used in the experiment, and the data collected from experiment under different water pressure and spatial arrangements of the sprinklers were used to calculate the Christiansen uniformity coefficient. We compared three spatial arrangements: triangle, square and hexagon. The irrigation-water depth curve under different pressure and arrangements was analyzed to calculate the irrigation uniformity.【Result】When water pressure remained constant, increasing sprinkler spacing scarified irrigation uniformity, while for a given sprinkler spacing, increasing water pressure benefited irrigation uniformity. There was no difference in irrigation uniformity between the three arrangements when the sprinkler spacing was <5.5 m, whereas when the spacing was >5.5 m, the irrigation of the triangle arrangement was most uniform.【Conclusion】Undesirable water pressure and sprinkler spacing could increase costing and jeopardize irrigation uniformity, and a rationale combination of sprinkler arrangement and water pressure is hence essential in designing sprinkler irrigation. From our experiment, spatially arranging the sprinklers in triangle with space between 5.5 m to 8.5 m and under water pressure between 200 kPa and 320 kPa appeared to be most efficient, increasing irrigation uniformity by more 80%. When the sprinkler spacing was less than 5.5 m, irrigation uniformity was almost same but the hexagon arrangement was preferred because it covered more area, thereby reducing cost compared to other two arrangements. © 2019 Journal of Irrigation and Drainage. All rights reserved.     

Mean Velocity Measuring Point of Semi-circular Open Channel Section Based on Flow Partitioning Theory北大核心CSCD

【Objective】The purpose of this paper is to reveal the characteristic position of the mean velocity along the normal line of the semi-circular channel wall and provided a novel method for flow measurement in non-uniformed channel, the physical model experiments were conducted in semi-circular channel.【Method】Based on the measuring results of flow velocity under different hydraulic conditions, the theoretical formula for calculating the mean velocity characteristic position along the normal line of the semi-circular channel wall was derived from flow partitioning theory. Considering the influence from side wall to friction velocity, the mean flow velocity of the entire semi-circular channel section can be obtained by measuring the flow velocity at the characteristic location.【Results】This paper proposed the theoretical formula and the measurements achieved a good agreement within 10% error. The formula can clearly reflect the distribution of the semi-circular open channel well. The velocity distribution should be analyzed along the normal line,thus the analysis methods was more reasonable. 【Conclusion】The theoretical results on determining mean velocity characteristic position fitted well the results from experiment and it indicated the formula has higher precision. The flow velocity at a specific position can be measured to further estimate the mean flow velocity of the section, so as to quickly and accurately determine the flow rate of the channel section in the project. © 2019 Journal of Irrigation and Drainage. All rights reserved.     

Numerical study on hydraulic performance of submerged propellers in oxidation ditch

The submerged propeller is an efficient diving mix device,which is applicable for oxidation ditch treatment in industry,city and village wastewater-treatment plant. The impeller structure and reasonable rotating speed are important factors that determine flow field distribution and energy conversion efficiency. So it is necessary to use modern design methods to develop new kinds of high efficiency submerged propellers,and research the flow field characteristics of submerged propellers. On the basis of the existing form drawing,three-dimensional model of submerged propellers and unstructured tetrahedral mesh were generated. Based on Navier- Stokes equations and standard k- ε turbulence model,the flow was simulated by using a simple algorithm. Through changing some design parameters of propellers,the corresponding numerical simulation results reveal that for the same impeller diameter and service area of submerged propellers,the power consumption could be reduced effectively by optimizing blade mounting angle,which can determine the best blade mounting angle and most suitable rotational speed under given conditions. The study can provide theoretical and project guidance for submerged propellers design.     

Effect of leading edge roughness oncavitation inception and development ona thin hydrofoil

Abstract:We investigate the cavitation incipience and development of water flow over a thin hydrofoilplaced in the test section of high一speed cavitation tunnel. Hydrofoils with smooth and rough leadingedge were tested for different upstream velocities and incidence angles. Our observations clearly re-vealed that cavitation incipience is enhanced by roughness at incidence angle below 20. This is in linewith the former reports，according to which roughness element decreases the wettability and traps a lar-ger amount of gas. As a result，surface nucleation is enhanced with an increased risk of cavitation.Surprisingly, for higher incidence angles(gt;30)，we have found that cavitation incipience is signifi-candy delayed by roughness while developed cavitation is almost the same for both smooth and roughhydrofoils. We argue that this unexpected incipience delay is related to the change in the boundary la-yer structure due to roughness. We also report a significant influence of roughness on the dynamic ofdeveloped cavitation and shedding of transient cavities.