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How to improve the flow in a centrifugal pump
22
Feature
WORLD PUMPS
January 2018
Research & development
How to improve the flow in a centrifugal pump T
he internal transient flow in the self-priming centrifugal pump is more complicated than in the common pump because of the reflux hole. This induces large pressure pulsations and axial force. Here, a new type of reflux hole structure was introduced to improve the flow.
The self-priming centrifugal pump is widely used in various fields, such as agriculture irrigation, chemical industry, municipal engineering, etc. Compared with the common pumps, it has many advantages, such as self-priming, simple structure, small volume and low cost. The self-priming centrifugal pump can normally operate after the selfpriming process, but the backflow from the reflux hole will destroy the uniform symmetrical flow structure in the
volute. The double vortical flow in the cross-section of the volute is changed into a single vortical flow. The special flow structures will always induce large pressure pulsations and axial force, which seriously affects the working stability of the pump. However, little research has been undertaken to improve the reflux hole structure. This work proposes a symmetric reflux hole structure for the
Figure 1. Structure diagram of improved self-priming centrifugal pump.
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self-priming centrifugal pump. The internal flow, pressure fluctuation and axial force are analyzed.
Pump geometry A drainage self-priming centrifugal pump (80ZW65-40) was used in this work as a research object. The main performance parameters are shown as follows: designed flow Qd=65 m3/h; head H=40 m; rotate speed n=2900 r/ min; reflux hole area A=400 mm2; angle of reflux hole position to the tongue θ=202°. The self-priming performance test results show that the pump can achieve the requirements of standard GB/T3216-2005. In order to improve the working problems induced by the reflux hole, this paper presents a symmetrical reflux hole structure, which can reduce pressure pulsation in the volute and the axial force of the impeller. As shown in Figure 1, a self-priming centrifugal pump with symmetrical reflux holes contains a suction chamber, an impeller, a volute, a gas-liquid separation chamber and a joint tube. Two reflux holes are symmetrically located at each side of the bottom of the 0262 1762/18 © 2018 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature January 2018
Figure 2. Grid construction.
volute. The joint tube plays a dual role. On the one hand, it links up the gasliquid separation chamber and the reflux hole. On the other hand, it has a drainage function.
Numerical method For the numerical simulation, the pump model should be discretized. Because of the complicated model structure, an unstructured tetrahedral
grid was used. As the best compromise between the solution accuracy requirements and the available computer resources, the total number of grids in the model pump is 1.92 million. Figure 2 shows the 3D models of the total domain and generated grids. The flow inside the pump can be considered as a three-dimensional incompressible turbulent flow. The finite volume method was adopted as the discrete method of governing equations (Reynolds-averaged Navier-Stokes equations). The standard k-ε turbulence model was used in the numerical simulation, which is a semi-empirical model based on the experimental data. The pressure-velocity coupling was calculated by means of the SIMPLEC algorithm. The momentum, turbulence kinetic energy and dissipation rate equations dispersed by the secondorder upwind. Velocity-inlet was adopted in the inlet, outflow boundary condition was used in the outlet, no slip conditions were used in the wall between liquid and solid, and the surface roughness was 0.02 mm. The steady calculation results were used as the initial value for unsteady calculation, and the time step Δt=0.0001724 s, totalled 120 time steps per impeller revolution.
Figure 3. Pressure monitoring points location.
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Feature
WORLD PUMPS
January 2018
Where Δp is the difference between instantaneous static pressure and mean pressure, ρ is the water density and u2 is the circumferential velocity at impeller outlet.
Performance curves The three-dimensional unsteady flow of the pump under various operating conditions was numerically simulated. The predicted pump performance curves were compared to the experimental data. As shown in Figure 4, good agreement between the simulation and experiment results was obtained. The largest head deviation under various flow rates was less than 5%, which indicates that the numerical calculation method was acceptable and could be used for the further research.
Figure 4. Comparison of performance curve between calculation and experiment.
As shown in Figure 3, monitoring points (P1~P8) are located at the mid-plane of the volute to record the pressure fluctuation. P1 is at the tongue tip, and P2 to P8 are distributed circumferentially every 45°.
Another point P9 is at the center of the reflux hole. A dimensionless pressure pulsation coefficient was introduced as follows:
Figure 5. Streamlines at the reflux hole in the volute cross-section.
(1)
Flow field analysis Figure 5 shows the streamlines in the volute cross-section under the design flow rate. As shown in Figure 5(a), a large vortex appears on the right side of the reflux hole in the pump, and its flow structure is asymmetric and evolving over time. However, as shown in Figure 5(b), the unilateral vortex disappears, and the backflow of both sides uniformly flow into the volute. The flow structure is almost symmetric in cross-section. Figure 6 shows the static pressure distribution of the conventional and new reflux hole. It presents a large pressure gradient in Figure 6(a), and a low pressure area occurs at the locations of the secondary flow vortex. However, as shown in Figure 6(b), the pressure distribution is almost symmetric in the cross-section of the volute with the new reflux hole. The pressure gradient is also uniform from both sides to the middle. From above, the flow field inside the volute improved dramatically.
Pressure fluctuation analysis
Figure 6. Static pressure distribution at the reflux hole in the volute cross-section.
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The calculation results showed that the pressure fluctuation intensity of all the monitoring points decreased to some extent, and the largest decrease occurred at monitor P9. Figure 7 shows the timedomain diagram of monitors P9 , which records data for two impeller rotation cycles. The trend of pressure fluctuation on both reflux holes is consistent, which shows periodic pulse. It has two peaks
WORLD PUMPS
Feature January 2018
impeller rotation cycles. It can be found that the axial force of the impeller is greatly reduced in the pump with the new reflux hole. The average and maximum value is reduced by 47.1% and 45.1%, respectively. Obviously, its improved effect is considerable, and large pumps can't ignore the axial force of impellers.
Conclusion The backflow in the new symmetric reflux holes presents a relatively stable flow structure, which greatly improves the asymmetric flow situation in the volute and especially reduces the vortex flow. Figure 7. Time-domain diagram of the monitor P9.
The symmetric backflow structure provides a more uniform pressure distribution, which can reduce the pressure pulsation and improve the pump working stability. The axial force of the impeller can be greatly reduced by the symmetry flow near the new reflux holes. This can increase the service life of the bearings and reduce the possibility of risk of axial force.
Acknowledgements
Figure 8. Frequency-domain diagram of the monitor P9.
and two valleys and the same number of blades. However, the pressure pulsation intensity of the improved pump is significantly low. Figure 8 shows the frequency-domain diagram obtained by the FFT (Fast Fourier Transform). It can be seen that there are mainly two harmonics in the frequency domain, including blade passing frequency 96.67 Hz and double blade passing frequency 193.34 Hz. It is obvious that the blade
passing frequency is the most critical one. It was also observed that the largest pressure pulsation amplitude of the pump is lesser than that of the model pump, and it increases from 0.0539 to 0.0357, with a decline of 33.8%.
The authors acknowledge the support of the National Natural Science Foundation of China (Grant No. 51709234), the Zhejiang Province Natural Science Foundation of China (Grant No. LQ17E090005), the Open Research Subject of Key Laboratory of Fluid and Power Machinery (Xihua University) (Grant No. szjj2016076) and Welfare Technology Applied Research Project of Zhejiang Province (Grant No. 2017C31025).
•
Analysis of axial force Figure 9 shows the changing of the impeller axial force over time in two
References Please contact the author directly for full references.
Contact Peijian Zhou Lecturer, College of Mechanical Engineering, Zhejiang University of Technology, PR China [email protected] Zhenxing Wu College of Mechanical Engineering, Zhejiang University of Technology, PR China Jiegang Mou College of Mechanical Engineering, Zhejiang University of Technology, PR China
Figure 9. Axial force of the impeller.
www.worldpumps.com
25
Feature
WORLD PUMPS
January 2018
Research & development
How to improve the flow in a centrifugal pump T
he internal transient flow in the self-priming centrifugal pump is more complicated than in the common pump because of the reflux hole. This induces large pressure pulsations and axial force. Here, a new type of reflux hole structure was introduced to improve the flow.
The self-priming centrifugal pump is widely used in various fields, such as agriculture irrigation, chemical industry, municipal engineering, etc. Compared with the common pumps, it has many advantages, such as self-priming, simple structure, small volume and low cost. The self-priming centrifugal pump can normally operate after the selfpriming process, but the backflow from the reflux hole will destroy the uniform symmetrical flow structure in the
volute. The double vortical flow in the cross-section of the volute is changed into a single vortical flow. The special flow structures will always induce large pressure pulsations and axial force, which seriously affects the working stability of the pump. However, little research has been undertaken to improve the reflux hole structure. This work proposes a symmetric reflux hole structure for the
Figure 1. Structure diagram of improved self-priming centrifugal pump.
www.worldpumps.com
self-priming centrifugal pump. The internal flow, pressure fluctuation and axial force are analyzed.
Pump geometry A drainage self-priming centrifugal pump (80ZW65-40) was used in this work as a research object. The main performance parameters are shown as follows: designed flow Qd=65 m3/h; head H=40 m; rotate speed n=2900 r/ min; reflux hole area A=400 mm2; angle of reflux hole position to the tongue θ=202°. The self-priming performance test results show that the pump can achieve the requirements of standard GB/T3216-2005. In order to improve the working problems induced by the reflux hole, this paper presents a symmetrical reflux hole structure, which can reduce pressure pulsation in the volute and the axial force of the impeller. As shown in Figure 1, a self-priming centrifugal pump with symmetrical reflux holes contains a suction chamber, an impeller, a volute, a gas-liquid separation chamber and a joint tube. Two reflux holes are symmetrically located at each side of the bottom of the 0262 1762/18 © 2018 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature January 2018
Figure 2. Grid construction.
volute. The joint tube plays a dual role. On the one hand, it links up the gasliquid separation chamber and the reflux hole. On the other hand, it has a drainage function.
Numerical method For the numerical simulation, the pump model should be discretized. Because of the complicated model structure, an unstructured tetrahedral
grid was used. As the best compromise between the solution accuracy requirements and the available computer resources, the total number of grids in the model pump is 1.92 million. Figure 2 shows the 3D models of the total domain and generated grids. The flow inside the pump can be considered as a three-dimensional incompressible turbulent flow. The finite volume method was adopted as the discrete method of governing equations (Reynolds-averaged Navier-Stokes equations). The standard k-ε turbulence model was used in the numerical simulation, which is a semi-empirical model based on the experimental data. The pressure-velocity coupling was calculated by means of the SIMPLEC algorithm. The momentum, turbulence kinetic energy and dissipation rate equations dispersed by the secondorder upwind. Velocity-inlet was adopted in the inlet, outflow boundary condition was used in the outlet, no slip conditions were used in the wall between liquid and solid, and the surface roughness was 0.02 mm. The steady calculation results were used as the initial value for unsteady calculation, and the time step Δt=0.0001724 s, totalled 120 time steps per impeller revolution.
Figure 3. Pressure monitoring points location.
www.worldpumps.com
23
24
Feature
WORLD PUMPS
January 2018
Where Δp is the difference between instantaneous static pressure and mean pressure, ρ is the water density and u2 is the circumferential velocity at impeller outlet.
Performance curves The three-dimensional unsteady flow of the pump under various operating conditions was numerically simulated. The predicted pump performance curves were compared to the experimental data. As shown in Figure 4, good agreement between the simulation and experiment results was obtained. The largest head deviation under various flow rates was less than 5%, which indicates that the numerical calculation method was acceptable and could be used for the further research.
Figure 4. Comparison of performance curve between calculation and experiment.
As shown in Figure 3, monitoring points (P1~P8) are located at the mid-plane of the volute to record the pressure fluctuation. P1 is at the tongue tip, and P2 to P8 are distributed circumferentially every 45°.
Another point P9 is at the center of the reflux hole. A dimensionless pressure pulsation coefficient was introduced as follows:
Figure 5. Streamlines at the reflux hole in the volute cross-section.
(1)
Flow field analysis Figure 5 shows the streamlines in the volute cross-section under the design flow rate. As shown in Figure 5(a), a large vortex appears on the right side of the reflux hole in the pump, and its flow structure is asymmetric and evolving over time. However, as shown in Figure 5(b), the unilateral vortex disappears, and the backflow of both sides uniformly flow into the volute. The flow structure is almost symmetric in cross-section. Figure 6 shows the static pressure distribution of the conventional and new reflux hole. It presents a large pressure gradient in Figure 6(a), and a low pressure area occurs at the locations of the secondary flow vortex. However, as shown in Figure 6(b), the pressure distribution is almost symmetric in the cross-section of the volute with the new reflux hole. The pressure gradient is also uniform from both sides to the middle. From above, the flow field inside the volute improved dramatically.
Pressure fluctuation analysis
Figure 6. Static pressure distribution at the reflux hole in the volute cross-section.
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The calculation results showed that the pressure fluctuation intensity of all the monitoring points decreased to some extent, and the largest decrease occurred at monitor P9. Figure 7 shows the timedomain diagram of monitors P9 , which records data for two impeller rotation cycles. The trend of pressure fluctuation on both reflux holes is consistent, which shows periodic pulse. It has two peaks
WORLD PUMPS
Feature January 2018
impeller rotation cycles. It can be found that the axial force of the impeller is greatly reduced in the pump with the new reflux hole. The average and maximum value is reduced by 47.1% and 45.1%, respectively. Obviously, its improved effect is considerable, and large pumps can't ignore the axial force of impellers.
Conclusion The backflow in the new symmetric reflux holes presents a relatively stable flow structure, which greatly improves the asymmetric flow situation in the volute and especially reduces the vortex flow. Figure 7. Time-domain diagram of the monitor P9.
The symmetric backflow structure provides a more uniform pressure distribution, which can reduce the pressure pulsation and improve the pump working stability. The axial force of the impeller can be greatly reduced by the symmetry flow near the new reflux holes. This can increase the service life of the bearings and reduce the possibility of risk of axial force.
Acknowledgements
Figure 8. Frequency-domain diagram of the monitor P9.
and two valleys and the same number of blades. However, the pressure pulsation intensity of the improved pump is significantly low. Figure 8 shows the frequency-domain diagram obtained by the FFT (Fast Fourier Transform). It can be seen that there are mainly two harmonics in the frequency domain, including blade passing frequency 96.67 Hz and double blade passing frequency 193.34 Hz. It is obvious that the blade
passing frequency is the most critical one. It was also observed that the largest pressure pulsation amplitude of the pump is lesser than that of the model pump, and it increases from 0.0539 to 0.0357, with a decline of 33.8%.
The authors acknowledge the support of the National Natural Science Foundation of China (Grant No. 51709234), the Zhejiang Province Natural Science Foundation of China (Grant No. LQ17E090005), the Open Research Subject of Key Laboratory of Fluid and Power Machinery (Xihua University) (Grant No. szjj2016076) and Welfare Technology Applied Research Project of Zhejiang Province (Grant No. 2017C31025).
•
Analysis of axial force Figure 9 shows the changing of the impeller axial force over time in two
References Please contact the author directly for full references.
Contact Peijian Zhou Lecturer, College of Mechanical Engineering, Zhejiang University of Technology, PR China [email protected] Zhenxing Wu College of Mechanical Engineering, Zhejiang University of Technology, PR China Jiegang Mou College of Mechanical Engineering, Zhejiang University of Technology, PR China
Figure 9. Axial force of the impeller.
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