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Impellers of low specific speed centrifugal pumpbased on the draughting technologyTo cite this article: C Hongxun et al 2010 IOP Conf. Ser.: Earth Environ. Sci. 12 012018

 

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Impellers of low specific speed centrifugal pump based onthe draughting technology

H Chen1, W Liu1, W Jian1 and P Wei 2 1Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, No.149 Yanchang Road, Shanghai, 200072, China 2Shanghai East Pump (Group) Co., LTD No.1588 Fulian Road, Shanghai, 201906, China

E-mail: chenhx@shu.edu.cn

Abstract. The authors analyzed the reasons of low efficiency under different operation condition based on the performance test and CFD numerical simulation approach. And the analysis focuses on the relationship between pump efficiency and inner flow characteristics. In order to improve the internal flow and increase efficiency of the pump, some draughting methods of improving the internal flow structure have been proposed, and some new impellers were developed by these methods. The main geometric parameters of the impellers, such as diameter, width and installation of the size, were consistent with the original impeller. The experimental results show that the efficiency of new impellers was improved significantly. The authors' work has opened up a new direction for further improving the efficiency of the low specific speed centrifugal pump.

1. Introduction Centrifugal pump has an extensive application fields among the vane pump. The Efficiency of the middle and

high specific speed centrifugal pump has reached a considerable level while that of the low specific speed centrifugal pump is relatively low. Many research works have been done on the low specific speed centrifugal pump and a lot of literatures have been published. Theoretical methods and experiences to improve the efficiency of the low specific speed centrifugal pump have been summarized. The performance of the low specific speed centrifugal pump has some improvement.

The design theory and method on the low specific speed centrifugal pump were properly summarized in Reference [1]. According to the traditional design method, many problems of the low specific speed centrifugal pump are shown: efficiency is low, hump exists in the head-discharge curve, and power rise rapidly with the discharge increased.

Experimental and theoretical studies have been done to improve the efficiency of the low specific speed centrifugal pump since 1970. Some improved design methods on the low specific speed centrifugal pump were put forward, mainly include: increased discharge design method, none overload design method, area ratio design method, splitter blade (or short blade or auxiliary blade) design method，and so on.

In the splitter blade offset design method, splitter blade is set among the long blade to change the velocity and pressure distributions, and the pump performance is improved. Many researches have been done on the centrifugal pump with splitter blade in recent years [2-6]. Numerical analysis, experiments, multi-scheme orthogonal experiments were used to study the effects of the length, shape and displacement of the splitter blade to the pump performance. G. Kergourlay, et al studied the splitter blade effects to the centrifugal pump performance, used unsteady CFD technology to study the flow structure within the pump and dynamic performance of the pump, and used pressure sensor to measure the pressure fluctuation. It is showed that when the splitter blade is set, the circumferential velocity and pressure becomes even, the vibration and the radiated noise are improved, and the effect to the pump performance sometimes is good or bad [7]. Researches both at home and abroad shown that the pump performance is not always improved when the offset blade is added and the efficiency can be improved about 1~2% only it is set properly. So the method of setting splitter blade does not improve the low specific number pump performance fundamentally.

25th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 12 (2010) 012018 doi:10.1088/1755-1315/12/1/012018

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Based on the theoretical analysis and many experiments, two new centrifugal pump blades were obtained, it is groove and gap drainage blades. The pump performance with the new type impeller is improved greatly, the operation range is enlarged, and the stability is somewhat improved, and the pump performance is not distorted, compared with that of pump with the conventional impeller. The invention has been accepted by State Intellectual Property Office of the People Republic of China (The invention patent application numbers are 200810042172.X and 200910052014.7 respectively).

2. Common low specific speed centrifugal pump 2.1 Analysis of the conventional designed blade

(1) Physical model and grids The parameters of the selected low specific speed centrifugal pump are: the diameter of the inlet is 40mm,

the outlet is 32mm, the diameter of the impeller is 320 mm, the number of the blade is 4 and the rated speed is 2900rpm, the specific speed of the pump is 25.58. Firstly, the pump was modeled based on the geometry size. Figure 1 shows the pump model with the blade designed by conventional method and the grids.

Hexahedral element was selected to mesh the whole pump, the total grids is 170,000. The first layer grids near the wall meet the y+ required by the algorithm.

Fig. 1 The pump model and the grids with the blade designed by conventional method

(2) Numerical simulation The finite volume method based on the finite element method in the ANSYS CFX11.0 was chosen to discrete

the governing equations, shape function was introduced to express the diffusion term and pressure gradient term. This method ensures the conservation of the finite volume method and the accuracy of the finite element method. High resolution scheme was adopted to solve the convection term.

Fully implicit and multi-grid coupled methods were adopted to solve the flow field. The momentum equation and the continuity equation were solved simultaneously while the repeated iteration, which is “assume pressure-solve-revise pressure （pressure-correction method）”, was avoided. The mesh scale can be effectively controlled and the solution speed can be improved when the multi-grid technique is introduced.

It can be found from Fig. 2 and Fig. 3 that there exists a very large vortex in every flow passage during the pump operation. The large vortex costs energy and blocks the flow passage, so this is the main reason that the pump efficiency is low.

2.2 Long-short splitter blade (1)Physical model In order to improve the efficiency, splitter short blade was set to suppress the large vortex within the pump to

improve the flow pattern and to make the flow passage unblocked. Two types of long-short splitter blade (the angle between the splitter blade and the blade suction is 45o and 36o respectively) were simulated and experimented. The model of the pump with the two types blades are shown in Fig. 4 and Fig. 5 respectively.

25th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 12 (2010) 012018 doi:10.1088/1755-1315/12/1/012018

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Fig. 2 Pressure distribution within the pump with blade designed by conventional method (left) Fig. 3 Streamline within the pump with blade designed by conventional method (right)

Fig. 4 The pump model long-short splitter blade (left is 45o)

Fig. 5 The pump model long-short splitter blade (right is 36o)

(2) Numerical simulation The pumps with the splitter blades (45o and 36o) were simulated by the same numerical method mentioned

above. Figure 6 and Fig. 7 show the pressure distribution and streamline within the pump with long-short splitter

blade (45o) respectively. From Fig. 6 and Fig. 7, it can be found that the flow pattern in the flow passage is improved and there is not

large vortex, but the streamline in front of the short blade is still a little distortion. Figure 8 and Fig. 9 show the pressure distribution and streamline within the pump with long-short splitter

blade (36o) respectively.

25th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 12 (2010) 012018 doi:10.1088/1755-1315/12/1/012018

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Fig. 6 Pressure distribution within the pump with long-short splitter blade (45o left)

Fig. 7 Streamline within the pump with long-short splitter blade (45o right)

From Fig. 8 and Fig. 9, it can be found that the flow pattern in the flow passage is improved more during the pump operation than that of the long-short splitter blade (45o); especially the distorted streamline in front of the blade is eliminated.

Fig. 8 Pressure distribution within the pump with long-short splitter blade (36o)

Fig. 9 Streamline within the pump with long-short splitter blade (36o)

(3) Performance test Experiment on the pump with the conventional blade and the long-short splitter blades were done, the

efficiency curves of the pump are shown in Fig. 10. From the comparison, we could conclude that the long-short splitter blade can improve the flow pattern and

make the best efficiency point move to large discharge. In other words, the contribution of the long-short splitter blade to the efficiency is obviously when pump operated in large discharge. And the pump efficiency is not improved in low discharge. The efficiency of the pump with long-short splitter blades (36o) is higher that of the pump with long-short splitter blades (45o) at large discharge.

25th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 12 (2010) 012018 doi:10.1088/1755-1315/12/1/012018

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Fig. 10 Performance of the pump with the conventional blade and the long-short splitter blades

3. Groove drainage blade (1) Physical model One method to prevent the boundary layer separation is to supply additional energy to the blocked flow in the

boundary layer. This can be obtained by setting special device in the body or use the energy of the main flow. Considering simplification processing, easily controlling and no additional energy losing, the energy of the main flow were directly used. In line with the thought that make effective use of and conduct the inflow of the pump itself to suppress the boundary layer separation, the groove drainage blade design method was proposed. The concrete controlling measure is to cut a groove between the pressure and suction side. Using the pressure difference in the two sides of the groove to conduct the jet flow to the separation point, the separation is suppressed and the efficiency of the pump is improved.

(2) Performance test The performance of the pump with the groove drainage blade was tested and compared with that of the blade

designed by conventional method and splitter blade (45o and 36o), Figure 11 shows the comparison. It can be found from Fig. 12 that the efficiency of the pump with the groove drainage blade is improved at

multi-operating points, and the effect is more obviously at large discharge point.

Fig. 11 discharge-efficiency comparison among the pump with the four types blade

25th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 12 (2010) 012018 doi:10.1088/1755-1315/12/1/012018

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4. Gap drainage blade (1) Physical model From the flow field of the pump with the conventional deigned blade, it can be found that the separation

begins at the vicinity of the blade inlet, this probably relates to incoming flow attack angle. The research shows that the incoming flow attack angle affects the pump performance significantly; the optimum incoming flow conditions can be obtained by adjusting the incoming flow attack angle; and the incoming flow attack angle is the reason why the separation of the boundary layer.

The method to control the separation of the boundary layer can be found to improve the pump performance in wide operating range under a certain incoming flow attack angle. Some modification at the inlet was adopted; a certain width gap was cut to control the incoming flow attack angle. When the incoming flow flows to the separation point through the gap to suppress the flow separation, the pump efficiency is improved. Based on the succession of the little attack angle, the shape of the gap was tried to change. The change can make more incoming flow flows to the separation area rapidly to further suppress the flow separation.

(2) Performance test The performance of the pump with the gap drainage blade (both little and large) were tested and compared

with that of the pump with conventional designed blade, long-short splitter blade (45o and 36o), gap drainage blade. Figure 12 and Fig. 13 show the comparisons.

From Fig. 12 it can be found that the efficiency of the gap drainage blade (little) improved greatly and obviously even in the little discharge. Figure 13 shows that the operation efficiency of the pump with the gap drainage blade (big) improved further in the whole operation range than that of the pump with the gap drainage blade (little). Figure 14 shows Specific speed (Ns)-efficiency curves of the pump with the six types blades, which proved further the effect of new type blades.

Fig. 12 Discharge-efficiency of the pump with the five types blades (left)

Fig. 13 Discharge-efficiency of the pump with the six types blades

5. Conclusion From a series of calculations and experiments, we can explicitly found that the pump performance in the

whole operating conditions is improved with the controlling measure changes continuously. Especially the effects of the groove drainage and gap drainage blade are obvious; the conception was achieved by effectively using and conducting the incoming flow to control the boundary layer to improve the efficiency. It is a great breakthrough in the study of the pump that the gap drainage blade was proposed. The flow controlling method proposed by the author not only improves the low specific speed pump performance but enriches and develops the centrifugal pump design theory.

25th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 12 (2010) 012018 doi:10.1088/1755-1315/12/1/012018

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Fig. 14 Specific speed Ns-efficiency curves of the pump with the six types blades

References [1] Zhang J 2008 Numerical Forecast and Research on the Design Method for Centrifugal Pumps with

Splitter Blades (Ph. D. Thesis Technique and Research Center of Fluid Machinery Engineering of Jiangsu University) (Zhenjiang Jiangsu Province China: Jiangsu University)

[2] Xu W and Yuan S 2006 Advances in Optimal Design of Low Specific Speed Centrifugal Pump Impellers J. of Fluid Machinery 34(2) 39-42

[3] Li Y B, Zhang D, Zhao W 2008 Influence of Blade Number and Splitter-Blade Position on Performance of Centrifugal Pumps J. of Lanzhou University of Technology 34(2) 45-48

[4] Cui B, Zhu Z, Chen Y and Lin Y 2006 Numerical simulation of inner flow field in centrifugal impeller with long, middle and short blades J. of Propulsion Technology 27(3) 243-247

[5] Shaojuan G, Chaoqun N, Weiguang H, Tao F and Ke L 2006 Numerical Study Of Unsteady Flow In Centrifugal Pump With Different Type Of Impellers Chinese Journal Of Mechanical Engineering 42(5) 27-31

[6] Chen S, Zhou Z, Qiang G, Dengfeng Y and Linsuo W 2005 Orthogonal Experimental Study on Centrifugal Pump with Deviated Splitter Vanes J. of Yangzhou University(Natural Science Edition) 8(4) 45-48

[7] Kergourlay G, Younsi M, Bakir F and R Rey 2007 Influence of Splitter Blades on the Flow Field of a Centrifugal Pump: Test-Analysis Comparison International Journal of Rotating Machinery 2007 1-13

25th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 12 (2010) 012018 doi:10.1088/1755-1315/12/1/012018

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