Journal of Thermal Science Vol.22, No.2 (2013) 117−122
DOI: 10.1007/s11630-013-0601-6
Article ID: 1003-2169(2013)02-0117-06
Influence of Blade Outlet Angle on Performance of Low-specific-speed Centrifugal Pump Cui Baoling, Wang Canfei, Zhu Zuchao, Jin Yingzi The Province Key Laboratory of Fluid Transmission Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China © Science Press and Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg 2013
In order to analyze the influence of blade outlet angle on inner flow field and performance of low-specific-speed centrifugal pump, the flow field in the pump with different blade outlet angles 32.5°and 39° was numerically calculated. The external performance experiment was also carried out on the pump. Based on SIMPLEC algorithm, time-average N-S equation and the rectified k-ε turbulent model were adopted during the process of computation. The distributions of velocity and pressure in pumps with different blade outlet angles were obtained by calculation. The numerical results show that backflow areas exist in the two impellers, while the inner flow has a little improvement in the impeller with larger blade outlet angle. Blade outlet angle has a certain influence on the static pressure near the long-blade leading edge and tongue, but it has little influence on the distribution of static pressure in the passages of impeller. The experiment results show that the low-specific-speed centrifugal pump with larger blade outlet angle has better hydraulic performance.
Keywords: centrifugal pump; blade outlet angle; numerical simulation; external characteristic
Introduction The blade outlet angle is one of the most important geometric parameters for the impeller of centrifugal pump, which has a significant influence on the pump head, efficiency and so on. Some researches had been done on the effect of blade outlet angle on the pump performance using theoretical analysis and experimental method. T. Shigemitsu et al. [1] studied three types of rotors with different outlet angles in the mini turbopumps. He investigated the effect of the blade outlet angle on performance and internal flow field of mini turbo-pumps. Also González et al. [2] found that different blade outlet angles have significant influence on the moment characteristics of the pump. Guangwen Li [3] measured the internal flow field accurately using two
dimensional laser Doppler velocimeter when the centrifugal pump delivering water with large blade outlet angle operated at the best and small flow conditions. Xianfang Wu et al [4] had analyzed the influence of blade outlet angle on performance characteristic of centrifugal pump with different specific speeds. Based on the multiple regression method, Xijie He [5, 6] researched on the effect degree and sequence of impeller geometric parameters on performance characteristic of centrifugal pump, and the results showed that blade outlet angle has significant influence on the pump head. With the rapid progress of computer technology and computational fluid dynamics, many numerical studies have been carried out on centrifugal pump [7, 8], but few are on the lowspecific-speed centrifugal pump. So, it is necessary to investigate the effect of different blade outlet angles on
Received: October 2012 CUI Baoling: Professor This investigation was supported by National Natural Science Foundation of China granted No.50976105, No.51276172 and Zhejiang Provincial Natural Science Foundation Granted No.R1100530. www.springerlink.com
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performance of low-specific-speed centrifugal pump. In this paper, to analysis the influence of blade outlet angle on performance and internal flow of low-specific-speed centrifugal pump, the flow field in the pump with different blade outlet angles is numerically calculated using commercial software Fluent. The external performance experiment is also carried out on the pump.
Computation model Geometrical model The design parameters of the low-specific-speed centrifugal pump studied are flowrate Q = 1.5m3/h, head H = 15m, the rotating speed n = 2900r/min. The specific speed ns=28. The impeller is a complex one with four long blades and eight short blades. To achieve better suction performance, a variable-pitch inducer is designed upstream of the impeller. The three dimensional model of pump is shown in Fig.1.
computational domains include impeller, inducer, the extension of inlet and outlet, volute and clearance between impeller with the front shroud and hub. To ensure the stability of calculation result, there is a proper extension at the outlet of impeller. The numerical grids are obtained by Gambit, and interfaces are formed between the two adjacent faces. Because the computational domains, which are inducer, impeller and volute, are in different levels of geometrical complexity, meshing is finished separately for different parts. Meanwhile, unstructured grid having strong adaptability is adopted. The numerical grid is shown in Fig.3.
Fig. 3
Fig. 1
The three dimensional model of pump
In the impeller(see Fig.2), inlet diameter D1 = 40 mm， outlet diameter D2 = 105mm, inlet width b1=11 mm, outlet width b2=4mm. Two impellers have the same parameters except for blade outlet angle. The blade outlet angles are β2=32.5°and β2=39° respectively.
Numerical grids
Calculation In the numerical analysis, the commercial software Fluent is used. Fluid is assumed under the steady condition and the RNG k-ε model is adopted as the turbulence model. The numerical calculation of whole flow field for the two different blades outlet angles is conducted at different flow rates based on the SIMPLEC algorithm which couples the pressure and velocity. The specific boundary conditions are as follows. 1) The inlet boundary condition: The constant velocity is given as the boundary condition at inlet and the axial velocity is determined by the law of mass conservation and the assumption of zero-entry swirl. 2) The outlet boundary condition: The outflow is used as the outlet boundary condition. Suppose the flow at the outlet is fully developed. 3) The wall condition: Non-slip boundary condition is adopted for the solid wall. The standard wall function is utilized for the domains near the wall.
Numerical results analysis (a) Centrifugal impeller
(b) Blade outlet angle
Fig. 2 Sketch Map of Centrifugal Impeller
Computational domain and grid In this research, the whole flow field is calculated. The
Pressure analysis on the mid-section In order to investigate the influence of blade outlet angle on the internal flow and performance of centrifugal pump, the numerical analyses are performed at design flow rates for different blade outlet angles β2=32.5° and 39° separately.
Cui Baoling et al.
Influence of Blade Outlet Angle on Performance of Low-specific-speed Centrifugal Pump
The static pressure distribution on the mid-section with two different blade outlet angles is shown in Fig4 (a) and (b). From Fig.4, it can be seen that the static pressure in two impellers both increases from the inlet to outlet, and the pressure on the pressure surface is higher than that on the suction surface at the same radius. The static pressure distribution in two impellers is uniform and regular while there is a little fluctuation near the impeller outlet because of the effect of the volute tongue. Low pressure regions appear near the leading edge on the suction surface of the four long blades and it is found that there are different size low pressure regions separately. The low pressure region at the suction side of the blade is also the place where is easy to occur cavitation.
(a) β2 = 32.5°
Fig. 5
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(b) β2 = 39°
Static pressure near the tongue
Circumferential pressure distribution The monitoring points are set on the interface between impeller outlet and volute inlet and near volute wall every 10 degrees. Therefore, there are 36 monitoring points along the circumference. The Ⅷ section of the volute is defined as circumferential angle 0°, and the positive rotation is counter-clockwise. Static and total pressure distribution on the interface (R = 52.6mm) between impeller outlet and volute inlet is shown in Fig.6. From Fig.6, it is found that the flow in
(a) β2 = 32.5°
(a) Static pressure distribution
(b) β2 = 39°
Fig. 4 Static pressure on the mid-section
Pressure analysis near the tongue The static pressure distribution near the tongue area is shown in Fig.5. It can be seen that the pressure distribution near the tongue is uneven, and there is an obvious pressure change from the tongue to the exit diffusion segment. The pressure fluctuation is also found at the tongue region. The low pressure area near the tongue is larger in Fig.5 (a), and the pressure near the wall of exit diffusion segment is relatively low. The low pressure near the tongue may be caused by the impact and backflow in the exit diffusion segment, which will result in certain hydraulic loss.
(b) Total pressure distribution
Fig. 6
Pressure distribution on the interface
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the impeller is unstable because of the rotor-stator interaction between impeller and volute. The pressure fluctuation distribution along the circumference is uneven and changes like sine signal. And the number of wave peak is nearly the same as the number of impeller blades, which means it produces rotor-stator interaction between blades and volute while the blade passes the volute. Also it can be seen that the static pressure and total pressure of β2=39° is larger than that of β2=32.5°. Besides, the fluctuation range of total pressure is larger than that of static pressure. The static and total pressure distribution near the volute wall is shown in Fig.7. It is found that the range of pressure fluctuation becomes very small compared with that on the interface, and the static pressure of β2=39° is higher. The static pressure near the wall increases with the increasing of circumferential angle because the dynamic pressure transforms into static pressure with the increasing of section area for spiral volute. Due to the hydraulic loss during the transformation the total pressure near the volute wall decreases gradually along with the circumference. The total pressure of 39° outlet angle is basically higher than that of β2=32.5°.
Streamline distribution on the mid-section The streamline distribution for the two different blade outlet angles on the mid-section is shown in Fig.8. It is found that the internal flow of the two impellers is non-uniform. There exist backflows at inlet of the impeller which may be caused by the uneven of the circumferential velocity at the edge of rotational blade. Besides, the backflows are also observed near the pressure side at blade outlet in two impellers. Compared with the streamline distribution in them, the larger blade outlet angle can improve the flow condition in the impeller so as to improve the discharge capacity of the passage.
(a) β2 = 32.5°
(a) Static pressure distribution (b) β2 = 39°
Fig. 8
(b) Total pressure distribution
Fig. 7
Pressure distribution near volute wall
Streamline distribution on the mid-section
Velocity distribution The circumferential and radial velocity distribution on the interface is shown in Fig.9. It is easy to find that the circumferential velocity is larger than the radial one, so the fluid on the interface flows along the volute in the helix direction. Compared with the circumferential velocity, there is negative value for the radial velocity near the tongue and the circumferential angle of 240°, which means the vortexes occur in the impeller passage because the fluid rotates with the impeller at high speed and brings the reverse fluid.
Cui Baoling et al.
Fig. 9
Influence of Blade Outlet Angle on Performance of Low-specific-speed Centrifugal Pump
(a) Radial velocity
(a) Radial velocity
(b) Circumferential velocity
(b) Circumferential velocity
Velocity distribution on the interface
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Fig. 10 Velocity distribution on the volute wall
From Fig.9 (b), it is found that there is certain fluctuation for the circumferential velocity and it decreases with the increasing of circumferential angle. Because of the spiral volute, the section area increases with the increasing of circumferential angle. The circumferential and radial velocity distribution near the volute wall is shown in Fig.10. Compared with the velocity distribution on the interface, the velocity fluctuation range near the volute wall becomes smaller. With the increasing of circumferential angle, the radial velocity approximates to a straight line, and it is basically the same for blade outlet angle 32.5°and 39°. Along the circumferential direction, the circumferential velocity reduces gradually. That is because the distance between the volute wall and impeller outlet is more and more far, and the force coming from impeller on the fluid near the wall is getting smaller and smaller.
Fig.11, it can be seen that the numerical result is close to the experimental one at different outlet angle conditions. The trend of the numerical result basically agrees with that of experimental result. From Fig.11 (a), the computational head at blade outlet angle β2=32.5° is higher than that of blade outlet angle β2=39° at small flow rate. And then when blade outlet angle β2=39°, it is higher with the increasing of flow rate. For the computational efficiency, there is little difference between two blade outlet angles when the flow rate is less than 1.2m3/h. At design point, when blade outlet angle β2=32.5°, the computational head Hs1 = 15.58m and efficiency ηs1 = 13.65%, while the experimental head Ht1 = 16.5m and efficiency ηt1 = 10.32%. When the blade outlet angle β2=39°, the computational head Hs2 = 16.58m and efficiency ηs2 = 14.57%, while the experimental head Ht2 =17.36m and efficiency ηt2 =12.64%.
External experiment
Conclusions
The characteristic performance curves of pump obtained by the experiment and the simulation are shown in Fig.11 when blade outlet angle β2=32.5° and 39°. From
To investigate the influence of blade outlet angle on the performance and internal flow, the centrifugal pump with complex impeller at different outlet angles is ana-
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Acknowledgement This investigation was supported by National Natural Science Foundation of China granted No.50976105, No.51276172 and Zhejiang Provincial Natural Science Foundation Granted No.R1100530.
References [1] T. Shigemitsu, J. Fukutomi , R. Nasada and K. Kaji. The Effect of Blade Outlet Angle on Performance and Internal Flow Condition of Mini Turbo-Pump. Journal of Thermal (a) H-Q curves
Science, vol.20, pp.32—38, (2011). [2] González J , Santolaria C. Unsteady flow structure and global variables in a centrifugal pump. ASME Journal of Fluids Engineering, vol.128, pp.937—946, (2006). [3] Guangwen Li. The flow measurement of centrifugal pump impeller with great outlet angle[J]. Journal of agricultural machinery, vol.30, pp.54—59, (1999). [4] Xianfang Wu, Minggao Tan, Houlin Liu, Yong Wang. Kai Wang. The influence of blade outlet angle on the centrifugal pump performance. Journal of Agricultural Mechanization Research, vol.9, pp.166—175, (2010). [5] Xijie He, Yuwen Huang, Yanxiao Zhao, Shuhong Li, Aixi Zhang. The influence of parameters in centrifugal pump
(b) η-Q curves
Fig. 11
Performance curves
lyzed by numerical simulation and experiment. The trend of the numerical result basically agrees with that of experimental result. The outlet angle has effect on the low pressure area at the suction side of long-blade leading edge and near the tongue, but has little influence on the pressure distribution in the passage of impeller. The larger blade outlet angle can improve the flow condition in the impeller so as to improve the discharge capacity of the passage. With the larger blade outlet angle, the low-specific-speed centrifugal pump achieves better hydraulic performance. Further, it is important to design the suitable blade outlet angle to improve the hydraulic performance of centrifugal pump.
on the efficiency. Drainage and Irrigation Machinery, vol.20, pp. 9—10, (2002). [6] Xijie He. The effect of main parameters on the low specific speed centrifugal pump. General Machinery, vol.1, pp. 63—65, (2004). [7] V. I. Veselov. Effect of the outlet angle β2 on the characteristics of low specific speed centrifugal pumps [J]. Power Technology and Engineering, vol.16, pp.267—273. (1982). [8] José Gonzúlez, Joaqun Fernández, Eduardo Blanco, et a1. Numerical Simulation of the Dynamic Effects due to Impeller—Volute Interaction in a Centrifugal Pump[J]. Transactions of the ASME, vol.124, pp.348 — 354, (2002).
