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Journal of Thermal Science Volume 22 Issue 2 2013 [Doi 10.1007%2Fs11630-013-0601-6] Baoling Cui, Canfei Wang, Zuchao Zhu, Yingzi Jin -- Influence of Blade Outlet Angle on Performance of Low-specific-speed Centrif

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  Journal of Thermal Science Vol.22, No.2 (2013) 117 − 122   Received: October 2012 CUI Baoling: Professor This investigation was supported by National Natural Science Foundation of China granted No.50976105, No.51276172 and Zheji-ang Provincial Natural Science Foundation Granted No.R1100530.   www.springerlink.com   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 cal-culated. 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 calcula-tion. 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 pres-sure 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 per-formance using theoretical analysis and experimental method. T. Shigemitsu et al. [1] studied three types of rotors with different outlet angles in the mini turbo-  pumps. He investigated the effect of the blade outlet an-gle 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 cen-trifugal 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 multi- ple regression method, Xijie He [5, 6] researched on the effect degree and sequence of impeller geometric pa-rameters 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 low- specific-speed centrifugal pump. So, it is necessary to investigate the effect of different blade outlet angles on  118   J. Therm. Sci., Vol.22, No.2, 2013  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 differ-ent 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 cen-trifugal pump studied are flowrate Q  = 1.5m 3 /h, head  H   = 15m, the rotating speed n = 2900r/min. The specific speed n s =28. The impeller is a complex one with four long blades and eight short blades. To achieve better suc-tion performance, a variable-pitch inducer is designed upstream of the impeller. The three dimensional model of  pump is shown in Fig.1. Fig. 1 The three dimensional model of pump In the impeller(see Fig.2), inlet diameter  D 1 = 40 mm , outlet diameter  D 2 = 105mm, inlet width  b 1 =11 mm, out-let width b 2 =4mm. Two impellers have the same parame-ters except for blade outlet angle. The blade outlet angles are  β  2 =32.5°and  β  2 =39° respectively. (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 computational domains include impeller, inducer, the extension of inlet and outlet, volute and clearance be-tween impeller with the front shroud and hub. To ensure the stability of calculation result, there is a proper exten-sion at the outlet of impeller. The numerical grids are obtained by Gambit, and interfaces are formed between the two adjacent faces. Because the computational do-mains, which are inducer, impeller and volute, are in dif-ferent levels of geometrical complexity, meshing is fin-ished separately for different parts. Meanwhile, unstruc-tured grid having strong adaptability is adopted. The nu-merical grid is shown in Fig.3.   Fig. 3  Numerical grids   Calculation In the numerical analysis, the commercial software Fluent is used. Fluid is assumed under the steady condi-tion 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 dif-ferent 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 Pressure analysis on the mid-section In order to investigate the influence of blade outlet an-gle on the internal flow and performance of centrifugal  pump, the numerical analyses are performed at design flow rates for different blade outlet angles  β  2 =32.5° and 39° separately.  Cui Baoling et al. Influence of Blade Outlet Angle on Performance of Low-specific-speed Centrifugal Pump   119 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 suc-tion 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)  β  2 = 32.5° (b)  β  2 = 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 distribu-tion 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 back-flow in the exit diffusion segment, which will result in certain hydraulic loss. (a)  β  2 = 32.5° (b)  β  2 = 39° Fig. 5 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) Static pressure distribution (b) Total pressure distribution Fig. 6 Pressure distribution on the interface  120   J. Therm. Sci., Vol.22, No.2, 2013 the impeller is unstable because of the rotor-stator inter-action between impeller and volute. The pressure fluctua-tion 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  β  2 =39° is larger than that of  β  2 =32.5°. Besides, the fluc-tuation range of total pressure is larger than that of static  pressure.   The static and total pressure distribution near the vo-lute 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  β  2 =39° is higher. The static pressure near the wall increases with the increasing of circumferential angle because the dy-namic pressure transforms into static pressure with the increasing of section area for spiral volute. Due to the hydraulic loss during the transformation the total pres-sure near the volute wall decreases gradually along with the circumference. The total pressure of 39° outlet angle is basically higher than that of  β  2 =32.5°. (a) Static pressure distribution (b) Total pressure distribution Fig. 7 Pressure distribution near volute wall 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 impel-ler which may be caused by the uneven of the circum-ferential 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 stream-line distribution in them, the larger blade outlet angle can improve the flow condition in the impeller so as to im- prove the discharge capacity of the passage. (a)  β  2 = 32.5° (b)  β  2 = 39° Fig. 8 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 ve-locity, 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.
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