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

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Journal of Thermal Science Vol.22, No.2 (2013) 117 −122

Received: October 2012 CUI Baoling: ProfessorThis 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-speedCentrifugal 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 importantgeometric parameters for the impeller of centrifugal

pump, which has a significant influence on the pumphead, efficiency and so on. Some researches had beendone on the effect of blade outlet angle on the pump per-formance using theoretical analysis and experimentalmethod. T. Shigemitsu et al. [1] studied three types ofrotors 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 miniturbo-pumps. Also González et al. [2] found that different

blade outlet angles have significant influence on themoment 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 outletangle operated at the best and small flow conditions.Xianfang Wu et al [4] had analyzed the influence of bladeoutlet angle on performance characteristic of centrifugal

pump with different specific speeds. Based on the multi- ple regression method, Xijie He [5, 6] researched on theeffect degree and sequence of impeller geometric pa-rameters on performance characteristic of centrifugal

pump, and the results showed that blade outlet angle hassignificant influence on the pump head. With the rapid

progress of computer technology and computational fluiddynamics, many numerical studies have been carried outon centrifugal pump [7, 8], but few are on the low-specific-speed centrifugal pump. So, it is necessary toinvestigate the effect of different blade outlet angles on



118 J. Therm. Sci., Vol.22, No.2, 2013

performance of low-specific-speed centrifugal pump. Inthis paper, to analysis the influence of blade outlet angleon performance and internal flow of low-specific-speedcentrifugal pump, the flow field in the pump with differ-

ent blade outlet angles is numerically calculated usingcommercial software Fluent. The external performanceexperiment 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 specificspeed ns=28. The impeller is a complex one with fourlong blades and eight short blades. To achieve better suc-

tion performance, a variable-pitch inducer is designedupstream 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 mmoutlet diameter D2 = 105mm, inlet width b 1=11 mm, out-let width b2=4mm. Two impellers have the same parame-ters except for blade outlet angle. The blade outlet anglesare 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, theextension of inlet and outlet, volute and clearance be-tween impeller with the front shroud and hub. To ensurethe stability of calculation result, there is a proper exten-

sion at the outlet of impeller. The numerical grids areobtained by Gambit, and interfaces are formed betweenthe 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 softwareFluent is used. Fluid is assumed under the steady condi-tion and the RNG k- ε model is adopted as the turbulencemodel. The numerical calculation of whole flow field forthe two different blades outlet angles is conducted at dif-ferent flow rates based on the SIMPLEC algorithm whichcouples the pressure and velocity. The specific boundaryconditions are as follows.

1) The inlet boundary condition: The constant velocityis given as the boundary condition at inlet and the axialvelocity is determined by the law of mass conservationand the assumption of zero-entry swirl.

2) The outlet boundary condition: The outflow is usedas 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 isutilized 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 designflow 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-sectionwith two different blade outlet angles is shown in Fig4 (a)and (b). From Fig.4, it can be seen that the static pressurein two impellers both increases from the inlet to outlet,

and the pressure on the pressure surface is higher thanthat on the suction surface at the same radius. The static

pressure distribution in two impellers is uniform andregular while there is a little fluctuation near the impelleroutlet 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 thatthere are different size low pressure regions separately.The low pressure region at the suction side of the blade isalso 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 isshown 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 diffusionsegment. The pressure fluctuation is also found at thetongue region. The low pressure area near the tongue islarger in Fig.5 (a), and the pressure near the wall of exitdiffusion segment is relatively low. The low pressurenear 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 betweenimpeller outlet and volute inlet and near volute wallevery 10 degrees. Therefore, there are 36 monitoring

points along the circumference. The Ⅷ section of thevolute 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 isshown 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 andchanges like sine signal. And the number of wave peak is

nearly the same as the number of impeller blades, whichmeans it produces rotor-stator interaction between bladesand 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 withthat on the interface, and the static pressure of β 2=39° ishigher. The static pressure near the wall increases withthe increasing of circumferential angle because the dy-namic pressure transforms into static pressure with theincreasing of section area for spiral volute. Due to thehydraulic loss during the transformation the total pres-sure near the volute wall decreases gradually along withthe circumference. The total pressure of 39° outlet angleis 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 bladeoutlet angles on the mid-section is shown in Fig.8. It isfound 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 canimprove 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 onthe interface is shown in Fig.9. It is easy to find that thecircumferential velocity is larger than the radial one, sothe fluid on the interface flows along the volute in thehelix direction. Compared with the circumferential ve-locity, there is negative value for the radial velocity nearthe tongue and the circumferential angle of 240°, whichmeans the vortexes occur in the impeller passage becausethe fluid rotates with the impeller at high speed and

brings the reverse fluid.



Cui Baoling et al. Influence of Blade Outlet Angle on Performance of Low-specific-speed Centrifugal Pump 121

(a) Radial velocity

(b) Circumferential velocity

Fig. 9 Velocity distribution on the interface

From Fig.9 (b), it is found that there is certain fluctua-tion for the circumferential velocity and it decreases withthe increasing of circumferential angle. Because of thespiral volute, the section area increases with the increas-ing of circumferential angle.

The circumferential and radial velocity distributionnear the volute wall is shown in Fig.10. Compared withthe velocity distribution on the interface, the velocityfluctuation range near the volute wall becomes smaller.With the increasing of circumferential angle, the radialvelocity approximates to a straight line, and it is basicallythe same for blade outlet angle 32.5°and 39°. Along thecircumferential direction, the circumferential velocityreduces gradually. That is because the distance betweenthe volute wall and impeller outlet is more and more far,and the force coming from impeller on the fluid near thewall is getting smaller and smaller.

External experiment

The characteristic performance curves of pump ob-tained by the experiment and the simulation are shown in

Fig.11 when blade outlet angle β 2=32.5° and 39°. From

(a) Radial velocity

(b) Circumferential velocity

Fig. 10 Velocity distribution on the volute wall

Fig.11, it can be seen that the numerical result is close tothe experimental one at different outlet angle conditions.The trend of the numerical result basically agrees withthat of experimental result. From Fig.11 (a), the compu-tational head at blade outlet angle β 2=32.5° is higher thanthat of blade outlet angle β 2=39° at small flow rate. Andthen when blade outlet angle β 2=39°, it is higher with theincreasing of flow rate. For the computational efficiency,there is little difference between two blade outlet angleswhen the flow rate is less than 1.2m 3/h. At design point,when blade outlet angle β 2=32.5°, the computationalhead H s1 = 15.58m and efficiency ηs1 = 13.65%, while theexperimental head H t1 = 16.5m and efficiency ηt1 =10.32%. When the blade outlet angle β 2=39°, the compu-tational head H s2 = 16.58m and efficiency ηs2 = 14.57%,while the experimental head H t2 =17.36m and efficiencyηt2 =12.64%.

Conclusions

To investigate the influence of blade outlet angle onthe performance and internal flow, the centrifugal pump

with complex impeller at different outlet angles is ana-



122 J. Therm. Sci., Vol.22, No.2, 2013

(a) H-Q curves

(b) η-Q curves

Fig. 11 Performance curves

lyzed by numerical simulation and experiment. The trendof the numerical result basically agrees with that of ex-

perimental result. The outlet angle has effect on the low pressure area at the suction side of long-blade leadingedge and near the tongue, but has little influence on the

pressure distribution in the passage of impeller. The lar-ger blade outlet angle can improve the flow condition inthe impeller so as to improve the discharge capacity ofthe passage. With the larger blade outlet angle, thelow-specific-speed centrifugal pump achieves better hy-

draulic performance. Further, it is important to design thesuitable blade outlet angle to improve the hydraulic per-formance of centrifugal pump.

Acknowledgement

This investigation was supported by National NaturalScience Foundation of China granted No.50976105,

No.51276172 and Zhejiang Provincial Natural ScienceFoundation Granted No.R1100530.

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Documents

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