Analysis of cavitation behaviour in a centrifugal pump

Transcription

1 IOP Conference Series: Earth and Environmental Science Analysis of cavitation behaviour in a centrifugal pump To cite this article: M He et al 2012 IOP Conf. Ser.: Earth Environ. Sci View the article online for updates and enhancements. Related content - Cavitating flow investigation inside centrifugal impellers for a condensate pump W Wei, X W Luo, B Ji et al. - Experimental investigation of pressure instabilities affected by cavitation for a double-suction centrifugal pump Z F Yao, F J Wang, R F Xiao et al. - Centrifugal pump performance drop due to leading edge cavitation X J Li, Z Y Pan, D Q Zhang et al. This content was downloaded from IP address on 07/12/2017 at 15:00

2 Analysis of cavitation behaviour in a centrifugal pump M He 1, L P Fu 2, L J Zhou 1, Q Guo 1 and Z W Wang 3 1 College of Water Conservancy and Civil Engineering, China Agricultural university Qinghuadonglu Street, Haidian District, Beijing, , China. 2 Hangzhou Resource Power Equipment Co., Ltd Weishan Road, Lingang Industrial area, Guali Town, Xiaoshan District, Hangzhou, Zhejiang, ,China. 3 Department of Thermal Engineering, Tsinghua University Qinghuayuan, Haidian District, Beijing, , China. zlj09@263.net Abstract. Cavitation is a well-known problem in centrifugal pumps, causing serious damage and substantial head losses. However, the reason for the sudden head drop in cavitation curves is not fully understood. In this paper, the transient three-dimensional cavitating flow field in a centrifugal pump was calculated using RNG k turbulence model and Rayleigh Plesset cavitation model. The NPSH-H curve and the cavitation development in the whole passage were predicted. The blade loading and energy transfer are analyzed for various cavitation conditions. The results show that the existing of the cavities changes the load distribution on blades. With the decrease of NPSH the loads on blades tend to increases in the rear part but decreases in the front part. If NPSH is not so low, sometimes the overall torque may increase slightly, thus the head may also increase slightly. But if the NPSH become low and reach a threshold value, the overall torque will also decrease. At the same time, the energy dissipation in the vortices increases greatly because of the growth of the cavities. These two reasons make the head drop rapidly. 1. Introduction Cavitation in centrifugal pumps can lead to substantial performance losses, strong unsteady forces, noise and erosion [1]. Many experiments and numerical simulations have been done on this problem over the last few years. Most of them focused on the cavitation inception, cavity visualizations, and/or the threshold corresponding to pump head drops [2-6]. For example, Medvitz Richard B. et al.[7] captured the Characteristic performance trends associated with off-design flow as well as the rapid drop in head coefficient at low cavitation numbers. Coutier-Delgosha, O. et al.[8] investigated the quasi-steady cavitating behavior of three pumps and successfully obtained the head drop and the vapor structures using the barotropic state law. However, the reason for the sudden head drop in cavitation curves is not fully understood. In this paper, the unsteady 3D cavitating simulation was performed by using the Rayleigh Plesset cavitation model in ANSYS-CFX.12. Much attention is paid to the influence of cavitation behavior on the pump characteristics and on the sudden head drops observed in the simulation result. 2. The calculation model Published under licence by Ltd 1

3 2.1. Computational domain and grids The calculation model is a single stage centrifugal pump with specific speed ns = 97.1, impeller diameter 180mm, and speed 2900rpm. The domain meshes consist of unstructed hybrid grid. Near the blade leading edges, tailing edges and the tongue of the volute, the mesh is refined. The grid is shown in Fig. 1(a). One blade named Blade 1 is selected to show the load variation under cavitaiton conditions, as shown in Fig. 1(b). Z=0.5b section blade1 (a) Calculation domain and mesh (b) Position of blade1 and Z=0 section Figure 1. Calculation domain 2.2. Physical model A homogeneous multiphase model was applied for the cavitation model [9]. The Rayleigh Plesset model, where the second order terms and the surface tension were neglected, was employed to describe the growth of a gas bubble. The saturated vapor pressure was 3574Pa. Turbulence effects were modeled using the RNGk turbulence model with standard wall functions and adiabatic wall boundary conditions. This turbulence model has been applied for this kind of calculations proving to be appropriate [10]. A second order scheme was used for the discretisation of The UNRANS Equations in the spatial domain. The first order scheme was used for the temporal term Boundary conditions Two sets of boundary conditions have been compared at first. One is that the velocity imposed upstream and the total pressure downstream. The other is that pressure imposed upstream and flow rate downstream. Little difference in the global NPSH-H curves is observed for the two set of boundary conditions tested. Thus the boundary condition was specified with a velocity inlet and a pressure outlet. At solid walls, the no-slip boundary condition was imposed in the appropriate frame of reference. 3. Analysis of cavitating flows 3.1. Cavitating curves Cavitating flows were simulated for three flow rate condition, 0.7Q O, 1.0Q O and 1.3Q O under different cavitation numbers(npsh). The NPSH-H curve was obtained by reducing the pump outlet pressure. As expected, with the decrease of the NPSH, the pump head H remains constant before the NPSH reach a certain value where the head H drops sharply, as shown in Fig.2 for 0.7Q O. For the other 2 discharges, the curves are similar and not present here. 2

4 Figure 2. NPSH-H curve (Q=0.7Q O ) Six conditions named A(NPSH=24.64m), B(NPSH=8.6m), C(NPSH=3.9m), D(NPSH=2.91m), E(NPSH=2.03) and F(NPSH=1.92m) was selected for flow field visualization. These points are plotted in Fig.2 by solid dot Flow field visualizations Fig. 3 illustrates the general development of cavities in the impeller channels corresponding to the six operating points (A, B, C, D, E and F) indicated in Fig. 2. The hub is colored in gray, the blades are represented in blue, and the two-phase areas appear in yellow (the surface drawn corresponds to a 5% void ratio).it shows that the cavity mainly grows on the blades suction side, while no vapor is present on the pressure side expect for point E and F. A B C D E F Figure 3. Cavity evolution on the blade suction side The cavity attached on the suction side grows progressively: the cavity first appears on the leading edge of the blade near hub (Fig.3 Cond. B, inception). And the cavities then grows downstream and they extend longer near the hub than near the shroud ( and C); then, the cavity length increases quickly (Condition D and E) and form great blockages in the upstream of the blade-to-blade channels. In Cond. F, the cavities even extend from pressure side to suction side of the opposite blade in some channels. Cond. D to F is the regime where a severe performance breakdown is observed as shown in Fig.2. Some typical conditions (A,B, D and E) are selected for the following analysis, in which Cond. A corresponds to non cavitation case. Cond. B is the inception of the cavities, Cond. E corresponds to the critical threshold NSPH according to the relative head drop at this discharge. D is a condition just before the sudden head drop Blade loading distribution Because of the influence of the volute tongue, the flow field inside the impeller shows obvious periodic variation; with dominate frequency of fn, where fn is the runner rotational frequency. That means, for each blade the pressure distribution on it repeats after one rotation. Define Ѳ =0 the 3

5 location where the blade trailing edge(on the pressure side), impeller center and tongue being in a straight line, as shown in Fig. 1(b) for Blade 1. The pressure distribution on Blade 1 at 6 special positions (Ѳ =0,60,120,180,240 and 300 ) are shown in Fig. 4 for above three conditions (A, D,E). NPSH=24.64m(A) NPSH=2.91m(D) NPSH=2.03m(E) Pressure surface Pressure surface pressure surface 4

6 Figure 4. The pressure distribution on Blade 1 at 6 special positions Fig. 3 and 4 shows that when the NPSH decreased from 24.64m(A) to 2.91m(D), obvious cavities develop along the blade suction side but the pressure distribution on blades is not significantly affected except for the position 0 and 60 where the pressure difference between the suction and pressure side is slightly increased. This causes a slight increase of torque. With a further reduce of the NPSH to 2.03m(E), the cavities grow rapidly and the pressure distribution on blades is obviously affected in all positions, with the load on blades increases in the rear part and decreases in the front part, especially for the position 0, 60 and 120. This make the overall torque decreases anyhow. It is interesting to compare the overall shaft power (P s ) and hydraulic power (P h ) curve during the cavitation evolution. They are plotted in Fig. 5. The curves in Fig. 5 show that the turning points in P s - H curve and the P h -H curve are the same. This implies that the head drop has close relation to the torque reduction, which is also noticed by an analysis based on the energy balance for an industrial inducer in Ref [2], indicating that major reasons of the head drop are related to the torque reduction. However for Cond. E, the hydraulic power shows a 3.7% decrease while the shaft power only shows a 1.1% decrease related to that for non-cavitation Cond. A. That means for the centrifugal pump in this case, there is other reason for the sudden head drop after Cond. D. E D F C B A E F D C B A Figure 5. Comparison of the overall shaft power (Ps) and hydraulic power (Ph) curves 3.4. Energy transfer inside the impeller The middle plane section that is perpendicular with the rotational axis (Z=0) was selected to investigate the variation of the total pressure inside the impeller. Fig. 6 shows the comparison for the 4 conditions (A, B, D and E). To avoid the roundoff error the total pressure increase (dh 1 ) relative to that on inlet is used. Another 4 section Ⅱ, Ⅲ, Ⅳ and Ⅴ along radial direction are also selected for plotting the contours of dh 1 as shown in Fig. 7. On all conditions, the overall energy increases along the streamline direction because of the work done by the blades. Obvious differences exist near the leading edges where the cavities appear. The blue area with low dh 1 value means there is small energy transfer in this area. It can be seen from Fig. 6-7 that the blue area grows large with decrease of NPSH and the growth of the cavities. It means that the energy transfer near leading edge is smaller for lower NPSH Conditions. 5

7 (Cond.A) (Cond.B) (Cond.D) (Cond.E) Figure 6. Comparison of total pressure increase dh 1 on the middle plane Ⅴ Cond. A Ⅳ Ⅲ Ⅱ Ⅴ Cond. B Ⅳ Ⅲ Ⅱ Cond. D Cond. E Ⅴ Ⅳ Ⅲ Ⅱ Ⅴ Ⅳ Ⅲ Ⅱ Figure 7. Comparison of total pressure increase dh1 on the cross sections To make a quantitative analysis the averaged dh 1 on these 4 sections on the 4 Conditions are plotted in Fig. 8, where the averaged dh 1 on the inlet (sectionⅠ) and outlet (section Ⅵ) of the calculation domain are also included. This Fig. again indicates that with the development of cavitation, the energy transfer tend to be more active in the rear part than in the front part and the loads on blades become heavier in the front part than rear part. 6

8 Figure 8. Averaged total pressure increase dh 1 from the cross sectionsⅠto Ⅵ It is also interesting to notice that with the growth of the cavities a high value area (in red color) appears on the suction side near section Ⅳ, as highlighted by the red circles in Fig. 7. On Cond. E this area occupies a significant part cross the passage and has much higher value than on Cond. D. Checking the flow structure inside the impeller, one can see that the cavities enhance the vortices on the suction side. With the growth of the cavities the vortices becomes large and extends towards trailing edge. Since the overall energy in volute is higher, the revised flow near the suction side caused by vortices carry part of the energy from the volute back into the impeller passage. Thus this part of fluid gains more energy from the work done by the blades. This is the reason for the high energy area shown in Fig. 7 by the red circles. (NPSH=8.6m, Cond. B) (NPSH=2.91m, Cond. D) (NPSH=2.03m, Cond. E) Figure 9. Streamline inside the impeller on different cavitation conditions The energy carry from the upstream maintains the swirling of the fluid and compensates the energy dissipation. If NPSH is not so low, the load distribution on blades changes in a way to become heavier in the front part than rear part. Since the radius increase from the leading to the trailing edge, sometimes this may cause a small increase of the overall torque. If the torque increase can compensates this part of energy, the head keep almost unchanged, as in the case of Cond. B. If the torque increase overcomes this part of energy it may even cause a slight increase of the Head (these cases are often observed in experiments). But if the NPSH become very low, the overall torque will finally decrease, as in our case on Cond. E. At the same time, the energy dissipation in the vortices increases greatly because of the growth of the cavities. These two reasons make the head drop rapidly. 4. Conclusions In this paper, the transient three-dimensional cavitating flow field in a centrifugal pump was calculated using RNG k turbulence model and Rayleigh Plesset cavitation model. The NPSH-H curve and the cavitation development in the whole passage were predicted. The blade loading and energy transfer are analyzed for various cavitation conditions. The results show that the existing of the cavities changes the load distribution on blades. With the decrease of NPSH the loads on blades tend 7

9 to increases in the rear part but decreases in the front part. If NPSH is not so low, sometimes the overall torque may increase slightly, thus the head may also increase slightly. But if the NPSH become low and reach a threshold value, the overall torque will also decrease. At the same time, the energy dissipation in the vortices increases greatly because of the growth of the cavities. These two reasons make the head drop rapidly. Acknowledgements The authors thank the National Natural Science Foundation of China (No , ) for supporting the present work. Nomenclature H pump head (m) NPSH Net pressure suction head(m) Pin Pv NPSH g v in absolute velocity of pump inlet (m/s) P in static pressure of pump inlet (Pa) P Tin inlet total pressure (Pa) P T outlet total pressure (Pa) P v saturated vapor pressure (Pa) Q flow rate (m 3 /s) T Torque(N m) pump rotation speed(rad/s) dh 1 total pressure increase (Pa), PT PTin P s overall shaft power(kw), P S T * / 1000 P h hydraulic power (kw), Ph dh1* Q / 1000 angle between blade and volute tongue( ) impeller rotational frequency (Hz) f n 2 vin 2g References [1] Coutier-Delgosha O, Fortes-Patella R, Reboud J L and Hakimi N Proc. ASME FEDSM (Quebec, Canada, 2002) [2] Ait Bouziad Y, Farhat M, Guennoun F, Kueny J L, Avellan F and Miyagawa K Proc. Fifth Int. Symp. on Cavitation (Osaka, Japan, 2003) [3] Ait Bouziad Y, Farhat M, Kueny J L, Avellan F and Miyagawa K Proc. 22th IARH Symp. on Hydraulic Machinery and Systems (Stockholm, Sweden, 2004) [4] Mejri I, Bakir F, Rey R and Belamri T 2006 J. ASME J. Fluids Eng [5] Li Jun, Liu Lijun and Li Guojun 2007 J. Joural of EngineEring Thermophysics [6] Coutier Delgosha O, Reboud JL and Fortes Patella R Proc. of the 4th Int. Symp. On Cavitation (Pasadena, USA, 2001) [7] Medvitz Richard B, Kunz Robert F and Boger David A 2002 J. ASME Fluids Eng [8] Coutier Delgosha O, Fortes Patella R, Reboud J L and Hakimi 2003 J. ASME J. Fluids Eng [9] Help Navigator ANSYS CFX, Release 11.0 [10] Bakir F, Rey R, Gerber A G, Belamri T and Hutchinson B 2004 J. International Journal of Rotating Machinery