Research on axial thrust of the waterjet pump based on CFD under cavitation conditions

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1 IOP Conference Series: Materials Science and Engineering OPEN ACCESS Research on axial thrust of the waterjet pump ased on CFD under caitation conditions To cite this article: Z H Shen and Z Y Pan 015 IOP Conf. Ser.: Mater. Sci. Eng Related content - Sponsorship Statement N Désy - Numerical analysis of head degrade law under caitation condition of contrarotating axial flow waterjet pump D Huang and Z Y Pan - Numerical simulation on the caitation of waterjet propulsion pump C Z Xia, L Cheng, Y N Shang et al. View the article online for updates and enhancements. This content was downloaded from IP address on 04/05/018 at 10:15

2 International Symposium of Caitation and Multiphase Flow (ISCM 014) IOP Pulishing Research on axial thrust of the waterjet pump ased on CFD under caitation conditions Z H Shen and Z Y Pan National Research Center of Pumps, Jiangsu Uniersity, Zhenjiang 101, China shenzhspar@16.com Astract. Based on RANS equations, performance of a contra-rotating axial-flow waterjet pump without hydrodynamic caitation state had een otained comined with shear stress transport turulence model. Its caitation hydrodynamic performance was calculated and analysed with mixture homogeneous flow caitation model ased on Rayleigh-Plesset equations. The results shows that the caitation causes axial thrust of waterjet pump to drop. Furthermore, axial thrust and head caitation characteristic cure is similar. Howeer, the drop point of the axial thrust is postponed y 5.1% comparing with one of head, and the critical point of the axial thrust is postponed y.6%. 1. Introduction Waterjet propulsion is also known as waterjet pump. It is a deice that uses reaction spraying flow to push essel forward y a pump. Due to the adantages of waterjet pump hydraulic performance, such as superior resistance to caitation characteristics, and structural characteristics of the deice make it more and more suitale and rapid deelopment momentum [1]. Waterjet pump power staility is of foundation to the ship staility so the research of waterjet thrust characteristics is essential. Liu Chengjiang, Wang Yongsheng et., introducing how to otain waterjet propulsion thrust theory, experiment and numerical calculation method, alidated CFD method is feasile and credile []. Jules W. Lindau, Christopher Pena et. analyzed of caitation performance AxWJ- type waterjet pump, discoered the relationship etween waterjet thrust roken and ules locked rotor runner []. And usually, waterjets during acceleration and cornering produces caitation [4]. But there is rarely studies on relationship etween waterjet caitation and thrust in terms of the internal flow at home and aroad [5-7]. In this paper, the use of CFD technology, a new type of rotary axial waterjet pump caitation was calculated at design conditions, and the predicted axial thrust and internal flow of caitation under different conditions, and preliminary analyzed of the relationship etween the two.. Research model and computation method Contra-rotating axial flow pump s design parameters used in this paper are: design flow Q d taken to 0. m / s, first stage impeller head taken to m, secondary impeller head taken to 8m. Contra-rotating axial flow pump specific speed 600. First stage impeller lade numer 5, secondary impeller lades numer 4, anes leaf numer is 7. Flow components joint model shown in figure 1 (figure 1 using Cartesian coordinate system),from the perspectie of imports to exports, first stage impeller speed of r/min, direction is clockwise, secondary impeller speed of 1450 r/min, direction is Content from this work may e used under the terms of the Creatie Commons Attriution.0 licence. Any further distriution of this work must maintain attriution to the author(s) and the title of the work, journal citation and DOI. Pulished under licence y IOP Pulishing Ltd 1

3 International Symposium of Caitation and Multiphase Flow (ISCM 014) IOP Pulishing counterclockwise. Propeller model was three-dimensional modeling using Pro-ENGINEER, calculating area from entrance to exit and no exit tue in ordinary waterjet simulation, this study's propeller was added an export tue with special use, as shown in figure 1. Figure 1. Model for entire pump passage: 1 Pump inlet. first stage impeller. axial clearance. 4 secondary impeller. 5 guide ane and nozzle. 6 pump outlet. The grids of the three-dimensional model was generated y the ICEM CFD (integrated computer engineering and manufacturing code for computational fluid dynamics).the structured grids in computational regions are shown figure. The total numer of grid elements is for all the domains. (a) first stage impeller () secondary impeller (c) guide ane (d) grid for entire pump passage Figure. Grid iew in computational region. The oundary conditions of steady flow simulation were almost the same under oth caitation and non-caitation conditions. The total pressure at the inlet of the suction pipe was applied, and then the outlet oundary condition was mass flow rate. The impeller surfaces were set up as a rotating wall. All the other walls were stationary.. Goerning equation and caitation model In the mixture model of the apor/liquid two-phase flow, the fluid is assumed homogeneous so that the multiphase fluid components were assumed to share the same elocity and pressure [8]. A caitation process is expressed y the mass transfer equation. Equation (1) has demonstrated the conseration equation of the apor olume fraction. Note that the source terms and represent eaporation and condensation. Where is olume fraction of one component. ui Re Rc (1) t x i 1 ()

4 International Symposium of Caitation and Multiphase Flow (ISCM 014) IOP Pulishing R e F N 4R e p p sign l p p () R c F N 4R c p p sign l p p (4) For the aporization, N is gien y N 1 4R d (5) For the condensation, N is gien y P N (6) 4R In equation () and equation (4), is apor pressure, is aporization coefficient, F c is condensation coefficient. Empirical coefficients used to represent the apor increasing or reducing time step. For the aporization, F e 50. For the condensation,. In the present study, SST turulence model was selected. F e F c Results and discussion Figure and 4 shows pump head and pump axial thrust of the caitation performance of the waterjet pump. With the decrease of the net positie suction head (NPSHA), the pump head decreases. NPSHA =.76 is approximated the net positie suction head critical (NPSHC) for the pump while head down.6%. Head decreases caused y increasing ules in the impeller. With the appearance of caitation, the flow inside the pump state changes and the change accumulate to a certain extent on the performance of a drop head. We find it similar to figure 4 and figure, i.e. the axial thrust of the pump also decreases with the decrease of the NPSHA, reaching a critical point decline intensified. The decline-starting point of the axial thrust delays 5.1% compared with the head decline-starting point, critical point delays.6%. When caitation occurs, the axial thrust decline is earlier than head fall; it is possile acuoles might hinder the interaction etween the lade and the working medium. But this does not mean caitation is the direct cause of head drop. Figure. Caitation characteristic cure of the pump at Q d.

5 International Symposium of Caitation and Multiphase Flow (ISCM 014) IOP Pulishing Figure 4. Caitation characteristic cure of the axial thrust at Q d Vapor ules distriution This section the design point of axial thrust decline cure was selected to inestigate the relationship etween the deelopment of the caitation distriution in impeller channel and the decline of the axial thrust in conjunction with the impeller caitation shown in Figure 5. It is analyzed the influence of caitation ased on 5 condition points (a~e) on the decreased cure of axial thrust in figure 4. Figure 5 shows the distriution of ules of fie operating points, while the caitation region is composited y the olume fraction of 10% iso-surface. When NPSHA was not reduced, slight caitation occur in the suction side of the first stage impeller tip as shown in figure 5(a). When NPSHA was 4.78 m, the caity area of the suction around the secondary impeller tip increased, while coered import to export side. Meanwhile, ules were found at pressure side near the hu of the first stage impeller, so was the suction side near the tip of the secondary impeller. When NPSHA dropped to.76 m, ules came out at the center of the suction side of the secondary impeller, which may e reasons and signs for the axial thrust decline eginning; When NPSHA was.99 m, ule had coered / of the suction side of the first stage impeller, while the surface area of the ule at the hu also further expanded, and at the same time the axial thrust had dropped 1.57%. (a) NPSHA=10.0 m () NPSHA=4.78 m (c) NPSHA=.76 m (d) NPSHA=.50 m (e) NPSHA=.99 m Figure 5. Distriution of the apor ules at Q d 4

6 International Symposium of Caitation and Multiphase Flow (ISCM 014) IOP Pulishing 4.. Impeller surface axial thrust Figure 6, X-axis represents NPSHA, where Y-axis represents the axial thrust of eery part of the impeller. With the NPSHA decreasing, the axial thrust of the impellers each surface reduced. The difference is that the axial thrust of the first impeller rose, when NPSHA reached the maximum was 4.78 m, an increase of 15.6%. Then it egan to decrease, when NPSHA was.76 m, the axial thrust and starting point (NPSHA = 10.0 m) flat, slightly decreased 0.04%. Secondary impeller axial thrust cure y a slow decline to a sharp decline in the critical point of.76 m, showing a similar trend in figure 4. This will also e consistent with the analysis of figure 5(c). (a) first stage impeller Figure 6. Axial thrust of impellers. () secondary impeller Figure 7, the X-axis represents the position of the lade, 0 said lade inlet, 1 lade outlet; Y-axis represents the pressure coefficient of the lade. Pressure coefficient C p is defined as C p p pin (7) 1 Where C p is pressure coefficient, p is lade surface static pressure, p in is pump inlet static pressure, ν in is the tip peripheral speed of first stage impeller inlet side. In figure 7(a), under conditions of no caitation, a "low-oltage spikes "later in the work surface of impeller inlet side came out. In figure 8, the low pressure region near the inlet of the lade face appeared extending from the hu. From the aoe results that caitation had occurred here, caitation flow lines tail twisted, ule extruded flow channel, changing the flow structure at the entrance. The suction side of the impeller lade pressure is the lowest part, high flow rate, and prone to caitation. With NPSHA = 4.78,.76 m, lade surface pressure decreased, ut the asic trend remained unchanged. When NPSHA =.99 m, the lade face pressure was further reduced, area of "low spike" increased and figure 5(e) showed caitation area near the hu of the first stage impeller further expansion. In this case the pressure at the suction side of the lade inlet increased, and smoothly extended to the rear of the lade. In figure 7(), when NPSHA reduced from 10.0 m to 4.78 m, pressure coefficient cure did not change significantly. NPSHA down to.50 m, the pressure coefficient decreased oerall, and NPSHA down to.99 m, the pressure coefficient sharply declined. 5. Conclusion and recommendations 1) Waterjet pump axial thrust declines caused y caitation. With NPSHA decreasing, the axial thrust and head of the pump appear a slight decline after the rise at eginning and decline intensified at the critical point. The decrease starting point of the axial thrust delays 5.1% compared with the head starting point, critical point delays.6%. in 5

7 International Symposium of Caitation and Multiphase Flow (ISCM 014) IOP Pulishing ) Axial thrust of the Impellers present caitation critical point, while the initial stage of caitation performance is of different nature. Caitation initial stage, axial thrust of the first stage impeller rose. When NPSHA = 4.78 m it reached a maximum and then egan to decline. The difference is that the initial stage of the axial thrust of the secondary impeller was gradually decreased. With NPSHA down to.76 m, the axial thrust appeared steep drop phenomenon. ) We find that the cure of axial thrust presents similar trend to head caitation characteristics, ut oth the starting drop point and the critical drop point is different. The next step is to explore the possile relationship etween the two. In addition, this paper only preliminary reeals changes in axial thrust under caitation conditions, ut also analyzes the relationship etween caitation and axial thrust from the effects of caitation on the flow structure. (a) first stage impeller Figure 7. Distriution of C p at 0.5 span. () secondary impeller Figure 8. distriution of the pressure and elocity of first stage impeller s work side at NPSHA=.5 m Acknowledgement This research was supported y the Stated Key Program of National Natural Science Foundation of China (Grant No ), and the Priority Academic Program Deelopment of Jiangsu Higher Education Institutions (PAPD). Reference [1] Bulten N W H 006 Numerical analysis of a waterjet propulsion system Dissertation Astracts Internationa 68 (0) [] Liu C J, Wang Y S and Zhang Z H 009 Chin. J. Computational Mech [] Lindau J W, Boger D A, Meditz R B and Kunz R F 005 J. Fluid Eng [4] Ding J M 01 J. Huazhong Uni. of Sci. and Tech. (Natural Science Edi.) [5] Wang D J and Cao S L 00 J. Hydroelectric Eng. E [6] Terwisgat K K and Ketal V 00 Performance and caitation analysis of a waterjet system in a caitation tunnel The 7th Int. Conf.on Oil Fast Sea Transportation (Ischia, Italy, Oct 6

8 International Symposium of Caitation and Multiphase Flow (ISCM 014) IOP Pulishing 00) pp 57-6 [7] Brennen C E 011 Hydrodynamics of pumps (Camridge UK: Camridge uniersity press) [8] Li X J 01 Mechanism and unsteady dynamic characteristics of leading edge caitation in a centrifugal pump (Zhenjiang: Jiangsu Uni.) 7