Impeller Comparsion

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Cavitation improvement of double suction centrifugal pump HPP Fuhren

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2012 IOP Conf. Ser.: Earth Environ. Sci. 15 022009

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Cavitation improvement of double suction centrifugal pump

HPP Fuhren

A kerlavaj1, M Titzschkau2, R Pavlin3, F Vehar3, P Menar1 and A Lipej11TRI Department, Turbointitut d.d., Rovnikova 7, 1000 Ljubljana, Slovenia2 Grimsel Hydro, Kraftwerke Oberhasli AG, Postfach 63, CH-3862 Innertkirchen,Switzerland3Pumps programme, Turbointitut d.d, Rovnikova 7, 1210 Ljubljana, Slovenia

E-mail: [email protected]

Abstract. A double suction storage pump has been refurbished because of the strong cavitation

which resulted in cavitation damage on blade and consequently in frequent repairs of the

impeller. The analyses of the old and the new impeller were done by the computational fluid

dynamics (CFD), performing transient simulations with the commercial solver Ansys CFX. Inthe simulations, the scale-adaptive-simulation with the curvature correction (SAS-CC)

turbulence model was used. No model tests were carried out. Additionally, observations with

the digital camera were made through the specially designed plexi-glass window, mounted at

the lid at the suction side. The predicted pump head at the operating point agrees well with thepump characteristics measurements, performed with the direct thermodynamic method. The

extent of the cavitation predicted by CFD is smaller than the observed one because the cloud

cavitation was not predicted. The observations of the cavitation extent show that the impeller

design is better than the old one, which was also possible to anticipate based on the CFDresults.

1. Introduction

The storage pump HPP Fuhren is a double suction pump with specific speed nq=30 which started tooperate in 1961. Due to cavitation damage the impeller needed to be repaired at almost every overhaul,which means every five years. The many overhauls decreased the performance of the impeller and, for

a few years, the impeller had to be changed each year because of severe cavitation damage.At the beginning of operation in the 1960s cavitation damage was merely visible on the suction

side of the leading edge. Now cavitation damage is visible on all blades of the drive end and non-driveend of the impeller. There is a significant area of cavitation erosion 150mm behind the leading edge onsuction side on every second blade. The worst damage occurs on the pressure side of the leading edge:there are holes through almost every second blade after approximately 1500 hours of operation, whichis a typical damage for the recirculation cavitation.

It was decided that the analysis of the existing pump and the development of the new impellershould be done by use of computational fluid dynamics (CFD). The pump station is of minor

importance in the complex hydropower system of the KWO so model tests as recommended for thistype of impeller were not planned in this refurbishment project. The development consisted of theimprovement of the flow in the pump inlet casings, as well as of the development of the new impeller.

In the present paper, the CFD simulation of cavitation on impeller blades and the comparison with itsvisualization will be presented.

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Figure 1.Cavitation damage at the old impeller after 2400 hours of operation.

2. Numerical method

The numerical computations were performed using a finite-volume commercial solver Ansys CFX [1].

The simulations were performed on the LSC Adria supercomputing centre located at Turbointitut,which consists of 256 IBM HS22 blade servers, each equipped with two quad-core Intel Xeonprocessors L5530 2.4GHz 8MB L2 and 16 GB RAM. For fast inter-node communication theInfiniband link with MPI protocol is used.

When compared with the characteristics determined by site tests it was apparent that transientcalculations were the only appropriate ones, as the prediction of the pump head obtained with the

steady-state results was approximately eight percent too high. Therefore, in our case the system of theReynolds-averaged Navier-Stokes equations was closed by using the scale-adaptive-simulation [2]turbulence model with the curvature-correction [3] option (SAS-CC). The reason for using such amodel was due to our positive experience regarding the simulations of vortical flow in pump intakes[46]. Namely, the initial research was dedicated to the flow in the suction chamber, where thevortical structures appear and were presumed to be one of the reasons for the damage of the impeller.

Figure 2. Computational domain of the pump. Flow direction is indicated by arrows. The approximateposition of the observation window is indicated by a red circle.

The computational domain (figure 2) consists of three unstructured computational meshes withhexa-cores: the inlet piping with suction chambers, the impeller, and the spiral casing with the diffuser.Several impellers were analysed before making the final decision for a suitable one. In this paper, only

the results obtained with the existing (old) configuration with 9 impeller blades and the final (new) onewith 7 impeller blades are presented. The number of elements for the computational meshes can befound in table 1. In meshing process, special attention was paid to the size of the elements in the

suction chambers, as well as at the impeller and diffuser leading and trailing edges (figure 3).

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Table 1. Sizes of computational meshes.

Number of elements Number of nodes

Computational mesh Oldconfiguration

Newconfiguration

Oldconfiguration

Newconfiguration

Piping with suctionchambers

7,890,000 7,890,000 4,046,000 4,046,000

Impeller (both sides) 8,219,000 9,164,000 2,676,000 3,137,000

Diffuser and spiralcasing

4,107,000 4,107,000 1,460,000 1,460,000

Total 20,216,000 21,161,000 8,182,000 8,643,000

Figure 3. Details of computational meshes: (a) Suction chambers and (b) impeller blade leading edge.The simulations were performed as transient ones, for the prototype size of the storage pump.

Time-step size corresponded to two degrees of the impeller rotation, with up to 30 iteration loops pertime step. The convergence criterion was 10-5 for the RMS of residuals. Usually, the criterion was metin approximately ten iteration loops. The maximum velocity residuals were below 6.10

-3. The average

Courant number for the time-step size was approximately 2.4. The overall time of simulations was

long enough to get a statistically averaged solution.For the simulations the usual boundary conditions were used: the total pressure was specified as the

inlet boundary condition and the mass flow was specified at the outlet. The 'high-resolution' advectionscheme was used, which is an upwind adaptive scheme, based on the Barth and Jespersen's limiter [7],and assures the boundedness of the solution. The second-order backward Euler transient scheme wasused as a time-stepping scheme in all cases.

2.1. Cavitation modelIn CFX [1], the cavitation model is based on a Rayleigh-Plesset equation, which describes the growthof a gas bubble in a liquid:

22

vB B

B 2

f B f

3 2

2

p pR dRR

t dt R

(1)

In equation (1), pv is the vapour pressure in the bubble, p is the pressure in the liquid around thebubble, RB represents the radius of the bubble, f is the density of the liquid and represents the

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surface tension coefficient at the bubble surface. The second order terms and the surface tension areneglected and the following equation is obtained:

vB

f

2

3

p pdR

dt

(2)

Based on the equation (2) a relatively simple derivation is applied to obtain the rate of change of

mass of one bubble, and also of the total interphase mass transfer rate per unit volume. It is importantto note that the final equations of the vaporization and of the condensation presume that the volumefraction of nucleation sites is 5.10

-4and the radius of nucleation sites (noncondensable gases) is 1 m.

3. Experimental method

The idea of the experiment was to observe the cavitation at the leading edge of the impeller. Thereforea lid intended to observe the blades easily from suction side was replaced by a modified lid. In the

centre of this new lid there is a plexi-glass window with a thickness of 20 mm and it is designed as canbe seen on figure 4. Due to the low suction head this seemed to be a safe way to get more informationabout the cavitation. To be sure that the equipment would withstand any kind of load all stresses werecalculated in ANSYS Workbench. The original lid is fitted nicely to the shape of the suction chamber;however, there was no influence detectable by the use of the flat experimental lid.

Figure 4. Design of the window and position at the inlet chamber at non-drive-end.The viewing distance from the window to the leading edges is approximately 65cm. For the

observation a stroboscopic light and an ordinary digital camera were used. It turned out, that somecameras have problems with the flashing light so different models were tested until one was found thatworked fine. A high-speed camera was tested too but it turned out that the lightning of the impellerwas too low so these results are not very useful.

4. Results

4.1. Experimental observation of the old impellerAs a result of the experimental observation the source of the significant damage at suction side wasdetected. It turned out that the cavitation cloud (figure 5a) propagated into the channel and (most ofthe time) collapsed behind the leading edge of the following blade.

A possible explanation for the severe damage at pressure side is that parts of this huge cavitationcloud are transported across the channel to pressure side due to recirculation. This circumstance wouldalso explain why the cavitation cloud is relatively small at the moment the bubbles cross the channel

(figure 5b). The definite process of the way, how the bubbles travel across the channel should beobserved in further investigation.

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Figure 5.Fully developed cavitation cloud (left) and impact of the cloud and travelling bubblesfrom suction to pressure side (right).

4.2. CFD resultsA new impeller for the pump was designed and optimised on the basis of the CFD simulations. Themain goal was to minimize the cavitation damage so that the impeller lifetime expectancy would belonger.

In order to minimize the cavitation damage the number of impeller blades was reduced and (amongthe new design in general) special care was taken in designing the inlet angle. The comparison of thepredicted extent of cavitation is depicted in figure 6. The predicted extent of the cavitationcorresponded to the length of the cavitation sheet, as will be discussed in section 4.3. The length of thecavitation sheet for the new impeller seems a bit longer (in the flow direction), but it is thinner than the

one predicted for the old impeller. Occasionally, for the old impeller, a small amount of the cavitationsheet was present on the pressure side of the blade (figure 6a, red circle), which is an indicator of the

weakly designed inlet angle (which could also be the result of the many impeller repairs).

Figure 6. Comparison of cavitation cloud (iso-surface of one percent of vapour volume fraction)between the old impeller (left) and the new impeller (right). The contour lines represent the wall

distance from the blade surface.The comparison of the predicted pump head is presented in figure 7. The site test [9], conducted

with the direct thermodynamic method for the old impeller, agrees with the CFD prediction at thenormal operating point, which is at Q = 2 m

3/s. The results of the CFD simulation with the new

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impeller have shown an increased value of pump head in comparison to the old impeller. Therefore,the new impeller was modified in the manufacturing process (based on the CFD results) to produce the

same head as the old one. The site test [10] has shown that the pump head of the old and the newimpeller is approximately the same.

1.4 1.6 1.8 2.0 2.2

170

180

190

200

210

220

H[

m]

Q [m3/s]

Old impeller, measurements

Old impeller, CFD

New impeller (modif.), measurements

New impeller, CFD

Figure 7. Comparison of the pump head. The pump operating point is at Q = 2.0 m3/s.

Apart from the results presented in figure 7, a NPSH-H curve was also calculated. The CFDsimulations, which were extremely time-consuming due to the transient nature of flow, have shownthat the 3 % drop in the pump head occurs between NPSH values 12.8 m and 13.3 m. The measured

curve was not available, but it is expected that the measured NPSH3% should be slightly above thisvalue. Namely, the numerical results usually predict lower NPSHr than measurements, probablybecause in CFD usually only the sheet cavitation is predicted correctly, whereas the extent of the

cavitation cloud is under-predicted, as it will be discussed in section 4.3.

4.3. Comparison of the CFD results with the observationsThe comparison between the CFD and the experimental observation of the cavitation is presented infigures 8 and 9. In both figures the direction of the view is approximately the same. The cavitationextent, observed in figure 8, consists of the cavitation sheet and the cavitation cloud. The mechanismof the formation of the cloud is explained in [11]: a re-entrant jet of water is formed at the end of thecavity and starts to travel below the cavitation sheet towards the leading edge. At one moment it cutsoff the cavity, which starts to travel along the blade in a form of a cloud. Figure 8 presents the

maximal size of the cavitation cloud in a longer time period of the observation, in order to get animpression of the improvement between the impellers. In figure 8a (old impeller) it seems that the partof the cavitation cloud (near the shroud) propagates in reverse direction in front of the impeller inlet(does not enter into the channel), which means that the pump is operating in the recirculation range.This is not the case with the new impeller (figure 8b).

In figure 9 the CFD prediction of the cavitation can be observed. It is obvious that the predictedextent is much smaller than in figure 8 because the cavitation cloud is not predicted. The imperfectprediction of the cavitation is a consequence of the too large predicted viscosity (which is a sum of the

molecular and turbulent viscosity) at the end of the cavitation sheet, which prohibits the formation ofre-entrant jet of the liquid below the cavitation sheet. It is shown in [12] that the cavitation modelapplied in CFX is able to predict the shedding of the transient cavities (the cavitation cloud) when

either an original DES model [8] is applied or if a correction of water properties for a two-phasehomogeneous model is used. The correction can change the viscosity at the liquid-vapour boundary

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and thus triggers the formation of the re-entrant jet below the cavitation sheet. Such non-linearcorrection function for the density is presented in [13].

In the current study the sheet cavitation extending from the blade leading edge was relativelysuccessfully predicted. The simulation with the old impeller was checked for the possible suctionrecirculation (at the final simulated time) but it was not predicted.

The experimental observation of the old impeller in figure 8 was done after approximately 1000hours of its operation, whereas for the new impeller it was performed after approximately 400 hours ofoperation. The information about the water temperature at time of the observations is not available, butbased on the date and time it is assumed that in figure 8 the water temperature was approximately fiveto eight degrees lower than the specified water temperature in CFD simulations, which was 13 C. Theinformation about the nucleation size was not available, and thus the default value (2

.10

-6m) was used.

Figure 8. The observed extreme size of the cavitation cloud at the pump impeller; (a) the old impellerat 1000 hours of operation; (b) new impeller at 400 hours of operation. LE indicates the leading edge.

Figure 9. The predicted size of cavitation by CFD; (a) the old impeller; (b) the new impeller. The iso-surfaces are coloured by water velocity. The circular area is semi-transparent to enable the observation

of the next blade. LE indicates the leading edge.

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5. Summary and conclusions

The design for the new impeller for the double suction centrifugal pump was based on the predictionsof transient CFD simulations. The comparison with the observations of the cavitation of the old andthe new impeller revealed that the CFD predicts smaller cavitation regions, especially in the case ofpump operation in the recirculation range (old impeller): the simulation predicted the sheet cavitation,but not the cloud cavitation. According to this, the cavitation damage on the pressure side of the blades,which eventually led to the holes through the blades, was not predicted by the CFD. Nevertheless,with the transient CFD simulations the pump head was predicted well and it was possible to anticipate

that for the new impeller the extent of the cavitation damage would be smaller because of betterhydraulic design at the inlet, resulting in lower extent of cloud cavitation, and operation out of therecirculation range.

Acknowledgements

The research was partially funded by the Slovenian Research Agency ARRS - Contract No. 1000-09-160263.

References

[1] Ansys 2010 Ansys CFX-Solver Theory Guide[2] Egorov Y and Menter F 2008Development and application of SST-SAS turbulence model in the

DESIDER project Advances in Hybrid RANS-LES Modelling ed. S-H Peng and W Haase(Heidelberg: Springer) pp 261270

[3] Smirnov P E and Menter F 2009J. Turbomach. 131(4) 041010[4] kerlavaj A, kerget L, Ravnik J and Lipej A 2011 Proc. IMechE Part A: J. Power and Energy

225 76478[5] kerlavaj A, Lipej A, Ravnik J and kerget L 2011 Surface vortex simulation at selected water

temperatures Proc. of AJTEC2011 (New York : ASME) p44268[6] kerlavaj A, Lipej A, Ravnik J and kerget L 2010IOP Conf. Ser.: Mater. Sci. Eng.12 012034[7] Barth T J and Jespersen D C 1989 The design and application of upwind schemes on

unstructured meshes Proc. of 27th Aerosp. Sci. Meet. (Reno: AIAA) 0366 (12pp)

[8] Strelets M 2001 Detached eddy simulation of massively separated flows Proc. of 39th Aerosp.Sci. Meet. Exhib. (Reno: AIAA) 0879 (18pp)

[9] Rus T and Janar B 2011 KW Fuhren Pump Efficiency Measurements with ThermodynamicMethod(Ljubljana: Turbointitut) Report 3018

[10] Rus T, Janar B and Pavlin R 2012 KW Fuhren Refurbished Pump HP-80d with NewTurboinstitut Impeller Characteristics Measurements with Thermodynamic Method(Ljubljana: Turbointitut) Report 3063

[11] De Lange D F and De Bruin G J 1998 Appl. Sci. Res.58 91114[12] Ait Bouziad Y 2005 Physical Modelling of Leading Edge Cavitation: Computational

Methodologies and Application to Hydraulic Machinery (Lausanne: EPFL) These 3353[13] Coutier-Delgosha O, Reboud J L and Delannoy Y 2003Int. J. Numer. Meth. Fluids42 52748

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