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Nuclear Engineering and Design 239 (2009) 899–912

Contents lists available at ScienceDirect

Nuclear Engineering and Design

journa l homepage: www.e lsev ier .com/ locate /nucengdes

HF experiment and CFD analysis in a 2 × 3 rod bundle with mixing vane

yung Soo Shin ∗, Soon Heung Changorea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea

r t i c l e i n f o

rticle history:eceived 10 February 2008eceived in revised form 31 December 2008ccepted 19 January 2009

a b s t r a c t

In this study, the CHF enhancement using various mixing vanes is evaluated and the flow characteristicsare investigated through the CHF experiments and CFD analysis.

CHF tests were performed using 2 × 2 and 2 × 3 rod bundles and with R-134a as the working fluid. Thetest section geometry was identical to that of commercial PWR fuel assembly not including the heatedlength (1.125 m) and number of fuel rods. From the CHF tests, it was found that the CHF enhancement usingmixing vanes under higher mass flux (1400 kg/m2 s) and lower pressure (15 bar) conditions is larger thanthe CHF enhancements under other conditions. Among the mixing vanes used in this study, the swirl vaneshowed the best performance under relatively low pressure (15 bar) and mass flux (300–1000 kg/m2 s)conditions and the hybrid vane performed best near the PWR operating conditions.

The detailed flow characteristics were also investigated by CFD analysis using the same conditions asthe CHF tests. To calculate the subcooled boiling flow, the wall partitioning model was applied to the wallboundary and various two-phase parameters were also considered. The reliability of the CFD analysisin the boiling analysis was confirmed by comparing the average void fractions of the analysis and the

experiments: the results agreed well. From the CFD analysis, the void fraction flattening as a result of thelateral velocity induced by the mixing vane was observed. By the lateral motion of the liquid, the voidfraction in the near wall was decreased and that of the core region was increased resulting in the voidfraction flattening. The decrease of the void fraction in the near wall region promoted liquid supply tothe wall and consequently the CHF increased. For the quantification of the void flatness, an index was

abilit

developed and the applic

. Introduction

In a PWR nuclear fuel assembly, a spacer grid is used toaintain a gap between the fuel rods. Maintaining the gap is

o thermal-hydraulically support uniform heat transfer and tohermal-hydraulically suppress the local occurrence of CHF. If a

ixing vane is attached to the spacer grid, the functions of thepacer grid are further reinforced, and the CHF is greatly enhanced.or example, a commercial nuclear fuel assembly with a mixingane can enhance the CHF by up to 10% and the economic benefitsre great due to this enhancement. Despite the demerits of a mix-ng vane, such as the increase of the pressure drop and FIV and theebris problem, most nuclear fuel development includes the devel-pment of a mixing vane because the economical benefits are too

reat to ignore.

In the existing research, the enhancement of CHF using a mixingane is one of many considerations in CHF studies on rod bundleeometry. ERPI, CE, and B&W correlations, the well-known corre-

∗ Corresponding author. Tel.: +82 42 869 3856; fax: +82 42 869 3810.E-mail address: byungsoo@kaist.ac.kr (B.S. Shin).

029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2009.01.011

y of the index in the CHF assessment was confirmed.© 2009 Elsevier B.V. All rights reserved.

lations for rod bundles, include the effect of a spacer grid, a guidethimble, the non-heating area locations, and so on. In this case, theeffect of the mixing vane is taken into consideration in the gridfactor (Chang and Baek, 1997).

Fortini and Veloso (2002) and Lee (2000) have attempted to pre-dict the CHF in rod bundles by applying the correction factor to theCHF correlation in a circular tube. They also included the effects ofthe mixing vanes as a correction factor. The reason that the studieshave propriety is because the shapes of the existing mixing vanesare similar to each other and the CHF enhancements were not vastlydifferent. Therefore, a correction factor that does not significantlyaffect the performance of the prediction should be used. Recently,Chen et al. (2004) and Akhtar et al. (2006) performed a CHF exper-iment using rod bundles with refrigerants: they showed that usinga refrigerant enables proper results to be obtained through fluid-to-fluid modeling.

Some research has investigated the detailed mechanism of theCHF enhancement caused by the mixing vane. Generally, it is well-

known that the CHF enhancement through the mixing vane is dueto the generation of swirl and cross flows, and the increase of theturbulent intensity. Chung et al. (1996) summarized the effects asshown in Table 1. de Crecy (1994) conducted a radial non-uniformheating CHF experiment to investigate the change of location and

900 B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912

Nomenclature

A interfacial area (m2)cP specific heat capacity (kJ/kg K)CD drag coefficientdW bubble departure diameter (m)E interfacial energy transfer (kJ/s)f bubble departure frequency (1/s)G mass flux (kg/m2 s)hQ quenching heat transfer coefficient (W/m2 K)H enthalpy (kJ/kg)k thermal conductivity (W/mK)Lh heating length (m)m evaporation rate (kg/s)M interfacial momentum transfer (kg m/s2)n nucleation site density (1/m2)qc critical heat flux (kW m−2)T temperature (K)U axial velocity (m/s)xlocel local quality

Greek letters˛ void fraction� density (kg/m3)� interfacial mass transfer (kg/s)� viscosity (N s/m2)

SubscriptsG vapouri inletk phase

torw

Table 1Mechanisms of CHF enhancement (Chung et al., 1996).

Region Effecting mechanism CHF

General Increase of enthalpy mixing IncreaseLow quality Breakup of bubble crowding Increase

Centrifugal acceleration Increase

L liquidW wall

he enhancement of the CHF value through the implementationf a mixing vane. According to Crecy’s study, the CHF appeared atandom radial locations and the CHF value increased in rod bundlesith a mixing vane.

Fig. 1. Experiment

High quality (Annular flow) Directing entrained liquids fromcore to the wall

Increase

Breakup of liquid film Decrease

Mixing vanes in nuclear fuel assemblies are used to increasethe thermal mixing and increase the DNB margin. To date, inmost research on mixing vanes, the shape of the mixing vane isnot important; rather the research only considers one parameter,such as the mixing vane factor, in the CHF correlation. However,due to the economical benefits gained by incorporating a mixingvane and the technical improvements in the CFD code, much morethermal-hydraulic research on mixing vanes has been undertakenrecently.

Using the CFD analysis of a single phase flow, Ikeda et al. (2006)predicted the location of the DNB occurrence. They established thatthe location of the maximum local enthalpy in a single-phase anal-ysis corresponded to the location of the DNB occurrence. However,the analysis could not directly predict the CHF value and could onlypredict the location of the DNB. Krepper et al. (2007) predicted var-ious two-phase variables in the subcooled flow boiling using a wallboiling model. This study shows the potential of predicting the CHFthrough the CFD analysis using the wall boiling model in a two-phase flow. The study establishes that the CFD analysis results inproper flow characteristics regarding the CHF prediction. However,the study only shows the analysis results for a single-phase flow: theCFD analysis should also be applied to a two-phase flow to examinethe detailed mechanism of the CHF enhancement

The primary objective of this paper is to find the representa-tive mixing factor to mean the performance of the mixing vane

on the CHF. To achieve the primary objective, a CFD analysis wasperformed under identical conditions to those of the CHF test. Thetwo-phase flow parameters under the CHF condition were calcu-lated using the CFD analysis. The CHF test was performed in a 2 × 3

al test loop.

B.S. Shin, S.H. Chang / Nuclear Engineerin

Fig. 2. Test section.

Fig. 3. Test mixing vane (a) split vane

g and Design 239 (2009) 899–912 901

rod bundle with a simple spacer grid and various mixing vanes atKAIST R-134a Loop. The R-134a was a working fluid and, for CFDanalysis, CFX 11.0 with a beta version of the wall boiling model wasused. By comparing the experimental and numerical results, therepresentative mixing factor related to the CHF enhancement wasobtained. An additional objective of this study was to investigate theCHF characteristics of various mixing vanes and the effect of vary-ing design parameters on the mixing vane. This study also proposesthe design direction and provides useful information for designingmixing vanes.

2. Experimental works

Fig. 1 shows the test loop for the experiment. The loop is closedloop and R-134a was used as the working fluid. The detailed spec-ifications of the loop can be found in the experimental apparatussection of Shin et al.’s study (Shin et al., 2005).

Fig. 2 shows the axial and cross sectional geometry and structureof the 2 × 3 rod bundle test section. The distance between heaterrods was 12.65 mm; the heater rod diameter was 9.5 mm; the dis-tance between the heater rod and the insulated wall was 1.575 mm;and the heating length was 1.125 mm. In Fig. 2, A and B representthe spacer grids. Spacer grid A was a simple supported grid withouta mixing vane and spacer grid B was a grid with a test mixing vane.The distance between spacer grid A and spacer grid B was 600 mm.Spacer grid B was located at 150 or 300 mm upward from the upper

section of the heater rod. Bakelite was used for the wall of the flowchannel for electrical and thermal insulation.

A heater rod is composed of a heating section (SUS-304) and anon-heating section (copper). In the inner section of a heater rod,two K-type thermocouples measure the surface temperature of the

(b) swirl vane (c) hybrid vane.

902 B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912

Table 2Test condition about 2 × 3 rod bundle.

Without spacer grid With spacer grid

Pressure (bar) 15–25 15–25Mass flux (kg/m2 s) 300–1400 300–1400Inlet subcooling (kJ/kg) 48.38–83.70 48.63–83.86Vane location from the

end of heater (mm)– 150 or 300

Table 3Test cases for validation of CFD analysis.

P (MPa) G (kg/m2 s) Q (W/m2) dTin (K) D (m) L (m) V (m/s)

Case 1 7 1000 1.0E+06 83.59 0.012 2 1.35CCC

hma

vFovssEflohTfl

oiet1aC

ase 2 11 1000 1.0E+06 58.39 0.012 2 1.49ase 3 7 1000 8.0E+05 55.73 0.012 2 1.35ase 4 7 2000 8.0E+05 55.73 0.012 2 2.70

eater rod. There are pressure transmitters and thermocouples thateasure the inlet and outlet pressures and temperatures at the inlet

nd outlet of the test section, respectively.In this study, three types of mixing vane were used as test mixing

anes. Fig. 3 shows a schematic diagram of the test mixing vanes.ig. 3(a) shows the split vane attached to the Vantage-5HTM devel-ped by Westinghouse Company (De Mario et al., 1986). The mixingane was developed to reduce the enthalpy difference between theubchannels by generating a cross flow. Fig. 3(b) represents thewirl vane (Chun et al., 2001) developed by KAERI (Korea Advancednergy Research Institute, Korea). The swirl vane generates a swirlow inside a subchannel, and the swirl flow lowers the amountf wall void fraction by centrifugal force. Fig. 3(c) also shows theybrid vane (Chun et al., 2005) that was also developed by KAERI.he hybrid mixing vane simultaneously generates swirl and crossows.

The experiments were performed in high-pressure conditions atver 90 bar (water converted pressure). The range of the mass fluxncluded a low mass flux and a high mass flux to investigate the

ffect of the mixing vane at various mass flux ranges. Table 2 showshe test conditions at the flow inlet. The pressure remained between5 and 25 bar, the mass flux was between 300 and 1400 kg/m2 s,nd the inlet subcooling was between 40 and 80 kJ/kg. The type ofHF was a departure from nucleate boiling. It is assumed that the

Fig. 4. Forced convective subcooled boiling.

Fig. 5. Mesh generation for CFD analysis.

CHF occurs when the temperature rise exceeds 5 ◦C/s or the walltemperature of a heater rod is more than 10 ◦C higher than otherheater rods.

3. Numerical method

3.1. Physical models

3.1.1. Two-fluid modelIn a two-phase flow, the vapor and liquid phases are treated as

continua. The phases may interact with each other through inter-facial forces as well as heat and mass transfer across the phaseinterfaces. The two-fluid model solves two sets of mass, momen-tum, and energy conservation equations, which are written for eachphase as:

(a) Continuity equation

∂

∂t(˛k�k) + ∇ · (˛k�kUk) = �k (1)

(b) Momentum equation

∂

∂t(˛k�kUk) + ∇ · (˛k�kUkUk) = ∇ · [˛k�e

k(∇Uk + (∇Uk)T )]

−˛k∇pk + ˛k�kg + Uki�k + Mk (2)

neering and Design 239 (2009) 899–912 903

Fv

Fv

Fa

B.S. Shin, S.H. Chang / Nuclear Engi

(c) Energy equation

∂(˛k�kHk) + ∇ · (˛k�kUkHk − ˛k�e

k∇Tk)

∂t

= ∂

∂t(˛kpk) + Q k + Hki�k + Ek (3)

ig. 6. Comparison of experimental works with numerical analysis on area averagedoid fraction: pressure effect.

ig. 7. Comparison of experimental works with numerical analysis on area averagedoid fraction: mass flux effect.

ig. 8. Comparisons of experimental works with numerical analysis on area aver-ged void fraction: heat flux effect.

Fig. 9. Variation of CHF in 2 × 3 rod bundle with simple spacer grid with majorparameters.

where � k, Mk, and Ek are the interfacial mass, momentum, andenergy transfer, respectively.

In the set of conservation equations, the interfacial mass transfer(� k), which is related to the phase change between vapor and liquidstates such as evaporation and condensation, is expressed as:

�lg = max(

hLG(Tsat − TL)ALG

HLG, 0

)for condensation (4)

�lg = max(

hLG(TL − Tsat)ALG

HLG, 0

)for evaporation (5)

The interfacial momentum transfer is composed of a drag forceand a non-drag force. The drag force depends on the relative velocity

904 B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912

Fig. 10. Comparison of experimental data with Bowring correlation.

Fig. 11. Variation of enhancement of CHF on Split vane with mass flux.

Fig. 12. Variation of enhancement of CHF on Split vane with inlet subcooling.

Fig. 13. Variation of enhancement of CHF on Split vane with pressure.

of the two phases, on the flow against the cross-sectional area, onthe density difference, and on the dynamic viscosity of the fluid.Thus, the drag is calculated by:

MdL = −Md

G = 34

CD

db˛G�L|UG − UL|(UG − UL) (6)

The other interfacial forces are of second order and influence thedistribution of the bubbles. These forces are primarily referred toas non-drag forces and include the turbulent dispersion force, liftforce, wall lubrication force, and virtual mass force. The numericalforms and coefficients of the non-drag forces have been presentedin (Krepper et al., 2007).

3.1.2. Subcooled boiling modelIn this simulation, together with the two-fluid model, the boil-

ing phenomenon in a subcooled boiling flow was considered. Fig. 4shows the general boiling phenomena in an internal flow. In this fig-ure, after the ONB point, there is a subcooled boiling region wherethe average liquid temperature is lower than the saturated temper-

ature, but the local liquid temperature near the wall has alreadyreached the saturated temperature. To simulate subcooled boil-ing, the heat partitioning model was applied to the wall boundaryconditions.

Fig. 14. Variation of Critical heat flux on Split vane with exit quality.

B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912 905

Fs

tata(

q

w1a

t

q

h

qE = m · (HG − HL) (10)

ig. 15. Variation of enhancement of CHF on three types of mixing vane with inletubcooling.

In the heat partitioning model, the wall heat flux is divided intohree types of heat flux: convection heat flux, quenching heat flux,nd evaporation heat flux. According to Krepper et al. (2007), theurbulent convection heat flux is calculated using the same temper-ture wall function as that of the pure liquid flow without boilingEgorov and Menter, 2004):

˙ F = A1�fcpuW

T+y+(L)

(TW − TL) (7)

here T+y+(L) is analytically calculated and non-dimensional (Kader,

981) at a non-dimensional distance from the near-wall cell, y+(L),nd uW is the friction velocity.

The quenching heat flux is calculated using the quenching heatransfer coefficient:

˙ F = A2hQ(TW − TL) (8)

In this equation, the quenching heat transfer coefficient is:

Q = 2kLf

√tW

�aL(9)

Fig. 16. Variation of enhancement of CHF on three types of mixing vane with pres-sure.

The evaporation heat flux is obtained via the evaporation massflux on the wall:

where the evaporation rate is expressed by:

m = �d3W

6�Gf · n (11)

906 B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912

Ffl

3

tgfia

ig. 17. Variation of enhancement of CHF on three types of mixing vane with massux.

.2. Modelling and numerical method

The geometrical shape of the numerical analysis model is iden-ical to the experimental test section: it had a 2 × 3 rod bundleeometry with various mixing vanes. For practical calculations andne mesh generation, the model was divided into three sectionsccording to the flow directions. Each section contained a bottom

Fig. 18. Variation of CHF enhancement ratio on the position of mixing vane with (a)mass flux (b) inlet subcooling (c) pressure.

area, a mixing vane area, and an upper area. Here, the meshes inthe bottom area and upper area were generated using the hexago-nal sweep methodology, but the mixing vane area was split using

tetrahedrons. Fig. 5 shows the mesh generations corresponding toeach area. The average mesh size was 1 mm for the cross sectionand 10 mm in the axial direction. The calculations were connectedusing the BC profiles.

B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912 907

baawdtiidf

bwr

3

mfeeiaf

dmo

Fig. 19. Axial locations of observed cross section area.

For the flow characteristics analysis with the CHF condition,oth the two-fluid model and the wall boiling model should bepplied. The working fluid was a liquid and gas phase of R-134a,nd each phase was set to a continuous fluid and dispersed fluidith the diameter depending on the fluid temperature. The stan-ard k-e model and dispersed phase zero equation was used asurbulent models about liquid and gas phases, respectively. Thenterfacial transfer in the two-phase flow was also considered. Var-ous coefficients and models of drag and non-drag forces wereetermined, and the phase change model was used as a mass trans-er model.

The CFD code used in the numerical analysis was the CFX 11.0eta version with the wall boiling model. The physical time scaleas 0.05 s, and the convergence criterion was 10−8 for the residual

ms value.

.3. Validation

To validate the applicability of the abovementioned numericalethods, a numerical analysis for a simple round tube was per-

ormed. A semicircle with a symmetric surface was treated forconomical calculations. The test case was determined based on thexperiments of Bartolomei and Chanturiya (1967). In these exper-ments, a subcooled convective flow boiling was considered in anxis-symmetric vertical heated pipe. Table 3 shows the test cases

or the validation of the CFD analysis.

The average void fraction along the equivalent quality for vali-ation is presented in Figs. 6–8. The application of the numericalethod, described in the previous section, shows the plausibility

f the calculated results. Therefore, this method is applied to the

Fig. 20. Contour of axial velocity in the case (a) with or (b) without mixing vanealong the axial locations.

numerical analysis concerning the CHF test for a rod bundle with amixing vane.

4. Results and discussions

4.1. Experimental results

In the CHF experiment with a 2 × 3 rod bundle and a simplespacer grid, the trends of the CHF regarding the major parameterswere investigated. Fig. 9 shows the parametric trends of the CHFregarding pressure, mass flux, and inlet subcooling. The CHF value

increased as the mass flux and inlet subcooling increased, and thepressure decreased. This trend is similar to that in a circular tube(Bartolomei and Chanturiya, 1967).

Fig. 10 presents a comparison of the experimental data in thisstudy with the Bowring correlation. For the comparison, the CHF

908 B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912

elocity distributions.

duartrpdt

dbsoo

aiotmtetcatbtItw

i

Fig. 21. Axial v

ata with R-134a was converted to the corresponding water datasing the Katto model (Moon et al., 1996) and the hydraulic equiv-lent diameter. From Fig. 10, it is seen that the value of the CHF in aod bundle is greater than that in a circular tube. It is assumed thathe CHF values increase due to more vigorous turbulent flows and aeduction in the enthalpy difference as a result of the macroscopiculsation in rod bundle (Katto, 1978). The CHF data in the rod bun-le with a simple spacer grid was used as the base data to estimatehe CHF enhancement of the spacer grids with a mixing vane.

In this study, The CHF experiment was performed with threeifferent mixing vanes. The CHF enhancement, i.e. the differenceetween the CHF value on mixing vane and that on the simplepacer grid, showed a common trend in the experimental resultsf the three different mixing vanes. Figs. 11–13 present the resultsf the CHF experiment for the split vane.

Fig. 11 shows the CHF enhancement with the mass flux vari-tions at 15, 20, and 25 bar. In the figure, the CHF enhancementncreases as the mass flux increases. The enlargement of the effectf the mixing vane with the increase of the mass flux can be relatedo the CHF enhancement mechanism presented in Table 1. If the

ass flux increases, the swirl flow and cross flow are enlarged byhe mixing vane and the turbulent intensity also increases. Thenlargement of the swirl flow strengthens the centrifugal force, andhe centrifugal force causes bubbles near the wall to move into theenter of the subchannel due to the different densities of the liquidnd vapor. Therefore, the wall void fraction is reduced. Furthermore,he enlargement of the cross flow reduces the enthalpy differenceetween and within the subchannels. The reduction in and betweenhe subchannels leads to a reduction of the enthalpy near the wall.

n addition, the enlargement of the turbulent intensity disperseshe bubble crowding near the wall. In a high mass flux, the effectill be more intense.

Fig. 12 shows the variation of the CHF enhancement with thenlet subcooling at 500, 1000, and 1400 kg/m2 s. In the figure, Fig. 22. Cross flow and swirl flow induced by mixing vane.

B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912 909

e and

ti

siidfm

q

x

i

Fig. 23. Void distributions at the bottom and top of mixing van

he enhancement of the CHF decreases as the inlet subcoolingncreases, but the decrease is relatively small.

Fig. 13 shows the variation of CHF enhancement with pres-ures of 300, 500, 1000, and 1400 kg/m2 s. At these pressures, thenlet subcooling was 60 kJ/kg. In the figure, the CHF enhancementncreases as the pressure increases. If the pressure decreases, theifference in density between the liquid and vapor enlarges. There-ore, the centrifugal force is enlarged and mixing by cross flow is

ore vigorous.Fig. 14 shows the CHF with variations in the exit quality. The exit

uality is calculated as:

local = 1HLG

(4qcLh

DhG− �Hi

)(12)

As seen in Fig. 14, the CHF enhancement has a high qualityncrease.

the location of CHF (a) w/o mixing vane (b) with mixing vane.

From the experimental results with the three different mixingvanes, the trends of the CHF enhancement according to each mix-ing vane are similar to the trends of the split vane. However, theperformance of each mixing vane differs to the others according tothe test conditions.

Fig. 15 shows the variation of the CHF enhancement with inletsubcooling at 20 bar with 500 and 1000 kg/m2 s pressure. The inletsubcooling did not have a significant effect on the performance ofthe mixing vane.

Fig. 16 shows the variation of the CHF enhancement at a pressureof 60 kJ/kg. At 15 bar, the swirl vane has a better performance, but

the hybrid vane gave a greater CHF enhancement at 25 bar.

Fig. 17 shows the variation of the CHF enhancement with amass flux of 60 kJ/kg. The values of the CHF enhancement havea local peak at 800 kg/m2 s and globally, the CHF enhancementincreases as the mass flux increases. The hybrid vane provided

9 neering and Design 239 (2009) 899–912

afl

bicmimc

4

atite

difs

Fa(wTsTTeCwit

ddtptte

ivFt

vcdorstt

Cvaab

10 B.S. Shin, S.H. Chang / Nuclear Engi

superior enhancement at a high pressure and a high massux.

Generally, the effect of the mixing vane is reduced as the distanceetween the mixing vane and the location of the CHF occurrence

ncreases (Shin et al., 2005). This is due to the decay of the swirl andross flow along the flow direction. Fig. 18 presents the effect of theixing vane location on the CHF enhancement. From the figure

t can be seen that the change of the CHF value according to theixing vane location is large in low inlet subcooling and pressure

onditions and a high mass flux conditions.

.2. CFD results

The boiling flow under the CHF conditions was simulated by CFDnalysis using the two-fluid model and wall boiling model. Here,he geometry for the simulation was identical to that of the exper-mental test section. The most promising test cases were selectedo investigate the CHF enhancement on the flow parameters in thexperiment.

The effect of the mixing vane on the flow parameters andistributions was investigated. Then, the primary parameters for

nvestigation are the axial and lateral velocity distributions, voidraction distributions, and turbulent kinetic energy (TKE). Fig. 19hows the axial locations of the observed cross section area.

First, the axial and lateral velocity distribution was examined.ig. 20 shows the contour of the axial velocity with and withoutmixing vane along the axial locations. Comparing Fig. 20(a) and

b), it was founded that the axial velocity distribution in the caseith a mixing vane was flattened: this is also confirmed in Fig. 21.

hen, the flatness of the velocity distribution began upstream of thepacer grid and the rate of flatness decayed along the flow direction.his shows that the flatness may disappear after a long distance.he flatness affects to the CHF enhancement in two ways. The firstffect is the decrease of the average enthalpy at the location of theHF occurrence resulting from the mixing between the subchannelshen the axial flow is converted to a lateral flow. The second effect

s the increase of the axial mass flux at the local weak location ifhe CHF point is inside a mixing region.

The lateral velocity distribution is related to the axial velocityistribution. That is, the local mass flow rate corresponding to theecrease of the axial velocity due to the flatness will be transferredo the mass flow rate in the lateral direction. This is a commonhenomenon in agreement with mass conservation. It is knownhat the lateral flow is the primary reason for the enhancement ofhe CHF, because the lateral flow induces the mass, momentum, andnthalpy transfer between the subchannels.

Fig. 22 shows the cross flow and swirl flow induced by the mix-ng vane. Then, at the bottom of the spacer grid with the mixingane, the cross flow caused by the dispersed lateral flow appears inig. 22(a). This occurs because the mixing vane becomes an obstacleo the flow.

Therefore, the estimation of the cross flow created by the mixingane requires the consideration of both upstream and downstreamonditions of the mixing vane. However, the swirl flow only appearsownstream of the mixing vane. The regions where a swirl flowr cross flow may occur are defined as mixing regions. In a mixingegion, the mass, momentum, and enthalpy are actively transferred,o the flow will be redistributed. Therefore, it can be assumed thathe region size and the mixing rate within the region are related tohe enhancement of the CHF.

The void fraction is an important variable used to estimate the

HF. In bubble crowding models such as the CHF model, the criticalalue of the local void fraction is suggested. The void distributionst the bottom and top of the mixing vane and the location of the CHFre shown in Fig. 23. The void distribution is flattened due to mixingy lateral velocity. However, unlike velocity distribution, void dis-

Fig. 24. Turbulent kinetic energy in case with and without mixing vane.

tribution does not change after redistribution. Therefore, the voiddistribution at the top of the mixing region can be important toenhance the CHF because the void distribution is not changed afterthe redistribution.

Turbulent intensity is an important parameter in CHF prediction.In isotropic turbulent flows, the turbulent intensity is expressedas a turbulent kinetic energy. Fig. 24 shows the turbulent kineticenergy with and without a mixing vane. In this figure, the TKE with-out a mixing vane has a local peak at the gap between the walland the rod, but it does not occur when a mixing vane is present.Fig. 25 shows the TKE changes before and after the mixing vane.In Fig. 25(b) and (c), it can be seen that the TKE decreases down-stream of the mixing vane. This results from the streamline beingmaintained strongly by the swirl and cross flows after the mixingvane. The increase of TKE at the CHF location is not due to the mix-ing vane, but rather to the bubble generation resulting from the wallboiling.

To find the parameters related to the CHF enhancement ratio,the momentum transfer ratio, swirl ratio, velocity flatness, and voidflatness were estimated. From Fig. 26, it can be seen that the indexof the void flatness is linearly related to the CHF enhancement ratio.The index is defined as follows:

˛avg√∑(˛ − ˛avg)2

If the index of the void flatness is increased, the void fractionat a local weak region, such as a corner subchannel, is decreased,which leads to the enhancement of the CHF. Therefore, the perfor-mance of mixing vanes can be compared with each other throughthe estimation of the void flatness.

B.S. Shin, S.H. Chang / Nuclear Engineering and Design 239 (2009) 899–912 911

m

((

(

Fig. 25. Turbulent kinetic energy changes before and after mixing vane.

The two-phase flow characteristics near the spacer grid are sum-arized as follows:

1) There is a mixing region around the spacer grid.

2) In the mixing region, a lateral velocity composed of the swirl

and cross flows is generated.3) Lateral velocity consists of the void fraction to be redistributed

(flatness of void fraction), and this effect accumulates along theflow direction.

Fig. 26. Effect of void flatness to CHF enhancement ratio.

(4) Lateral velocity decays along the flow direction, but the redis-tributed void fraction is maintained.

(5) Turbulent kinetic energy is increased by bubble generation, butis not increased by the lateral velocity. Therefore, the turbulentincrease resulting from the mixing vane is not dominant in atwo-phase flow.

5. Conclusions

The major conclusions from the experimental work and numer-ical analyses are as follows:

(1) The CHF enhancement using mixing vanes was enlarged underrelatively high mass flux and low pressure conditions. Undera high mass flux condition, the enlarged swirl and cross flowsled to the reduction of the wall void fraction and enthalpy nearthe wall. Under a low pressure condition, the effect of the swirland cross flows strengthened due to the high density ratio (liq-uid/vapor).

(2) Among the mixing vanes used in this study, the swirl vaneshowed the best performance under a lower pressure and massflux conditions compared with the PWR operating conditions.Near the PWR operating conditions, the hybrid vane showedthe best CHF enhancement.

(3) The lateral velocity induced by the mixing vane redistributedthe void fraction downstream of the mixing vane. The max-imum void fraction was reduced by the redistribution and,consequently, the CHF was enhanced.

(4) To quantify the void fraction distribution, a void flatness indexwas developed. The void flatness index is directly proportionalto the CHF enhancement ratio and, thus, the index can be usedto assess the mixing vane performance in CHF.

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