Nb

ED

a

ARRA

1

lcCtpcscstcmcit

floed

0d

Nuclear Engineering and Design 239 (2009) 888–898

Contents lists available at ScienceDirect

Nuclear Engineering and Design

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

on-intrusive experimental investigation of flow behavior inside a 5 × 5 rodundle with spacer grids using PIV and MIR

lvis E. Dominguez-Ontiveros, Yassin A. Hassan ∗

epartment of Nuclear Engineering, Texas A&M University, College Station, TX 77843, USA

r t i c l e i n f o

rticle history:eceived 30 November 2008eceived in revised form 25 January 2009ccepted 26 January 2009

a b s t r a c t

The validity of the simulation results from computational fluid dynamics (CFD) is still under scrutiny.Some existing CFD closure models for complex flow produce results that are generally recognized asbeing inaccurate. Development of improved models for complex flow simulation requires an improvedunderstanding of the detailed flow structure evolution with dynamic interaction of the flow multi-scales.Thus, the goal of this work is to contribute to a better understanding of presupposed and existent eventsthat could affect the safety of nuclear power plants. The fundamental phenomena of fluid flow in rod

bundles with spacer grids can be elucidated by using state-of-the-art measurement techniques. Thisstudy aims to develop an experimental data base with high spatial and temporal resolution of fluid flowvelocity inside a 5 × 5 rod bundles with spacer grids. The full-field detailed data base is intended to validateCFD codes at various temporal-spatial scales. Measurements are carried out using dynamic particle imagevelocimetry (DPIV) technique inside an optically transparent rod bundle utilizing the matching index of

. Thise flow

refraction (MIR) approachbundle under single phas

. Introduction

Fluid flow around circular cylinders is one of the classical prob-ems of fluid mechanics and has been well studied because of itsommon occurrence in many forms and in different applications.ylinder-like structures can be found both alone and in groups inhe designs for heat exchangers, cooling systems for nuclear powerlants, offshore structures, power lines, struts, grids, screens, andables, in both single and multi-phase flows. A complete under-tanding of the fluid dynamics for the flow around a circularylinder includes such fundamental subjects as the boundary layer,eparation, the free shear layer, the wake, and the dynamics of vor-ices. The flow field of multiple-cylinder configurations involvesomplex interactions between the shear layers, vortices and Kar-an vortex streets (Zdravkovich,1987). The problem is further

omplicated by the large number of configurations encounteredn practice, resulting in different flow patterns, and by the effect ofheir interactions.

One of the applications of paramount importance in this study is

uid flow in fuel rod bundles of water nuclear reactors. In this typef nuclear reactor, optimum heat removal from the surface of fuellements is the subject of many studies for researchers in order toetermine reactor thermal margin and safety. In these reactors, the

∗ Corresponding author.E-mail address: Hassan@ne.tamu.edu (Y.A. Hassan).

029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2009.01.009

work presents full field velocity vectors and turbulence statistics for a rodconditions.

© 2009 Elsevier B.V. All rights reserved.

spacer grids which support the fuel assembly are used as an effec-tive mixing device by attaching various types of flow deflectors.Several recent works are focused on the development of numericalsimulations that predict the complex behavior of fluid flow close togrid spacers and between fuel assemblies.

The validity of the produced results from computational fluiddynamics (CFD) is still under scrutiny for several applications inreal practical cases. Moreover, the existing models for multiphaseflows produce results that are generally recognized as unreliable.Development of better models for multiphase simulation requiresan improved understanding of the flow evolution with dynamicinteraction of the flow multi-scales. Therefore, experimental datais urgently needed for validation of the CFD models.

In order to establish reliable design and performance criteriafor tube bundle models, better velocity data is needed. Simoninand Barcouda (1988) conducted experiments using laser Doppleranemometry (LDA) within a specific tube arrangement. Althoughthey gathered some velocity data in cross-flow over a tube bundle,the data points were limited and detailed velocity distributions, orwhole flow field data was not available due to the nature of the LDAtechnique. Chang et al. (2008) used 2D-LDA measurements in a 5 × 5rod bundle array scaled to be 2.6 times larger than the actual bundle

size. This work focused on the performance and mixing character-istics of two kinds of spacers with turbulence enhancement vanes.The used spacer grids were of a typical split and swirl type for pres-surized water reactors (PWR). The experiments were performed at acondition of Reynolds number of Re = 48,000 and pressure of 1.5 bar.

E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear Engineering and Design 239 (2009) 888–898 889

erime

ToautcadittfhTtrttacvw

Fig. 1. Schematic diagram of the exp

hey reported the velocity at various axial positions ranging fromne hydraulic diameter to 16 hydraulic diameters. The reportedccuracy of the velocity measurements is about 1.6%. However, these of laser Doppler velocimetry (LDV) methods was restricted byhe existence of invisible regions in fuel rod bundles and insuffi-ient spatial resolution for the narrow gaps in rod bundles. Ikedand Hoshi (2006) developed a miniaturized LDV system embed-ed in a fuel cladding. The rod-embedded fiber LDV can be inserted

n an arbitrary grid cell instead of a fuel rod without disturbinghe flow. They obtained flow velocity data in a 5 × 5 rod bundleypical of a PWR fuel assembly. Measurements were carried outor a central rod with a pitch-to-diameter ratio P/D = 1.326 with aydraulic diameter, Dh = 9.75 mm. The Re number used was 57,000.he reported uncertainty in velocity data was 2.1%. In their work,he conditioning grid was without mixing vanes. They showedesults of axial and cross flow velocity. The cross flow data indicatedhat positions close to the grid had velocities 40% greater than posi-ions farther from the grid. The velocity fluctuations also increased

s much as 100% at positions far from the grid compared to positionslose to the grid. In the case of the axial-flow velocity, the mean flowelocity at X = 3.1Dh was smaller than at X = 20.5Dh and fluctuationsere greater closer to the grid. It is found that for this type of grid

Fig. 2. Test section showing the optica

ntal hydraulic loop and test section.

the flow recovers after a distance of X/Dh = 10 for axial flow velocity.For the case of cross flow situations the fluctuations remain con-stant after X/Dh = 10 but the mean value of the velocity componentdecreases with distance from the grid. Mean cross flow velocityand turbulence in the fuel bundle are bigger near the spacer grid;recovery of mean flow velocity and a reduction in turbulence wereobserved downstream. These flow behaviors were assumed to bemainly due to the mixing vanes. The results indicate that the mixingvane effect has a strong influence to around X = 10Dh downstreamof the spacer grid. Conner et al. (2005) presents the experimentalresults of a 5 × 5 fuel bundle with spacer grids. Several spacer typestypical of PWR’s were tested in an air loop with fully heated rods.The work focuses on heat transfer measurements using a speciallydesign thermocouple holder than can be moved axially inside thebundle. The Re number tested ranged from 15,000 to 37,000 basedon the hydraulic diameter, Dh = 11.77 mm. The results were calcu-lated based on temperature measurements at discrete positionsinside the bundle. Conner et al. showed an improvement in the heat

transfer after the spacer-grids but did not provide any explanationabout the probable mechanism behind these improvements.

Yang and Chung (1998) have studied the influence of the spacergrids on the turbulent mixing within square sub-channel geome-

l arrangement and PIV system.

890 E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear Engineering and Design 239 (2009) 888–898

urem

tfltauapmpr

mTPbsmfmHpaios(rri

Fig. 3. Meas

ry. They analyzed the generation and decay of turbulent energyrom LDA measurement results. Neti et al. (1982) measured theateral velocity profiles within sub channels using 2D-LDA andhey claimed that the magnitudes were less than 1% of the aver-ged axial velocity. However, detailed secondary flow features werenclear. Rehme (1987) performed measurements using hot wirenemometry (HWA) in order to confirm the existence of lateral flowulsations between the sub-channels. It is concluded that the ther-al mixing in sub-channel geometry is mostly influenced by almost

eriodic macro scale flow pulsations caused by the flow instabilityather than the secondary flow from the Reynolds stress gradients.

Regarding computational work, several attempts have beenade to model the thermal hydraulic behavior of fuel rod bundles.

zanos (2004) simulated the flow in a 7 × 7 rod bundle typical of aWR. The spacers were modeled as plates with no thickness and thelockage ratio of the spacer was used as controlling parameter. Theimulations were made using various turbulence models with sym-etric and periodic boundary conditions. Tzanos concluded that far

rom components that cause significant flow deflections the agree-ent of mean velocity predictions with measurements was good.owever, near such components the discrepancy between velocityredictions and measurements could be significant. These discrep-ncies were attributed to shortcomings of the k–ε models. Evenn rod bundles without flow deflectors, the turbulence predictionsf standard k–ε models showed significant discrepancy with mea-

urements. Ikeno and Kajishima (2006) used large Eddy simulationLES) coupled with the immersed boundary method in an effort toepresent the effect of flow geometry and compared some of theiresults with particle image velocimetry (PIV) focusing on develop-ng swirl. The Re number for this investigations was Re = 4100. The

Fig. 4. Cross sectional configuration

ent system.

results indicated the effects of: the mixing-vanes which caused theswirl with large-scale fluctuation and enhancement of heat trans-fer. The produced vortices in the rod gap caused enthalpy mixingenhancement between channels. The model successfully explainedthe mechanism of decaying swirl in the rod bundle: a develop-ing vortex in the rod gap and the decreasing wake behind themixing-vanes promoted the decay of swirl more strongly than ina pipe. Lee and Choi (2007) focused on the computational analysisof turbulence intensities, maximum surface temperatures of the rodbundle, heat transfer coefficients and pressure drops of four kindsof mixing vanes. The authors used a 17 × 17 rod bundle since theyconsidered that vortex size effect may not be represented correctlywith a smaller rod array. Caraghiaur and Anglart (2007) measuredthe axial velocity in three different sub-channels of 5 × 5 rod bun-dle with spacers using LDV. The results were compared with CFDpredictions using a k–ε and SSG turbulence models. It should benoted that the spacer did not have any turbulence enhancementdevice such as mixing vanes. In this study both turbulence modelsover-predicted the rate of the velocity downstream of the spacer.In addition, the root mean square values of the axial velocity didnot agree with the CFD prediction. Caraghiaur and Anglart con-cluded that the pressure drop over the spacer was under-predictedby 20–30% in comparison with CFD code calculation. There was areasonable agreement between axial velocity distributions down-stream of the spacer with a larger CFD model i.e. with half of the

bundle geometry to remove the influence of symmetry conditionsimposed on sub-channel boundaries in the CFD predictions.

Verification and validation (V&V) are the primary means toassess the accuracy and reliability of computational simulations.V&V methods and procedures have fundamentally improved the

of the rod bundle assembly.

E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear Engineering and Design 239 (2009) 888–898 891

g a fue

ctobmpt

Fig. 5. A 5 × 5 spacer grid segment filled with FEP tubes emulatin

redibility of simulations in several fields, such as aerospace indus-ry. The field of nuclear reactor safety has placed great emphasis

n developing validation benchmarks. Many of these validationsenchmarks are closely related to the overall behavior for deter-ination of velocity, temperature, and void fraction among other

arameters. However, detailed information of local velocity andemperature in full-field are not available for CFD validation.

Fig. 6. Test section showing the optical transparency achieved inside the

l-bundle. (a) Pipes immersed in water. (b) Pipes immersed in air.

The importance of computer simulations in the design andperformance assessment of engineered systems has increased dra-

matically during the last three decades. To have justified confidencein this evolving field, there should be major improvements in thetransparency and maturity of the computer code used, the clarity ofthe physics included or excluded in the modeling, and the compre-hensiveness of the uncertainty assessment performed. CFD codes

bundle by matching the refractive index of the FEP rods and water.

892 E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear Engineering and Design 239 (2009) 888–898

Ft

sa

taaplbmi

ig. 7. Horizontal laser sheet passing across the rod bundle without distortion dueo a good matching index of refraction.

hould be applied with care to the flows where dominant physicsre clarified.

Thus, the ultimate goal of this work is to contribute to a bet-er understanding of presupposed and existent events that couldffect the safety of nuclear power plants by using state-of-the-rt measurement techniques that may elucidate the fundamentalhysics of fluid flow in rod bundle with spacer grids. In particu-

ar, this work concerns the development of an experimental dataase with high spatial and temporal resolution of flow measure-ents inside 5 × 5 rod bundles with spacer grids. The data base is

ntended to validate CFD codes at various scales. Measurements are

Fig. 8. Multi-scale m

Fig. 9. Top view of rod bundle showing the planes selected for the PIV measure-ments.

carried out using dynamic particle image velocimetry (DPIV) insidean optically transparent rod bundle utilizing the matching indexof refraction (MIR) approach (Hassan and Dominguez-Ontiveros,2008). This work presents results showing full field velocity vec-tors and turbulence statistics for the bundle under single phase flowconditions.

2. Experimental set-up

The experiments were conducted in the test loop of the opticalmulti-phase flow research laboratory (OMRL) at Texas A&M univer-sity (TAMU) allowing for test pressures up to 3 atm. A schematic ofthe hydraulic flow loop is shown in Fig. 1. The flow loop consisted of

a variable speed pump, storage tank, flow meter, flow straightener,mesh, and test section. Water was used as the working fluid, and itflowed from the bottom of the test section in an upward direction.The flow rate was controlled by the rotational speed of the pump

easurements.

E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear Engineering and Design 239 (2009) 888–898 893

uotle

cddswt

Toaplcfltw

dle with spacer grids was installed in the aluminum square duct.

Fig. 10. Typical instantaneous velocity fields.

sing a frequency controller and a control valve in the main loop inrder to assure reproducibility of flow rates and constant torque inhe pump impeller. The fluid passed through a series of filters thatimited the size of suspended particles in the fluid to 50 �m duringxperiments.

The instrumentation and control stages for the hydraulic loopontained turbine type flow meters and rotameters. Pressure trans-ucers were installed along the test section to measure the localifferential pressures across the spacer grids and the loop pres-ure. The data was collected using a data acquisition unit NI-SCXIith the 1000/1600/1314 modules for signal conditioning, analog

o digital conversion, and transmission.A dynamic PIV system was used to measure full velocity fields.

he system was comprised of a high repetition Nd:YaG twin laser,ptics, high accuracy pulse generator, motorized position stages,nd high speed cameras as shown in Fig. 2. The maximum laserulse frequency was 20,000 pulses per second with a 527 nm wave-

ength and maximum energy of 10 mJ per pulse. Three high speed

ameras with up to 120,000 fps were used to accurately capture theuid behavior in the test section. A laser sheet with about 1 mmhickness was produced using a set of optical lenses and mirrorshich impinged perpendicular to the lateral face of the channel.

Fig. 11. Average velocity fields.

Polymer seeding particles with diameter distribution of 6–9 �mand specific gravity of 1.05 were added to the fluid in a volume con-centration of 1.8e−2% for the PIV measurements. Data was obtainedat various planes of the test section. Cross-flow measurements wereobtained by using a second laser sheet to form a horizontal planeparallel to the spacer grid span. The horizontal laser sheet with a2.5 mm thickness was shaped using a 50% mirror arrangement asshown in Fig. 3. A third high speed camera was placed in the test sec-tion with an inclination of 55◦ in order to capture the data obtainedusing the horizontal laser sheet.

2.1. Test section

For observation of the flow structure the test section houseda square-arrayed 5 × 5 rod bundle. The test assembly consisted of24 fuel rods with a diameter of D = 10.25 mm placed inside theegg-crated spaces of the spacer subsection and one square rodwith a side length of 12.5 mm as shown in Fig. 4. The rods werearranged in a square array with a P/D = 1.26. This 5 × 5 rod bun-

Both ends of the rod bundle were fixed with the same type ofspacer grids. The square duct was made of an aluminum verti-cal channel with dimensions 101.6 mm × 101.6 mm × 1828 mm. Thechannel had four observation windows in order to have optical

894 E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear Engineering and Design 239 (2009) 888–898

aw5dTia1aTrrrt1wisft

i

(e.g., Hassan and Dominguez-Ontiveros, 2008). Although match-

Fig. 12. Average velocity fields with calculated streamlines.

ccess to the channel’s interior faces. A set of four spacer gridsere located inside the channel with a distance between them of08 mm. However, the grid located at the end of the bundle on theownstream side was 250 mm from the edge of the previous spacer.he special shallow plastic rods are located inside the spacer gridn order to simulate the fuel rods. The rods were fabricated usingfluorinated ethylene-propylene (FEP) plastic with dimensions of

0.25 mm OD × 10 mm ID × 1270 mm long with a ±0.07 mm toler-nce in all dimensions (Dominguez-Ontiveros and Hassan, 2007).hese dimensions followed closely the dimensions of the real fuelods used in the UltraflowTM spacers (Kraemer et al., 1995). Theods were sealed at each end with a conical aluminum cap. Theods were filled with a solution of 5% chlorine in water. This solu-ion kept the refractive index of water close to its original value of.33 and prevented the growth of bacteria and algae in the fluid,hich could alter the optical transparency of the stagnant solution

nside the rods. The measurements were taken around the secondpacer grid, with the first spacer grid serving as a conditioning grid

or the measurement zone. The spacer grids had mixing devices ofhe swirl type attached to the edge of the grid.

The velocity components and their fluctuations were measuredn the test section at several locations upstream and downstream

Fig. 13. Typical instantaneous velocity fields.

the grid by particle image velocimetry. The local turbulence quan-tities were integrated over the fluid section to obtain the value ofturbulence in different sections. This value could then be used asboundary condition for CFD simulation. The flow velocities weremeasured using varying axial elevations from 0.1 to 6 hydraulicdiameters, with the top edge of the spacer-grid as the referencepoint.

2.2. Matching index of refraction

Optical flow studies of complex geometries such as fuel bundleswere shown to be possible by matching the index of refraction ofthe solid test section with that of the working fluid. When lightpasses, through materials with a different index of refraction, thelight bends at the interface of the materials causing distortion orhidden areas. However, by matching the refractive index of thematerials, the bending of the light was minimized or eliminated,making the field of interest accessible and the test section opti-cally transparent. MIR techniques have been used in past studies

ing the refractive index of the solid and liquid was not difficult initself, however, other concerns such as fluid price, viscosity, density,toxicity, flammability, compatibility, etc. complicated the selectionprocess. In this work, optical transparency was achieved by match-

E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear E

iipoiwnw

cwtoFsstaotbtm

The mean streamwise velocity determined in this fashion was

Fig. 14. Average velocity fields.

ng the index of refraction. The immersed plastic rods have a similarndex of refraction as water. The plastic rods are made from FEPolymer with a refractive index of 1.338, and filled with a solutionf water and chlorine with a refractive index of 1.333. By match-ng the index of refraction of both materials optical transparencyas achieved. The working fluid for the tests was water, thereforeo correction is necessary and the transparency of the test sectionas assured.

Fig. 5 shows an emulated hot-rod (red color) embedded in theentral rod position. The rod is optically accessible when the bundleas submerged in water. The matched refractive index of water and

he FEP pipes allowed for the visualization of the central rod with-ut considerable disturbances from the rod-layers surrounding it.ig. 6 presents another pictorial of the achieved transparency in theetup. The vertical channel was partially filled with water to demon-trate the immediate clearness and accessibility of any point insidehe grid spacer. The unobstructed path of the blue laser light sheets it passed through the rod bundle is demonstrated in Fig. 7. It wasbserved that light was not significantly distorted demonstrating

he optical transparency obtained by this technique. As indicatedy Hassan and Dominguez-Ontiveros (2008), the wall-thickness ofhe cylindrical rod must be chosen to achieve an optimal compro-

ise between rod strength and optical transparency. Consequently,

ngineering and Design 239 (2009) 888–898 895

the FEP shallow rods were fabricated with a thin wall for trans-parency and filled with a solution of water and chlorine to enhancerod strength and prevent bacteria growing.

3. Results

The measurements for axial velocity at the investigated regionwere performed at several planes using a multi-scale approach.This approach consisted of performing measurements on variousviewing areas of the test section. The scales were selected based onthe physical constrictions and the desired resolution of the velocityfields. Fig. 8 shows the different scales used in this work. The firstscale examined the flow behavior in the planes that cover the fiverods and its respective sub-channels which are denoted by the yel-low dashed line. At this scale the interaction between sub-channelsand general characteristics of the flow were captured. The equiva-lent viewing area of the cameras for this scale was 70 mm × 60 mm.The second scale is denoted by the area marked with a solid redline in Fig. 8. This scale captured the flow characteristics of acentral sub-channel with an equivalent camera viewing area of15 mm × 12 mm. At this scale the flow evolution in the sub-channelcould be observed. The third scale is denoted by the red dashedline in Fig. 8 and it has an equivalent viewing area of 5 mm × 4 mm.This scale was intended to resolve the local flow behavior, such asboundary layer thickness, turbulence energy production, turbulentenergy dissipation, eddy size, coherent structures identification,among other important parameters of interest.

The data collection process included the measurement of everyscale at various planes inside the 5 × 5 bundle. The selected planeswere parallel to the frontal channels’ window and they varied indistance from the wall to the center of the bundle by changing theposition of the optical set-up and cameras using automatic motor-ized positioning stages as shown in Fig. 9. Fig. 10 shows the PIVmeasurement result of the axial velocity vectors in the investigat-ing region downstream and upstream the spacer grid. The velocityfields show the whole velocity distribution in the region from 0 to1.2Dh for both cases.

3.1. Velocity field around spacer

Several different techniques were used to analyze the behav-ior of the channel flow. Velocity statistics were obtained from PIVmeasurements in the x–y plane. These velocity statistics were deter-mined by ensemble averaging over the number of velocity fieldsusing Eq. (1),

A =

N∑i=1

Ai(X̄)

N(1)

where A is the quantity being averaged, X̄ is the position vector, andN is the total number of realizations.

The result from Eq. (1) was a two-dimensional average vec-tor field. The fluctuating velocity fields were obtained using theReynolds decomposition for each vector in each instantaneousvelocity field using the following expression,

u = U + u′ (2)

where U is the mean velocity and u′ is the velocity fluctuation.

subtracted from each instantaneous velocity field to obtain thefluctuating velocity fields using Eqs. (3) and (4). These fluctuatingvelocity fields were then ensemble averaged to obtain the stream-wise and normal intensities as well as the Reynolds shear stresses

896 E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuclear Engineering and Design 239 (2009) 888–898

resses

u

u

v

u

wl

Fig. 15. Turbulence intensities and Reynolds st

sing Eq. (5).

′i(x, y) = 1

N

N∑i=1

[ui(x, y) − U(x, y)] (3)

′i(x, y) = 1

N

N∑i=1

[ui(x, y) − U(x, y)] (4)

′v′(x, y) = 1N∑{[

u′(x, y)] [

v′(x, y)]}

(5)

N

i=1

i i

Instantaneous velocity vectors are shown in Fig. 10. The swirlas caused by the flow around the mixing-vane. The swirl contains

arge-scale fluctuation, which affected the downstream turbulence.

below and above the spacer grid for Re = 6500.

Structure and intensity of the downstream turbulence are impor-tant for the promotion of heat transfer on the rod surface. As shownin Fig. 10, the velocity field along the rod surface was captured rea-sonably by the PIV measurements. The flow fields are presented forboth positions: downstream and upstream the spacer grid. Theseinstantaneous velocity fields show non-uniform direction due tothe swirling motion of the fluid after having passed through thespacer. There is a change in the maximum velocity attained in thechannel of 43% with respect to the maximum measured down-stream from the spacer. To obtain the mean value the data was

averaged from more than 45,000 instantaneous vectors. Axial evo-lutions of the mean streamwise velocity are shown in Fig. 11. Asshown in Fig. 11 the flow was accelerated through the sudden nar-rowing channel spacer. The position of local maximum velocity waslocated at the center of the channel with a distance from the spacer

lear Engineering and Design 239 (2009) 888–898 897

eotssoia

vatleFfibtfavnoituoasFicpctisottcE

u

dirwottoteistg

iltris

boundary layer caused by the mixing vanes which seemed to pro-duce an attenuation of the vorticity magnitude. Although vorticitywas always present in the flow, there seemed to be a counteract-

E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuc

dge X = 6Dh. The averaged data shows a uniform direction profilef the velocity field after having passed the spacer which elucidateshe effect of averaging. On the other hand, the velocity profile down-tream of the spacer grid showed a similar profile to that found inhear flow along no-slip wall conditions. One of the advantagesf the DPIV obtained measurements was the direct comparison ofnstantaneous data with CFD which was previously done using anveraging approach.

The plane at a distance of X = 0.3Dh intersected with the mixinganes, where the flow was the wake flow behind the mixing-vane,local acceleration in the narrow gap between the rod and the

ip of mixing-vane is shown in Fig. 12. Streamtraces were calcu-ated from the instantaneous and averaged velocity fields in anffort to visualize the flow behavior below and above the grid.ig. 12 shows the streamtraces of the velocity averaged velocityelds. It was observed that the deflection of the flow was causedy the mixing vanes. The results showed the complex influence ofhe spacer grid in the sub-channel formed between the wall sur-aces of two adjacent rods. The results of 2D velocity fields showed

considerable reduction in the axial velocity component of theelocity vector at the center of the channel. This change in mag-itude is compensated by an increase in the radial componentf the velocity vector. Flow visualization demonstrated swirlingmmediately above the spacer grid. Figs. 13 and 14 show the instan-aneous and averaged velocity fields respectively downstream andpstream the spacers in the second plane measured. As previ-usly explained in this section, the PIV measures were performedt various planes using motorized positioning stages. The resultshown in Figs. 13 and 14 correspond to the plane 2 illustrated inig. 9. Since the rods are optically transparent when the waters flowing through the channel, the instantaneous velocity fieldsould be obtained. It was observed that measured vectors in thislane had strong direction changes caused by the mixing vanesompared to the uniform vector direction seen downstream fromhe grid. Once again, the averaged velocity fields are illustratedn Fig. 14. Axial evolutions of turbulence intensities and Reynoldstresses are shown in Fig. 15. The turbulence intensity was basedn the deviation from the mean velocity. Vortex cores were iden-ified using the Galilean decomposition approach. In the Galileanransformation the total velocity is represented as the sum of aonstant convection velocity, Uc, plus the deviation as shown inq. (6)

= Uc + uc (6)

In an effort to locate vortex cores, Adrian et al. (2000), selectedifferent fractions of the centerline velocity as the convection veloc-

ty implemented in the Galilean transformation technique. Theesulted vortex cores and length scale are illustrated in Fig. 15ith red circles. The averaged fluctuating velocity vectors are

verlapped in the contour plot showing the magnitude of theurbulence intensities. This figure shows the reduction in vor-ex cores as a direct consequence of the spacer grid’s influencen the flow behavior. The swirl decreased with distance fromhe mixing-vanes. Predicting this decrease rate is important tovaluate heat transfer at the rod surface. The mean axial veloc-ty decreased with an increase in turbulence when passing thepacer grid. The axial flow recovered after several diameters fromhe grid by settling down the flow field with distance from therid.

The results indicate that the flow field around the spacerncluded a large-scale unsteadiness and an undeveloped turbu-

ent flow. The unsteadiness was caused by a separation abovehe mixing vane attached to the spacer. The separation caused aapid increase of the pressure drop against inclination of the mix-ng vanes. The undeveloped turbulence appears in the inlet of thepacer, where the developed main flow in the sub-channel cen-

Fig. 16. Vorticity modification below and above the spacer grid for a Re = 6500. Thevelocity field spans for about 1Dh from the edge of the spacer.

ter was divided by the spacer edge. Turbulence models based on adeveloped turbulent flow were fundamentally inapplicable to theseflows.

3.2. Vorticity modification

Vorticity could be calculated using the average, the instanta-neous, or the fluctuating velocity field. One of the advantages ofPIV measurements is that it offered information about the vorticityfield from the spatial distribution of velocity. In this case, since mea-surements were carried out in the x–y plane, only one component(spanwise) of the vorticity vector could be obtained. Therefore, theexpression for instantaneous spanwise vorticity is

ωz =(

∂u

∂y− ∂v

∂x

)(7)

The derivative calculation was carried out using central dif-ference scheme. Substituting the fluctuating components of thevelocity in Eq. (7), the fluctuating spanwise vorticity was obtained.Likewise, the substitution of the average velocity componentsyielded the average vorticity field. In turbulent flows, the existenceof a high average vorticity is well known. This high level of vortic-ity was caused by the extension or “stretching” of vortex filaments(Tennekes and Lumley, 1974). Vortex stretching, has been regardedas the cause of the high rate of dissipation associated with turbu-lent motions. The effect of the spacer grid was a modification of the

ing phenomenon that suppressed the stretching in the flow. Fig. 16shows the average vorticity fields obtained for single phase flowbelow and above the spacer grid. It was observed that the close tothe wall there is a pronounced decrease in the vorticity as the flowpasses through the spacer grid.

8 lear E

4

loisimsttsw

dotncsstdfl

twldsqsutn

98 E.E. Dominguez-Ontiveros, Y.A. Hassan / Nuc

. Conclusions

The effect of the spacer grid was a modification of the boundaryayer caused by the mixing vanes which produced an attenuationf the vorticity magnitude. Although vorticity was always presentn the flow, there seemed to be a counteracting phenomenon thatuppressed the stretching in the flow. There is a change in the max-mum velocity attained in the channel of 43% with respect to the

aximum measured downstream of the spacer. The averaged datahowed a uniform direction profile of the velocity field after passinghe spacer which elucidates the effect of averaging on the results. Inhe other hand, the velocity profile downstream of the spacer gridhows a similar profile to those found in shear flow along a no-slipall.

One of the advantages of the DPIV obtained measurements is theirect comparison of instantaneous data with CFD which was previ-usly done using an averaging approach. The results indicated thathe flow field around the spacer included a large-scale unsteadi-ess and an undeveloped turbulent flow. The unsteadiness wasaused by a separation above the mixing vane attached on thepacer. The undeveloped turbulence appeared in the inlet of thepacer, where the developed main flow in the sub-channel cen-er was divided by the spacer edge. Turbulence models based on aeveloped turbulent flow are fundamentally inapplicable to theseows.

Successful use of PIV in addition to matched index of refrac-ion was used for full field velocity fields in a complex geometryithout flow disturbance. The results showed that turbulence evo-

ution can be observed using the DPIV technique. Good qualityata with high spatial and temporal resolution open the pos-ibility of CFD benchmarking and validation among important

uantification of the turbulent structures present in the flow abovepacer grids. The experimental results from this work may beseful for the development of a turbulence model for applica-ions such as sub-channel geometries, where the flow is highlyon-isotropic.

ngineering and Design 239 (2009) 888–898

References

Adrian, R.J., Christensen, K.T., Liu, Z., 2000. Analysis and interpretation of instanta-neous turbulent velocity fields. Experiments in Fluids 29, 275–290.

Caraghiaur, D., Anglart, H., 2007. Measurements and CFD Predictions of Velocity,Turbulence Intensity and Pressure Development in BWR Fuel Rod Assemblywith Spacers. Proceedings of the 12th International Meeting on Nuclear ReactorThermal Hydraulics (NURETH12), Pittsburgh, PA, September 30–October 4.

Chang, S.K., Moon, S.K., Baek, W.P., Choi, Y.D., 2008. Phenomenological investigationson the turbulent flow structures in a rod bundle array with mixing devices.Nuclear Engineering and Design 238, 600–609.

Conner, M.E., Smith, L.D., Holloway, M.V., Beasley, D.E., 2005. Heat Transfer Coeffi-cient Testing in Nuclear Fuel Rod Bundles with Mixing Vane Grids, Water ReactorFuel Performance Meeting, Kyoto, Japan, October 2–6.

Dominguez-Ontiveros, E.E., Hassan, Y.A., 2007. Jet flow mixing measurements in thevicinity of rod bundles using PIV. Transactions of the American Nuclear SocietyWinter Meeting, Washington, DC, November 11–15.

Hassan, Y.A., Dominguez-Ontiveros, E.E., 2008. Flow visualization in a pebble bedreactor experiment using PIV and refractive index matching techniques. NuclearEngineering and Design 238, 3080–3085.

Ikeda, K., Hoshi, M., 2006. Development of rod-embedded fiber LDV to measurevelocity in fuel rod bundles. Journal of Nuclear Science and Technology 43,150–158.

Ikeno, T., Kajishima, T., 2006. Decay of swirling turbulent flow in rod-bundle. Journalof Fluid Science and Technology 1, 36–47.

Kraemer, W., Proebstle, G., Uebelhack, W., Keheley, T., Tsuda, K., Kato, S., 1995. TheULTRAFLOW spacer—an advanced feature of ATRIUM fuel assemblies for boilingwater reactors. Nuclear Engineering and Design 154, 17–21.

Lee, C.M., Choi, Y.D., 2007. Comparison of thermo-hydraulic performances of largescale vortex flow (LSVF) and small scale vortex flow (SSVF) mixing vanes in17 × 17 nuclear rod bundle. Nuclear Engineering and Design 237, 2322–2331.

Neti, S., Eichhorn, R., Hahn, O.J., 1982. Laser Doppler measurements of flow in a rodbundle. Nuclear Engineering and Design 74, 105–116.

Rehme, K., 1987. The structure of turbulent flow through rod bundles. Nuclear Engi-neering and Design 74, 105–154.

Simonin, O., Barcouda, M., 1988. Measurements and Prediction of Turbulent FlowEntering a Staggered Tube Bundle. Proceedings of the 4th international sympo-sium on applications of laser anemometry to fluid mechanics, Lisbon, Portugal,July.

Tennekes, H., Lumley, J.L., 1974. A First Course in Turbulence. MIT Press, Cambridge,Massachusetts.

Tzanos, C., 2004. Computational fluid dynamics for the analysis of light water reactorflows. Nuclear Technology 147, 181–190.

Yang, S.K., Chung, M.K., 1998. Turbulent flow through spacer grids in rod bundles.Journal of Fluids Engineering ASME 120, 786–791.

Zdravkovich, M.M., 1987. The effects of interface between circular cylinders in crossflow. Journal of Fluids and Structures 1, 239–261.