PIV Experimental Investigation of Entranc[1]

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PIV experimental investigation of entrance configurationon flow maldistribution in plate-fin heat exchanger

Jian Wen *, Yanzhong Li *, Aimin Zhou, Ke Zhang, Jiang Wang

Department of Refrigeration and Cryogenics Engineering, School of Energy and Power Engineering, Xian Jiaotong University, Xian 710049, China

Received 30 March 2005; received in revised form 5 September 2005; accepted 21 October 2005

Abstract

Flow characteristics of flow field in the entrance of plate-fin heat exchanger have been investigated by means of particle image veloc-imetry (PIV). The velocity fields were measured using the two-frame cross-correlation technique. A series of velocity vector and stream-line graphs of different cross-sections are achieved in the experiment. The experimental results indicate that performance of fluidmaldistribution in conventional entrance configuration is very serious, while the improved entrance configuration with punched bafflecan effectively improve the performance of fluid flow distribution in the entrance. Based on the analysis of the fluid flow maldistribution,a baffle with small holes is recommended to install in the entrance configuration in order to improve the performance of flow distribution.When the punched baffle is proper in length, the small holes is distributed in staggered arrangement, and the punched ratio graduallyincreases from central axis to the boundary along with the baffle length, the performance of flow distribution in plate-fin heat exchangeris effectively improved by the optimum design of the entrance configuration. The flow maldistribution parameter S in plate-fin heatexchanger has been reduced from 1.21 to 0.209 and the ratio of the maximum velocity to the minimum h is reduced from 23.2 to1.76 by installing the punched baffle. The results validate that PIV is well suitable to investigate complex flow pattern and the conclusion

of this paper is of great significance in the optimum design of plate-fin heat exchanger. 2005 Elsevier Ltd. All rights reserved.

Keywords: Plate-fin heat exchanger; PIV; Entrance configuration; Optimum design

1. Introduction

Plate-fin heat exchangers are widely used in cryogenicplants such as air separation industries to exchange heatenergy among more than two fluids with different supplytemperatures because of their high efficiency, more com-

pact structure and lower costs than two-stream heatexchanger networks [14]. The use of multi-stream plate-fin heat exchangers is more cost-effective and can offer sig-nificant advantages over conventional two-stream heatexchangers in certain applications, especially for cryogenicplants. The longitudinal wall heat conduction, the nonuni-

formity of temperature field and fluid flow could becoupled, which is the main reason that causes the deterio-ration of heat exchanger efficiency, especially for the com-pact heat exchanger. The effect of fluid flow nonuniformityon heat exchanger efficiency is of first-order importancebecause it can intensify longitudinal wall heat conduction

and the maldistribution of interior temperature. The fluidflow maldistribution in plate-fin heat exchangers may becaused by many factors, such as the improper entranceconfiguration, various manufacturing tolerances, the heattransfer process and so on.

Most of previous works mainly investigated the effect offlow nonuniformity on the heat exchangers performancedeterioration based on their own flow maldistributionmodel. Fleming [5] set up a flow maldistribution model inpaired-channel heat exchangers and investigated the effectof flow maldistribution on the performance deterioration.

0011-2275/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cryogenics.2005.10.010

* Corresponding authors. Tel.: +86 29 8266 3725; fax: +86 29 8266 8725(J. Wen), tel.: +86 29 8266 8725; fax: +86 29 8266 8789 (Y. Li).

E-mail addresses: [email protected] (J. Wen), [email protected] (Y. Li).

www.elsevier.com/locate/cryogenics

Cryogenics 46 (2006) 3748
mailto:[email protected]:yzli-epe@mail.%20xjtu.edu.cnmailto:yzli-epe@mail.%20xjtu.edu.cnmailto:yzli-epe@mail.%20xjtu.edu.cnmailto:yzli-epe@mail.%20xjtu.edu.cnmailto:[email protected]


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Chiou [6] set up a continuous flow distribution model

and studied the thermal performance deterioration incross-flow heat exchangers. The combined effects of walllongitudinal heat conduction, inlet flow nonuniformityand temperature nonuniformity on compact plate-fin heatexchangers using finite element method have been investi-gated by Ranganayakulu and Seetharamu [7]. In fact, aflow distribution model that can precisely describe the realflow distribution in plate-fin heat exchanger does not exist.A few literature of experimental investigation on flow mal-distribution in plate heat exchangers can be found in therecent years, Lalot and Florent [8] etc. investigated theeffect of flow nonuniformity on the performance of plate

heat exchangers. They also found the optimum locationof perforated grid in header and reverse flow could occurwith poor inlet header design. The literature of improvedconfiguration to enhance the flow uniformity in plate-finheat exchanger is limited in recent years. Jiao [9] investi-gated experimentally and analyzed theoretically the com-bined effects of distributors inlet angle and mass flowrate on flow maldistribution. Some distributors with differ-ent inlet angles were studied under similar conditions andthe optimum performance had been obtained at inlet angleof 45. Zhang [10] proposed a structure of two-stage-distri-bution and the numerical investigation shows the flow dis-tribution in plate-fin heat exchanger is more uniform if theratio of outlet and inlet equivalent diameters for both head-ers is equal. Wen [11] employed CFD technique to simulateand analyze the performance of fluid flow distribution andpressure drop in the header of plate-fin heat exchanger. Abaffle with small-size holes is recommended to install inthe header in order to improve the performance of flowdistribution.

Although some work has been carried out to study ofthe effects of entrance configuration, more detailed infor-mation in the entrance part of a heat exchanger is stillunknown. The characteristics and phenomena of flow fieldare needed to master for the improvement of flow distribu-

tion. On the basis of numerical simulation [11], flow pat-

terns of flow field in the header of plate-fin heat

exchanger have been investigated by means of particleimage velocimetry (PIV) in this paper. The velocity fieldswere measured using the two-frame cross-correlation tech-nique. A series of velocity vector and streamline graphs ofdifferent cross-sections are achieved.

PIV is an instantaneous whole-field measurement tech-nique, which uses a pulsed light-sheet to illuminate a gasflow seeded with tracer particles. Four to six successiveimages of the tracer particles are recorded photographi-cally and the pictures digitized. It can potentially providemore information about flow mechanism than that fromconventional measurement techniques. PIV has already

had quite a lot of applications in studies of fluid mechanics,marine hydrodynamics and aerodynamics [1216]. It hasbroken through some limitations of single point measure-ment of conventional measuring technology. It is able notmerely to measure the instantaneous velocity field of fluidfrom low-speed to supersonic range, but also succeeds inapplying to the measurement of multiphase flow, thermo-fluid and turbulent structures, etc. So in this paper PIVtechnology is used to study the characters of fluid distri-bution in the header of plate-fin heat exchangers and getbetter header configuration. The difference of flow charac-teristic between air and low-temperature fluids is limited.The effect of entrance configuration on flow field unifor-mity has little correlation with the fluid species for thatwe mainly investigate the effect of configuration changeson the fluid field uniformity, which is a relative quantity.It concentrates on the qualitative comparison of fluid fieldbefore and after the configuration improvement in thispaper. So the air of room temperature is adopted as theworking fluid in the experiment of this paper.

2. Experimental system and evaluation of flow

maldistribution

Fig. 1 shows the schematic drawing of experimental sys-

tem, which consists of wind path system and PIV system.

Nomenclature

A area of header outlet (mm2)A0 area ratio of medium-size holesA1 total area of small-size holes (mm

2)

A2 total area of medium-size holes (mm

2

)A3 total area of big-size holes (mm2)

D1 diameter of header inlet tube (mm)D2 diameter of pipe in the wind path (mm)L length of the header (mm)N the number of cross-sectionsR radius of small holes (mm)R1 radius of header (mm)Re Reynolds number

S flow maldistribution parameterv fluid flow rate (m/s)Z distance from the header central axis (mm)d

punched ratioh velocity ratio

Subscripts

ave average valuech channeli serial number of passagemax maximum valuemin minimum value

38 J. Wen et al. / Cryogenics 46 (2006) 3748


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And the arrangement of them is also illustrated in thediagram.

2.1. Wind path system

In the wind path system, the air is supplied by a upstream

fan and the flux is measuredby an orifice flow meterinstalledin the pipeline. Radial pressure port is used in the pressuremeasurement. The upstream pressure port is one diameterfar away from the orifice, while the downstream pressureport is 1/2 diameter from the orifice. The straight pipesbefore and afterthe orifice flow meter should be enough longto guarantee the correct measurement of the flow.

Think of the symmetry of header configuration, half theheader length is measured in the experiment. Eleven cross-sections are measured from the central axis to the endboundary along the direction of header length (z-direc-tion). The flow field of only four typical cross-sections is

analyzed in the paper as illustrated in Fig. 2. Cross-section1 is located at the central axis, straightly facing the inlettube. Cross-sections 2 and 3 are deviated from the centralaxis and the distance increases gradually. And cross-section4 is in the out end of header and is the farthest away fromthe central axis of header.

2.2. Header configurations

It has been found that the best position for the punchedbaffle is midway between the inlet tube and the core of the

header [8]. So a baffle with small holes is put forward toinstall at the 1/2 height of the header symmetrically, whichis demonstrated in Fig. 3. The flow maldistribution of theoutlet along the header length (z-direction) for the conven-tional header (denoted as configuration A) is very serious.Because the header has larger dimension comparing to the

inlet tube diameter, the fluid tends to go preferentially intothe channels in the center. The velocity distribution is basi-cally symmetrical and decreases from the axis line to theboth ends. They are higher in the center and lower in theboundary. So for the uniformity of flow distribution, differ-ent flow area should be selected to the fluid field accordingto the velocity distribution. Small flow area should beapplied to high-speed flow zone and large flow area shouldbe applied to low-speed fluid. Under ideal condition, thedot product v.dA should be a constant value for each infin-itesimal area. Thus the total flow field will be distributedmore uniformly if the integral of v.dA is equal to con-

stant.A baffle with small holes according to this designing idea

is put forward to meet the distribution requirement. Thesmall holes are spotted in the baffle according to the veloc-ity distribution of conventional header of CFD simulationand the punched ratio is gradually increasing in symmetryfrom the central axis to the boundary [11]. Three differentkinds of holes in diameter are punched on the differentbaffle configurations. There are spotted symmetricallysmall (2R = 10 mm), medium (2R = 20 mm) and big(2R = 30 mm) holes from the axial line to the boundaryon the baffle. And the punched zone of the small-size holes(2R = 10 mm) is the center where it is corresponding to thecross-section of the inlet flow pipe. The baffles shown inFigs. 46 have the dimension of 600 260 5 mm3

(length width thickness), and the holes are distributedwith in-line arrangement. The total area of big-size holeson the baffle shown in Fig. 4 (denoted as configurationB) is the largest. And it decreases gradually shown inFig. 5 (denoted as configuration C) and Fig. 6 (denotedas configuration D). The dimension of baffles shown inFigs. 7 and 8 is 670 260 5 mm3 (length width thick-ness). The small holes shown in Fig. 7 are distributed within-line arrangement (denoted as configuration E), while inFig. 8 they are distributed in staggered arrangement

(denoted as configuration F).Fig. 2. Schematic diagram of test section location.

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The experimental model used in PIV experiment must bemade of transparent materials in order to let the laser sheet

illuminate the area of the measured velocity field and help

CCD camera to capture the pictures. So the melon peelstructure of header was made of lucite cylinder with the

thickness of 8 mm. The outlet microchannels and the

Baffle

Fig. 3. Definition of the baffle position in the header.

Fig. 4. Schematic drawing of baffle construction of configuration B.

Fig. 5. Schematic drawing of baffle construction of configuration C.

Fig. 6. Schematic drawing of baffle construction of configuration D.



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punched baffles were made of lucite plate of 5 mm. All theabove test models were made according to the original size,guaranteeing the performance of the prototype can bereflected as accurate as possible by the models.

Because the size of the model is large, the pulsed laserlight-sheet of PIV system is not too large to illuminatethe whole test section. And the area of measured velocityfield captured by CCD camera is only about 150 mm 150 mm. Due to the limitation of the laser light-sheet widthand the minimum object size that the CCD camera cancapture, so half the header cross-section is chosen as themeasured section (shown in Fig. 1). Fluid distribution con-ditions of different position of header cross-sections alongthe direction of header length (z-direction) are measuredby PIV system in the experiment.

2.3. PIV system

The measurement of the velocity field in the header wascarried out with a two-dimensional PIV equipment, whichconsists of illumination system, image acquisition system,

synchronous controlling system and image processing sys-

tem. Light source is supplied by a large power harmonicNd-YAG twin laser system (wavelength k = 532 mm, pulseenergy = 50 MJ). The pulsed laser and the light-sheetoptics are part of the illumination system. The light-sheetoptics includes cylindrical and spherical lenses. The cylin-drical lens diverges the laser beam in one direction butkeeps it the same thickness as the laser beam in the otherdirection. This gives a light-sheet that has some heightbut is thin. The spherical lens controls the light-sheet thick-ness. For the measurements, a light-sheet thickness of lessthan 1 mm is desired. The spherical lens creates a beamwaist at its focal distance. Moving away from the focal dis-tance, the light-sheet is thicker. With the proper selection oflight-sheet optics, the laser light-sheet can be set to thecorrect size for experiment. The flow in a PIV experimentis illuminated with the pulsed laser light-sheet of a thick-ness of 1 mm. The camera usually is focused near thelight-sheet waist, at the focal length of the spherical lens.The image signal acquisition is carried out by PIVCAM-30 of type 630046, whose resolution factor is 1018 1018pixel. The fastest acquisition speed is 30 frames per second.

The CCD camera is a combination of cross-frame and

Fig. 7. Schematic drawing of baffle construction of configuration E.

Fig. 8. Schematic drawing of baffle construction of configuration F.



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rapid-output of image signal matrix that enabled capturingtwo images with minimum time separation of 1 ls. Syn-chronizer is used to control the output of laser pulse andthe sequence of image acquisition, which guarantees allparts are coordinated and gone on according to the regularorder. The software of controlling PIV system and data

analysis is Insight NT. It can capture 1000 frame picturesin succession. Two-frame cross-correlation and fast Fou-rier transforms are adopted in the data processing. Instan-taneous velocity vector field and some other parametersdistribution also can be obtained swiftly through batchprocessing.

The selection and seeding of tracer particles is one of thekey problems of success or failure for PIV system measure-ment, especially for the case where air is adopted as theworking medium. Accompanying characteristic, scatteringcharacteristic and uniformly sowing are the three aspectsthat the tracer particles should be considered of at the sametime. The density of tracer particles should be proper in

order to realize the flow field measurement by particleimage. That is to say, the number of tracer particles shouldbe abundant, but limited not to form the scattered spotwithin certain area of the measured velocity field. In thisexperiment, Rosco1600 fog generator is used to generatetracer particles and fuming liquid especially used for airsystem is supplied by TSI Ltd. The average diameter ofspherical drops is between 1 lm and 2 lm, which meetsthe demands of PIV system for tracer particles. In orderto ensure the uniformly scattering of tracer particles andreduce their interference to original flow field, the seedingof tracer particles is performed at the fan inlet, which is

far away from the test area. Thus, tracer particles and airenter the pipeline at the same time and have a uniformmixing at the test section.

2.4. Evaluation of flow maldistribution

Two parameters are introduced in this paper to evalu-ate the maldistribution, namely, flow maldistributionparameter S and velocity ratio h, which are defined asfollows:

S

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

N 1 XN

i1

Vi Vave2

vuut 1

h Vmax

Vmin2

They disclosure the dispersion degree of experimental re-sults and reflect the flow maldistribution conditions underdifferent working conditions, different kinds of header con-figuration parameters and so on. The smaller the absolutevalue is, the more uniform the distribution is. Where Nstands for the passage number, Vi stands for the velocityof each passage and Vave stands for the average velocityof all the passages. Vmax and Vmin are the maximum velo-city and minimum velocity of all the passages, respec-

tively.

3. Experimental results and discussion

3.1. The comparison of flow patterns

3.1.1. The experimental results of conventional header

(configuration A)

Cross-section 1 shown in Fig. 9 is located at the centralaxis and facing the inlet tube. The flow maldistribution isserious in the y-direction of the cross-section. The velocityis high and the upstream streamlines flow to the export inparallel, where the spot is facing the inlet tube of headerin the y-direction. While at the spot which is deviated fromthe inlet tube, fluid mainly flows from upstream and vortexis generated. A dead zone is formed for the reverse flow,thus fluid is distributed very nonuniformly in the y-direc-tion. It is because of the suddenly enlargement of fluidboundary geometry that flow behavior varies rapidly andthe main flow separates from the surface to form vortex.The vortex can result in the loss of mechanical energy by

converting mechanical energy into thermal energy. Thereis also mass transfer between the vortex and the mainstream besides friction, so the energy loss is much.

The cross-section 2 (shown in Fig. 10) and cross-section3 (shown in Fig. 11) is deviated from the inlet tube. As thedistance from the inlet tube increases, small vortex enlargesgradually into large-scale vortex. So it can be concludedthat this portion of fluid in the header are mainly distrib-uted by the diffusion of vortex. It is because that fluid flowsto the two sides of the header by the transverse pressuregradient at the outlet cross-section. Minority of the fluidreaches micro-channels and goes through, while the major-

ity impacts on the boundary and is blocked. The fluid sep-arates from the surface to form vortex and diffuses allaround. As the distance from the inlet tube further

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increases, the vortex shrinks for the viscous dissipationwith all around fluid (shown in Fig. 11).

The cross-section 4 illustrated in Fig. 12 is at the bound-ary of the header and is the farthest away from the inlettube. The majority of fluid has been branched off beforeit reaches the section and fluid kinetic energy is consumedby vortex, which results in the decrease of fluid velocity. Sothe fluid flux through the micro-channels nearby is very lowand the section becomes the low-speed zone of the header.And for the viscous dissipation between the fluid and theboundary, vortex vanishes and the streamlines are parallelto the outlet micro-channels.

Based on the analytical and experimental investigations

presented above, it can be concluded that it is mainly the

fluid vortex and transverse pressure gradient that resultin the fluid distribution in the conventional header. Thusthe velocity near the inlet tube is high, while the velocityis low at the boundary. Besides the fluid distribution non-uniformly, the energy loss caused by the vortex is high.So the header configuration should be improved to makethe fluid distributed more uniformly.

3.1.2. The experimental results of improved header(Configuration F)

The fluid from the inlet tube flows in front of thepunched baffle. The circulation area expands suddenlyand it causes the pressure reduction. Two kinds of factorscause the pressure drop, one is the irreversible effect, theother is the velocity variation. There are two kinds of con-dition that fluid reaches the baffle: (1) fluid reaches thesmall holes and goes through directly, which is the throt-tling expansion; (2) fluid reaches the baffle wall and isblocked. According to the Bernoulli equation, pressurerises and transverse pressure gradient is formed, whichresults in fluid flowing all around and going through fromother small holes. And the different diameters of smallholes are of great benefit to fluid uniform distribution.The small-size holes in the center part reduce the flux goingthrough from the micro-channels adjacent to the centralline. And the medium-size and large-size holes augmentthe flux that flows from the two side micro-channels. Thusthe fluid is distributed evenly before it reaches the headeroutlet.

As shown in Fig. 13, the fluid is distributed more uni-formly in the y-direction and the dead zone vanishes inthe cross-section 1 after the improvement of header config-uration. In the y-direction, the velocity facing the inlet tube

decreases significantly, while that deviated from the inlet

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tube increases, comparing to that of conventional headerconfiguration A. The diameter of small holes is the smallestin the center of the baffle, which creates much resistance tothe fluid and transverse pressure gradient is formed tooblige fluid flow all around. In the same way, the diameterof the holes at the edge of the baffle is larger so that thebranch fluid can get through with no trouble. So the fluidof these cross-sections is distributed more uniformly in

the y-direction. And the vortex in front of the punched baf-fle vanishes compared to that of conventional header con-figuration A. That is because the addition of baffle, whichhalves the header height and the space is too narrow toform vortex. There are some vortices behind the punchedbaffle, facing the small holes. The velocity of the fluidincreases after going through the small holes and it initiatesviscous shear with around low-speed fluid, so the vortexesare formed.

The cross-section 2 in Fig. 14 is deviated from the inlettube. There is a big vortex in front of the baffle and thefluid is distributed uniformly after going through the baffle.Fluid from upstream is obstructed by the header wall andseparates from the surface, thus an obvious vortex creates.For the cross-section does not face the inlet tube, so thereare only medium-size and big-size holes in the punched baf-fle. Thus the resistance of baffle to fluid is low and the fluidis easy to go through. And fluid is distributed more uni-formly after branched off by the small holes in the baffle.

The cross-section 3 as shown in Fig. 15 is far away fromthe inlet tube and there is no baffle. The vortex vanishesfor the viscous dissipation with around low-speed fluid.For the resistance of baffle wall, all the fluid going throughfrom the baffle is less comparing to that of conventionalheader configuration A, which results in more fluid flowing

from the cross-section and the velocities increase.

As illustrated in Fig. 16, the velocities in cross-section 4also increase and the uniformity of fluid distribution isimproved greatly after the addition of the punched baffle.The streamlines are perpendicular to the outlet micro-chan-nels. And fluid goes through the micro-channels directly.That is to say, the fluid is distributed more uniformly forthe increase of flux far away from the center axis withthe addition of punched baffle.

The results of experiment illustrates that the fluid is dis-tributed more uniformly. With the addition of punchedbaffle, fluid is distributed by the baffle and then flows to

the header outlet. Not only in the axial direction (z-direc-

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tion), but also in the y-direction, the condition of fluid dis-tribution is improved greatly. The number of big vortexdecreases compared to that of conventional header config-uration A. So the energy loss caused by turbulent dissipa-tion reduces, which can compensates for some of thepressure loss caused by the addition of the punched baffle.As simulated from reference [11], the pressure loss inducedby the baffle is about 400 Pa as its maximum, which isacceptable for a plate-fin heat exchanger.

3.1.3. The comparison of fluid distribution

Eleven cross-sections are measured in the PIV experi-ment and the header configuration is symmetrical. So it ispresumed that fluid in the header is distributed symmetri-cally and then the serials number is 21. Fig. 17 shows thefluid distribution condition in the axial direction (z-direc-tion) of the two headers, which is at the same inlet Rey-

nolds number (Re = 6.0 104), where the average valuefor the y-direction is adopted. The fluid distribution iseffectively improved for the improved header configurationwith punched baffle. The flow maldistribution of the out-let along the header length for the conventional headerconfiguration A is very serious. The velocities decrease

from the axis line to the boundary, they are high in the cen-ter part and low in the boundary. Because the flow headerhas larger dimension comparing to the inlet tube diameter,the fluid tends to go preferentially into the channels in thecenter. The velocities in the two sides of the header arelower for the fluid has been branched off before it reachesthe end of the header. The ratio of the maximum velocityto the minimum h (stated in Eq. (2)) is equal to 23.2 andthe flow maldistribution parameter S (stated in Eq. (1)) isequal to 1.21. While for the improved header configurationF, the velocities of center outlet decrease and that of twosides increase obviously, which uniform the fluid distribu-tion in the header. So the velocity ratio h is reduced to

1.76 and the flow maldistribution parameter S is reducedto 0.209.

Not only in the y-direction, but also in the direction ofheader length (z-direction), the fluid distribution isimproved greatly for the improved header configurationwith punched baffle. It can be concluded from above inves-tigation that it is mainly the header improper configurationthat results in the flow maldistribution. So the conventionalheader configuration A should be improved to enhancethe uniformity of flow distribution in plate-fin heat exchan-gers.

3.2. Improvement of header configuration

For the improved header configurations, the velocitiesincrease in the zone of two ends of header and decreasein the zone near the axial line. Thus, the fluid flow is dis-tributed more uniformly. Unfortunately, the pressure dropmay increase for the fluid resistance added by the bafflewall, which is inevitable but not anticipated. So it is obligedto get the suitable baffle configuration, such as the dimen-sion, the area ratio of different holes and so on, for gettingthe optimum point of flow uniform distribution.

3.2.1. The effect of total area ratio

The velocities near the center are large and that near theboundary are small for the conventional header configura-tion A. So the punched zone of the small-size holes(2R = 10 mm) is the center where it is corresponding tothe cross-section of the inlet flow pipe. It is concentratedon the relationship between the total area of medium-sizeholes (2R = 20 mm) and that of big-size holes (2R =30 mm) in the following. The dimension of configurationB, configuration C and configuration D is 670 260 5 mm3 and the number of medium-size holes increases grad-ually. A1, A2 and A3 are denoted as the total area of small,medium and big-size holes, respectively. And the area ratio

A 0 is defined as follows:

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Fig. 17. Velocity distribution of different headers at Re = 6.0 104.

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A0

A2

A1 A2 A33

The area ratio A 0 of the configuration B, C and D is 0.26,0.33 and 0.40, respectively. The experimental results of theeffect caused by parameters A 0 are illustrated in Fig. 18,where the average inlet Reynolds number is equal to

6.0 104. As the area ratio increases, the maldistributionparameters S of the three configurations are 0.381, 0.306and 0.384 respectively, which are close to each other. Theuniformity of flow distribution of configuration C (arearatio A 0 = 0.33) is the best. As the area ratio increases, thetotal area of medium-size holes increases while that ofbig-size decreases, so the number of holes increases andthe influence of the baffle on flow distribution increases.On the other hand, the increase of area ratio results in thedecrease of punched ratio and the flow resistance increases,so a combinative consideration of distribution uniformityand flow resistance should be done together. For the header

with configuration C, it is suitable when area ratio is equalto 0.33, namely the total area of medium-size holes takes up1/3 of the whole area of all the holes.

3.2.2. The effect of baffle dimension

In the experiment, the location where the baffle isinstalled has been determined to the 1/2 height of theheader, and the thickness of the baffle is determined to5 mm, so the baffle is 260 mm in width. The diameters ofthe three kinds of holes are the same as described above.Fig. 19 shows the flow distribution performance along withthe change of baffle length at Re = 6.0 104, where config-uration C is 2/3 of the header length and configuration E is

3/4 of the header length.The flow maldistribution parameter S of configuration

C is 0.306 and that of configuration E is reduced to0.279. As the baffle length increases, the effect of baffleon flow distribution is strengthened, so fluid is distributedmore uniformly. But on the other hand, the baffle lengthincrease may result in the increase of flow resistance and

bring about the increase of pressure drops. The influenceof baffle on flow distribution is significant along with theincrease of baffle length since the maldistribution parame-ter S decreases and the flow is distributed more uniformly.Combinative consideration of the two factors, the bafflelength is selected as 670 mm in length, which is just 3/4of the header length.

3.2.3. The effect of holes distribution

The distribution characteristic of flow velocity and theirdifferences of configuration E and configuration F is shownin Fig. 20 to compare the effects of different holes distribu-tion. The small holes in configuration E is in-line arrange-

ment, while that of configuration F is in staggeredarrangement. The inlet conditions are the same for differentheaders at Re = 6.0 104.

When the small holes distribution is changed from in-line arrangement to in staggered arrangement, the uni-formity of flow distribution is enhanced effectively. Thevelocities of center outlet decrease and that of two sides

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Fig. 18. Velocity distribution versus area ratio of medium-size holes at

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2.0

2.5

3.0

Velocity(m/s)

Serials number

Configuration CConfiguration E

Fig. 19. Velocity distribution versus baffle length at Re = 6.0 104.

0 5 10 15 20 25

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Velocity(m/s)

Serials number

Configuration F

Configuration E

Fig. 20. Velocity distribution versus small holes arrangement at

Re = 6.0 104.



11/12

increase obviously. It is easily understood that the varia-tion of small holes distribution increase the number ofsmall holes in the baffle, which intensifies the effect of baffleon the flow uniformly distribution. And when the holes dis-tribution in baffle is changed from in-line arrangement to instaggered arrangement, the punched ratio on baffle will

increase from 47% to 53%, which leads to the increase offlow area on the baffle and the flow resistance broughtabout by baffle necessarily decreases. It can be concludedfrom above investigation that the baffle with staggeredarranged holes is suitable to improve the flow distributionin header. The flow distribution is more uniform and thepressure drops are smaller. The improvement of headerconfiguration with a stagger arranged baffle should beselected firstly.

3.2.4. The effect of punched ratio

When the baffle configuration is determined as configu-ration F, the fluid is distributed more uniformly than that

of conventional header, the highest fluid velocity decreasesand the lowest velocity is enhanced greatly. For configura-tion F, the velocity distribution is mostly concentrated inthe range of 1.0 and 1.5 m/s, which takes about 72.0% ofthe whole passages at Re = 6.0 104. The percentage ofvelocities between 0.8 and 1.0 m/s is 23.8%. And the veloc-ity ratio h is 1.76. While for configuration A, the velocitydistribution is concentrated between 0.1 and 1.0 m/s, whichtakes about 66.7% of the numbers of whole passages. Thevelocity ratio h is 23.2. From above discussion, the effect ofheader configurations on flow maldistribution is promi-nent. The flow velocity of the passages near the boundary

can be enhanced effectively by changing the header config-uration from A to F. The fluid distribution of configurationF gives the most uniform result among the cases consideredin this paper.

From above discussion, it can be concluded that thedetermination of baffle configuration has the relationshipwith the diameter of inlet tube, the length and diameter ofheader, the diameters and distribution of holes when thebaffle installation location has been defined. Fig. 21 showsthe distribution of punched ratio, in which the staircasecurve is drawn according to the realistic condition (configu-ration F) and the smooth curve is simulated from the for-mer. The correlation of punched ratio along with theposition in the direction of baffle length (z-direction) underthe ideal situation should be established as follows:

d 47:75 Z

100

24

where d stands for the punched ratio (%) and Z stands forthe Zposition along with the baffle length (mm). In practi-cal condition, if the holes on baffle can be punched accord-ing to the simulated smooth curve, we can get v.dA = constand the flow can be distributed more uniformly than theconfiguration F used here.

In a word, the improved header installed with baffle can

effectively enhance the flow uniformity. It is noticeable that

the baffle configuration has the relationship with headerconfiguration and fluid flow condition, which must be inconsideration in the optimization of baffle configuration.It can be foreseen that fluid flow can be distributed uni-formly when the baffle is suitable in length, the holespunched on it are distributed in stagger arrangement andthe punched ratio increases gradually from the axial lineto the boundary.

4. Conclusions

Flow characteristics of flow field in the header of plate-

fin heat exchanger have been investigated by means of par-ticle image velocimetry (PIV), which breaks through theshortages of conventional measurement of single point,high workload and so on. The velocity fields were mea-sured using the two-frame cross-correlation technique,which is carried out swiftly, reliably and correctly. A seriesof velocity vector and streamline graphs of different cross-sections are achieved in the experiment. The experimentalresults indicate that performance of fluid maldistributionis very serious in conventional header used in industrydue to poor header configuration, while the improvedheader configuration with punched baffle can effectivelyimprove the performance of fluid flow distribution in theheader both in the axial direction (z-direction) and in they-direction. When the baffle is proper in length, the holesare distributed in staggered arrangement, and the punchedratio gradually increases from central axis along with thebaffle length, the flow maldistribution parameter S inplate-fin heat exchanger has been reduced from 1.21 to0.209, and the velocity ratio h is reduced from 23.2 to1.76 by installing the baffle. The fluid flow distribution inplate-fin heat exchanger is more uniform by the optimumdesign of the header configuration. The number of vortexdecreases compared to that of conventional header config-uration and then the energy loss caused by turbulent dissi-

pation reduces, which can compensates for some of the

-400 -300 -200 -100 0 100 200 300 400

40

50

60

70

Actual curveExpected curve

Punchedratio(%)

Z position (mm)

Fig. 21. Punched ratio versus z position along with the baffle length forconfiguration F.



12/12

pressure loss caused by the addition of the punched baffle.The results validate that PIV is well suitable to investigatecomplex flow pattern. The baffle is lower in cost and con-venient in assembly, while the effect of the fluid flow distri-bution uniformly by the improved configuration isobvious. The conclusion of this paper is of great signifi-

cance in the improvement of plate-fin heat exchanger.

Acknowledgements

The paper is supported by the Doctor Foundation ofXian Jiaotong University (DFXJTU2004-02). The authorswould like to thank the Ministry of Education, PR Chinaand Xian Jiaotong University for their financial support.

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PIV Experimental Investigation of Entranc[1]