NH3 condensation in a plate heat exchanger

Delft University of Technology

NH3 condensation in a plate heat exchanger

Experimental investigation on flow patterns, heat transfer and frictional pressure drop

Tao, Xuan; Dahlgren, Elias; Leichsenring, Maaike; Infante Ferreira, Carlos A.

DOI

10.1016/j.ijheatmasstransfer.2020.119374

Publication date

2020

Document Version

Final published version

Published in

International Journal of Heat and Mass Transfer

Citation (APA)

Tao, X., Dahlgren, E., Leichsenring, M., & Infante Ferreira, C. A. (2020). NH3 condensation in a plate heat

exchanger: Experimental investigation on flow patterns, heat transfer and frictional pressure drop.

International Journal of Heat and Mass Transfer, 151, [119374].

https://doi.org/10.1016/j.ijheatmasstransfer.2020.119374

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drop

Xuan

Tao

∗

,

Elias

Dahlgren,

Maaike

Leichsenring,

Carlos A.

Infante

Ferreira

Process and Energy Laboratory, Delft University of Technology, Leeghwaterstraat 39, 2628 CB, Delft, the Netherlands

a

r

t

i

c

l

e

i

n

f

o

Article history: Received 16 November 2019 Revised 3 January 2020 Accepted 12 January 2020 Keywords:

Condensation heat transfer Two-phase frictional pressure drop Flow patterns

Experimental investigation Plate heat exchanger NH 3

a

b

s

t

r

a

c

t

ThispaperinvestigatesNH3condensationinaplateheatexchangerbyvisualizingtheflowpatternsand

measuring heattransfercoefficients and frictionalpressure drop.Visualization experiments were con-ductedbetween20and100kgm−2_s−1_{.Fullfilmflowtakesplaceatlargemassfluxesandintermediate}

massfluxesoflowvaporqualities,whilepartialfilmflowoccursatsmallmassfluxesandintermediate massfluxesathighvaporqualities.Theheattransferandfrictionalpressuredropexperimentscoverthe massfluxesof21~78kgm−2_s−1_{,theaveragedvaporqualitiesof0.05~0.65andthesaturatedpressureof}

630to930kPa.Vaporqualitieshavesignificantinfluencesonheattransferandfrictionalpressuredrop. Inthetestedranges,theeffectofmassfluxesisnoticeableonfrictionalpressuredrop,butismoderate onheattransfer.Theimpactofsaturatedpressureissmall.Theheattransferreflectsthechangeofflow patterns.Thefrictionalpressuredropshowsthecharacteristicsofseparatedflow.

© 2020 The Authors. Published by Elsevier Ltd. ThisisanopenaccessarticleundertheCCBY-NC-NDlicense. (http://creativecommons.org/licenses/by-nc-nd/4.0/)

1. Introduction

Inrecentdecades,theutilizationoflowgradeheatattracts in-creasinginterestgivenitsenormousamountandeasy availability. The main resources include solarenergy, geothermal energy, the coldenergyofLNG,wasteheatofindustryandseawater(the tem-peraturedifferencebetweenwarmsurfaceseawaterandcolddeep seawater).Theconversionoflowgradeheattoshaftpowercanbe reachedbyOrganicRankineCyclesorKalinaCycles,andisusually implementedinlargeplantstoimprovethefinancialreturn [1,2,3]. Limitedbythesmalldrivingtemperaturedifference,largeamount ofheatneedstobetransferred,whichrequireshighlyefficientheat exchangers andlargecharge ofworkingfluid.The heatexchanger effectivenessdominatestheoverallcycleefficiency.

NH3 has favorable transport properties and large latent heat.

Plateheatexchangers(PHEs)canhandlelargeflowratesandhave largeheattransferareasperunitvolume.ThecombinationofPHEs andNH3 ispromising forlarge and intermediate heatloads. The

compact structures reduce the charge ofworking fluidand other

∗ _{Corresponding author.}

E-mail addresses: x.tao@tudelft.nl (X. Tao), c.a.infanteferreira@tudelft.nl (C.A. In- fante Ferreira).

equipmentinvestments.PHEsarewidelyusedgiventheirsuperior heattransferperformance [4,5,6,7].

PHEsmostlyhavehydraulicdiametersof2~5mm [8],whichare closetothe criticaldiameters ofmicroorminichannels [9]. The transitionfrommacro-channels to micro-channelshappenswhen surfacetensiondominatesover gravity.It isdetermined by chan-nelsizesandfluidproperties.ThelargesurfacetensionofNH3

en-hancesthe micro-channelcharacteristics duringthe condensation inPHEs [10].Intheconfinedchannels,therelativemagnitudesof surfacetension,gravity andshearforce differfromthoseinlarge smooth tubes. The two-phase interface and wetting characteris-tics are consequently different.Heat transfer and frictional pres-suredropare functionsof theoccurring flowpatterns. NH3

con-densationinPHEsneeds tobeinvestigatedforwhatconcerns the mechanismofenergyandmomentumtransport.

The flow patterns in PHEs have been experimentally inves-tigated by several researchers. These studies are summarized in Table 1. This paper is focused on two-phase vertical downward flow, whichis thecommon flowdirectionof condensers and ab-sorbers. Taoetal. [11]presented a detailedoverviewofprevious studies up to2018. More recently,Buscher [12] carriedout visu-alizationexperiments on air-waterflowingin a PHE horizontally, vertically upward and downward.The flow directiontakes effect onlyatsmallmassflowrates.Duringverticaldownwardflow, big

https://doi.org/10.1016/j.ijheatmasstransfer.2020.119374

Nomenclature

Symbols

A actualheattransferarea[m2_]

cp specificheatcapacity[Jkg−1K−1]

dh hydraulicdiameter[mm]

dp platethickness[mm]

g gravitationalconstant[ms−2]

G massflux[kgm−2s−1]

h enthalpy[Jkg−1]

Lp port-to-portplatelength[mm]

˙ma massflowrate[kgs−1]

Nu Nusseltnumber[-]

P pressure[kPa]

Pr Prandtlnumber[-] ˙

Q heatflowrate[W]

Re Reynoldsnumber[-]

T temperature[◦C]

U overallheattransfercoefficient[Wm−2K−1]

v specificvolume[m3_kg−1_]

x vaporquality[-]

Greeksymbols

α

heattransfercoefficient[Wm−2K−1]

β

chevronangletoflowdirection[°]



difference

ρ

density[kgm−3]

λ

thermalconductivity[Wm−1K−1]

Subscripts

a ammonia

after aftercooler

av averaged

de deceleration

ele elevation

exp measuredvalue

fri frictionalpressuredrop

G gasorvapor

in inletoftestsection

L liquid

mix mixingprocess

out outletoftestsection

port portsoftestsection

sat atsaturatedconditions

test testsection

tube connectedtube

w water

wall platewall

bubbles stagnate behind the contactpoints when the mass flow ratesofliquidandvaporareverylow.Injectionnozzleswere spe-ciallydesignedtopromote uniformtwo-phase distributionacross thechannel width.The homogeneousvoid fractionmodelis cho-sentopredictthetransitionamongbubblyflow,intermittentflow andfull filmflow. The bubblebreakagedetermines thetransition amongroundish bubbles,irregularblob-likebubblesandstagnant gas phase. The bubble breakage is modelled according to turbu-lent kinetic energy. Slug flow and churn flow are not observed. Buscher’s [12]flowpatternmodelexcludesphasechangeandmass transfer.JiangandBai [13]visualized air-waterflow ina capsule-type PHE, and found that the main flow patterns are film flow, plugflowandchurn flow.Theyproposed transitioncriteriaofthe flowpatterns.Filmflowchangesintoplugorchurnflowwhenthe wavesgrowlargeenough toblock thegas core.It happenswhen theliquidflow rateincreasesandoccupieslargerflow area.

Con-Ta b le 1 Flo w patt ern s tudies fo r tw o-phase ve rt ic a l do wn w a rd flo w in PHEs. St u d y Plat e type Che v ro n angle Wo rk in g fluid Flo w patt erns Tr ib b e & Müller -S teinhag e n [32] Cor rug at e d 30/30, 30/60, 60/60 Air /W at er Re g u la r bubbl y , irr e gular bubbl y , tr ansition b e tw een bubbl y and ch u rn , ch u rn , film, partial film Vlasogiannis et al. [33] Cor rug at e d 60/60 Air /W at er Bubbl y , tr ansition b e tw een bubbl y and ri vule t, slug, ri vule t Wink elman [34] Cor rug at e d 25.7/25.7 Air /W at er Churn, wav y , film, partial film Nilpueng and Wo n g w is e s [31] Cor rug at e d 35/80 Air /W at er Slug, annular liq u id b ri d ge , annular liq u id bridg e/air alone Gr ab ens tein and Kab e lac [29] Cor rug at e d 63/63/63/63, 27/27/27/27 Air /W at er , R365mfc Bubbl y , slug, film Gr ab ens tein et al. [30] Cor rug at e d 63/63/63/63, 27/27/27/27 Air /W at er , R365mfc Bubbl y , irr e gular bubbl y , slug, film Busc her [12] Cor rug at e d 75/75 Air /W at er R o undish bubble, irr e gular blob-lik e bubble, Ta y lo r-li k e bubble, he te ro g e neous, partial film, full film, st agnant ga s phase Jiang and Bai [13] Capsule-type 23.5/66.5 a Air /W at er Fi lm , plug, ch u rn a These ar e the angles b e tw een capsules and the ve rt ic a l dir ection.

Fig. 1. Diagram of the experimental setup including the working fluid cycle, cold water cycle and hot water cycle.

sequently,thegaspressuredecreases. Thepressureatthebottom of thewaves islarger than atthe top, enlargingthe waves.Plug flow changesintochurn flowwhen thebreakingforces dominate over coalescenceforces,which aredetermined byturbulence and surfacetension,respectively.

In general, the visualizationexperiments ofPHEs mostlyused air-water and were without phase change. The flow patterns of condensing NH3 are different because of the distinct two-phase

densityratio andsurfacetension incomparisonto air-water. Ad-ditionally,thevaporqualitiesdecreasefromtheinlettotheoutlet duringdiabaticflow,graduallychangingtheflowpatterns.

DespitethepromisingperformanceofPHEsinNH3systems,the

researchoncondensationislimited.ThecondensationofHFCs, hy-drocarbonsandHFOsinPHEshasbeenwidelyinvestigatedby ex-periments, but reports on NH3 data are scarce [8]. NH3 is

char-acterizedbylargethermalconductivity,surfacetensionandlatent heat.Theheattransferandfrictionalpressuredropcorrelations de-rived from HFCsor hydrocarbons are hardly applicable for NH3.

The deviations depend on the channel sizes. These correlations over-predict the experimental heat transfer coefficients (HTCs)of alargediametertube(8mm),butunder-predicttheHTCsofsmall diametertubes(0.98,1.44and2.16mm) [14,15,16].

ThispaperinvestigatesNH3condensationinPHEsandpresents

the experimental dataof two-phase flow resulting fromthe dis-tinct fluidpropertiesofNH3. Sections2and 3describethe

exper-imental setupandmethods. Section4discusses theflowpatterns includingfullfilmflowandpartialfilmflow.In Section5,the ex-perimentalresultsofheattransferandfrictionalpressuredropare analysed based on flow patterns. The flow patterns give insight into the characteristics ofseparated flow andreveal thephysical processesduringNH3 condensationinPHEs,andinthiswayform

the basis to develop mechanistic models of HTCs and frictional pressuredrop.

2. Experimentalsetup

Theexperimentalsetupwasoriginallybuiltasanoceanthermal energyconversionsystem,andcanoperateasOrganicRankine Cy-cleorKalinaCyclewithNH3andNH3/H2O,respectively.Thesetup

iscomposedofaworkingfluidcycle,ahotwatercycleandacold watercycle.

2.1. Workingfluidandwatercycles

Fig. 1 showsthe process flow diagram. NH3 is heated by hot

waterin an evaporator,where itis partiallyevaporated. The two phasesflowintoaseparator,afterwhichthevapor flowsthrough

anexpansionvalve withthepressurebeingreduced.Theliquidis subcooledina recuperatorandflows throughanexpansionvalve. Thetwophasesarecontrolledtobeatthesamepressureandmix upbeforeenteringthetestsection.Thefluidispartiallycondensed inthetestsectionby thecoldwater,andthen issubcooledinan aftercooler.Afterwards,itflowsintoabuffertank.Theliquidlevel in the buffer tank decreases for large mass flow rate, providing theupperlimitofthemassflowrate.Avariablespeeddiaphragm pump controls the flow rate of the NH3. Diaphragm pumps are

suitable for corrosive fluids like NH3, and are able to cope with

cavitation.Inordertoreduce thefluctuationofflow rateand op-eratingpressure,a damperfilled withnitrogenisinstalled atthe outletoftheworkingfluidpump.Themassflowratebecomes un-stablefor smallvalues, limitingthe lowest mass flow rate. Next, NH3 flowsthroughtherecuperatorintotheevaporator.

Threebrazedplateheatexchangersactastheevaporator, recu-peratorandaftercooler.Asightglassisinsertedintotheseparator to observe the liquidlevel. Two relieve valvesare installed after thepump andbefore theseparator. Thesevalves open when the absolutepressure is above 1600 kPa. The setup is wrapped with alayer ofinsulationmaterial of20mm, whilethetest section is insulatedwithtwolayers.

Thecoldwatercyclefeedsboththetestsectionandaftercooler inparallel.Thecoldwaterispumpedandseparatedtothesetwo heat exchangers. The waterpump isa centrifugal pump. The in-letsofthetestsectionandaftercoolerhavethesametemperature andpressure,and sharesensors. Theflow rates ofboth lines are measured to determinethe heat transfer rate. Wateris mixedin atank.Theflow rateandtemperatureattheheatexchangerinlet arecontrolledbythewaterpumpandarefrigerationcycle.During theexperiments, the waterside mass flow ratewasclose to the maximum attainable value by the pump to increase this HTC. A PIDtemperaturecontrollerisimplementedtoachieve thedesired watertemperature. Thecoldwatercyclehasacoolingcapacityof about6000Wat8◦C.

Thehotwatercycleiscomposedofatank,anelectricalheater, acentrifugalpumpandsensors.Theheattransferratetothe evap-oratorisdeterminedbythewaterflowrateandtemperatureglide. The flow rate and temperature at the evaporator inlet are con-trolledbythewaterpumpandheater.Theheaterhasamaximum capacityof6000W.

2.2.Measurementinstrumentation

Temperatureandpressurearemeasured atseveralpositionsof theset up. Forsimplicity, Fig.1 only showsthe sensors that are relevantforthediscussionofthispaper.Thetemperatureand

pres-Table 2

Specification of the sensors.

Sensors Type Sensor uncertainty Range Temperature PT-100 ±0.05 ◦_{C 2 /50} ◦_C NH 3 pressure Absolute ±0.05% FS 0/1000 kPa NH 3 pressure Gauge ±0.5% FS 0/1000 kPa Water pressure Gauge ±0.5% FS 0/250 kPa Differential pressure SITRANS P DS ±0.3% FS 1.6 /160 kPa NH 3 overall flow Coriolis ±0.2% RD 0 /0.02 kgs −1 NH 3 liquid flow Coriolis ±0.2% RD 0 /0.013 kgs −1 Water flow Turbine ±1% FS 30 /3000 Lh −1 ±7.2 Lh −1a

NH 3 density Anton Paar ±0.1 kgm −3 –

a These sensors have an uncertainty of ±1% FS in the full range. The measured value of F03 is kept almost constant at 432 Lh −1 _{during experiment. F03 is cali-} brated in this range with an uncertainty of ± 7.2 Lh −1 _.

sureof NH3 are measured at the following positions: the vapor

andliquidinlets ofthe testsection, aswell asthe outletsof the testsectionandaftercooler.TheNH3pressuredropwithinthetest

sectionisalsomeasured.Theoverallandliquidmassflowratesof theworkingfluidaremeasured attheinletoftheevaporatorand attheliquidline.Theratioofmassflowratesisusedtocheckthe vaporquality atthetest sectioninlet. Thetemperatureand pres-sureofthecoldwateraremeasuredattheinletandoutletofthe testsectionandaftercooler,aswell asitsvolumeflowrate. Den-sityismeasured by D01 atthesame location astheoverall NH3

mass flow rate. When the setup is operated with NH3/H2O, the

densityisusedtodeterminethebulkNH3concentration.

Table2liststhesensorfeatures.Themeasurementsystemwas originally described by Tao et al. [17], andwas further modified to improve the accuracy and to extend the measurement range. Temperatureismeasuredwithfour-wirePT-100ofhighprecision. Theuncertaintyis±0.05◦_{Cintheoperatingranges.NH3pressure}

is measured by two types of sensors. P03 and P08are absolute pressuresensors,andtheuncertaintyis±0.05%FS.TheotherNH3

pressuresensorsaregagepressuresensorsandhaveanuncertainty of±0.5%FS.Thewaterpressuresensorsarealsogagepressure sen-sorswith±0.5% FS uncertainty.Thepressure dropofNH3 within

the test section is measured by a differential pressure sensor of ± 0.3% FS uncertainty. The NH3 mass flow rate is measured by

Coriolistypemassflowmetersof±0.2%RDuncertainty.Thewater flowmetersoriginallyhaveanuncertaintyof_±1%FSand consider-ablycontributetotheuncertaintyofcondensationHTCs.Thewater flowratewaskeptatthemaximum valueavailable by thepump andwasalmost constant.F03 isspeciallycalibratedinthe tested rangeandhasanuncertaintyof±7.2Lh−1.Theuncertaintyofthe NH3 densitysensoris±0.1kgm−3.

Data are acquired and recorded withLabVIEW. It also imple-ments control and safety protection. A program is developed to analyzethemeasureddata.Inordertodeterminethesteadystate, thetimederivativesofall themeasureddata arecalculated.Both the slopes and amplitudes are assessed. Pressureand mass flow ratefluctuatemostlyaftertheworkingfluidpump,andare stabi-lizedby adjusting the damper.The steadystate iskept for more than10 min,andthe time averaged valuesare used fordata re-duction.

2.3. Testsection:gasketedplateheatexchangerandtransparentplate

Thetestsectionisagasketedplateheatexchanger(GPHE)with theplatenumberadjustable.Inorderto avoidtheflow maldistri-butionamongchannels,theGPHEconsistsofthreeplatesforming two channelsduring theexperiments. Onlyone plate iseffective forheat transfer. NH3 flows vertically downwardin one channel,

whilecold waterflows upward intheother channel. Thesethree platesarepressedbyframeplatesfrombothsides. Fig.3illustrates theplateoftheGPHE,and Table3givesthegeometrical parame-ters.Taoetal. [11]definetheseparameters.TheGPHEisassembled from Alloy 316 plates and EPDM gaskets to be compatible with NH3.

The plate has triangular inlet and outlet areas, where flow is distributed from the inlet port or into the outlet port. The plate surface is composed of sinusoidal corrugations, where the groovescan be considered asbranching sub-channels in parallel. Thegroovesononeplatehaveanoppositedirectionwiththoseon theadjacentplate.Fluidflowsalongthegrooves, whichissimilar toinclineddownwardflow.Meanwhile,thefluidalsopassesacross thegroovesatthecontactpointsofadjacentplates.

As shownin Fig.2,a transparentplateis fabricatedto visual-izethecondensingflow. Thisplatehasthesamegeometryasthe metalplatesexceptthattheplatethicknessis25mm.The corru-gatedsurfaceisreplicatedsothattheflowpatternsduring conden-sationare abletoreappear. Thevisualization testsection is com-posed of two metal plates and the transparent plate. The frame plateadjacenttothetransparentplatehasthreeobservation win-dows.Thevisualizationexperimentisdiabatic.Theflowdirections of NH3 andcold water are the same asduringthe heat transfer

experiments.

Thetransparentmaterialisrequiredtobemachinableand com-patiblewithNH3.Taoetal. [11]summarizedthetransparent

ma-terials used to manufacture PHEs applicable for air-water, which arenot compatiblewithNH3.Kim etal. [18]andNakamuraetal. [19]3Dprintedtransparentplatesofsmallsizes.Leeetal. [20] fab-ricatedatransparentplatewithpolycarbonate.daSilvaLimaetal. [21] and Arima et al. [22] used sight glass to fabricate smooth tubes andsmooth platesfor NH3 visualization, butsight glass is

not machinable.Inordertotest thecompatibility,thesamplesof severalpolymerswereimmersedintoliquidNH3.Polystyrene(PS)

has better compatibility compared with polymethylmethacrylaat (PMMA), polyvinylchloride (PVC),polycarbonate (PC) and tereph-thalateglycol(PETG).ButPSmaydecayafterlongtimeoperation. Leichsenring [23]andLeichsenringetal. [24]presentedthedetails ofmaterialselectionandstrengthanalysis.Thetransparentplateis manufacturedbyglueingalayerofPSandalayerofPMMA.ThePS platecontactsNH3,which hasathicknessof5mmandwas

ma-chined. The PMMA plate improves the mechanical strength. It is 20mmthick andispressed bytheframe plate.The visualization experiment lasted for 2 weeks.After then, the transparent plate startedtodecaysotheexperimenthadtobestopped.

Flow patterns were captured with a high speed camera from the front, with a frame rate of 3000 fps and a resolution of 1024×1024 pixels. The transparent plate was wrapped with an LED-stripon thesides circumferentially.Thechannel is well illu-minatedasshownin Fig.2(Right).

Table 3

Geometrical parameters of the test section. Plate number Port-to-port plate length In gasket plate width Heat transfer area Hydraulic diameter Chevron angle Enlargement factor Channel gap Plate thickness Corrugation wavelength – mm mm m 2 _{mm – mm mm mm} 3 668 95 0.640 2.99 63 ° 1.15 1.72 0.58 6.67

Fig. 2. Pictures of the experimental setup and visualization section.

Fig. 3. Pictures of the stainless steel plate and transparent plate.

3. Datareduction

In Eqs.(1)and (2),the inletandoutletNH3 enthalpies ofthe

testsection, ha,inandha,out,aredetermined fromtheenergy

bal-anceoftheaftercoolerandtestsection.Theinletandoutletvapor qualitiesaredeterminedreferringtoRefprop9.1 [25].Theaveraged vapor quality is used to analyze the flow patterns, heat transfer andfrictionalpressuredropdata.Theinletandoutletvapor qual-ities are also used to illustrate the flow patterns. The condensa-tion HTC,

α

a, is calculated using Eq.(3). The condensation

tem-perature,Ta,changes slightlyfromthe inlettotheoutletbecause

ofthefrictionalpressuredrop.ThewaterHTC,

α

w,ispresentedin Eq.(4)[26].It hasbeenderivedfromH2O–H2Oexperimentsusing

Wilson-plotmethod. ha,out=ha,04+ ˙ Qa f ter ˙ m_a,01 = h_a,04+ cp,wm˙w,04

(

Tw,07− Tw,05

)

˙ m_a,01 (1) h_a,in=ha,out+ ˙ Qtest ˙ ma,01 = ha,out+cp,wm˙w,03_m

(

_˙Tw,06− Tw,05

)

a,01 (2) ˙ Qtest=m˙w,03cp,w

(

Tw,06− Tw,05

)

=m˙a,01



h_a,in− ha,out



=UA

(

Ta,in− Tw,06

)

−

(

Ta,out− Tw,05

)

ln Ta,in−Tw,06 Ta,out−Tw,05 = A 1 αa+ dp λwall +α1w

(

Ta,in− Tw,06

)

−

(

Ta,out− Tw,05

)

ln Ta,in−Tw,06 Ta,out−Tw,05 (3) Nuw=

α

w dh

λ

w = 0.275Re0.7 w Pr1w/3 320≤ Rew≤ 2600 (4)

Eq.(5)calculates thefrictional pressuredrop ofcondensation,



Pfri,a, by subtracting the other components from the measured

pressuredrop,



Pexp,a.In Eqs. (6)and(7), thepressure dropsof

inletandoutletports,



Pinport,aand



Poutport,a,aretreatedas

sud-denenlargementandsudden contraction oftwo-phase flow [27]. In Eqs. (8) and (9), the deceleration pressure rise,



P_de,a, and elevationpressure rise,



Pele,a,use the homogeneous two-phase

model [27].Themixingpressuredrop,



Pmix,a,regardsthe

spray-ingofliquidintovaporthroughorifices,andiscalculated accord-ingtoJankowskietal. [28].Thedetailsofthecalculationare spec-ifiedinTaoetal. [26].Thefrictionalpressuredropofcondensation accountsformorethan90%ofthemeasuredpressuredrop.



Pf ri,a=



Pexp,a−



Pinport,a−



Pout port,a−



Pmix,a

+



Pde,a+



Pele,a (5)



Pinport,a=G2tube

v

L Atube Aport



1−Atube Aport



1+

v

G−

v

L

v

L x



(6)



Poutport,a= G2 tube

v

L 2



₁ 0.591− 1



2



1+

v

G−

v

L

v

L x



(7)



P_de,a=G2

₍

_v

G−

v

L

)

(

xin− xout

)

(8)



P_ele,a=

ρ

avgLp (9)

4. Flowvisualizationresults

Visualizationexperiments ofNH3 were conductedatthe

con-densationpressureof690kPa.Theflowpatternschangedwith va-porqualities and mass fluxes. Only film flow has beendetected. Air-waterexperimentsobservedbubblyflow,slugflow,churnflow, film flow or the equivalent flow patterns [12,29,30,31,32,33,34]. Bubblyflow isknownto be limitedatlargemass fluxesandlow vapor qualities. The kinetic energy is strongenough to breakup thebubbles, andthebubble breakupovercomes coalescence.The condensingNH3inthepresentpaperwasbelow100kgm−2s−1.By

contrast,bubblyflow ofair andwaterhappensabove theoverall massfluxofapproximately100kgm−2s−1[12,29,30,32,33].Buscher [12]didnot observe slugflowor churnflow, which is character-ized by the phenomenon that thewhole channel widthis occu-pied by gas or liquid periodically. In tubes, bubbles move faster

Fig. 4. Full film flow with G = 81 kgm −2 s −1 and x _{av = 0.30 at the top, middle and} bottom windows. The corresponding videos are included in the Supplementary ma- terials of the online version.

withgrowingsizes,andcoalescemoreintensively.However,inthe corrugatedchannels,thecentrifugalforcesweakenbubble acceler-ationandsuppresstwo-phase separation. Theslugor churnflow inother researchesismainlyattributedtothegas-liquidinjection [12].

The test section could not be insulated fromthe front, which results in some heat leak. According to uncertainty analysis, the heatleakhasminorinfluences onvaporquality,butbrings about significanterrorsto HTCs.Thus thecondensationHTCscannot be derived.Theoutletvaporqualitiesaredeterminedfromtheenergy balanceoftheaftercooler,andarenotinfluencedbytheheatleak. The inletvapor qualities are based on the energybalance of the testsection,andareaffected bytheheat leak.The effectissmall sinceNH3haslargelatentheat.Thishasbeencheckedmakinguse

oftheoverallandliquidflowmeters.

4.1.Flowpatterns

The termsfilmflowandpartialfilmflow areusedinprevious papers.Inordertoclarifythewettingcharacteristics,inthispaper,

fullfilmflowdescribestheflowpatternforwhichthewallis com-pletelywetted, whilepartialfilmflowindicatespartofthewall is dry.Filmflowisthesumoffullfilmflowandpartialfilmflow.

Fig. 4showsthe pictures offull film flow, whichis similarto annularflowintubes.Thevideosoffullfilmflowareprovidedin theSupplementary materials of the online version of this paper. Fig.5 illustrates the distribution of liquidand vapor. Continuous liquidfilmflowsonthewall,whilevaporcoreflowsinthecenter ofthegroove.Becauseofthecorrugatedgroove,thetwo-phase in-terfaceisroughevenatlow massfluxes. Wavesandripplesmove attheinterface.Thewavesaredispersedwithvaryingwavelengths andamplitudes,andgrowlargerwithincreasingmassfluxes. Var-iousflow phenomenaare observed, andare shownin Fig.5. For superposedwaves,severalwavesinteractwitheachother,making the flow more chaotic. Continuous waves have connected waves that flow at the same speed. These waves extend over adjacent grooves. Periodicwavesflow in the samepaths and havealmost thesameshapes.Therestwavesarediscreteandareindependent oftheotherwaves.

Thewavesgraduallydecreaseinamplitudefromtheinlettothe outlet.Intheinletregion,theliquidandvaporaredistributedinto adjacentgrooves,andpassacrossgroovesintensively.Additionally, thevaporqualitydecreasesalongtheflowdirection,whichreduces thekinetic energygradually. The vapor phase haslarger velocity than theliquid phase, andtears liquid droplets off thefilm. The channelofthePHEissonarrowthatdropletsareeasilydeposited

Fig. 5. Schematic of full film flow. Indicating different kinds of waves.

Fig. 6. Partial film flow with G = 43 kgm −2 _s −1 _{and x av = 0.46 at the top, middle} and bottom windows. The corresponding videos are included in the Supplementary materials of the online version.

on the other side of the wall. Consequently, no stable entrained liquiddropletsareformedinthevaporcore.

Fig.6 showspartial filmflow. It occurswhen the liquidmass fluxisreduced.Lessliquidisattributedtohighervaporqualityor smallermassflux.Theliquidisnotenoughtowettheentirewall, andpartsof thewall becomedry. NH3 haslarge surfacetension,

whose influence becomesprominentin small diameterchannels. Largecontactangles reduce thewetting ability,break upthe liq-uid film andchange the flow pattern [35]. Large surfacetension makestheliquidfilmdiscontinuous [36].Additionally,surface ten-siontends tostabilizethe flowandsuppressthewaves [37].The videosofpartialfilmflowareprovidedintheSupplementary ma-terialsoftheonlineversion.

Asshownin Fig.7,two typesofdryzonescoexist.Inthefirst type, whole pieces of zones become dry, and the areas ofthese zonesincreasewithdecreasingliquidmassfluxes.Intheinlet re-gion next to the port, liquid and vapor are distributed into the parallel grooves. When the areas of these zones are small, dry zonesare locatedclose toone edge ofthe plate,andhave trian-gular shapes.When theseareasincrease,thedryzonesgrow un-tilcovering mostpartof theplate. In thesecond type, dryspots arediscretelydistributedinthecorrugatedgrooves.Undergravity force, these dry zonestend to appear at the top of the grooves.

Fig. 7. Schematic of partial film flow. Indicating different types of dry zones.

Thisflowissimilar tostratifiedflowininclineddownwardtubes. Thepropertiesofthewall surfaceaffectthefluidmotion,andthe flow direction of the fluid changes when the surface is inhomo-geneous [38].The temperaturedistributionofthecorrugated sur-faceisslightlyinhomogeneousbecausethecoldwatercrossesthe contact points andforms vortices in the grooves within the ad-jacentchannel [39].The variationofwall temperaturemakes the liquiddistributeunevenly.Inbothtypes,vaporcondensesdirectly atthewall. Thesezonesseemtobe coveredwithathin conden-satelayer, whichmakesitdifferentfromadiabaticflow. Thelayer issothinthattheobservationcanbesubjectiveandneedsfurther discussion.Vaporcondensatesonthewallaslongasthewall tem-perature is below the saturated temperature [40,41]. During the presentexperiments,theinletoftheworkingfluid isinthe two-phase regionandisnotsuperheated. Theaveraged wall tempera-ture islower than the saturated temperatureformore than 1 K. Theoretically, condensationtakes place oncethe vapor is in con-tactwiththe wall.Moreover, theoccurrenceof condensationcan be proved fromthe opposite perspective. The dryzones become ineffectiveifcondensationdoesnottakeplace,andtheheat trans-ferdeterioratesbecausetheeffectiveareaisreduced.However,the heattransferisenhancedforpartialfilmflow.Thiswillbefurther discussedin Section5.1.

The waves forpartial film flow are weaker than for full film flow because of the smaller liquid mass flux. The liquid film of partial film flow isrelatively thin. Thin film tends tobe laminar, and the structure oftwo-phase interface is smoother.Deposition ofentrainedliquiddropletshasbeenobservedatthedryzones.

4.2. Flowpathsandflowdistribution

Flow paths wereoriginally usedtodescribe theflow direction of single phase flow [[42,39]. During two-phase flow, the liquid phase is traced by the movement of waves. Fig. 8 schematically showstheflowpathsoftheliquidphase.Crossingflowgoesfrom one side of the plate to the other side, andreflects at the edge oftheplate,changingdirection.Wavesaremoreintensivecloseto theedge.Wavylongitudinalflowcirclesthecontactpointsoftwo adjacentplates.Theobservedflowisacombinationofbothpaths, andcrossingflowisdominant.Theliquidflowsalonggrooves,and gradually goesacrossthegrooves. Thedirectionshiftsdownward. The combinedflow paths makethe flowturbulent atsmallmass

Fig. 8. Schematic of liquid flow paths. The observed flow path is a combination of crossing flow and wavy longitudinal flow.

fluxes,soPHEshavelargerHTCsandfrictionalpressuredropthan tubes.

Buscher [12]andTribbeandMüller-Steinhagen [32] tracedthe flowpaths ofbubbles duringbubblyflow. Crossingflow is domi-nantwithsmallchevronangles(30°),whilewavylongitudinalflow prevailswithlargechevron angles(60° and75°).Thechevron an-gleoftheplatesusedinthisstudyis63°,andcrossingflow domi-natesduringfilmflow.Itisexpectedtobewavylongitudinalflow ifbubblyflow occurs. The differencebetween homogeneousflow andseparatedflow plays arole.Bubbly flowisconsidered as ho-mogeneousflow, andis similarto singlephase flow.Film flow is separatedflow,andshowsdistinctcharacteristics.

Themaldistributionofliquidandvaporismoreseverecloseto the inlet port. The liquid phase has larger momentum than va-por, andthe flow splits among the grooves. Liquid preferentially flows into the front groovesdirectly connected to the inlet port, whilevapor isdistributedinto allthe grooves. Theflow distribu-tionbecomesuniforminthedownwarddirection.Theflowreflects andrecirculatesattheedge,whichimprovesthetwo-phase distri-bution.Inthedownwarddirection, thewave amplitudesdecrease andthe two-phase interface becomessmoother. Similar distribu-tionwasobservedduringthevisualizationofair-waterflow with-outphasechange [34].

4.3.Flowpatternmap

Fig.9presentstheflowpatternmapplottedwiththemassflux andvapor quality,whichindicates the condensation process.The map includestheflow patternsatthe inlet,middleandoutletof thetestsection,whichwereobservedfromthethreewindows.The data points are assigned to the inlet, averaged and outletvapor qualities,respectively. These three flow patterns foreach operat-ingconditionareconnectedbylines.Thecondensationisfromthe rightendtotheleftendofthelines.Thechangeofvaporqualities issmall.Atlarge andsmallmassfluxes,theflow patternsremain thesamefromthe inlettooutlet,whilethe flowpatternschange intheflowdirectionforintermediatemassfluxes.

Full film flow occurs atlarge mass fluxes orthe combination ofintermediate massfluxesandlow vaporqualities.On the con-trary,partialfilmflow isatsmallmass fluxesorthecombination ofintermediatemassfluxesandhighvaporqualities.Flowpatterns dependon theliquidmassflux.A certainamountof liquidis re-quiredtocompletelywetthewallsurfaces,otherwisethethin liq-uidfilm breaks up. The minimumwetting mass flux dependson

Fig. 9. Flow pattern map of condensing flow in the PHE, with the vapor quality decreasing from the inlet to the outlet. The flow patterns at the inlet, middle and outlet windows of the same operating condition are connected by lines.

thesurfacetension,liquiddensityandliquidviscosity. The liquid velocitydistributionisnotuniformespeciallyforlargecontact an-gles [43,44]. At intermediate mass fluxes of low vapor qualities, partialfilm flowchanges intofull film flowfromthe inletto the outlet.Thesedatapointsareconsideredtobeinthetransition re-gion. The vapor quality decreases in the flow direction with in-creasingliquidmassfluxes.Moreover,theflowdistributionamong groovesbecomesmoreuniformafterreflectionattheplateedges. This phenomenon illustrates the influence of the inlet distribut-ingregiononflowpatterns.Partialfilmflowoccursatsmallmass fluxes. Inthe top window, dryzonesare continuous andstretch. Inthemiddleandbottomwindows,liquidtends tobe distributed amonggroovesuniformly.Dryspotsarediscreteineachgroove.

5. Experimentaldataanddiscussion

CondensationexperimentsofNH3arecarriedout.The

sensitiv-ityof HTCs and frictional pressure dropto vapor qualities, mass fluxesandsaturatedpressuresisinvestigated.Becauseofthelarge latentheat,thechangeofthevaporqualitiesissmall,andthe fol-lowingdiscussionisrepresentedwiththeaveragedvaporqualities. ThemeasuredHTCsandfrictionalpressuredropareconsideredto bequasi-local.Duringtheexperiments,somedatapointswere re-peatedby increasing anddecreasing theheatingcapacity, respec-tively.Therepeatabilityisprovedtobereliable.

5.1.Heattransfercoefficient

The condensationHTCs are large,and the corresponding heat transferresistances are relatively smallcompared withthe water side. In order to improve the measurement accuracy, the water sidemassfluxeswerekeptlargetoreducethecorrespondingheat transferresistance.Inthetestedranges, theratioofheattransfer resistancebetweenthecondensationsidetowatersidehasan av-eragedvalueof1.6.Additionally,theaccuratemeasurementofthe heatdutyrequireslargeriseofthecoldwatertemperature,whose averaged value is 2.8 K during testing. HTCs larger than 30,000 Wm−2K−1areexcludedbecauseoftheirlargeuncertainty.

Fig.10illustratestheHTCsvariationwithaveragedvapor qual-itiesandmassfluxes. Thesaturatedpressureis690kPa.Itcovers theaveragedvaporqualitiesof0.05~0.65andmassfluxesof21~78 kgm−2s−1.Limitedbythecoolingcapacityofthesetup,higher

va-Fig. 10. Condensation HTCs with varying averaged vapor quality at different mass fluxes.

porqualitycannot beobtained. HTCsincreasesharplyandalmost linearlywithvaporqualities.For42kgm−2s−1,theHTCs increase 2.5timeswhentheaveragedvaporqualitiesrisefrom0.2to0.65. A slight rise of the vapor quality increases the volume flux no-ticeably.Thetransitionofflowpatternshasbeenobservedduring thevisualizationexperiments.Withincreasingvaporqualities,the vapor velocity increasesand the film thickness decreases. Larger shearforcedisturbsthetwo-phaseinterfaceandenhancestheheat transfer.Moreover,athighvaporqualities,theliquidfilmislocally extremely thin.The vaporcondensesdirectlyon thewallandthe heattransferresistanceissmall.SinceNH3hasalarge two-phase

densityratio, thesensitivity tovapor quality is stronger thanfor otherrefrigerantssuchasR134aandR410A [45].

The influenceof mass fluxesisnot monotonic. HTCs decrease slightly with mass fluxes in the range of 21~42 kgm−2s−1, and increase moderatelyfrom 42 to 78 kgm−2s−1. It is attributed to the change of flow patterns. At smallmass fluxes, a part of the wall surfaceisdryduetosmallamounts ofliquid.Thevapor ve-locityislow,andonly limitedshearforce isexerted onthe two-phaseinterface. Thus theliquidfilmislaminar. Thecondensation process isdivided into two parts.Some vapor condenses directly on the wall, while other condenses on the two-phase interface where the wall is wetted. Gravity-controlled condensation dom-inates over convective condensation. For a certain vapor quality, theportion ofwettedsurfaceincreaseswithmassfluxesbecause ofmore liquid.Consequently, theheat transfer resistanceslightly increases.Atlargemassfluxes,thewallsurfaceiscompletely wet-ted.Shearforceisdominant,andconvectivecondensationapplies. Foracertain vaporquality,themassfluxesofbothvaporand liq-uidincrease.Highervaporvelocitiesintensifyshearforceand pro-moteripplesonthetwo-phaseinterface.Ontheonehand,the liq-uid film becomes turbulent with increasing mass fluxes. But on theother hand,thickerliquid filmincreasesthe heat transfer re-sistance. As a consequence, HTCsincrease moderatelywith mass fluxesatlargevalues.In Fig.10,theaverageduncertaintyofHTCs is ±14.4%. The measurement uncertainty is contributed by tem-perature sensors andflow meters. The temperaturedifference is smallerforhigherHTCs,andtherelativeerrorbecomessignificant, whichcontributesprominentlytotheuncertaintypropagation.

In Fig.11,theinfluenceofsaturatedpressure isshownattwo massfluxes.The trendsatthesemassfluxesaresimilar,andonly the values at 52kgm−2s−1 are discussed. HTCs decrease slightly

Fig. 11. Condensation HTCs with varying averaged vapor quality at different con- densation pressures.

Fig. 12. Condensation HTCs with varying liquid and vapor mass fluxes.

withincreasing saturatedpressure, andbecome about20% lower from690to 930kPa.Higher saturatedpressure hassmaller two-phasedensityratio,andtheshearforceattheinterfaceisreduced. The thermal conductivityof liquidalso decreases athigher pres-sure. Despitetheminoreffectofsaturatedpressure,theinfluence of vapor qualities is still significant. The averaged uncertaintyof HTCsis±14.1%accordingtotheuncertaintyanalysis.

Fig.12furtherpresentstheinfluenceofliquidandvapormass fluxes. The visualization experiments have indicated separated flow. Forthis reason the two-phase mass fluxes are investigated separately.

TheslopeofHTCsforacertain vapormassflux isdividedinto two parts. HTCs decrease noticeably withincreasing liquid mass fluxes atsmall values,and stay almost constant at larger values. Theliquidfilmisthemainheattransferresistanceduring conden-sation.Atsmallliquidmassfluxes,partialfilmflowapplies,anda partofthewall surfaceisnot wetted.Vaporcondensatesdirectly on thewall. The area ofthe dryzonesdecreases withincreasing amountofliquid,andtheHTCdecreasessignificantly.Atacertain

Fig. 13. Frictional pressure drop with varying averaged vapor quality at different mass fluxes.

value,the wall gets completelywetted. It becomesfull filmflow. Furtherincreaseofmassfluxthickens theliquidfilm.Meanwhile, theliquidflow becomesmore turbulent.Thesetwo effects coun-teracteach other.Consequently, HTCsremain constant. Theslope ofHTCschangesattheliquidmassfluxesofaround40kgm−2s−1. PartialfilmflowhaslargerHTCsthanfullfilmflow. Thisconfirms thatcondensationtakesplaceinthedryzonesandindicatesthat alltheheattransferareaiseffectiveduringpartialfilmflow.

Largervapormassfluxesenhancetheheattransfer.Higher va-por velocities intensify the two-phase shear and thin the liquid film.The enhancement islarger forpartial filmflow. Droplet en-trainmentispromotedbythehighervaporvelocitiesandtendsto increasetheheattransfer.

5.2.Frictionalpressuredrop

Fig. 13 shows the influence of averaged vapor qualities and massfluxesonfrictionalpressuredrop.Atcertainmassfluxes, fric-tionalpressuredropincreasesalmostlinearlywithvaporqualities. The increase is sharper atlarge mass fluxes. When theaveraged vapor qualities have an increment of 0.1, the frictional pressure dropincreasesabout10kPam−1 for71kgm−2s−1 andincreases2 kPam−1 for30kgm−2s−1. Theflow patternshaverelatively small influences on frictionalpressure drop comparedwith heat trans-fer [20]. Both full film flow and partial film flow are separated flows.Liquid andvapor flowseparately andinteractonthe inter-face. The two-phase pressure drop is contributedby the follow-ingshearforces: betweenthevapor andwall,betweentheliquid andwallandatthetwo-phase interface.Thevolumefluxand av-eraged velocity increase significantly withvapor quality,and the shearforcesaregreatlyenhanced.

Frictional pressure drop increases sharply with mass fluxes. Largerliquidmass flux thickens thefilm thicknessandpromotes turbulence.Vaporphasehaslargervelocitythanliquidandshaves the two-phase interface. Larger vapor mass flux intensifies the shearforcesanddissipates themomentum intensively.In Fig.13, theaveraged uncertaintyoffrictionalpressuredrop is_±15.0%.As shownin Table 2,the differentialpressure sensor hashigh accu-racy.Butthepressuredropfluctuates duetothe operationofthe workingfluidpump.Anextrauncertaintyof±10.0%isincluded.In ordertoensuretheaccuracy,frictionalpressuredropvalueslower than3.5kPam−1 areexcluded.

Fig. 14. Frictional pressure drop with varying averaged vapor quality at different condensation pressure.

Fig. 15. Frictional pressure drop with varying liquid and vapor mass fluxes.

Fig. 14showsthe sensitivitytosaturated pressures.The influ-enceisnegligibleatlowvaporqualities,andbecomesnoticeableat highvaporqualities.Itisattributedtothechangeofvapordensity. Higher saturatedpressure has insignificant effectson liquid den-sity,butincreasesvapor densityremarkably.At highvapor quali-ties,highersaturatedpressuredecreasestheaveragedvelocity be-causeof the larger vapor density, and consequently reduces the momentum dissipation. Frictional pressure drop has an averaged uncertaintyof±14.2%.

Fig. 15 shows the influence of liquid and vapor mass fluxes separately, and both phases contribute to the frictional pressure drop.At different vapor mass fluxes, the frictional pressure drop increaseswithliquidmassfluxes, andthe trendissimilar. When the vapor mass flux increases from 13.5 to 21.5 kgm−2s−1, the frictionalpressure dropmorethan doubles.In thesamerange of liquidmass flux, the frictional pressure dropincreases about 1.2 times. This is in agreement with the phenomenon of separated flow.The vaporflow dominatesthefrictional pressuredropsince the large velocity intensifies the momentum dissipation. During partialfilmflow,thetrendlineswiththevariationofliquidmass fluxesareslightlysteeperthanduringfullfilmflow. Thefrictional

pressuredropislessaffectedbythetransitionfromfull filmflow topartialfilm flow,whilethe transitionhassignificant influences onheattransfer.

6. Conclusions

This paper experimentally investigates NH3 condensation in

PHEs,andtheconclusionsaresummarizedbelow.

• In the testedranges, the flow patternsare full film flow and partialfilmflow.Thetransitionmainlydependsonthewetting characteristics.Full film flow occurs atlarge liquidmass flux, while partial film flow takes place at small liquid mass flux. Theinlet distribution alsoplays a role. The liquidis unevenly distributedamonggroovesclosetotheinlet,andbecomes uni-formalongtheflowdirection.

• HTCsincreasesignificantlywithvaporqualities.HTCsdecrease withmass fluxesfrom 21 to 42kgm−2s−1,and then increase withmass fluxesin 42~78kgm−2s−1. The transitionis atthe overallmassflux of42kgm−2s−1.HTCsdecrease slightlywith increased saturated pressures. Heat transfer is enhanced dur-ingpartialfilmflowsincevaporcondenses directlyonthedry zones.

• Frictionalpressure dropincreasessharplywithvapor qualities and mass fluxes. Larger two-phase mass fluxes enhance the shearforcesattheinterfaceandatthewall.Frictionalpressure dropslightlydecreaseswithincreasingsaturatedpressures.

TheHTCsandfrictionalpressuredropshow thecharacteristics ofseparatedflow. Theflow patternsresultfromthefluid proper-tiesofNH3 andformthebasis forthedevelopmentofpredicting

modelsforNH3condensationinPHEs.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

CRediTauthorshipcontributionstatement

XuanTao:Conceptualization,Methodology,Formalanalysis, In-vestigation,Datacuration,Writing-originaldraft.EliasDahlgren: Visualization, Data curation, Writing - review & editing. Maaike Leichsenring: Visualization, Data curation, Formal analysis, Writ-ing -review& editing.CarlosA. InfanteFerreira: Conceptualiza-tion,Resources, Project administration, Fundingacquisition, Writ-ing-review&editing,Supervision.

Acknowledgments

ThisprojecthasbeendevelopedincooperationwithAllseas En-gineeringB.V.Theauthorsacknowledgethefinancialsupportfrom the China Scholarship Council andfrom theKoude Groep Delft / Wageningen.Theauthorsare gratefultoEdwinOvermarsfor pro-vidingthehighspeedcamera.Thetransparentplatewas manufac-turedintheEWIDEMOofTUDelft.

Supplementarymaterials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ijheatmasstransfer. 2020.119374.

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