fuel Experimental study on the accretion and release of ice in aviation jet Aerospace Science and Technology

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Aerospace Science and Technology

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Experimental study on the accretion and release of ice in aviation jet fuel

Mathias Schmitz

^∗

, Gerhard Schmitz

HamburgUniversityofTechnology,InstituteofEngineeringThermodynamics,Denickestr.15,21073Hamburg,Germany

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received10May2018

Receivedinrevisedform25August2018 Accepted27August2018

Availableonline7September2018

Keywords:

Fuelsystem Aviationfuel Lowtemperatures Water

Iceaccretion Iceformation

Ice formations inaircraft fuelsystemspose aserious safety threat withpotentially disastrous conse- quences,whenrestrictingthefuelflowtowardstheengines.Thisisanongoingchallengeintheaerospace industry.Inthisworkexperimentalstudieshavebeenperformedtoinvestigatetheeffectsoftempera- ture,flowrateandsurfacepropertiesontheaccretionandreleaseoficeinflowingfuel.Atestrigwith aglass-windowedpipehasbeenemployedtoquantitativelymeasurethetransienticingprocessunder controlledconditions.Theaccretediceexhibitedsoftandfluffycharacteristicsandwas mostlikelythe result ofimpingingsolidiceparticlesthatwereentrainedinthefuelflow.Theiceparticlesweremost stickyinatemperaturerangebetween−^6◦Cand−^{20◦C.Thethicknessofaccretedicedecreasedwith} roughnessonaluminiumsurfacesandtherewasasignificantreductiononpolytetrafluoroethylene(PTFE) and polymethyl methacrylate(PMMA) incomparisonto aluminium,copper orstainlesssteel surfaces.

Comparisonof thethicknessofaccretedice withthe iceadhesion strengthreportedinthe literature showed aclearcorrelation.Theexperimental resultswillhelptogainbetterunderstanding oftheice accretionprocessinflowingfuelandmayserveasbasisfordesignguidelinestominimizeiceformation withinanaircraftfuelsystem.

©^{2018TheAuthors.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCC} BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Aviation fuels are known to contain small quantities of dis- solved water. The solubility decreases withtemperature and any excesswaterisprecipitatedout whenthefueliscooled[1–3].As jet aircraftareexposed to verylow temperatures,the watermay leadtoiceformationswithin thefuelsystemwithpossibledetri- mental effect upon the fuel flow. Numerous corrective measures were developed to prevent this, which include the use of anti- icingfueladditives,fuelheaters,improvedtemperaturemonitoring [4] andbypassesaroundmeshstrainers[5].Thesemeasureseffec- tivelyaddresstheproblem,butdonotnecessarilyaddresstheroot cause[5].Afterthe investigationoftheBoeing777crash landing atHeathrowairportinJanuary2008,theAirAccidentInvestigation Branch(AAIB)expressedconcernsaboutthelackofunderstanding ofthenatureandbehaviourofwaterinjetfuelandmaderecom- mendationsforfurtherresearch[6,5].

Murray et al. [7] investigated the freezing behaviour of mi- cronsizedwaterdropletsimmersedinJet A-1 aviation fuel.They found thatthe majorityofthesedroplets were ableto remain in

*

Correspondingauthor.

E-mailaddress:mathias.schmitz@tuhh.de(M. Schmitz).

ametastablesupercooledstatetillaround−^{36◦C.Lametal.made} similarobservationsforsomewaterdropletsthatprecipitatedout fromamodelfuel(toluene)below−^30◦C[8].

Laoetal.[9] studiedthetransitionfromdissolvedwatertothe depositionoficeonsubcooledsurfacesusingasimulatedfueltank.

Theyobservedafairlyuniformthinlayerofsphericaldroplets/par- ticles that formed on top ofan aluminiumblock. Thedeposition seemedtogrowdirectlyfromthedissolvedphase.Theycompared the growth characteristics with the initial frost formation period described in[10] and suggestedthat theice deposition was due to theBergeronprocess. Similarly,Lametal.[8] observedamass transfer frommetastable (spherical)iceparticlesto hexagonalice particlesandattributedthistotheaugmentedWegener–Bergeron–

FindeisenprocessandtheOstwaldripeningprocess.

Lametal.[11] describedtheicethataccretedonsubcooledsur- facesassoftandfluffy,muchakintofreshsnow.Theicepossessed averyhighporosityandlittleadhesionstrength.

After the incident at Heathrow in 2008, Boeing‘s Kent Fuels Laboratory andtheNorthBoeingFieldPropulsionLaboratorycon- ducted experiments on ice accretion within fuel lines using a mock-upoftheaircraft‘sfueldeliverysystem[6].Theirresultssug- gesteda“sticky”regionbetweentemperaturesrangingfrom−^5◦C and−^{20◦C,inwhichiceparticlestend toadheretosurfacesand} toeachother.In[12,4] suchstickybehaviourisdescribedbetween https://doi.org/10.1016/j.ast.2018.08.034

1270-9638/©^{2018TheAuthors.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense} (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Nomenclature Abbreviations

AAIB AirAccidentInvestigationBranch

AL aluminium

CU copper

PMMA polymethylmethacrylate PTFE polytetrafluoroethylene SST stainlesssteel

Latin symbols A areainm² d diameterinm

D_h hydraulicdiameterinm h heightinm

l lengthinm

Ra arithmeticaverageroughnessinm ta icethicknessinm

ta averageicethicknessinm U velocityinm/s

V˙ volumeflowinl/h

w widthinm

Greek symbols

γ

masscontentinppm_mass ϑ temperaturein^◦C

temperatureinK

a normalizedmaximumaverageicethickness

τ

timeins icereleaseratio Subscripts

a accretionofice

b bulk

C testbodycentrepiece CH chiller

D duct

H testbodyholder

r release

ref reference

s steadystate,solubility

t target(temperatureorvolumeflowrate),total(water content)

TS testsection

u undissolved(watercontent)

w water

# ^targettemperaturereached ^{startvolumeflowincrease endvolumeflowincrease}

♦ ^starttemperatureincrease

temperatures of −^{12◦C and} −^{18◦C. Murray et al.} [7] suggested thatthestickyiceparticlesareinfactsupercooledwaterdroplets thatfreezeheterogeneouslywhencomingintocontactwithasolid surface.

Theaccreted ice foundin the AAIBtestswas softand mobile [6]. Considerable amounts could be shed off the inner pipe sur- faces by increasing the flow rate. The iceshedding resulted in a highparticle concentration that was able to clog a fuel-oil heat exchangerandhencerestrictthe fuelflow. Thesuddenreleaseof accretediceisoftenreferredtoas“snowshower”[11,13].

In summary, the icing of aircraft fuel systems is affected by a complexinteraction of waterprecipitated, mass transfer, phase change,accretionontosurfacesaswellasthesubsequentrelease.

Althoughenhanced effortshavebeendevotedto theresearch re- cently,theunderlyingmechanismsarestillpoorlyunderstood.The main objective of this study is to gain further understanding of thetransient iceaccretionandreleaseprocess inflowingfueland ofthe effectofthe temperature,flow rate,surface roughnessand surfacematerial.

2. Materialsandmethods 2.1.Testrig

Fig.1showsaschematicofthetestrigemployedforthisstudy.

Thefuelflowisfedfromaweldedaluminiumtankof100 lcapac- ityand driven by a gear pump (Typ KF-F 16RF 2-ATEX, Kracht, Germany). The pump speed iscontrolled by a PID controller im- plementedinthecentraldataacquisitionandcontrolsystem(Lab- VIEW).Aplatetypeheatexchangerisusedtocoolthecirculating fuel.The requiredcooling capacity is provided by a chilling sys- tem(IntegralProcessThermostatsT10000,Lauda,Germany)which isconnectedtothefuelsystemthroughasecondarycooling loop withamonoethyleneglycol-watermixture.

Keycomponent oftheapparatus isthe test section.Here, the fuel passesthrough a straight rectangularduct (l_D=^{840 mm in} length,wD=¹⁵.8 mm inwidth,hD=¹⁸.0 mm inheight).Thetwo

opposing sidewallsare madeoftriple-glazedinsulating windows.

This permits visual observation of the icing process within the cold fuel flow as well asthe application ofnon-intrusive optical measurement techniques.In ordertoavoidanyview blockingice accretiononthe glasswindows,the inner(fuel wetted)sidesare coveredwithathinlayerofacrylicglass(polymethylmethacrylate, PMMA), which,inpreliminarystudies,was found tohavelimited affinitytoice.

After traversing the test section, the fuel flows back into the tank. The test rig piping consists of stainless steel pipes having an inside diameter of 19.6 mm. The entire test rig is thermally insulatedwith38 mmArmaflex/AF(Armacell,Luxembourg).

The test rig is instrumented with a total number of 19 type T thermocouples.They are located, forinstance, atthe inlet and outlet of the test section, in the tank, after the heat exchanger andafterthepump.ACoriolisflowmeter(FCMassfloMass2100, Siemens,Germany)isusedto measurethevolumeflow rate.The totalwatercontentofthefuelisdeterminedbyKarlFischertitra- tion(TitroLine7500KFtrace,SIAnalytics,USA)usingthesampling bypassasshowninFig.1.

Thetestrigisdesignedtooperateataminimumfueltemper- ature of −^{22◦C and a maximum volume flow rate of 1200 l/h.}

According to thefuel propertiesgiven in[1], thiscorresponds to a bulk velocity in the test section of Ub,TS =1.172 m/s and a Reynoldsnumber ofReTS=^{4665,the latteris basedon thebulk} velocityandthehydraulicdiameterDh,TS= _w^2w_D+^Dh_h^D_D=¹⁶.83 mm.

2.2. Testbodies

In this work, the ice accretion is studied using exchangeable testbodies.Thesetestbodiesare mountedtransverselytothedi- rectionofflowbetweenthetransparentsidewallsofthetestsec- tion,seeFig.2.Thetestbodiescreateadeflectionoftheoncoming fuel flow and therefore promote the deposition of entrained ice particles.Inrealaircraftfuelsystems,suchsignificantflowdeflec- tions canbefound, forexample,insharppipebends,connectors, valves,pumpinletscreensorfeedejectorpumporifices.

Fig. 1.Simplified schematic of the icing test rig.

Fig. 2.Geometry and arrangement of the wedge shaped test body within the test section.

Fig. 3.Sketchofthewedgeshapedtestbodyusedinthisstudy,:centrepiece, :holder.

Thetest bodiesare an assemblyofthreeparts: acentrepiece andtwo holders, see Fig.3. While thecentre piece isthe actual object against which the ice is supposed to accrete, the holders arerequiredtokeepthecentrepieceinposition,however,without obstructing the side view towards the centre piece. They there- forehaveaslightlysmallercross-sectionand,inordertoavoidany viewblockingiceaccretions,theyaremadeofacrylicglasssuchas the duct’s sidewalls. The test body assemblies are placed at the duct’scentre line about550 mm after the test section entrance.

Thiscorrespondsto33timesthehydraulicdiameter.

Twosets of blunt, wedgeshaped test bodiesare prepared, as showninFig.3.Thefirstsetisusedtostudytheeffectofsurface roughness. For this, the surface of four aluminium centre pieces arealteredmechanically:bypolishingtoamirrorfinish(POL),by glassbeadblasting (GBB)andby abrading withcoarse(A400) or withfine(A3000)abrasivepaper.AsillustratedinFig.3,thesedif- ferenttreatmentsareonlyappliedtotheupperhalfofeachwedge, i.e.tothe“samplesurface”, whereasthelower halfofall wedges aretreatedbyglassbeadblasting(GBB).Thelatterservesasa“ref- erencesurface”. Simultaneousobservationofbothhalvesallowsa direct,comparativeevaluationoficingbehaviour.

The second set of wedges is prepared with the aim to study the effect of surface material. The centre pieces of five wedges are made out of: aluminium (AL), copper (CU), stainless steel (SST), polymethyl methacrylate (PMMA) and polytetrafluoroethy-

lene (PTFE). To create similar and hence comparable surface to- pographies,allofthesamplesurfacesarepolished.

All surfaces are characterised both, quantitatively in terms of surface roughness measured by a contact profilometer (XR20/

GD120,Mahr,Germany)aswellasqualitativelybymicroscopicim- agestakenwithInfiniteFocusG4(Alicona,Austria).

2.3. Iceaccretionmeasurement

Quantitative measurement of the iceaccretionprocess isper- formed by an imagingsystemin combinationwith digitalimage processingtechniques.Theimagingsystemcomprisesa4608 pix× 3288 pix CMOS camera (acA4600-uc10, Basler, Germany) and a telecentric lens (CCTV LM1125TC, Kowa, Japan). The camera is aligned perpendicular to the test body in order to obtain a two dimensionalprojectedviewofthelateralextentsoftest body(in- cludingiceformations),asillustratedinFig.2.Onlythefrontalpart ofthe wedgeshaped test bodiesiscaptured.The field ofview is 14.798 mm×¹⁰.559 mm,whichcorrespondstoapixelresolution of3.211 μm/pix.Auniformbackgroundilluminationisprovidedby acustommadeledpanel.Theimagesarecalibratedbytheknown dimensionsofthetestbodies.

A chain of digital image processing techniquesare applied to automatically determine the ice accretion from the raw images.

Thisinvolvestwomajorsteps:segmentation andgauging.Byseg- mentation theobjectofinterest,inthiscasetheice, isseparated from the background,which is basically done by subtracting the currentlyinvestigatedimagefromareferenceimage.Thereference image is typically the first image of an experiment showing the naked testbody.Inthisway,changesfromtheinitialstate,which must be due to the presence of ice, become visible. In the dif- ference image the pixels are categorized as “ice” or “no ice” by applyingafixedthresholdgrayscalevalue.Ithastobenoted,how- ever,that thismethodofsegmentationwouldproduce significant errorsifthereareeven smallmisalignments betweentheinvesti- gatedandthereferenceimage.Suchmisalignmentsmaybecaused by displacements between the camera and the test body during theexperiment,forinstanceduetothermalexpansionsorfluiddy- namicpressures.Asacountermeasure,misalignmentsaredetected

priortosubtractionbyanintensity-basedimageoptimisationfunc- tionand, if necessary, compensatedby a correctivedigital image shift.

After segmentation the ice is quantified in terms of a total cross-sectionalareaofice, A_a,simplybycountingthepixelrepre- sentingice.Furthermore,theiceisquantifiedintermsofacircum- ferentialicethicknessta.Forthis, the(iced)test body contouris tracedbyaMoore–Neighbortracingalgorithmand, aftertransfor- mationintoalocalcoordinatesystem,subtractedfromthecontour ofthenakedtestbody.

ForsomeexperimentstheCMOScameraisalsoequippedwith along-distancemicroscope(K2DistaMax,Infinity,USA)inorderto capturedetailedfeaturesoftheiceformations.

2.4.Iceparticlessizeandmorphologyanalysis

Aparticlesizeandshapeanalyser(EyeTech,Ankersmid,Nether- lands)is(temporally)installedwithinthefuelsamplingbypassin order to visualise and measure the entrained water/ice particles (cf. Fig. 1). The particle analyser captures 345.6 μmײ⁵⁹.2 μm (0.540 μm/pix)microscopic imagesofthefuelstreamwhilepass- ingthroughacuvette.Astreamofnitrogenisblownacrosstheex- ternalsurfacesofthecuvette topreventcondensationfromform- ingatthe lowoperation temperatures.Particle segmentation and gaugingproceduresareperformedinpostprocessingusingtheIm- ageJdistributionFiji[14].

2.5.Experimentalprocedure

Thetankisfilledwithabout60 lofJet A-1,producedbyama- jorUK oil refiner.Forthis studytwodifferentfillings (I & II)are used.Foreachfilling,thefluiddynamicproperties,namelydensity andviscosity, weremeasured bytheCoriolisflow meterinstalled in the test rig (FC Massflo Mass 2100, Siemens, Germany) and an Ubbelohde viscometer (50101-0a, SI Analytics, USA), respec- tively.ThewatersolubilitywasdeterminedbyKarlFischertitration (TitroLine7500KFtrace,SIAnalytics,USA)inasimilarprocedure asdescribedin[2].

Theicingexperimentsarecarriedoutwiththefuelasreceived.

Noadditionalwaterisinjected beforeorduring theexperiments, which means that any formations ofice arise solely fromwater that occurs naturally in the fuel, i.e. from water that is precip- itated out during cooling. Injection of additional water as in [6, 15,11,16,13] would increase the yield of iceaccretion andhence enhancevisualisation,butnumerousadditionalfactorsanduncer- taintiesrelatedtosuchaninjection(method,location,time)would artificiallybeintroduced,whichmightdeterioratereproducibility.

Thetest bodies are cleanedwithisopropyl alcoholandblown drypriortoinstallation.

Atemperatureandvolumeflowrateprofile,asshowninFig.4, is used for the experiments. Starting from a chiller temperature ofϑCH= ^{20◦C the fuelcirculates foratleast tenminutes inor-} der to homogenise the fuel and to reach steady state condition.

Afterwards, the fuel is cooled at a rate of ^ϑCH/t=⁰.5^◦C/min until the desired target temperature ϑCH,t is reached. This tem- perature is maintained for at least 3 h and then, in order to evaluatethestrength oftheaccreted ice, thevolume flowrateis gradually increased at a rate of ^V˙/t=^{20 l}/h/min (^Ub,TS/t= 0.0195 m/s/min) up to V˙ =^{1000 l}/h (U_b,TS=⁰.977 m/s). Af- terfiveminutes, the volume flowrate isreduced to its previous valueandanothertenminuteslatertheheatingstartswitharate 1.0^◦C/min.Aslightlyhigherfinaltemperatureof25^◦Cischosento further raise the water solubilityand hence accelerate the reab- sorptionof precipitatedwater. Betweentwo experiments a reab- sorptiontimeofatleasttwelvehoursisprovided.

Fig. 4.Temperatureandvolumeflowrateprofileusedintheexperiments,shown hereforϑCH,t= −18^◦C andV˙t=650 l/h. :chillertemperature,––:volumeflow rate,#^:targettemperaturereached,/^{:start/endvolumeflowrateincrease,}♦^: starttemperatureincrease.

Theeffectsofthreeparametersontheaccretionandreleaseof icearestudied.ThetestconditionsaresummarizedinTable1.The fuel fillingis not changed when investigatinga parameter effect.

Atleastthreeexperimentsarecarriedoutforeachcondition.

Theevaluationoficeaccretionisprimarily basedonthemax- imumaverageicethicknessthatisobtainedontheupstreamside ofatestbodyduringanexperiment(between#^andinFig.4):

ta,max

=

^max

t_a,

#

...

(1) Furthermore, the normalized release of icecaused by the in- creasingvolumeflowrate(between^{andinFig.4)isusedas} ameasurefortheicestrength:

a

=

_releaseta

t_a,

=

^ta,

−

ta,

t_a, (2)

Thehydraulic diameteroftherectangulartestsection(D_h,TS= 16.83 mm≈⁰.66) iswithintherangeofpipediametersthatare typicallyusedinanaircraft’sfueldeliverysystem(≈⁰.5–2.5).

Theflowandtemperatureconditionsarequitemoderate.They are believed to be ofparticular relevance. The investigated tem- peratures are withinthe stickyiceregion andthemoderateflow ratesareexpectedtoyieldsignificanticeformationsasaccretedice seemstobesoftandmobile[6] withverylittleadhesionstrength [9,11].

The overall fuel cooling rate in this work is of the same or- derofmagnitudeasthatinarealaircraftfueltankduringtypical missions[17,15].Withintheheatexchanger,theskintemperature canbesignificantlylower thanthatoftheinflowing fuel.¹ Dueto this, thefuelmayexperiencea comparativelyrapidcoolingofup to ϑ=⁵.3^◦C (220 l/h)andϑ=².3^◦C (650 l/h)whilepassing through theheatexchanger. Thismightapply a thermalshockto partofthefuel.Duringtheinvestigations,however,therewereno indications for a possible effect on the general iceaccretion be- haviourpresented hereafter,e.g.when therewas achange inthe volumefloworthecoolingrate.

1 Thetemperaturedifferencebetweentheinflowingfuelandtheheatexchanger platesisestimatedfromthefueltemperatureinthetankandthetemperatureof thecoolingfluidwhenenteringtheheatexchanger.Themaximumdifferenceap- pearsattheendofthecoolingphase(indicatedby#^{inFig.4)andcanasmuch} asϑ=⁶.9^◦C for220 l/handϑ=³.1^◦C for650 l/h.

Table 1 Testconditions.

No influence of... ϑCH,tin^◦C V˙tin l/h test bodies fuel filling

1 temperature on ice accretion −^22–−^{6 6502} wedge shaped I

2 surface roughness on ice accretion & release −18 220¹& 650² wedge shaped, set 1 II

3 material on ice accretion & release −18 220¹& 650² wedge shaped, set 2 II

1 V˙t=^{220 l}/h≡^Ub,TS=.215 m/s≡^ReTS=^{1014 at}ϑTS,s= −¹⁷.3^◦C.

2 V˙t=650 l/h≡Ub,TS=0.635 m/s≡ReTS=2996 atϑTS,s= −17.3^◦C.

Fig. 5.Growthoficeonaluminiumwithglassbeadblasted(GBB)surfacefinish.Av- erageicethicknesstaforϑTS,s= −¹⁷.3^◦C andV˙t=^{650 l}/h.#^:targettemperature reached,^{/:start/endvolumeflowrateincrease,}♦^:starttemperatureincrease.

3. Resultsanddiscussion

3.1. Descriptionoficeaccretionprocess

The typical ice accretion process is illustrated for a reference experiment.Here,atest bodyisexposedtoafuelflowof650 l/h (U_b,TS=⁰.635 m/s).ThechillertargettemperatureissettoϑCH,t=

−^{18◦C,whichresultsinaslightlyhighersteadystatefueltemper-} atureinthetestsectionofϑTS,s= −¹⁷.3^◦C.TheReynoldsnumber isRe_TS=^{2996.The referenceexperimentwas carriedout several} timesbeforeandaftertheactualexperimentspresentedhereafter inordertoassurereproducibility.Unlessstatedotherwise,thefol- lowingresultsaretheaverageofeightindividual runs. Errorbars andpatchesareusedtoindicatethestandarduncertainty,e.g.the experimentalstandarddeviationofthemean[18].

InFig. 5the averageicethickness t_a onthe upstream side of testbodyisplottedagainsttime.Fig.6showsthetotalwatercon- tent

γ

w,talongwiththecorrespondingtheoreticalundissolvedwa- tercontent

γ

w,u.Thelatterwasderivedusingthefueltemperature and the measured water solubility. The uncertainty is estimated fromrepeatedmeasurementsthatwerecarriedoutmultipletimes during certain points in time, i.e. during the warming-up phase andshortlybeforetheincreaseofthevolumeflow.

Theobservationsmadeduringtheexperimentareasfollows:

00:30 Water beginstoseparate fromthefuel asthe temperature dropsbelow10^◦C.Micronsized, sphericaldroplets ranging fromabout2 μmto10 μmappearintheparticleanalyser.

00:50 Asthetemperaturereaches0^◦C thewaterdropletscould – atleastinprinciple–solidifyintoiceparticles.Theiractual stateis,however,undeterminedasthereisnovisualchange intheirappearance.

01:26 A few minutes after reaching the target temperature wa- terdroplets/iceparticlesstarttoadheretoeachother.They turninto fluffy, snowflake-likeice particles up to 250 μm insize. Atthe sametime, particles begintoadhere tothe testbody surface,ascanbe seenfromt_a inFig.5.Ingen- eral,the visualappearanceoftheaccreted particlesisvery

Fig. 6.WatercontentinJetA-1γwfor ϑTS,s= −17.3^◦C andV˙t=650 l/h.#/ : totalwatercontentγw,t,^/:theoreticalundissolvedwatercontentγw,u(filled/not filled:first/lastexperimentofthisseriesofexperiments).

similartothoseobservedinthefuelflow.Theirmorpholog- ical features aswell astheir sometimes partlyinterrupted contacttothesurface(cf. Fig.7)isanimportantcluewhich suggests that these particles were already of solid nature whenstrikingthesurface.Fig.7illustratestheicingprocess inaseriesofimagestakenwiththelongrangemicroscope.

Afterdeposition, thesingle particle inFig. 7ais firstcom- pressed fromits initially ≈^{70 μmto} ≈^{50 μm (cf. Fig.7c),} presumably dueto the fluid dynamic forcesacting on the (iced) surface. With further impingement of smaller and largerparticles, theparticleprogressivelygrows insize(cf.

Fig.7dto 7h)andeventually,inanongoing process ofde- position anddeformation,a thin, irregularlayeris formed.

The accreted icecanbest be described assoft, deformable andmobile. It seems to be similar to theice described in [11,6].

02:30 Most oftheavailable undissolvedwateris“consumed”due to deposition within the system. The ice thickness curve flattensandthefreewatercontenttendstowardszero.

04:26 Now,asaresultofthegraduallyincreasingvolumeflowrate between 04:26and 04:48(indicated by ^{and) partof} theaccreted iceisshedoff,i.e.theicethicknessdecreases (a=⁰.103).

04:59 The remaining iceis shed off while the fuel is heatedup again,mostlybetweentemperaturesof−^5–−^{2◦C (}≈^05:15).

Thiscanbeseenfromtheicethicknessandthewatercon- tent.

Comparison of the water content measured at the beginning (#^{)andtheend( )ofthisseriesof}experiments(cf. Fig.6)reveals nosignificantchanges.Thismeansthatevenafterfourmonthsand severalrunswiththesamefuelfilling(II),analmostidenticalwa- tercontentwasachievedeachtime.Thisisinagreementwiththe smalldeviationsintheindividualicethicknesscurves,asindicated by the standard uncertainty. In general, the measurement of the iceaccretionshowsresultswithgoodreproducibility.Theyconfirm

Fig. 7.Growthoficeonaluminiumwithglassbeadblasted(GBB)surfacefinish.The microscopicimagesshowthefrontalpartofthewedgeshapedtestbody(cf. Fig.3) atdifferentpointsintime(cf. Fig.4)forϑTS,s= −17.3^◦C andV˙t=650 l/h.

thesuitabilityofthemethodappliedinthisworktosystematically studytheeffectofvariousparameters.

3.2.Influenceoftemperature

Following the procedure described above, experiments were carriedout at differenttarget temperatures ranging fromϑCH,t=

−^{6◦C to}−^{22◦C.Fig.8showsthemaximumaverageice}thicknesses obtainedduringthe experiments,ta,max,asafunction ofthe cor- respondingsteadystatefueltemperatureinthetest sectionϑTS,s. Whilenoaccretionoficecanbeobservedabove−^{6◦C,thereisan} almost linearrelationship in a temperaturerange between−^6◦C and−^{15◦C.Theincrease inicethicknesscanbe partlyattributed} toadecreasingwatersolubility,becausefallingtemperatureslead toahigheravailabilityofundissolvedwaterasmorewatercomes out ofsolution.At test temperatures below−^{15◦C theicethick-} nessgraduallylevelsoff.Theiceseemstoloseitsabilitytoadhere tothetest body.IncaseofϑTS,s= −²⁰.8^◦C,it wasobserved that largefractionsoftheinitiallyaccretediceweresubsequentlyshed off.Thisbehaviouriswellreflectedinthefuelwatercontent.Fig.9 showstheremainingtotalwatercontentattheendofthecooling period(^{inFig.4).Acomparisonwiththewatersolubility(solid} line)revealsadramaticincreaseintheundissolvedwatercontent fortemperatures below−^{20◦C. Thismeans, a much higher frac-} tionoftheprecipitatedwaterwas notable topermanentlysettle oraccretewithinthesystemandthusstillcirculatedwiththefuel flow.

It seems unlikely that the deterioration in iceaccretion arose fromthemoderatechangesinviscosityanddensity.Measurements conductedwithsameReynolds numbersinstead ofsame volume flowratesgaveinfactalmostidenticalresults.

Thepresentresultsare ingoodagreement withtheBoing and AAIBinvestigations that found a stickyrange between−^{5◦C and}

−^{20◦C, where ice crystals were most likely to adhere to their}

Fig. 8.Maximumaverageicethicknessta,max(Eq.(1))onaluminiumwithglassbead blasted(GBB)surfacefinishinrelationtothesteadystatetestsectionfueltemper- atureϑTS,sforV˙t=^{650 l}/h.

Fig. 9.WatercontentinJetA-1γwattheendofthecoolingperiod(inFig.4)in relationtothesteadystatetestsectionfueltemperatureϑTS,sforV˙t=650 l/h.#: totalwatercontentγw,t, :measuredwatersolubilityγw,s.

surroundings[6].In[12,4] suchastickybehaviourisdescribedbe- tween−^{12◦C and}−^18◦C.

The change inaccretion behaviour below −^{20◦C seems to be} a consequenceofchanging icepropertiesandmightbe linked to theiceadhesionstrength.Archeretal.[19] measured thetensile interface strengthoficeon aluminiumsubstratesandobserveda dramaticdropatatemperatureof−^{20◦C.Theyexplainedthisby} the thinliquid-likelayer that is knownto existbetween iceand structural solids. Due to its damping effect, this layer would in- crease thestress requiredto separate theice substrateinterface.

Between −^{25◦C and} −^{30◦C, the liquid layer and therefore the} damping effect disappears. The interface strength approached an almostconstant value.Similar relationshipswere alsoreportedin [20] and[21].

Incontrasttothis,Dongetal.[22] measuredaninterfaceshear strength of ice on aluminium and copper plates that increased withdecreasingtemperature.SimilartoArcheretal.[19],however, therewasnofurtherchangewithtemperaturebelow−^20◦C.Dong etal.alsoexplainedthisbytheliquid-likelayer,butthegradually growing layer was believed toweaken theice adhesionwith in- creasingtemperaturesduetolubricatingeffects[22].

The role of a liquid like layer with regard to ice adhesion is notyetunderstood[23].Athighertemperaturesthereisageneral agreementintheliteraturethattheiceadhesionstrengthincreases withtemperaturedecrease[23].Thismightbeanothercontribut- ingfactortotheincreasingicethicknessbetween−^{6◦C and}−^15◦C inFig.8,apartfromadecreasingwatersolubility.

Fig. 10.Micrographsshowingthesurfacetextureofthealuminiumsamplesurfaces usedtostudytheinfluenceofsurfacetopography.

Table 2

ArithmeticaverageroughnessRaofthealuminium(ENAW-AlMgSi)samplesurfaces usedtostudytheinfluenceofsurfacetopography.

name method of processing Rain μm

GBB glass bead blasted^1,2 2.475±0.149

A400 abrasive paper, P400 1.210±0.020

A3000 abrasive paper, P3000 0.187±⁰.003

POL mirror polished 0.067±⁰.003

1 Referencesurfacetreatment.

2 Glassbeadof90–150 μm.

3.3. Influenceofsurfacetopography

Fig. 10 shows representative high magnification views of the testbodysurfacesusedtostudytheeffectofthesurfacetopogra- phy.Thecorresponding roughnessvaluesR_a are giveninTable2.

The roughest surface was obtained by glass beadblasting (GBB).

The topographyis characterisedby large peaksand valleys rang- ingfromapproximately10 μm–80 μm.Themirrorpolishedsurface (POL)hasthelowestroughness.Here,onlythinscratchesarevis- ible.The surfacesabradedwithemery paperpossessa roughness betweenthesetwo.Bothshowsimilarcharacteristics,i.e.unidirec- tionalgroovesinthedirectionofabrasion.Thegroovesaresmaller insizeforthefineremerypaper(A3000)thanforthecoarserone (A400).

Inthissection,theicethicknessresultsarenormalizedwiththe referenceicethicknessaccordingto

_a,max

=

^ta,max ta,max,ref

(3)

Fig. 11.Maximumaverageicethicknessratioa,max(Eq.(3))onaluminiuminrela- tiontotheroughnessRaforϑTS,s= −17.2^◦C.#:V˙t=220 l/h,:V˙t=650 l/h.

Fig. 12.Icereleaseratioa(Eq.(2))onaluminiuminrelationtooftheroughness RaforϑTS,s= −¹⁷.2^◦C.#^:V˙t=^{220 l}/h,^:V˙t=^{650 l}/h.

whereta,maxisthemaximumaverageicethicknessonthesurface underinvestigation (upperhalf ofthewedge)andta,max,ref isthe maximum average ice thickness on the corresponding reference surfaces(lowerhalfofthewedge),cf. Fig.3.Sincebothmeasure- ments areobtainedsimultaneouslyduring eachrunandtherefore undersameconditions,thisdirectcomparisonfurtherreducesthe influenceofexperimentaluncertainties.

InFig.11themaximumnormalizedicethicknessa,maxisplot- tedagainstthesurface roughnessRa.Thedashedlinesare drawn to guidetheeyeanddonotmean thatthereis alinearrelation- ship. Bothvolume flowratesshow thesametrend:thesmoother the surface, thelesstheamount ofice. Incaseofthe glassbead blastedsurfaces (GBB)a,max isclose toone. Thisisexpectedas the topography of this sample surface is identical to that of the alsoglassbeadblastedreferencesurfaces.Forthepolishedsurface (POL),a,maxdropsto0.704(220 l/h)and0.831(650 l/h),respec- tively.Theeffectofsurfaceroughnessismorepronouncedforthe higherflowrate.Thiscanbeattributedtothehighershearforces actingonthe(iced)testbodysurfaces.Highershearorseparating forcesmayenhancethesignificanceoftheeffectivecounteracting ice-solidinterfacestrength.

The releaseoficedueto theincreasingvolume flowrate(be- tween ^{andinFig.4) is}illustrated inFig.12in termsofthe icereleaseratioa.Thefigureshowsthatahigherfractionofice isshedawayfromsmoothersurfaces.Thismeanstheabilityofthe alreadyloweramountsoficetowithstandtheincreasingfluiddy- namicforcesdeteriorates.

Asforthetemperaturedependence,thecorrelationbetweenice thicknessandsurface roughnessmaybe explainedbythe icead-

Table 3

ArithmeticaverageroughnessRaofthepolishedsamplesurfacesusedtostudythe influenceofmaterial(wedgeshapedtestbodies,set2).

name material Rain μm

AL aluminium alloy¹ 0.111±⁰.004

SST stainless steel² 0.017±⁰.003

CU copper³ 0.137±⁰.006

PMMA polymethyl methacrylate 0.219±⁰.012

PTFE polytetrafluoroethylene 0.186±⁰.007

1 ENAW-AlMgSi ² X5CrNi18-10 ³ Cu-ETP

hesion strength. Despitethe very differentmethods employed in theirstudies,variousauthorsfoundasignificantlyhighericeadhe- sionstrength onmechanicallyroughened aluminiumsamples[19, 20,24–26].Zouetal.[24],Susoffetal.[25] andWuetal.[20] ex- plainedthis,inpart,withthelargereffectiveice-solidcontactarea thatisprovidedonroughersurfaces.Inaddition,accordingto[25, 20,19], thelarger peaksand valleys onrough surfaces contribute tothe adhesion strength asthey provide a greater likelihood for icetomechanicallyinterlockoractuallyanchoritself.Itshouldbe notedthatinthesestudiestheice-solidinterfacesweremadefrom watersamplesor(supercooled)water dropletsthat frozedirectly ontothesurfaces.Thus,theicestrengthmighthavebeenalsoaf- fectedbythesurfacewettabilityorthenumberofnucleationsites foricegrowth.

Inthisstudy,however,theicelayeris theresultofimpinging solid ice particles, as the above observations suggest (cf. Fig. 7).

Nevertheless,some ofthemechanismsdescribedinliterature can becarriedover:The topologyoftheglassbeadblasted(GBB)and coarselyabraded(A400)surfacesarecharacterisedbylargevalleys and grooves that are similar in size compared to the ice parti- clesentrainedinthefuel.Hence,iceparticlesareinprincipleable tobecometrapped bythesurface structures. Thiswouldincrease theireffectiveice-solidcontactarea, mightenablemechanicalin- terlockingand,consequently,yieldhigheradhesionstrength.

3.4.Influenceofmaterial

Threemetals (Aluminium,AL;stainless steel, SST;copper, CU) andtwo plastics (PTFE; PMMA) were used to evaluate the influ- ence of material. To reduce effects due to variations in surface roughness,ahomogenous,smoothsurface wascreatedforall test surfacesbymeans ofpolishing. Table3showsthesurface rough- ness Raobtainedforeachmaterial.Reflectingontheresultswith regardtotheinfluenceofsurfaceroughnessinFig.11,theremain- ingdifferencesinroughnessarebelievedtobetoosmalltomask thematerialdependence.

Possibleeffectsduetothedifferentthermalpropertiesarealso negligible. Prior to the actual experiments, additional tests were carried out in order to evaluate the dynamic cooling behaviour of different test bodies, in particular the delay withwhich they followthefueltemperature.Forthis,modifiedcylindricaltestbod- ies (dC=^{8 mm)made of PMMA andaluminium were equipped} withthermocouples to measure their core and surface tempera- tures.Additionally, asimulationmodelbasedon[27] wasusedto determine the dynamic temperature distribution within the ma- terial.Both, theexperimental andsimulationresults showedthat because of the moderate fuel cooling rate together with a high convectiveheat transferbetweenfuel andtest body,thesurfaces ofall test bodieswere ableto followthe fueltemperature with- outsignificantdelay,i.e.<0.1 K.Itthereforecanbeassumed that each ofthe test body surfaces hadreachedthe steady state temperaturebeforeanyiceaccretiontookplace.

Fig.13presentstherelationshipbetweenthemaximumaverage icethickness t_a,max andthe different materials. For both volume flow rates, the ice thickness is much lower on plastics (PMMA,

Fig. 13.Maximumaverageicethicknessta,max(Eq.(1))ondifferentmaterialswith polished(POL)surfacefinishforϑTS,s= −17.2^◦C.#:V˙t=220 l/h,:V˙t=650 l/h.

PTFE)in comparisontometals (AL,CU andSST).The average re- duction is49% (220 l/h)and77% (650 l/h),respectively. Between the metals, no clear trend exists. Possible material effects seem tobesmallinregardtotheexperimentaluncertainties. Thesame holdstruewiththetwoplastics.

The results onthe release ofice are not shownin thispaper asthey reveal noclearrelationship. Thisis aconsequenceof the verysmallamounts oficeinvolvedespecially incaseoftheplas- tics. Their detachment behaviour is subject to wide fluctuations andhencelargemeasurementuncertainties.

The dramatic reduction in ice accretion in the case of PTFE seemstobetheresultofitsso-calledicephobicproperties.PTFEis characterisedbyaverylowsurfaceenergy.Ithasaverylowaffin- itytowardsbothwaterandice[28].Coatings ofPTFEwerefound toreducetheiceadhesionstrengthbyafactorof5–7withrespect tothe barealuminiumsubstrates[29,30,25]. InMeninietal.[31]

theiceshearstrengthonaluminiumsubstrateswasreducedbya factorof almost 2.5when coatedwithPTFE andYang et al.[32]

reportedashearstrengthforpristine(solid) PTFEplatesthatwas 5%ofthatofbarealuminium.

For PMMA, the reports on the iceadhesion strength are less conclusive.In Archeretal.[19] theiceadhesionstrengthwas re- duced by a factor of1.44 when bare aluminiumsubstrates were coatedwith PMMA.They attributedthisto the fewernumber of bondingsites.Meuleretal.[33] foundareductionby asmuchas afactorof1.51withrespecttountreatedbaresteel.Bharathidasan et al. [34], on the other hand,observed a 1.43 times higher ice adhesionforaPMMAcoatingonuntreatedaluminium.This,how- ever, could alsobe explained by the fact that thePMMA surface possessedasignificantlyhigherroughness.

Raraty et al. measured the ice adhesion strength on various metals.Theresultsforbrassandaluminiumalloy weresimilar to thoseforstainlesssteel[35].Thisisinagreementwiththealmost identical valuesforaluminium andstainless steelshown in[36].

Sonwalker etal.[37] reported thelargeststrength oficeon tita- nium,followedbycopper,stainlesssteel,aluminiumandPTFE.The differencesbetweenthemetals,however,were smallincompari- sontoPTFE.

With theexception ofPMMA, all theseobservations correlate withtheicethicknessfoundinthisstudy.

4. Conclusions

Atestrigwasdesignedtomimictheicingconditionsinflowing fuel.Anopticalmeasurementsystemwasimplementedtoquantify iceformations downto afew micrometres inthickness. Thisal- lowedstudyingtheicingprocesswithoutanyadditionalinjection of waterand hencewith highreproducibility. A total number of