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Characterizationofjetsundersupercriticalconditions

GuillaumeRibert1andJean-BernardBlaisot2

NormandieUniv.,INSARouenNormandie,UNIROUEN,CNRS,CORIA76000Rouen,France

1. INTRODUCTION Thesuggestiontousesupercriticalfluids(SFs) isacenturyold,butthemainprogressin

investigatingandutilizingsupercriticalfluidshasoccurredmostlyinthepast20years[1].Theyhavebeenused (or proposed for use) as solvents, reactionmedia, processingmedia, andpropellants. As example supercritical fluids can be effectively used in many industrialapplicationssuchasextractions(ofvitamins,perfumes,etc.),syntheses,andprocessingsteps.SFs are used to remove unwantedmaterials, such as caffeine and cholesterol from foodproducts. They are also successfully used in many instances of analytical laboratoryseparationsandethaneisoneofthemostusedfluidsforextractionandchromatography.SFsarenowreplacingorganic solvents in industrialprocesses. Indeed,manyof thesesolventshaveadverseenvironmentalandhealtheffectsandsupercriticalCO2orwaterareattractivealternativesbecausetheyareinexpensiveandofferminimalthreattoboththeenvironmentandhumanhealth.SFsmaybealso found intheelectronic industrywhereminiaturizationeffortsforthenanoscaletechnologyrecommendtheuseoffluidswithzero-surface-tensionpropertythatareSFs.SFsareindeedbecomingmoreessentialnowadays.Anotherdomainofinterestofthepresentproposalconcernspropulsionwithapplicationtothe automotive and aerospace science and technology where SFs may be considered aspropellants [2]. Indeed, many fluid mechanical devices involve thermodynamic phasetransitionfromasubcriticaltoasupercriticalstate.Asexample,thehigh-pressurecombustionchambers of rocket engines operate at pressures and temperatures well above thethermodynamiccriticalpointsoftheinjectedpropellants,i.e.inasupercriticalstate(seeFig.1).Suchenginesoperateatveryhighpressure,typicallyoftheorderof10.0MPawhereasthecriticalpressureofhydrogenandoxygenisrespectively1.3MPaand5.0MPa.Afinalexampleconcernsthecleancoalcombustionandtechnologyoftransformation.Indeed,coal, themost abundant energy resource,will continue to bedominant in China’s energyscheme for a very long time [3]. Therefore, sustainable development requires thedevelopment and deployment of clean coal technologies such as supercritical and ultra-supercritical boilers, circulating fluidized bed combustion, and integrated gasificationcombinedcycles.CleanCoalTechnologies(CCTs)aretechnologieswhichfacilitatetheuseofcoal in a viable economic and human environment. A basic idea for the CCTs is thedevelopmentofefficientsystemsinordertodecreasetheamountofcoalusedtogeneratethe same amount of power. Some CCTs have already seen commercial application in

1 Ph.D. supervisor. 2 Ph.D.co-supervisor.

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industrializedcountries.Advancedelectricpowergenerationsystemsthatgenerateelectricitywithgreaterefficiencyandfewerenvironmentalconsequencesareundergoingdevelopmentinmanycountries.

Figure1:Pressure-Temperaturediagramforethanespecies.Interestofthepresentstudy:arrows.

Inparticular,thepulverizedcoalcombustionwithsupercriticalsteamcycles,togetherwithflue gas cleaning units is under current research and development. However, units withsubcritical boilers tend to have somewhat higher emissions of sulphur oxides (SOx) andnitroxides(NOx)perkWhthanunitswithsupercriticalboilers.Indeed,subcriticalsystemsarelessefficientgeneratinglesskWhperunitofcoalinput.Toimprovetheplantperformance,the introduction of supercritical or ultra-supercritical steam conditions and the use ofemissioncontroldevicesisthennecessary.

2. OBJECTIVES Mixing is one of the most important phenomena in combustion devices because it

determines combustion efficiency and stability alongwith heat transfer characteristics. Inaddition, most of the current understanding of turbulence and mixing is the result ofatmospheric-pressurestudiesprovidingnumerousnumericalandexperimentaldatabasesareavailable. A similar work has to be undertaken for sub-, trans- and super-critical flowsinvestigation,i.e.real-gaseffectshavetobeconsidered[4,5].Indeed,thisresearchfieldlacksof indisputableexperimentaldata,usefulforthenumericalcodevalidation.Simulationhasmadeseriousprogressinthelastdecadebutstillneedwell-documentedexperimentaltestcases.Actually,onlyfewexperimentaltest-benchesareabletoreach/runsupercriticalcases.Threeare located in theUSA (PennsylvaniaStateUniv.,Univ.ofFloridaandAFRL), two inGermany(M51andP8atDLR)andoneinFranceatONERA(Mascotte).ExceptfacilitiesfromUniv.ofFlorida[6],theyareallrunninghotfireconfigurationsdedicatedtorocketissuesthatalready are highly complex systems. In addition, no convincing experimental data areavailable.ThedeclaredobjectiveofREFINEistofilloutthisneedbythemeanofsimplewell-definedexperimentationswithveryadvanceddiagnosticstodeliverthefinestinformation.REFINEaddressesthenBasicResearch.

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Theseinvestigationsshouldprovideabetterunderstandingofatomization,mixingandflowdynamicsforhigh-pressuresubcriticalandsupercriticaljets.Itisexpectedthatthesestudieswill also lead to an enhancement of physicalmodels,whichwill be used for achieving animproveddesignmethodology.Forthisreason,cryogenicandsupersonicflowsareruledoutsinceasubstantialdeviceandsensitivetechnologyarerequired.StudyingafluidflowinjectionclosetothecriticalpointisbeyondtheobjectivesofREFINEastheconditionsofinjectionmaynotbewellcontrolledandtheuseoftheproposeddiagnosticsnottotallyguaranteed.TheREFINEprojectwillbeconfrontedtoseriouschallenges.Thefirstoneistherealizationofa clean high-pressure experiment. Even if the CORIA lab has a great experience in high-pressureinjection,itisalwaysachallengewhenextendingtheareaofresearch.Secondly,theX-ray diagnostics setting-up, which is the project keystone, has to deliver a non-polluteddensitymeasurement andGREMI is perhaps theonly French Lab able to realize this task.Among the potential candidates that can be used as a supercritical fluid, ethane (C2H6) isselectedbecauseitscriticalproperties,Tc=32.18°CandPc=4.8174MPa,considerablysimplifytheexperimentaldevicesetting-upusedtoproducewell-controlledsubcritical,transcriticalandsupercriticalconditions;itisoneofthesimplesthydrocarbonssothatthetheoryofitsbehavioriseasiertoestablish;itisanhydrocarbonandhydrocarbonsareusedeverywhere;finally,combinedwithHelium,itallowsagreatcontrastforX-raydiagnostics,keypointofthemethodology. Once the X-ray diagnostics calibrated, other classical techniques(shadowgraphy,Schlieren,etc.)couldbeincorporatedmoreeasilyforcomparisonstotheX-ray data. The CORIA Lab has the objective to become the scientific reference for high-pressuresystemsbyprovidingtothescientificcommunityacomprehensivedatabase.ThiscriterionwillbepertinenttoappreciatethesuccessofREFINE.Finally,afirstattempthasbeenperformedveryrecently(seeFig.2)leadingtoverypromisingresults.

Figure2: Injection of ethane into nitrogen at supercritical pressure.

Ph.D.Schedule:Year1:Test bench and injector design, preliminary tests and experimental programelaboration.Bibliography.Year2:Shadowgraphvisualizationsetupandmeasurements.X-Raysetupthenmeasurementsonsub-trans-andsupercriticalconditions.

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Year3:Data analysis (shadowgraph images treatments, X-Ray images treatments),participationtoscientificcongress,dissertationwriting.

3. BIBLIOGRAPHY [1] Joseph C. Oefelein and Vigor Yang, Combustion science and technology special issue: supercritical fluidtransportandcombustion,CombustionScienceandTechnology,178(2006)i–iii.[2]RainerN.Dahms,JosephC.Oefelein,Liquidjetbreakupregimesatsupercriticalpressures,CombustionandFlame162(2015)3648–3657.[3]WenyingChen,RuinaXu,CleancoaltechnologydevelopmentinChina,EnergyPolicy38(2010)2123–2130.[4]YangV.,Modelingofsupercriticalvaporization,mixingandcombustionprocessesinliquid-fueledpropulsionsystems,ProceedingsoftheCombustionInstitute,28(2000)925-942.[5]BellanJ.,Supercritical(andsubcritical)fluidbehaviorandmodeling:drops,streams,shearandmixinglayers,jetsandsprays,ProgressinEnergyandCombustionScience,26(2000)329-366.[6]SegalC.andPolikhovS.,Subcriticaltosupercriticalmixing,PhysicsofFluids,20,052101(2008).