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A review of Tunnel Fire Research from Edinburgh

Citation for published version:Carvel, R 2016, 'A review of Tunnel Fire Research from Edinburgh', Fire Safety Journal.

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AREVIEWOFTUNNELFIRERESEARCHFROMEDINBURGH

RickyCarvelBRECentreforFireSafetyEngineering,SchoolofEngineering,

UniversityofEdinburgh,EH93JL,Edinburgh,UK.Email:Ricky.Carvel@ed.ac.uk

ABSTRACTTheUniversityofEdinburghanditsalumnihavemadesignificantcontributionstoknowledgeinthefieldoftunnelfiresafetyengineering.Thispapersummarisesthesituationoftunnelfiresafety in theearly1970s,whenthedepartmentof fireengineeringwas foundedandbrieflydiscussesallthecontributionstoknowledgeinthefield,madebyEdinburghanditsalumniinthe past four decades. Research carried out at Edinburgh has changed theway the tunnelsafety industry estimates heat release rates in tunnels, has influencedway design fires arespecifiedandhaschallengedindustryopinionabouttheuseofwaterspraysintunnels.ThispaperispartofacelebrationoffourdecadesoffireresearchatEdinburgh.Keywordstunnelfires,ventilation,heatreleaserate,firespreadINTRODUCTIONThefireresearchgroupattheUniversityofEdinburghwasestablishedin1974andbecametheUK’sfirstacademiccentrededicatedtothestudyoffire.Overthepastfourdecades,thegrouphasgrowntobecomeaninternationallyrecognisedauthorityonfiredynamicsandfiresafetyengineering.Oneofthemanyareasofstudy,withinthebroaderfieldoffiresafety,towhichtheUniversityofEdinburghhascontributedisthesubjectoffiresafetyintunnels.ThispaperisareviewoftheworkpublishedbyEdinburgh,orbyEdinburghalumni,inthefieldoftunnel fire safety researchwith the aimof demonstratinghow theUniversity ofEdinburghhasinfluencedandtransformedthisspecificfieldofresearch.Becauseofthisintentthepaperit is clear that this cannot be a comprehensive review of the subject, indeed it will be aselectiveandsomewhatbiasedreviewof this fieldof study.However,given that thispaperwaswrittenaspartofacelebrationof fortyyearsof fireresearchatEdinburgh,thehopeisthatthereaderwillforgiveusforthisindulgence.TUNNELFIRESAFETYINTHEEARLY1970sBeforethefireresearchgroupwasestablishedattheUniversityofEdinburgh,theproblemoffiresafetyintunnelswasgenerallyaddressedbyusingventilationtomovesmokeintheeventofafire.Ventilationsystemsaretheoriginalsafetysystemfortransporttunnelsandformostof the 20th Century were the only fire safety system in most tunnels. The first ventilationsystem installed in a railway tunnel was in the Edge Hill tunnel, Liverpool, UK, in 1870(althoughmechanicalventilationhadbeencommonplaceinminetunnelnetworksforatleastthreecenturiesbeforethis[1]).Thiswasanexhaustfanfortheremovalofsmokefromsteamengines.In1927,theHollandTunnelintheUSAbecamethefirstroadtunnelequippedwitha(fully) transverseventilationsystem, that is,asystemofductsandopeningsprovided freshair into the tunnelatperiodic locationsalong its length,whilea secondsystemofopeningsandducts extractedpolluted air from the tunnel at periodic locations.While these systemswere originally conceived asmeans of replacing polluted air with fresh air, these systemsbegantobeunderstoodasameansofcontrollingsmokeintheeventofafireinatunnel.

Theprimaryfiresafetyquestionfordesignersoftransverseventilationsystemsinthe1970swas therefore“howmuchsmokedoes thesystemneedtoextract?”Assmokeproduction isbroadly proportional to heat release rate, this question appeared to be answered to thesatisfactionoftheindustrybyapaperpresentedatthe2ndconferenceonAerodynamicsandVentilationofVehicleTunnels,heldinCambridgein1976.A.J.M.Heselden’spaper“StudiesofFireandSmokeBehaviourRelevanttoTunnels”[2]presented,amongotherthings,somedatafromlargepoolfireexperimentscarriedoutintunnels.Oneofthepoolfireswasestimatedtohaveaheatreleaserate(HRR)ofabout20MW.Unfortunately,HeseldencommentedthatthiswasequivalenttoaHGVfireandtheindustrynowhaditsanswer;aHGVfireinatunnelwasconsideredtobe20MWfromthatmomenton,andforovertwodecadesafterthis,the‘designfire’usedtodefinethecapacityofsmokemanagementsystemsintunnelswasa20MWHGVfire[3].Theearly70swasnotwithoutinnovationinthefieldoftunnelfiresafety;1971sawthefirsttunneltobeequippedwithlongitudinalventilationusingjetfans,theBargagli-FerrieretunnelinItaly.Thistypeoftunnelventilationsystemhascometodominatetheindustryinthefourdecadessincethen,andhasbroughtwithitsomefiresafetyproblemsofitsown,asweshallsee.In the 21st Century, ventilation systems are only one of several safety systems used in theevent of a tunnel fire. Passive structural protection and water spray technologies areincreasinglybeingusedtomitigatetheeffectsoffiresintunnels.Butintheearly1970s,suchsystemswereratherrare.Indeed,atthistimeonlyJapanusedsprinklersforfireprotectioninasmallnumberofitstunnels.Wewillconsiderthesetopicsinmoredetailbelow.FOURDECADESOFTUNNELFIRESSince the early1970s therehavebeennumerousminor and severalmajor fire incidents inunderground transportation systems. As engineering in general, and safety engineering inparticular, is driven primarily through learning from failure, it is appropriate to chart theincidentswhichhaveoccurredinthepastfourdecades,aswellastoconsiderthechangesinengineeringpracticeandadvances inknowledgewhichhaveresulted fromthese.This isanabbreviatedlist.Acomprehensivelistofincidents,includingfurtherdetailsofthemajorityofincidentsmentionedhere,canbefoundintheliterature[4].The most common kind of tunnel fire in the 1970s appears to have been fires in masstransit/metrosystems.ThefatalincidentsinvolvingfiresinNewYorkCity(1970),Montreal(1971),Paris(1973),MexicoCity(1975),London(1975)andSanFrancisco(1979)showedtheterribleconsequencesoffiresinundergroundrailwaysystems.Followingtheseincidents,the majority of which involved train collisions as initiating events, a number of measureswere imposedonmetrosystemswhichreduced the likelihoodofcollisions insuchsystemsandhavethuslargelypreventedthiskindofeventinmorerecentdecades.Oftheothertunnelfireincidentsinthe1970s,thefireintheNihonzakatunnel(1979)hadthegreatest influence over tunnel fire safety practice. The accidentwhich involved four trucksand two passenger cars led to a fire which ultimately killed seven people and spread toinvolve 189 vehicles [5]. However, it was the failure of the installed sprinkler system tocontainthefire(thewatertanksrandrybeforethefirewasundercontrol)whichinfluencedpractice. Primarily on the basis of this incident, the tunnel safety industry took a stance

against the installation of sprinklers in road tunnels,whichwould not be overturned untilaftertheturnofthecentury.ThefireinvolvingafueltankerintheCaldecottTunnel,USA(1982),whichresultedinsevenfatalities,butwassuccessfullyextinguishedinundertwohours,wasprobablyinstrumentalinchangingtheindustry’sperceptionofhazardousgoodstransportintunnels.However,thefirein the Summit Tunnel, UK (1984), which also involved liquid fuel tankers, proved muchharder to extinguish, burned for over a day, and resulted in the tunnel closure for severalmonths.Byfarthemostinfluentialtunnelfireincidentofthe1980s,intheUKatleast,wasthefireinKing’sCrossUndergroundstationinLondon(1987)[6].Asisnowwelldocumented,thisfireexhibited the fire dynamics ‘trench effect’which led to very rapid fire growth andbroughtaboutflashoverinthestation’stickethallinonlyafewmoments,resultingin31fatalitiesandmanyother injuries. Itwas the investigationof this incident that first involved researchersfrom theUniversityofEdinburgh in research into tunnel firephenomena.Research carriedoutatEdinburgh, ledbyDrDougalDrysdale,wasamongthe firstworktodemonstrateandexperimentallystudythetrencheffect[7]aswellasstudyitnumerically[8].The largest loss of life in any tunnel fire incident occurred in the metro system in Baku.Azerbaijanin1995[9].Over200peoplediedduetothefireandsmokeinhalation,largelyasaconsequenceofpooregressprovisionandthedecisiontochangethedirectionofventilationduringtheevacuationprocess.Aconsequenceofthisincident,istheindustryconsensusthatanemergencyventilationstrategyshouldbeestablishedearlyduringanincident,andthattheventilationshouldnotbechangeduntilevacuationiscomplete.Wewillconsiderthisstrategyinmoredetail,below.ThefirstfireintheChannelTunnelbetweenUKandFranceoccurredin1996.Thishasbeenfollowedbyothersignificantfireincidentsinthetunnelin2006,2008,2011,2012and2015,ofwhichthe2008firewasthelargest intermsofvehiclesdestroyedanddamagecausedtothetunnelstructure.Nobodyhasdiedasaconsequenceofanyofthesesixincidents.Analysisof the first threeof these incidentshasbeen carriedout atEdinburgh, aswill bediscussedbelow. We have also recently participated in the investigation of the 2015 fire, but thesefindingshavenotbeenpublishedyet.ThespateofroadtunnelfireswhichoccurredattheturnofthecenturyinEuropehasbeenwelldocumentedintheliterature.ThefiresintheMontBlancTunnel(1999),TauernTunnel(1999), StGotthardTunnel (2001) and theFréjusTunnel (2005) together resulted in sixtyfatalities, over a hundred vehicles destroyed, and several years of tunnel closures. Theseincidentsforcedtheroadtunnelsafetyauthoritiestoreconsidertheirsafetypoliciesandledto a massive investment in tunnel fire research and development, at both national andinternational scales.As a consequenceof these incidents, questionsof fire suppression andmeansofescapeprovisionintunnelshavebeenraised,andtherehasbeenashiftinopinionsonthesetopicsintheindustry.The 2007 Burnley Tunnel fire further changed international opinion on the subject of firesuppression in tunnel fires,when the application of awater spray system appears to haveeffectivelycontrolledapotentiallylargefireinvolvingtwoHGVandacar.Othernotablefireincidentsinthe2000sweretheKitzsteinhornfunicularrailwayfire(2000)whichresultedin155 fatalities and the arson attack in the Daegu metro (2003) which led to nearly 200

fatalities. So far in the 2010s, there have been few fatal fire incidents, although one fireinvolvingtwomethanolfueltankersinaChinesetunnelledto31fatalitiesinMarch2014.FIRESPREADINTUNNELSIn theearly1990sDrAlanBeard, thena researchassociateat theUniversityofEdinburgh,beganresearchintothequestionoffirespreadbetweenvehiclesinthetunnelenvironment.Theworkwasinitiallybasedonearlierresearchintoflashoverinbuildings[10],butgrewtoinclude questions of the influence of flame impingement and longitudinal ventilation onwhetherornotfirewouldspread.ThisresearchledtotheFIRE-SPRINTmodel[11,12,13,14].Thisworkremainstheonlymodelabletodescribetheconditionsunderwhichfirecanspreadbetweenvehicleswhichare100sofmetresapart,suchasoccurredintheMontBlancTunnelfireincident.FIRESIZEINTUNNELSAs noted above, the industry consensus from the 1970s onward was that a HGV fire in atunnelwouldbeabout20MW.ThisillusionwasshatteredwhentheEUREKAEU499projectwascarriedoutintheearly1990s[15].TheonlyfiretesttodateofaHGVtractorandtrailerwithafullcargooffurniturewascarriedoutinaminetunnelinthenorthofNorwayon12thNovember1992.Oneofthecrucialquestionsregardingthisfiretestconcernedwhattheheatreleaserateofthefireactuallywas.VariousmethodsofestimatingtheHRRonthebasisoftherecorded data (flow velocity measurements, gas concentrations, temperature, etc.) wereattemptedbyteamsofresearchersfromNorway,GermanyandtheUniversityofEdinburgh.The estimates of HRR varied considerably between the various teams of researchers, seeFigure1,butitistheHRRcalculatedbyDrGeorgeGrantandDrDougalDrysdale[16]whichhasbeenadoptedasthemostrealistic,andthemethodusedbyEdinburghhasnoweffectivelybecometheindustrystandardforestimatingheatreleaserateintunnelfires.ThusUniversityofEdinburghresearchwasinstrumentalinchangingtheperceptionofafireinatunnelfrombeing20MWuptoabout120MW.In the late1990s,Beard continuedhis research into tunnel firedynamicsby recruiting theauthor and turning their attention to the question of fire size. The study considered theinfluenceoflongitudinalventilationonfiresizeinaprobabilisticmanner,andconcludedthatfires could grow to bemuch larger than 120MW, depending on the ventilation conditions.They identifiedageneral trend towards larger fireswith increasing longitudinalventilationvelocity [17]. Rather than fixating on absolute HRR estimates, the probabilistic studyquantified the effect of velocitybymeansof amultiplier, relative to the expectedHRRof asimilar vehicle in anunventilated tunnel.While thismethodologyhasbeenquestioned andcriticisedbysome[18,19], thepredicted trends inbehaviourhavebeenpartiallyconfirmedandvalidatedbysubsequentstudiesandfireexperiments[20,21].

Figure1 EstimatesoftheheatreleaserateoftheEUREKAEU499HGVfiretest,adaptedfrom

[15]CRITICALVENTILATIONVELOCITYCritical ventilation velocity (CVV) remains themost studiedphenomenon in the tunnel fireliterature [22].The fundamental conceptof smokemanagement in longitudinallyventilatedtunnelsisthat,foragivensizeoffire,thereexistsa‘critical’ventilationvelocitysufficienttoblowall thesmokeproducedbya firetoonesideofthefire locationonly. If theventilationflow is below this level, a layer of smoke may extend away from the fire location in theupstream direction, this is commonly referred to as “backlayering”. Earlier attempts toquantifythevariationofCVVwithfiresizehadenteredindustrypracticeinthe1980slargelythrough themodeldevisedbyDanzigerandKennedy [23]whichwas incorporated into theSubwayEnvironmental Simulator (SES)model [24]. Thismodelwas based on only a smallnumberofexperimentaldata.Edinburgh alumni Dr Graham Atkinson and Dr YajueWu studied CVV in detail in the late1990s.Atkinson’sresearchwithYasushiOkaledtotheir1995paper[25]whichisprobablythemost influentialpaper inthe literatureandwasthefirst toadequatelydefinethe ‘supercritical’ventilationvelocity(SCVV)concept.Intheirexperiments,OkaandAtkinsonobservedthat there isa relationshipbetween firesizeandcriticalventilationvelocityup toacertainlimit,but thatbeyond this limitno increase inventilationwouldbe required to control thesmokefromfireswithlargerheatreleaserates(HRR).ThiscanbeseenclearlyinFigure2.

Figure2 AnexampleofthevariationofCVVwithHRR,basedonOkaandAtkinson[25]Formany typical road tunnels, the SCVV is found to be about 3ms-1, somany emergencyventilationstrategiesforlongitudinallyventilatedtunnelsaimtoachievealongitudinalflowofabout3ms-1 in theeventofany fire, inorder tocontrolsmoke.Ventilationstudiessince1995 have tended to build on the work of Oka and Atkinson, adding various complexitiesrelating to features such as tunnel slope, considered by Atkinson &Wu [26], aspect ratio,investigatedbyWuandothers[27,28],andthepresenceofblockages,investigatedinpartbyEdinburghalumnusDrMarkTsai[29].TheSCVVconcepthasbecomewidelyacceptedintheindustry.However,whentheindustryrewordeditsrequirementsforsmokecontrolintermsofcriticalventilation velocity, it may have inadvertently overlooked an important fire dynamicsphenomenon,the‘throttlingeffect’,whichwewillconsiderbelow.DESIGNFIRESAsnotedabove,the‘designfire’forsmokecontrolintunnelswasformanyyearsconsideredto be 20MW. The experience of the EUREKAHGV fire test elevated this estimate, in somecases, to about 120MW.But thedifferencebetween a 20MW fire and a 120MW firewasgenerallytakentobeafunctionofthenatureofthefuelpresent.Carvel&Beard’sresearch,mentionedabove,castthequestioninanewlightasithighlightedtheeffectthatventilationhasonfiregrowthandpeakfiresize.In past decades the ventilation designer would select a design fire, calculate the criticalventilationvelocityrequiredforsmokecontrolandspecifythenumberofventilationdevicesrequiredtoachieve this flow.Recentworkhasshownthe flaw in thisreasoning; increasingtunnelventilationuptocriticalventilationflowmayhavetheeffectofenhancingthefireandcausing it to burn with a higher heat release rate. In that case, more ventilation may berequired[30].Thesedaysatunnelventilationdesignerhastoconsiderthatthecapacityandcharacteristicsoftheventilationsystemwillpotentiallyinfluencethebehaviourofanyfiresinthetunnel,intermsoffiregrowthrate,peakfiresizeandpropensitytospread,asdiscussedbelow.

VENTILATIONANDFIREBEHAVIOURWe have already seen how ventilation can influence the peak size of a fire in a tunnel.Research at Edinburgh also touched upon the questions of fire growth and fire spread. In2008,theauthorfirstobservedthatexperimentalfiresinlongitudinallyventilatedtunnelsdonot exhibit ‘t2’ fire growth behaviour, as commonly expected in compartment fires [31].Rather, tunnel fire experiments generally seem to grow following a two step linear growthmodel;thefirststageofwhich(oftenreferredtoasthe‘incipient’stage)ischaracterisedbyarelativelyslowrateofburning,andthesecondstageischaracterisedbyaveryrapidgrowth,oftenatarateabove5MWperminute.Theseobservationsweremadeonthebasisofastudyof 12 different full scale fire experiments as discussed in [31]. A graph of the apparentrelationshipbetweenfiregrowthrateandlongitudinalventilationvelocityisshowninFigure3. From the presenteddata it is clear that low ventilation velocities (below1ms-1) exhibitcomparativelylowgrowthrates,below5MW/min,whilehighventilationvelocities(about6ms-1) exhibit higher growth rates, about 10 MW/min. However, the most remarkableobservationfromthisdataisthatsomefiresventilatedwithratesclosetoabout3ms-1,thatis,closetotheemergencyventilationvelocityusedinmanytunnelfirestrategies,exhibitveryhigh growth rates of 20 MW/min and above. This apparent relationship has yet to berigorouslyproven,butifthisbehaviourisfoundtobevalidingeneral,thenitwouldseemthatusingtypical‘emergency’ventilationcouldresultintheworstcaseforfiregrowth.

Figure3 The apparent relationship between ventilation velocity and fire growth, adapted

from [31]. The line should not be understood to be anything other than a simpletrendline,withafairlypoor‘fit’(R2=0.5)duetothescatterofthedata.

Theauthoralsobrieflyinvestigatedtheinfluenceoflongitudinalventilationonflametiltandextension,demonstrating(notsurprisingly)that increasedlongitudinalventilationgenerallyincreasesthelikelihoodofflamesfromavehiclefireimpingingonanadjacent,downstream,vehicle[32].TheUniversityofEdinburghwereinvitedbytheRailAccidentInvestigationBranch(RAIB)toassist in their part of the investigation into the 2008 Channel Tunnel Fire. The primaryquestioninvestigatedwasthemechanismwhichcausedveryrapidfirespreadintheincident,

froma localised fireat the time the incident train came toa stop, toa fire involving tenormore carriages about half an hour later. Analysis of the first three Channel Tunnel fireincidents suggested that themaindriving force in the rapid fire spreadwas the reversal inairflowdirection,whichoccurredontwooccasionsduringthe1996and2008fires,butnotduringthe2006incident,whichinpartexplainswhythefirestayedlocalisedonthatoccasion[33].At the timeofwriting, theUniversityofEdinburghareagainassistingRAIBwith theirinvestigationsintothe2015ChannelTunnelfireincident.VENTILATIONANDWATERMISTSince the spate of catastrophic tunnel fires at the turn of the century, the tunnel safetyindustryhaschanged itsstanceon theuseof fixedwater-basedsuppressionsystems in thetunnel environment. While not explicitly tunnel related, a review published by Dr GeorgeGrantetal.in2000[34]hasbecomeastandardreferencedemonstratingthebenefitsofwatersprays for fire suppression,both for tunnel applications and inbuildings.Grantwenton towork with Eurotunnel in their early testing of on-board water mist systems for the HGVshuttle trains[35].While thetestsweregenerallysuccessful, thesuppressionsystemswereneverinstalledinpractice,primarilyforeconomicreasons.However, theUniversityofEdinburghwereamong the first to express cautionwhenwatermistsystemsbecamecommonlyproposed foruse invehicle tunnels.WorkbyDrGuillermoReinetal.[36,37]suggestedthattheuseofwatermistsystemsintunnelsforfireprotectionwasincompatiblewiththeuseofhighlongitudinalventilationduringthesameincidents,asthe smallest (andmost effective)watermist droplets could be carried tens or hundreds ofmetresdown the tunnelbefore reaching the roaddeck, andsowould,most likely,miss thetargetfirealtogether.AnexampleofresultsfromRein’sstudy[36]areshowninFigure4.

Figure4 Calculateddroplettrajectoriesfordifferentsizesofwatermistdroplets,subjecttoa

3ms-1longitudinalairflow,adaptedfrom[36].The gauntlet thrown down by this work was taken up during the German funded SOLIT2project[38],whichdemonstratedtheeffectivenessofwatermistsystemsinblockingradiantheat,preventingfirespreadandprotectingthetunnelstructure.Whileitremainstruethatthelightestdropletsmaybeblownawaybythewind,theydoappeartoprovideeffectivethermalmanagementastheypass[39].(Whilethebenefitsofwatermistsystemsforthermalmanagement,protectionofpeopleandprotection of structures have been demonstrated through large scale testing, including theSOLIT2project,thecurrentuseofterminologysuchas“FixedFireFightingSystems”and“FireSuppressionSystems”remainscontroversial,asdiscussedbytheauthorinakeynoteaddressdeliveredatthe2012InternationalSymposiumonTunnelSafetyandSecurity[40].)

VENTILATIONANDEGRESSAsdiscussedabove,ventilationinfluencesfiregrowth,spreadandpeaksize.Ventilationalsoinfluences smoke production and behaviour. The inter-relation of these factors is oftenunclearwithoutdetailedstudy.A recentwork byMichaelWinkler (a postgraduate student on the two year “InternationalMaster of Science in Fire Safety Engineering” degree programme, taught jointly at theUniversityofEdinburgh,GhentUniversity inBelgium,andLundUniversity inSweden[41])investigated all the relevant interactions between ventilation, fire growth, peak fire size,smoke production, smoke toxicity, and passenger egress time for the case of fires onpassenger trains stopped in tunnels [42,43 ,44]. Various fire location scenarios wereconsidered, Figure 5 compares the predicted carbonmonoxide levels in the smoky egresspathsforescapingpassengersinthescenarioofafireonthesecondcarriageofthetrain,andpassengersinthescenarioofafireatthemid-pointofthetrain,forbothnaturallyventilatedandmechanicallyventilatedstrategies(theegresspathbeginsatthedooroncarriage1andextendsawayfromthefiretowardsacross-passageinallcases).Thesedata,andotherswentinto calculations of fractional effective dose (FED), which suggest that the majority ofpassengersescapingthroughthesmokewouldbecomeincapacitatedifforcedventilationwasused, while it appears unlikely that any would become incapacitated, escaping in eitherdirection,ifnaturalventilationwasadopted.

Figure5 COconcentrationsexperiencedinthetunnelbythefirstpassengerescapingfromthe

train,foreachofthescenariosconsidered,adaptedfrom[42].Note,theegresspathsaredifferent lengths inthetwoscenariosconsidered,whichiswhythedashedlinesendafter8minutes,butthesolidlinesextendto12minutes.

THETHROTTLINGEFFECTA collaboration between the University of Edinburgh and Politecnico di Torino, Italy, sawground-breaking work in ‘multi-scale’ modelling of tunnel fires being carried out by DrFrancesco Colella et al. [45,46,47]. This work enabled the ventilation performance of full

tunnelnetworks tobe analysed indetail in a computationally efficientmanner for the firsttime.Aswellasprovidinga framework for tunnelventilationanalysis for industry,oneby-product of the studywas the ‘rediscovery’ of the “throttling effect”, an interactionbetweentunnelfiresandventilationflowswhichappearstohavebeenreasonablywellknowninthe1960sand70s,butvanished from the tunnel fire safety literatureafter thatas the focusofinterestmovedtowardscriticalventilationvelocity,asdiscussedabove.In essence, the throttling effect is the tendency of a fire in a tunnel to resist longitudinalairflow;thelargerthefire,thegreatertheresistance.Thus,whilecriticalventilationstudieshaveshownthatnoincreasein longitudinal flowvelocity isrequiredtocontrolsmokefromfires larger than the ‘super critical’ limit, in practice, an increasing number of ventilationdevicesarerequiredtoachievethisflow,andhencecontrolthesmokefromafire,asthefiresizegrows.A recent study by Edinburgh undergraduate Arnas Vaitkevicius, together with Colella andCarvel,examinednumericallythethrottlingeffectphenomenon[48,49].Theeffectwasclearlydemonstratedintheirresults,asshowninFigure6.

Figure6 Variation of critical ventilation velocity and the number of jet fans required to

generateitwithincreasingfiresize,adaptedfrom[48].Theabovehasbeenpresentedtogiveaflavourofsomeofthecontributionsofthe‘firegroup’at the University of Edinburgh to advances in tunnel fire safety. Other relevantworks notdescribed above have also ranged from experimental flammability studies of asphaltroadways [50] to pioneering use of CFD in tunnel fire studies [51]. At the time ofwriting,studiesintosmokemanagementintunnelsunderconstructionandthebehaviourofconcretetunnelstructuresunderfire loadingareongoing,otherfuturetunnel firesafetyprojectsareintended.

CONCLUSIONTheUniversityofEdinburghanditsalumnihavemadesignificantcontributionstoknowledgeinthefieldoftunnelfiresafetyengineeringoverthepastfourdecades.Edinburghhasledthewayinreducedscaleexperimentalstudies,analysisoffullscaledataanduseofcomputationalfluiddynamicsintunnelfirestudies.Asnewchallengesariseinthefieldoftunnelfiresafety,researchersfromtheUniversityofEdinburghwillcontinuetostudythem,andtoadvancethestateoftheartinknowledgeabouttunnelfirebehaviour.ACKNOWLEDGEMENTSThispaperwould,ofcourse,nothavebeenpossiblewithoutthehardworkofgenerationsofresearchers at the university, including somenot even cited herein. Itwould also not havebeenpossiblewithout funding fromandco-operationwithmanypartnersovermanyyearsincluding, but not limited to, BRE, Arup, The Ove Arup Foundation, the Health & SafetyLaboratory, The Royal Academy of Engineering, EPSRC, IFIC Forensics, and ScottishEnterprise.Thankyoutoeveryonewhohashelpedmakeuswhatweare!REFERENCES1 McPherson,M.J.(1993)SubsurfaceVentilationandEnvironmentalEngineering.Springer,1993.

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