8/3/2019 A Guide to the Design of Air Bubblers

1/11

Chapter39A GUIDETOTHE DESIGNOP AIRBUBBLERSFOR MELTINGICE

SimonInceHydraulicsSection,NationalResearchCouncilOttawa,CanadaINTRODUCTION

Theuseofair bubblersformaintainingice-freeareasinlakesandintheseahasbeenreportedabundantlyinthetechnicalliterature. Thisauthor(1962)reportedhisobservationsontwoair bubblerinstallationsintheCanadianArctictotheEighthInternationalConferenceonCoastalEngineering.Theresultsoftheseinvestigationswere,atthattime,stillinconclusive.oday,someofthemysteryisresolvedanditistheauthor'sopinionthattheexistenceofaheatreserveistheanswertotheproblem.asedonthispremise,anattemptismadeheretodevelopsomeguidelinesfortheproper utilizationofthisthermalreserve.

AIR BUBBLERSYSTEMSAreviewoftheliteratureandevaluationofexperi-mentsandobservationsbringoutthefollowingsalientpoints.

1. PREVENTIONOPICE-COVERFORMATIONTopreventiceformationtheoperationoftheairbubblermustbeginbeforefreeze-up. Ifalargethermalreserveduetostratificationexists,thentheupwardtrans-portandmixingofthewarmerstratasuppliestothesurfacelayerswateraboveitsfreezing pointandcompensatesfortheheatlossestotheatmosphere.hesizeoftheice-freeareadepends-asidefromatmosphericconditions-uponthe

temperaturestructureofthewater,thequantityofwatertransportedtothesurface,themixingintheverticalplumeandthetemperatureandvelocitydecayofthesurfacecurrent.Ifthereisinsufficientthermalreservetocompensateforheatlosses,thesystemmaystillworktoalimitedex-tent,providedthereisenoughturbulencecreatedonthesur-facetopreventtheformationofasolidicecover. Theheatlossesinthiscasearecompensatedbytheformationoffrazilice,eachgram ofwhichliberatesuponfreezingit slatentheatoffusion. Thefrazilparticlesarecarriedaway

bythecurrentsanddepositedontheundersideoftheadjacenticesheets. Theprinciplesinvolvedinthisprocessarethesameinfreshorseawater.600

8/3/2019 A Guide to the Design of Air Bubblers

2/11

AGUIDETOTHEDESIGNOFAIRBUBBLERSFORMELTING ICE0 12 . MELTINGOPANEXISTINGICECOVER

Themeltingofanicecover whenalargethermalreserveexistsi safunctionoftheflowfactorsoutlinedintheprevioussection. Ifthereisinsufficientthermalre-servewithintherangeofinfluenceoftheairbubblers,onlyalimitedamountoficecanberemovedfromtheundersideofthecover. Therearereports,however,thatbyusingunder-waterp u r i p s ,undercertainconditionsitispossiblet o"erode"theseaicebythemechanicalactionofabrasionratherthan meltitbythermalenergy. Thedisturbingpartoftheseclaimsi sthattheefficiencyoft h esystemisreportedtobemuchgreaterthan100percent,i.e.theenergyinputissmallerthanthatrequiredtomeltthevolumeofice. Thisisundoubtedlyanareawhich willrequiremoreresearch. Basedonreportsoftheoperationofthispump,i ti stheopinionofthisauthorthattwofactorsmighthavecontributedtot h esuccessoftheoperation.( a ) Theremusthavebeenintheregionasmallthermalreservenotdetectedbyconventionaloceanographicinstruments. Withtheverylargeamountsofwatercirculatedbythepumpthismayhavebeensufficienttohoneycombtheicecoverandcauseitsdisintegration.( b ) Theexperimentswereconductedinrelativelymildweather ( about0C)atwhichtimethestrengthofseaicehaddecreasedconsiderably,thuscontributingto

therapiddecay.SOMEAPPROXIMATERULESFORTHEDESIGNOP AIR-BUBBLERSYSTEMSOnthepremisethaticeremovalisduetothermaleffects,anattemptwillbemadet ogivesimpleworkingrulesforthepracticingengineer,toguidehimintheefficientdesignofairbubblerinstallations.Thehydrodynamicsofbubblecurtainsinhomogeneouswater hasbeenstudiedquiteextensively. Forthepractic-ingengineertheresultsofBulson(1961)andAbrahamand

Burgh( 1 9 6 i j . )aremostuseful.InsummaryandreferringtoFigure1 ,thefollowingrelationshipsarefoundt obevalid withinreasonablelimits.

( a )hemaximumhorizontalvelocity0ccursapproxi-matelyatadistance =d/2 1 ) ( b )0=1.2(g.qa)l/3 2 ) (c) dA 3 ) where

8/3/2019 A Guide to the Design of Air Bubblers

3/11

602 COASTALNGINEERING

o

U J oDC Q.>-h;OO_lUi>oz S >A ~

6UJ h-t- > ^i

8/3/2019 A Guide to the Design of Air Bubblers

10/11

AGUIDETOTHEDESIGN OFAIRBUBBLERSFORMELTING ICE09Ifthebubbleristobeinstalledatthebottom(d- 20m.),i tseemsreasonabletoassumethatthetemperatureofthehorizontaljetat =d/2illbeppproximately =1 C . Conveniently, Ts=0

With q = 0.0612cal/cm2/sec.X = 2000c m .k s 0.00133cal/sec.cm/CTl"Ts = 1C.V = 1.86 10"2cm2/sec.h- = 500c m .

equation(11)yields VQ=3 2cm/sec.Promequation(2), q 1920c m - ' / s e c / m .Theenergypersecondavailableintheairleavingthemani-fold,assumingisothermalconditionsandanairpressurejustsufficienttoovercomethestatichead,isgivenby

wherespecific weightofwaterYwHQ=atmosphericpressureinc m .ofwater.

Substitutionofvaluesyields,EA =207watts/m.

Forthesameconditions,ifthemanifoldissuspendedatadepthof10m , db =-r 250c m ,andweget m3

V0=b$ cm/sec,a 5 1 + 6 0j /m,and E.=373watts/m.CONCLUSIONS

Thepaperi sanattempttoputonamorerationalbasisthedesignofairbubblersformeltingtheicecoverandmaintainingice-freeareasinlakesandinthesea.incethereisnotsufficientexperimentaldatat osupportthevalidityofsomeoftherelationshipsusedfromthewall-jetanalogy,cautionshouldbeexercisedin puttingtoomuch

8/3/2019 A Guide to the Design of Air Bubblers

11/11

610 OASTALENGINEERINGfaithinexactnumericalresults.evertheless,itisbe-lievedthattheprocedureoutlinedabovemightbeusefulinestimatingtheorderofmagnitudeoftheairsupplyandpowerrequirements,onceinformationisavailableabouttheatmos-phericandoceanicorlimnologicenvironment.

REFERENCESInce,S.(1962).interregimeofatidalinletintheArcticandtheuseofairbubblersfortheprotectionofwharfstructures:EighthInternationalConferenceonCoastalEngineering,Mexico,pp.521-32.Bulson,P.S.(1961). Currentsproducedbyanaircurtainindeepwater:TheDockandHarbourAuthority,May,pp.15-22.Abraham,G.andBurgh,P.v.d.( I 9 6 I ] . ) . Pneumaticreductionofsaltintrusionthroughlocks:Proc.Am.Soc.CivilEngrs.,vol.90,No.HY1,January1 9 6 1 ; ,part1,pp.83-119.Sigalla,A.(1958)xperimentaldataonturbulentwalljets:AircraftEngineering,vol.30,pp.131-131+.Sidorov,A.(1957)*herelationofsurfacefrictionandheattransfer: SovietPhysics,vol.2,No.3,pp.l j . 9 9 - 5 > O l j . .