6.Suction capability and cavitation

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1、6SuctioncapabilityandcavitationDefinitionofterms:“Cavitation”meansthepartialevaporationofliquidinaflowsystem.Acavityfilledwithvaporiscreatedwhenthestaticpressureinaflowlocallydropstothevaporpressureoftheliquidduetoexcessvelocities,sothatsomefluidevaporatesandatwo-phaseflowiscreatedinasma

2、lldomainoftheflowfield.Thevaporcondensessuddenly(“implodes”)assoonasitistrans-porteddownstreamintozoneswherethestaticpressureagainexceedsthevaporpressure.Withincreasingextensionofthecavitatingzonewithtwo-phaseflow,theheadandefficiencyofthepumpmaybeimpaired,noiseandvibrationsexcitedandcom

3、ponentsdamagedthroughcavitationerosionundercertainconditions.Whenusingtheterm“cavitation”the“cavitatingflow”–i.e.theoccurrenceoflo-calzoneswithtwo-phaseflow–and“cavitationerosion”orcavitationdamagemustbewelldistinguished.Theterm“hydrodynamiccavitationintensity”isusedforthetotalityoftheim

4、-plosionenergyofallbubbles.Ifthisexceedsthe“cavitationresistance”,thema-terialwillbedamagedby“cavitationerosion”ifitisexposedsufficientlylongtotheattack.Thecavitationresistanceisamaterialpropertywhichisindependentoftheflowsystem.Thepossibilityofemployingcavitationprocessesforseparat-ingd

5、issolvedgasesfromliquidswasinvestigatedin[6.51].6.1Cavitationphysics6.1.1GrowthandimplosionofvaporbubblesinaflowingliquidAnysubstancecanexistinthephasessolid,liquidandgaseous.Thetransitionfromonephasetoanother(e.g.“formationofice”or“evaporation”)isdescribedthroughphaseequilibriumsinap-T-

6、diagram.Accordingly,thevaporpressurecurvepv(T)characterizestheequilibrium(the“saturationstatus”)betweenliquidandvaporfromthetriplepointtothecriticalpoint(Fig.6.1).Aliquidinconditionp1,T1withp1>pv(T1)canbeevaporated,ifitisheatedatconstantpressuretothesaturationtemperatureTv(p1)(pointV),or

7、expandedtothevaporpressurepv(T1)atconstanttemperature(pointKinFig.6.1).Duringphaseconversionfromliquidtovaportheevaporationenthalpymustbeadded.Inthereverseprocess,i.e.con-densation,theevaporationenthalpyisliberated.Duringcavitationasmallportionoftheliquidisexpandedatconst