Experimentsonflows,boilingandheattransferinporousmedia:EmphasisonbottominjectionOriginalResearchArticle

Thispaperdealswithbasicexperimentsconductedtoanalysetheeffectoftheparticles’shapeandsizedistributiononintrinsicpropertiesofporousbedsaswellastwo-phaseflowandheattransferintheseporousmedia. Structural,transportproperties,flowlawsandheattransferwithphase-changephenomenainseveralkindsofporousmediaarepresentedanddiscussed.Theporosityofstacksconstitutedbyspheresofvarioussizesisanalysed.Avariationlawoftheporosityasafunctionofthestandarddeviationoftheparticlesizedistributionisproposed.Theporosity,tortuosity,permeabilityandinertialcoefficientoftheflowlawinrandomlystackedfibresareestablishedexperimentallyandtheoretically.Theporosityofsuchmediaisfoundtovaryfrom0.35to0.92accordingtothefibreaspectratio.TortuosityandKozeny–Carmanparametersaredeterminedbybothelectricandhydrodynamicmethods.Theseparametersarefoundtovarywiththeporosityofthefibrousbed.Newrelationsofpermeabilityandinertialcoefficientarederivedfromexperimentalresults.Finally,apressuredroprelationisproposedforfibrousbeds. Convectiveboilingphenomena,withemphasistoapplicationonbottominjection,areexperimentallydeterminedforfibrousporousmedia.Temperaturefielddeterminationevidencestheformationofthreedistinctzonesintheporousmedium:aliquidzone,atwo-phasezoneandasuperheatedzone.Forhigherheatfluxdensity,afourthzoneisfoundinwhichvapourandliquidareinthermalnon-equilibrium.Aone-dimensionalanalyticalmodelofpressuredropintwo-phaseconfigurationhasbeenperformed.Comparisonswithexperimentaldataarefoundingoodagreementwiththeresultsofthismodelformoderateheatfluxes.Forhigherheatfluxvalues,discrepanciesarefound.Thesecasescorrespondtotheappearanceandtheevolutionofthethermalnon-equilibriumtwo-phasezone.Heattransfercharacteristicsattheheatedwallsareanalysed.Formationofvapourintheneighbourhoodoftheheatedwallshasastronginfluenceontheheattransfercoefficient.Thisbehaviourmayberelatedtothecriticalheatfluxphenomenonencounteredinusualducts.