IL08 NEW UNDERSTANDING OF THE INFLUENCE OF STOKES NUMBER ON PARTICLE-LADEN JETS FROM SIMULTANEOUS PLANAR MEASUREMENTS OF NUMBER DENSITY AND VELOCITY

Abstract

Abstract The influence of Stokes number on the evolution ofthe distributions of particle concentration and velocityfrom their exit values is reported for a co-flowing jetissuing from a long pipe. The data are obtained bysimultaneous Planar Nephelometry and Particle ImageVelocimetry using four cameras to provide high resolu-tionthroughthefirst30jetdiametersandalsocorrectionforopticalattenuation. Thesedataprovidemuchmoredetailedinformationthanisavailablefrompreviousmea-surements. TheseprovidenewunderstandingofhowtheStokes number influences the flow at the jet exit planeandhowthisinfluencepropagatesthroughoutthejet. 1. INTRODUCTION Particle-laden turbulent jets, typically consisting ofsolidparticlessuspendedinaconveyingfluid,areanim-portantclassofflow,notonlybecauseoftheirfundamen-talsignificance,butalsobecauseoftheirwiderelevancetomanyapplicationssuchasintheflamesofboilersandfurnaces. Thecombustionintheseprocessestypicallyoc-cursintheturbulentregime,sothatparticle-flowinter-actionsinfluencethedistributionsofbothresidencetimeandstoichiometry. Hencetheyalsoinfluencethein-flamedistributionsoftemperatureandspeciesconcentrations,which in turn influence the key performance character-isticsoftheflamedimensions,radiantheattransferandpollutant emissions [1]. Therefore, an understanding ofparticle-laden flows is crucial to optimising combustionprocesses in industrial systems to enable the design ofsafer,moreenergyefficientcombustionwithloweremis-sions. Thesignificanceoftheseflowshasresultedincon-siderableefforttounderstandthemovertheyears,withtheresultthatmuchisnowknownaboutparticle-ladenturbulent jets [2–12]. However, the greater complexityoftheseflowsovertheirsingle-phasecounterpart,whichincludestheintroductionofmanyadditionalparameterstogether with the many additional experimental chal-lenges introduced both by the conveying fluid and themeasurement of particle-laden flows, means that manygaps in understanding persist. The overall aim of thepresentinvestigationistomeetthisneedthroughade-tailedandsystematicexperimentalinvestigationofwell-characterisedturbulent,particle-ladenjets.Thefullcharacterisationofparticle-ladenturbulentjets requires the simultaneous measurement of the ve-locity field of both phases, together with the local, in-stantaneousparticlesizeandnumberdensity. Preferablysuchmeasurementsshouldalsobeperformedsimultane-ously in multiple dimensions and with sufficient spatialand temporal resolution owing to the turbulent natureof the flow. This challenge is so great that previousexperiments of particle-laden jets have each performedonly limited measurements that provide incomplete de-tail of the flow and/or address only a small number offlow conditions. Additionally, the different experimentsperformed in each laboratory have all been performedwith different initial and/or boundary conditions, suchas particle size distribution, particle mass loading andnozzlegeometry. Furthermore,nopreviousinvestigationhas provided a complete description of the inflow con-ditions, such as detailed profiles of the mean and fluc-tuating components of the velocity and scalar fields atthe nozzle exit. Hence there is a need for detailed andcomprehensive measurements of the velocity and scalarfieldsofturbulentjetsspanningawiderangeofcontrolvariables,suchastheStokesnumber.Previous measurements of the velocity and scalarfields of particle-laden turbulent jets are shown in Ta-ble1. Ofthepreviousmeasurementsofthevelocityfield[2–12], only five [8–12] also report the concentration ofthedispersedphase. Ofthese,nonereporttheimportantStokesnumber(