ASTM_E_2872_-_14.pdf

Designation:E287214Standard Guide forDetermining Cross-Section Averaged Characteristics of aSpray Using Laser-Diffraction Instruments in a Wind TunnelApparatus1This standard is issued under the fixed designation E2872;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.INTRODUCTIONIn this guide,test methodologies are described specifically relating to the use of laser diffraction(LD)instrumentation to estimate the droplet-size distribution for liquid sprays released into movingair streams.This guide presented is primarily applicable to aerial agricultural spraying,aerial forestsprays,or air-blast spraying.Cases in which the spray is ejected into a quiescent gas environment thatlacks the unifying effect of a well-defined gas co-flow may require different techniques orinstrumentation or both.In this guide,an average droplet size distribution for the entire spray isdetermined.It requires that the spray be statistically steady in time,but it may be polydisperse andspatially non-uniform.The droplet-size distribution used for characterization of a moving spray source must be determinedfrom a“flux-sensitive sample”or equivalent.This is because a flux-sensitive sample provides thefraction of the total liquid flow rate contributed by each size class of droplets and,therefore,is directlyrelated to the spray coverage.In contrast,the LD instrument derives its droplet-size distribution froma“spatial sample,”and therefore,its use for spray characterization is limited to test conditions underwhich equivalence between flux-sensitive samples and spatial samples can be established.Suchequivalence exists when the velocity of all droplets of the spray is equal and creating these conditionsis the basis of this guide.All tests relating to this guide require a wind tunnel with a test section of sufficient size that itcontains the entire spray plume up to the plane of measurement without droplets impacting the testsection walls under the prescribed operating conditions.The unobstructed wind tunnel air stream shallbe uniform and free of turbulence.The test air speed shall be chosen to match the relative speed ofthe sprayer to the ambient conditions.1.Scope1.1 The purpose of this guide is to define a test procedurefor applying the laser diffraction(LD)method to estimate anaverage droplet size distribution that characterizes the flux ofliquid droplets produced by a specified spray generation deviceunder specified gas co-flow conditions using a specified liquid.The intended scope is limited to artificially generated sprayswith high speed co-flow.The droplets are assumed to be in thesize range of 1 to 2000 m in diameter and occur in sprays thatare contained within a volume as small as a few cubiccentimetres or as large as a cubic metre.The droplet sizes areassumed to be distributed non-uniformly within the sprayvolume.1.2 This guide is intended primarily to guide measurementof performance of nozzles and atomizers using LD instru-ments.1.3 Non-uniform sprays require measurements across theentire spray cross section or through several chords providinga representative sample of the overall spray cross section.Theaim of multiple-chord measurements is to obtain a singledroplet size distribution that characterizes the whole sprayrather than values from a single chordal measurement.1This guide is under the jurisdiction of ASTM Committee E29 on Particle andSpray Characterization and is the direct responsibility of Subcommittee E29.02 onNon-Sieving Methods.Current edition approved April 1,2014.Published May 2014.DOI:10.1520/E2872-14.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 1.4 Use of this guide requires that the instrument does notinterfere with spray production and does not significantlyimpinge upon or disturb the co-flow of gas and the spray.Thistechnique is,therefore,considered non-intrusive.1.5 The computation of droplet size distributions from thelight-scattering distributions is done using Mie scatteringtheory or Fraunhofer diffraction approximation.The use of Mietheory accounts for light refracted through the droplet and thereis a specific requirement for knowledge of both real(refractive)and imaginary(absorptive)components of the complex indexof refraction.Mie theory also relies on an assumption ofdroplet homogeneity.The Fraunhofer diffraction approxima-tion does not account for light refracted through the droplet anddoes not require knowledge of the index of refraction.1.6 The instruments shall include data-processing capabili-ties to convert the LD scattering intensities into droplet sizedistribution parameters in accordance with Practice E799 andTest Method E1260.1.7 The spray is visible and accessible to the collimatedbeam produ