ASTM International

Standard Guide for Determining Cross-Section Averaged Characteristics of a Spray Using Laser-Diffraction Instruments in a Wind Tunnel Apparatus
 N E2872

Annotation

The purpose of this guide is to define a test procedure for applying the laser diffraction (LD) method to estimate an average droplet size distribution that characterizes the flux of liquid droplets produced by a specified spray generation device under specified gas co-flow conditions using a specified liquid. The intended scope is limited to artificially generated sprays with high speed co-flow. The droplets are assumed to be in the size range of 1 to 2000 m in diameter and occur in sprays that are contained within a volume as small as a few cubic centimetres or as large as a cubic metre. The droplet sizes are assumed to be distributed non-uniformly within the spray volume.

This guide is intended primarily to guide measurement of performance of nozzles and atomizers using LD instruments.

Non-uniform sprays require measurements across the entire spray cross section or through several chords providing a representative sample of the overall spray cross section. The aim of multiple-chord measurements is to obtain a single droplet size distribution that characterizes the whole spray rather than values from a single chordal measurement.

Use of this guide requires that the instrument does not interfere with spray production and does not significantly impinge upon or disturb the co-flow of gas and the spray. This technique is, therefore, considered non-intrusive.

The computation of droplet size distributions from the light-scattering distributions is done using Mie scattering theory or Fraunhofer diffraction approximation. The use of Mie theory accounts for light refracted through the droplet and there is a specific requirement for knowledge of both real (refractive) and imaginary (absorptive) components of the complex index of refraction. Mie theory also relies on an assumption of droplet homogeneity. The Fraunhofer diffraction approximation does not account for light refracted through the droplet and does not require knowledge of the index of refraction.