ASTM E2872-14(2024)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E2872 − 14 (Reapproved 2024)
Standard Guide for
Determining Cross-Section Averaged Characteristics of a
Spray Using Laser-Diffraction Instruments in a Wind Tunnel
Apparatus
This standard is issued under the fixed designation E2872; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
In 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 moving
air streams. This guide presented is primarily applicable to aerial agricultural spraying, aerial forest
sprays, or air-blast spraying. Cases in which the spray is ejected into a quiescent gas environment that
lacks the unifying effect of a well-defined gas co-flow may require different techniques or
instrumentation or both. In this guide, an average droplet size distribution for the entire spray is
determined. It requires that the spray be statistically steady in time, but it may be polydisperse and
spatially non-uniform.
The droplet-size distribution used for characterization of a moving spray source must be determined
from a “flux-sensitive sample” or equivalent. This is because a flux-sensitive sample provides the
fraction of the total liquid flow rate contributed by each size class of droplets and, therefore, is directly
related to the spray coverage. In contrast, the LD instrument derives its droplet-size distribution from
a “spatial sample,” and therefore, its use for spray characterization is limited to test conditions under
which equivalence between flux-sensitive samples and spatial samples can be established. Such
equivalence exists when the velocity of all droplets of the spray is equal and creating these conditions
is the basis of this guide.
All tests relating to this guide require a wind tunnel with a test section of sufficient size that it
contains the entire spray plume up to the plane of measurement without droplets impacting the test
section walls under the prescribed operating conditions. The unobstructed wind tunnel air stream shall
be uniform and free of turbulence. The test air speed shall be chosen to match the relative speed of
the sprayer to the ambient conditions.
1. Scope centimetres or as large as a cubic metre. The droplet sizes are
assumed to be distributed non-uniformly within the spray
1.1 The purpose of this guide is to define a test procedure
volume.
for applying the laser diffraction (LD) method to estimate an
average droplet size distribution that characterizes the flux of
1.2 This guide is intended primarily to guide measurement
liquid droplets produced by a specified spray generation device
of performance of nozzles and atomizers using LD instru-
under specified gas co-flow conditions using a specified liquid.
ments.
The intended scope is limited to artificially generated sprays
1.3 Non-uniform sprays require measurements across the
with high speed co-flow. The droplets are assumed to be in the
entire spray cross section or through several chords providing
size range of 1 μm to 2000 μm in diameter and occur in sprays
a representative sample of the overall spray cross section. The
that are contained within a volume as small as a few cubic
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.
This guide is under the jurisdiction of ASTM Committee E29 on Particle and
Spray Characterization and is the direct responsibility of Subcommittee E29.02 on
1.4 Use of this guide requires that the instrument does not
Non-Sieving Methods.
interfere with spray production and does not significantly
Current edition approved Feb. 1, 2024. Published March 2024. Originally
impinge upon or disturb the co-flow of gas and the spray. This
approved in 2014. Last previous edition approved in 2019 as E2872 – 14 (2019).
DOI: 10.1520/E2872-14R24. technique is, therefore, considered non-intrusive.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2872 − 14 (2024)
1.5 The computation of droplet size distributions from the 2.2 ISO Standards:
light-scattering distributions is done using Mie scattering ISO 13320:2009 Particle Size Analysis — Laser Diffraction
Methods, General Principles
theory or Fraunhofer diffraction approximation. The use of Mie
theory accounts for light refracted through the droplet and there
3. Terminology
is a specific requirement for knowledge of both real (refractive)
and imaginary (absorptive) components of the complex index
3.1 Definitions—For definitions of terms used in this
of refraction. Mie theory also relies on an assumption of
standard, refer to Terminology E1620 and ISO 13320:2009.
droplet homogeneity. The Fraunhofer diffraction approxima-
3.2 Definitions of Terms Specific to This Standard:
tion does not account for light refracted through the droplet and
3.2.1 aerial spraying, n—practice of delivering spray via an
does not require knowledge of the index of refraction.
airborne vehicle such as a fixed-wing aircraft or helicopter.
3.2.2 atomizer, n—spray generation apparatus.
1.6 The instruments shall include data-processing capabili-
3.2.2.1 Discussion—Various definitions for “atomizer” are
ties to convert the LD scattering intensities into droplet size
defined in Terminology E1620 by construction and atomization
distribution parameters in accordance with Practice E799 and
method.
Test Method E1260.
3.2.3 co-flow, n—coherent, moving gas phase surrounding a
1.7 The spray is visible and accessible to the collimated
plume of spray droplets that significantly influences the direc-
beam produced by the transmitter optics of the LD instrument.
tion of movement of droplets in a spray plume.
The shape and size of the spray shall be contained within the
3.2.4 co-flow generation device, n—wind tunnel or other
working distance of the LD system optics as specified by the
device that creates a steady, uniform air stream in the plane of
instrument manufacturer.
measurement.
1.8 The size range of the LD optic should be appropriate to
3.2.5 concentration sensitive, adj—statistical quantity de-
the spray generation device under study. For example, the
rived from a spatial sample.
upper bound of the smallest droplet size class reported by the
3.2.6 droplet size distribution, DSD, n—mathematical or
instrument shall be not more than ⁄4 the size of D .
V0.1
graphical representation of droplet sizes of a given spray
1.9 The values stated in SI units are to be regarded as
frequently shown as a volume fraction, number fraction, or
standard. No other units of measurement are included in this
cumulative fraction distributions.
standard.
3.2.7 laser diffraction, LD, n—used in this guide to refer to
1.10 This standard may involve hazardous materials,
a class of laser droplet-sizing instruments known collectively
operations, and equipment. This standard does not purport to
as laser diffraction instruments, also used to qualify data
address all of the safety concerns, if any, associated with its
gathered using an instrument of this type.
use. It is the responsibility of the user of this standard to
3.2.8 monodisperse, adj—refers to a spray in which all
establish appropriate safety, health, and environmental prac-
droplets have identical size.
tices and determine the applicability of regulatory limitations
3.2.9 nozzle, n—spray generation apparatus.
prior to use.
3.2.9.1 Discussion—Various definitions for “nozzle” are
1.11 This international standard was developed in accor-
defined in Terminology E1620 by construction and atomization
dance with internationally recognized principles on standard-
method.
ization established in the Decision on Principles for the
3.2.10 number concentration, n—number of particles in a
Development of International Standards, Guides and Recom-
unit volume of space.
mendations issued by the World Trade Organization Technical
3.2.11 obscuration, n—percentage or fraction of incident
Barriers to Trade (TBT) Committee.
light that is attenuated as a result of extinction (scattering or
2. Referenced Documents absorption or both) by droplets.
3.2.12 obstructed, adj—refers to co-flow generation device
2.1 ASTM Standards:
when the spray generation device is mounted such that it
E799 Practice for Determining Data Criteria and Processing
interferes with the gas-phase co-flow.
for Liquid Drop Size Analysis
E1260 Test Method for Determining Liquid Drop Size
3.2.13 plume, n—ensemble of droplets that constitutes a
Characteristics in a Spray Using Optical Nonimaging spray.
Light-Scattering Instruments
3.2.14 polydisperse, adj—refers to a spray in which droplets
E1620 Terminology Relating to Liquid Particles and Atomi-
have different sizes.
zation
3.2.15 sample distance, n—separation between the sample
volume of the LD system and the spray nozzle.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
E2872 − 14 (2024)
3.2.16 sample volume, n—intersection of LD beam and the manufacturer’s specification should be consulted regarding
portion of the spray plume containing a measurable concen- vignetting and multiple scattering limitations of a particular
tration of droplets. instrument.
3.2.17 model liquid, n—fluid used to simulate the properties
5. Significance and Use
of density, viscosity, and surface tension of another fluid.
5.1 This guide provides a means of using an LD instrument
3.2.17.1 Discussion—Typically used to replace sprays that
to obtain a droplet size distribution from a spray in gas co-flow
are flammable, toxic, or otherwise deemed too dangerous to
that approximates a flux-sensitive sample.
use in a spray test.
5.2 In many sprays, the experimenter shall account for
3.2.18 spatial segregation, n—spatial non-uniformity of
spatial segregation of droplets by size. This guide provides a
droplet sizes resulting from aerodynamic forces or atomization
means of spatial averaging the droplet distribution.
characteristics or both.
5.3 The results obtained will be statistical in nature and refer
3.2.19 spray characterization, n—process of describing a
to the time average of droplet size distribution of the entire
spray based on a theory of measurement in terms of parameters
spray.
such as liquid flow rate, flux, patternation, particle size, and
5.4 This guide is used to calibrate a spray generation device
velocity.
to produce a desired droplet size distribution under prespecified
3.2.20 spray generation apparatus, n—device specially de-
environmental and co-flow conditions or characterize an un-
signed to transform a bulk liquid into droplets.
known spray while minimizing the uncertainty in the measure-
3.2.21 traverse, n—device used to move beam from one ment.
position to another in space with documented precision and
6. Apparatus
accuracy.
6.1 The measurement apparatus includes an LD system.
3.2.22 traverse, v—act of moving laboratory equipment in
This system should provide means for producing a collimated
space.
laser beam that passes through a region of the spray, a detector,
3.2.23 vignetting, n—in the context of this guide, refers to
or detectors for recording scattered light from droplets and a
the inability of an LD instrument to accurately estimate the size
means for transforming the observations into statistical droplet
of those droplets in a spectrum whose contribution to the
size spectrum.
diffraction pattern falls outside the reach of the LD receiver
6.2 Spray generation apparatuses vary widely and provision
optics.
for their mounting depends on the type of spray they produce
3.2.24 volume concentration, n—volume of droplets in a
and the conditions under which the spray is typically used. The
unit volume of space.
spray generation apparatus should be mounted in the test
section of a wind tunnel that provides a constant, uniform,
3.2.25 working distance, n—distance within which a droplet
low-turbulence, incident gas stream of a size sufficient to
of the minimum diameter of the range—as defined by the LD
enclose the entire spray generation apparatus, its aerodynamic
system manufacturer—of a given optical arrangement is said to
wake, and the plume up to the plane of measurement.
have been measured accurately by the LD instrument.
6.3 Gas phase velocity at the measurement plane shall be
4. Summary of Guide measured for uniformity and steadiness with respect to turbu-
lence intensity. Any number of available instruments including,
4.1 A description of the principles of LD measurements is
but not limited to, pitot tubes, hot-wire anemometers, and
provided in ISO 13320:2009.
ultrasonic anemometers may be used, Such equipment shall be
4.2 A method of data interpretation for LD data analysis is calibrated against an appropriate primary standard.
provided in Practice E799.
6.4 The wind tunnel used to enclose the spray shall provide
gas co-flow velocities representative of relative velocity be-
4.3 A typical LD sample volume is idealized as a long, thin
tween the sprayer and the environment in the simulated spray
cylinder passing through the spray plume. The sample volume
application.
is delineated by the diameter of the laser beam and the edges
of the spray plume. The procedure in this guide covers methods
6.5 Optical access to the spray may be direct or via
of traversing the sample volume across the spray plume
viewports (approved by the LD manufacturer), slots, or holes
suitable for LD measurements of spatially irregular and non-
in the walls of the test section. Wherever possible, the LD
uniform sprays. The aim of the procedure is to determine a
instrumentation should be mounted such that its housing is
single droplet size distribution that is equivalent to a flux
entirely outside the spray and co-flow region or, at the very
sensitive sample.
least, in a location where it does not significantly impinge on
the spray plume or uniform co-flow region. In situations in
4.4 It is important to position the LD instrument at an
which aerodynamic fairing or waterproofing or both is applied
appropriate axial distance from the nozzle or atomizer along
the mean direction of co-flow to ensure complete primary and
secondary droplet breakup, minimal droplet velocity variation,
Bagherpour et al., “Droplet Sizing and Velocimetry in the Wake of Rotary Cage
and avoidance of vignetting and multiple scattering. The Atomizers,” Transactions of the ASABE, Vol 55, No. 3, 2012, pp. 579–772.
E2872 − 14 (2024)
to the LD device to minimize the effect of the obstruction, care measurements of known calibration standards. The range of
shall be taken to prevent any accumulation and shedding of obscuration for the stated uncertainties shall be respected in all
droplets from the obstruction into the LD beam path. testing.
6.6 The spray may remain stationary in the center of the air
7. Reagents and Materials
stream and the beam traversed relative to it, or the beam may
7.1 In many cases, the spray generation device is designed
remain stationary and the nozzle or atomizer traversed relative
to operate with a single specific liquid. This may be of any kind
to the beam. Choice of traversing method should reflect the
including flammable, toxic, or otherwise hazardous substances.
size of the spray and the dimension of the uniform gas phase
Such formulations are manufactured in bulk by companies and
velocity region. At no time may a spray be traversed to a
are typically marketed under a brand name. All testing fluids
location where wind tunnel walls or boundary layers alter the
should be accompanied by information sheets (Material Safety
spray plume or the aerodynamic wake of the spray generation
Data Sheet [MSDS] and Workplace Hazardous Materials
device. Traversing systems for either case should be robust,
Information System [WHMIS]).
enable position repeatability to 60.5 %, and preserve align-
7.2 For environmental or safety reasons, it may be desirable
ment of the LD system within the manufacturer’s specification.
to use an alternate model liquid that simulates the physical
6.7 LD systems are very sensitive to changes in optical
properties of the specified liquid such as viscosity, surface
alignment, and wherever possible, the instrument should be
tension, and density. These fluid properties are central in
clamped to a rigid optical table or rail to ensure alignment of
determining the size of the droplets that will be produced under
the transmitter and receiver throughout a given test. Changes in
a given set of ambient conditions.
optical alignment can result from non instrument-related influ-
7.3 Whatever liquid is used for testing purposes, its physical
ences such as excessive vibration, surface variation in
properties shall be carefully measured and noted as part of the
viewports, and significant changes in the gas refractive index
test record. It is advisable to maintain the test liquid at a
from heating or the presence of volatiles. Care should be taken
controlled temperature since temperature affects density and
to avoid causes of misalignment and correct problems in the
viscosity, which in turn affect the droplet sizes produced by a
apparatus wherever possible. Remedial action for vibration-
given device.
caused misalignment is described in Section 9. Bear in mind
that LD may not be an appropriate droplet-sizing method if the
8. Calibration and Standardization
wind tunnel apparatus cannot be made to accommodate align-
8.1 Calibration standards are necessary to verify the correct
ment sensitivity restrictions.
operation of the LD instrument, software and internal align-
6.8 When optical access is through viewports, it may be
ment of optical components is according to specification.
necessary to evaluate a background measurement at each
8.2 Correct operation of the LD instrument shall be con-
traverse location to account for the variation in optical path at
firmed using the LD system manufacturer’s current
each position. At no time may the spray droplets impinge on
specification, at the manufacturer- recommended frequency.
the windows during a test. If the viewports cause beam
Certification documents shall be kept on file.
misalignment to the extent that inner become disabled or if the
8.3 The instrument shall be fully serviced per manufacturer
incident intensity of the laser beam is reduced by more than
recommendation, and its performance verified by a
20 % or both, as compared to beam intensity in the absence of
manufacturer-certified technician on a regular basis. Instru-
viewports, the viewports shall be cleaned or replaced.
ment performance shall be verified by a manufacturer-certified
6.9 In situations with evident and high-amplitude vibration
technician in the event of a major disturbance or effect.
that causes misalignment of the apparatus, there should be
8.4 Wind tunnel velocity measurement should be periodi-
provision on the optical bench for vibration isolation.
cal
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