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MAY 1 6 19&& LANGLEY MESEARCH CUHE~

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NASA Contractor Report 174636 3 1176 01324 3564 __ _

Analys1s and Test1ng of a New Method for Drop S1ze Measurement Us1ng Laser L1ght Scatter Interferometry

w. D. 6acha16 and M. J. Houser

Aerometr1cs, lric. Mounta1n V\ew. Ca11forn1a

(NASA-C~-174Lj6) dJAlY~lS AN~ I~Sll~G Uf A ~l:IO/ i1ETdlJ;:; FI.)l\ Li,Ll' S ... Z;': i'iLA;jr.;J.·L1";;~'I U511,G :'ASEH "Cid.'IEh 11 • .i:LHiEl-.C:1ETH £'llldl L;;port

----_.---

(lleroilli:.u:ics, LIC.) 6d p he 1\04/i11' Aul UIlel.l;;:

August 1984

Prepared for'

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION lew's Research Center

(~C~ 14c G3/35 20187

., Under Contract NAS3-23684

I" t 'rl

NOMENCLATURE ...

1.0 INTRODUCTIO;\;

Table of Contents

2.0 THEORETICAL DESCRIPTION

2.1 Light Scattering Theory ...... .

2.2 Phase/Doppler Spray Analyzer Technique

3.0 EXPERIMENT . . . . . . . . .

3.1 Description of tbe Apparatus .

3.2 Experimental Verification of the Theory

3.3 Signal Quality Effects ...... :.

4.0 St;MMARY A~D CO;'l;CLUSIOr-;S

REFERE~CES . . . . . . . . . . . . . . .

. iii

1

5

·5

13

16

16

20

24

28

30

This Page Intentionally Left Blank

NOMENCLATURE

C(m.O) proportionality constant

E elect ric field

lInt light b('am intensity

10 light beam intensity at the measurement station

L optical path length

S number of drops per unit volume

p pov.w

Qc:1 extinction coefficient

QUQI light scattering coefficient

S(o. m. OJ scattering amplitude functions

\' visibility

l'mill' \ 'm:n sign al \'olt agp levels

a drop radiu~

d drop diamt'tt'T

db Ia~er b('am diameter at transmittpr lens

d:;. beam waist diam('ter

J focallrngtb

11. I:! squares of tb(' moduli of the amplitudp runction~

j = V=-l imaginary number

m refractiH'indrx

n Ilumber of particles in size class

r polar coordinate

s b('am s('paration at the transrnitt('t It'ns

timt'

X,Y cartt'siall coordinates

iii

PRECEDING PACE BLANK NOT FILMED i)M'G: \ \JIIi'!"("I\'}'I"IJA1! \I ,' ... \.'~' ~-Ltw*-,~ n .... ij Vn 1..4.1 .lL.I-II<!,

o = i.e/I).. size parameter

') beam intersection angh'

b fringe spacing

'7 phase due to optical path length

(J scattering angJp

).. laser wavelength

r. 3.142

(1 combined phase due to focal lines, optical path, and reflection

., light ray incident angle to the surface tangent

.,., refracted ray

o phase shift of th(' Doppler signah;

tv

1.0 INTRODUCTION

ORIG!N,l\L P/\G;:: l~]

OF POOR QWHJn

A resea.rcb progra.m was conducted to eva.JuatE' an innovative concept for spray drop

siZl> and velocity cbaracterizat ion. This program was motivated by the increasing demands

on diagnostic techniques which have accelerated with the need to rorm spray drops having

controlled size distributions, spray patterns, and mass flow rates. The goa.l or this program

is to develop an instrument for fuel spray combustion applications. However, the required

capability oC efficiently and accurately characterizing sprays is associated with a much

larg('f rang!:' of technologies. These applications include coal-oil and coal-water slurry

combustion, nuclear reactor saCety, aircraft icing, a~icultural sprays, meteorology and a

variety of industrial processes.

Thl> development of advanced laoSer-based instrumentation remains of importance to

tbe progress of experiml'ntaJ research andpr{'dictive code development in fuel spray com­

bustion .. Increa..<;ed Knowledge and undel"Standing of the fluid dyna.mics and combustion

chemistry are required in the development of aircraft engine combustors to enabll' con­

tinuous and efficient burning of fuel at high rates ""it'ha, minimum of soot' and other'

pollutant formation. Prest'ntly thert' is considerable interest in obtaining measurem('nb

of thf' fuel ane! air motions within the spray dropl(·t combustors. It is known that tbe tur­

bul('Ilt motions infiuenr(' th" rea('tions by increa.sing the oxygen supply to the burning fup!.

The rela.t i\'e velor it i('S between t he gas pbase and liquid droplets not only provid('s oxygPli

to tb(· burning droplet but also serves to remove tbe products of ('ombustion and bell(,(',

affects tbe evaporation and burning ratt'S and the poilutant formation. Thus, tt\:' drop!!'!

size distribution and velocity are importa.nt data to be obtained when characterizing the

combust ion process.

Basic r('Searcb ha.s also been conduded on two-pha.<-e turbulent j€'ts1.:!.3,4 to furni~h

a ba..<;is for the development of predirtin codes tbat may ultimately describ(' the' fluid

dyna.mics of spray rombustioD. This work has revealed that the turbulence structure of tll(·

continuous pha.se is altered by the presence of tbe dispersed phase. As may be exppcled.

the dispersed pha.<;e reduces the turbulense spectral intensity. in particular tlH' hibhl'r

frt'quencies. siure the dispersed pbase droplets do not tra~k the turbulent eddy fluctuatioIlS

and thus, increase th!:' dissipation of turbulent kim .. tic en (;rf:,,)·. Experimental in"esligati0n~

1

)

" /

._~ _..: ... : .. I. ::::{

uHICNi~,L. 'Pj':.~~.,: ','J

OF po on QU;\:...iC\/

<> ........ ~.~~" •• ,'- \--':"

have been carried out to measure the turbulence chara.cteristics with solid monodispersed

particulate. The need to obtain a complete data base using monodisperse and poly disperse

liquid droplets requires a. reliable means for drop size and velocity.measurement and (or

the separation of the gas pbase and liquid droplet velocity measurements.

The need ror simultaneous size and velocitv ~ea.suremeDts bas been recognized even

for the general characterization of sprays. This is especially true when studying nozzles

that generate droplets that a.re moving at a. relatively high velocity and subsequently relax

to the ambient Bow speed in accord witb their initiai momentum. The differing flux of the

various size classes can affect the measured size distributions depending on the metbod of

measurement.

In general, droplet sizing imposes unique constraints upon the techniques that may be

utilized. Droplets are deformable, break up under aerodynamic forces and collisions with

probes, contaminate surfaces, and are often present at high Dumber densities. Sampling

and otb!;'r material probe methods are therefol'e less desirable for implementation in spray

measurements. Optical methods consisting of im?-ging and laser light scatter detection

have been applied with varying degrees of success. The relative success of a method is

largely dependent upon the measurement environment, droplet size and number density,

type of matE.'rial forming the droplets, and othE.'r physical constraints of th,e test apparatus.

Instrument limitations are dependent upon the physical concepts incorporated Gto the

measurement de\'ice and how these cODcepts are affected by mteraction with the surround­

ing spray, aerodynamic and temperature fields. Techniques that have proven reliable in

ODe measurement situation bave produced erroneous results in others.

Because of the high droplet Du:nber densities in\'olved, most light scattering method~

classified as single particle counter systems will not function properly in spray environ­

ments. Basic methods utilizing absolute light scattering intensity measurements will ex­

perience significa.nt errors due to partial extinction of the laser beam and scattered light.

For these rE.'asons, the small angle scatter detection methods developed by Dobbins et

a\.5 and Swithellbank et a1.6 bave been favorably received. tJnfortunately, instrumC'nts

based on these concepts can only measure the Sauter mean diamet<.>r (S~ID) dirl'ctly.

Furthermore, these measurements are made_o~er' the entiret'xposed path of tb(' laser beam

2

,~/

k: __ . ':' .. : .... \., \ •.• -,:t I -:'~'

OflIGINN •• f),e.c.': ,j OF POOR QUj:~L1TY

which resul1s in poor spatial resolution. The methods cannot produce inCormation on th(·

droplet number density or the velocity.

Simultaneous measurement of the droplet size and velocity bas led to the combinat ion

oC the laser Doppler velocimeter (LDV) witb particle sizing methods. Particle sizing

interferometry developed by Farmer7 was a significant attempt in achieving this' goal. The

method utilized a standard LDV optical arrangement with on-axis light scatter detection.

Signal visibility was processed to obtain information relatable to the particle diametE't. The

method was however. limited to very low number density particle or droplet environmenb.

YulE'. et al8 attE'mpted to make simultaneous size and velocity measurements using an LDV

with small off-axis angle scatter detection. The droplet size inrormation was obtained from

th(' amplitude of the s('attered light intensity. The difficulties involved with the Gaussian

beam intensity distribution notwithstanding, the measurements were also affected by the

partial extinction of the laser beams and the scattered light.

Bachalo9 described a method that eliminated many of the aforementioned diflieulti('s

by producing a rigorous an:;dysis of the dual beam laser light scatter for large off-axic;

r('ceiYE'T angles. By using large angle light scatter detection, the measurement region couid

be reduced by as much as two orders of magnitude compared with the on-axis approach

This contribution suggested that the measurement of the size and velocity oC individual

droplet~ might be possible in dense sprays. Cse of the light scattered by reflection and

refraction allowed the measurement of dropl~ts as large as several millimeters in diame'ter.

The Droplet Sizing Interferometer based on the technique proved t.o be successful

in producing droplet size and velocity and size-velocity corrE'lations measurements under

limited spray conditibns. However, because of the signal processing methods used. th(·

system had some serious limitations. These included the need Cor extremely cardul

alignment, a size rangr sensitivity of a decade or less witb reduced size sensitivity at thl'

small size end of the range, a narrow Doppler frequency response range and sensitivity to

laser beam quality and relative intensity of the two beams.

Because of the significant potential advantage's of the light scattering interferometry

method, a new means fur processing the interference information was derived. This ~~

f!

innovation promised to increase the dynamic range of the syst~m while reducing the

possibility of measurement error resulting from imperfect alignment or beam intensity

differences at the probe volume. The concept resulted in a linear response to the drop size

with a r.elath·ely uniform sensitivity over the entire measurement range. A measurement

range of o\"('r a fa~tor of 30 can be attained while a factor of 100 may be possible without

optical adjustment but with only a ~haDge in photodetector gain.

In the following sections, a theoretical description of the concept is presented and the

experimental setup of the device is described. The test procedures used to evaluate the

method are outlined and test results are presented.

4

"\

.. ) -.

2.0 THEORETICAL DESCRIPTION

2.1 Light Scattering Theory

The scatt.ering of light by homogeneous dielectric spberes of arbitrary size is described

exactly by the well-known Loren~-Mie theory. However, even with large computers, the

computation time rE:'quired to calculate the scattering coefficients for a range or drop sizes

can bE:' prohibitive. The number of terms needed to be computed in the series solution is

proportional to the size parameter 0 = lid/A. Fortunately, for drop sizes greater than

thE:' light wavE:'length A, simpler asymptotic theories may be used. For spherical scatterers

that can be regarded as small spherical lenses or mirrors, the geometrical optics methods

produce accurate results.

van de Hulst 10 showed that for spheres much larger than the light wavelength and

with refractive index sufficiently different from the surroundings, the amplitude functions

derived from the geometrical optics approa('h were, in the asymptotic limit, equal to the

~1ie ampJit ude ru))('tions. Comparisons have been made to the exact Mie theory by a , ' . " 'I ~

number of research~r~ll:l:! to demonstrate the accuracy of the geometrical optics methods.

Yery good agreement was dE:'monstratedby Glantschnig and Chen12 for drops as small a::;

5 micrometers ..... an de Hulst demonstrated that for 0 > 10, the scattering of light can be

separated into the simplified theories of diffraction, refraction, and reflection.

Sca.ttered Intensity

Light scattered by diffraction which is described by the following expression

S. (0,8) -:- 02

A[J1(OSin 8)] d,!! 47r 0 ~in e (1)

where Jl is the Bessel's {unction and 0 is the scattering aUble is concentrated in a lobe

centered about the transmitted beam. This forward scattered light becom(:s more intense

by the diameter squared and smaller in angular distribution with increasing drop size.

Hodkinson and Greenleaver11 have shown that. for 0 > 15, diffraction becomes insignificant

at scattering angles greater than 10°. For spheres of refractive index m - 1.5 and

diameters as stnall as 2 micrometers, diffractive scatter is less than 10% of the total light

scattered at 45°. Thus, the light s('attered by diffraction canbe avoided when desired, by

proper placement of the r(>cciver.

5

;J '.<

l

" '"' .

ORIGINt\L rf{f:~:': ;", OF POOR QUt\!_:',

Light sC'aHered by refi('rtion and refraction is best described in terms of rays and the

use of tht' simple laws of refiedion and refraction, When a ray impinges on the surface of

a transparent sphere it produces a reflected and refracted uy, figure 1. The direction of

the refracted ray follows from Snell's law:

cos T = m cos r' (2)

whNe T and.T' are the angles between the surface tangent and the incident and refracted

rays. respectively. The partitioningof the energy into the two rays follows from the Fresnel

The emerging rays are characterized by two parameters, tbe angle T of the incident

ra\" and the integer p o( the interface, from which it emerges, That is, p = 0 (or the first . ,

surfacr reflection. p = 1 for the transmitted ray and p = 2 for the ray emerging after

one internal reflection, The energy in the remaining re~t'ctions is insignificant. The angl~""

betw('en thp incident ray and tbe pth emergent ray is given by

8 = 2(pi' - i) (3)

The fra("~ion of the incident intensity contained in tbe emergent rays can be obtained from

the Frbnrl coefficients. The details of derivation of the closed form 8-dependent intensity

functions are given in GJantschnig and Chen and only the results will be quoted herE',

In thr- notation of van de Hulst, the scattered light is described in terms or two

amplitude functions, SI(O', m, 0) and S2(0', m, 8) for the perpendicular (to the scattering

plane) and parallel incident polarizations, respectively. van de Hulst demonstrated that a

Jargespbere scatters over. flO% of the incident light in the rorward direction with gg.SC'(·

of the for,,'ard scattered light emerging from tb(· first two interfaces (p = 0 and p = 1),

Thus, only the first two terms of the series describing the rays emerging from the various

interfaces need be considered for most practical applications.

Glantscbnig and Chen obtained the following expressions for the amplitude function

6

)

ORIG'Nlil !:;:iI''''.'''' .... • ,... •• -IUi>::' ~~~ .

OF POOR QUALlTV

drs('rihingthr rays rd1c·('tNI from tllr first surfu("\, of the sph<'T(':

. 0 C' ;'''---:1 7f [ ( )] (I). Sin;, - V m~ - (,OS"-.) 1. r. . 0 S I (n. 111. 0) = 0 .. ··-···~·---·---=;=oo !:. - ex p ) .... + 20 Sill ..

. 0 C'" ,., 0 ? ? ? .' Sill ~ + V rn~ - ros~ 2 - - -

(.t)

(S)

wht'rl' suprrs('ript 1 implies thr p = 0 refir('tioll anQsubscripts 1 and 2 repres('nt til,'

pNp('ndic-ular and parallel polarizatiolls respertivdy. Th(' amplitude tundions for tlH' r:ly,;

enwrgillg from th(' second surfac-e aftl'r retraction by the sphl'f(' aregi\'en as follllws:

sil( 0 m.O) = + -C I 75~~i;,~s=t rlF':~~n~~; :o:,t~ill~~~i'j;~) (.1

X .x+( t" -20f +-~,~-=;';;;~sOl

Summing thp arnpliludp functions. thr total tor forward light scalterill~ is gin'lI a~

8)0,771,0) = Sdil 1(0,0) + S,)(o, m, OJ + 87(H. m. OJ

The dirnensionlrss intensities ij are simply

ij(I}. m, 0) = IS,;(o. 711, 0)1 2

7

j = ],2

.1

ORIGlNtl.L FF,'::;~ ~':r OF pom~ QU;\UYY

These expressions produce accurate results with the exception of s('attering at grazing or

nearly grazing incidence and for scattering into the rainbow angles as indicated by van de

Hulst. Th(' geometrical optics results can be used for values of Q~ 15 depending upon the \

information (what s('attering angles) and accuracy required. Comparisons of an ~xample

calculation with Mie theory (from Ref. 12) are sLown in figure 2. The agreement for thC'.

5J1m diamett>r droplets is very good even j hough this is near the minimum size wherein

the geomet.rical optics approximation applies.

For spheres large enough (d > 3J1m) to satisfy the above critNia. the scatt('red light

intensity appears as an obvious means for size analysis. The square of the diameter of the

spherir;ll scatt t>rE'C ('an be assumed to be proport ional to the scattered light intensity!

/..cadd, m, 0) = e(m, 0)· d'2 (10)

This rplat ionship is correct provided that the receiver fino. is smal: enough to form an

awrage over the angular fluctuations resulting fcom interference !}etween the ceflected

(p = 0) and refracted (p = 1) light. The measured light intensity is also dependent

upon tht' incident intensity 10 which is muitipl",d by the scatterir.'g coefficient, Qn,li'

In practic('. the optical collection efficiency and losses in the system are determined by

calibrations using spherical particles or drops of known size. t:nfortunately in realistic

droplet environments, the droplets wili attenuate the transmitted bea;;..:> and the scattered

light by an indeterminate amount.

For example, the light intensity scattered by a cloud of drops is

k

Qua/.N = 2: niCdi)I~cadm, 0, dil i=l

(11)

where the ni are the number of parti('les of size d j . The loss of power due to transmbsion of

the beam through a droplet. fi{'ld of path length, L is given by the wdl known Lambt'rt-Brer

lawa.s

dP = -kextlAdx

8

( 12)

..

ORIGINAL PtJ.GE ;~3 OF POOR QU.::\LITY

and the integrat.ion over the path length L gives

( 1:3)

whE.'re Jint is the incidE.'nt. intensity where x = O. It is known that for large Q the extinct ion

co('ffici('nt approarhps the value of 2 asymptotically. Tbat is,

lim Qcxt = 2 a"" co

(14)

This is often referred to as tbe extinction anomaly since large spheres would be expected

to block th(' light falling on there cross-sectional area. However diffraction whk!: i" an

edge efft'ct also scatters light proportional to the drop cross-sE.'ction;l] area. In tIl<' abs('n('p

of any absorption,

( 1:))

WhNt' a is th(' avcrage drop radius in the field. Benre,

( IG)

where'\" is the numher of droplets pet unit volum'~ and L is the optical path length through

the spray. For examplE.'. a spray with a mean drop size of 50llm at a number dE.'nsity of 100

drops/cc and a widt h 0(20 em will have an attenuation of lo/Iinc = o.gz. Assuming. thaI

Q,.cal is proportion31 to d'!., the error in tbe determination of d is approximately .1('c.";\t a

droplet concentration of 500 drops/ee, the mE.'a.sured drop size would be 18(~c smal!E.'r tlwn

it actually was barring any otiJE.'r attE.'nuations in th,:, optical path. TIH'numbE.'r dpn-;iti,>s

used in these examples are not atypical of those found in actual sprays. Thus, the simpl\'

u~eors('attered lil~ht intE.'nsity may not be a reliable m(>thod for measuring sprays. EH'n

with frE.'quE.'nt calibrations, these uncE.'rtaintips dur to attpnuation by drops in til(' L":1m

will occur.

Phase Dlle (0 Opti('aJ PatIi

ThE.' light rays emerOging from the sphere will bare diffE'TNlt path h>ngths d('Iwlldin;;

upon th(> angle of s('atter and th~ path lengths through til(> spll(>T(" For this reason, ttl('

9

'0

, <: I,

.'

>.

,. .

~1 . ".;

OR!G!Nr"lL ?~~(~:j:.: ; ..... , OF POOR Q:J,\:"'LY

.. .

~." ~ '"

intensities of the outgoing beams cannot be added directly. The beams which all originated

from the same coherent incident wave must have their complex a.mplitudes added and the

squared modulus then becomes the correct intensity.

In order to compute the phases from ray optics, van de Hulst references the actual ray

to a hypothetical ray scattered without .8. phase lag at the center of the sphere. l'\eglecting

phase shifts of 1r at reBE'ction and phase shift!> of r./2 at focal lines (all of which cancel

from the subsequent analysis), a simple geometric analysis results in the expression

'1 = 20'(sin T - pm sin r') (17)

for the phase shift witb respect to the reference ray. It is of interest to note that the phase is

directly proportional to 0' = ¥ which implies that the number of extrema in the scattering

pattern is also proportional tv the dimensionless size, 0', The change in phase which is

independE'nt of the incident intensity or scattering amplitudes but is directly proportional

to tbe drop diameter is a more practical means for obtaining the size information.

Dua.l Beam Light Scattering

One approach for extracting size information from the phase shift is to utilize th(>

dual beam seattering arrangement of the familiar laser Doppler velocimeter (LDV), figurE'

3, Assuming linearly polarized light, the amplitude functions associatl'd with scattering

from beams land 2 are

Sll(m.8,d} = V;; exp(jerd (IS)

(lg)

where the double subscript indkates only polarization 1 is considered, j is the imaginary

value /1 = --1 and (J = tJ neglecting phases shifts of 1r due to reflection, 1r/2 due to focal

. lines and the Fresnel coefficients. When a spherical partkle passes th,rough the intersection

of the two beams, it will scatter light trom e.ach beam as if the other beam W3.S not there.

Thus, the scattered ljght waves may be described as

10

... ,. ,. ~.' . ~ t';' >:.:-

~. '

1:1.,.

ORICfNAL PA1~;i7';' OF POO,'? QUA<L"n:';'

''',',.,

'E ( B .1,) - S ( 0 d)exp(-jkr + jWtt) 1 m, ,," - 11 m". 'k

J r

E ( () d) = S ( B d)exp(-jkr + jW2t) 2 m, , 12 m, , 'k '

J 'r

(20)

(21)

The total scatter is obtained by summing the complex amplitudes from each beam and

then determining the intensity which is

(22)

were 0' if the phase difference between the scattered fields, In this expression, the cross

product term 21EIIIE21 cosO' corresponds to the sinusoidaliDten~ity .... ariation of the fringE', .. ,'

pattern while the IEte + IE~d2 terms are the d,c. or pedestal components. The visibility

of the scattered fringe pattern is simply equal to the ratio of these two terms.

v == 21£111 E 21 cosO' IElI~ + IE212

(23)

Confusion can occur here by the use of the terms visibility and signal visibility. In th(>

former tase. ~1ichelson 's definition is implied which is a measure of the distinctness of th<:>

fringes formed in the space surrounding the drop, figure 4. If a point, detector was moyed

normal to the resultant fringe pattern, a signal proportional to the local light intensity

'would be produced and the visibility of the resultant sinusoidal signal would be

v = l';nax - Vmin Vrnax + Vmin

(24)

The signal risibility is the relative modulation of the signal received through the detection

optics and photodetector. This is usually produced by the integration of the scattered

fringe pattern over the area of the receiver lens. In general, the signal visibility depends

on the receiver aperture, drop size, the beam intNsection angl€', and li:;ht wavelength as

11

well as other param('ters and will always be less than or equalto the scattered interference

fringe visibility.

SiDce the rays from beams 1 and 2 intersect at a small angle. ')'( < 10°), at the sphere,

the scattering angles 81 and 82 for these pairs of ra.ys reaching a common point in space

and interfering are approximately equal. Thus, the amplitude. functions 51 and Sz are

also approximately equal. From equations 20, 21, and 23, it follows that the visibility of

thE'fl'inge pattern is approximately unity. The spatial frequency of the fringe pattern is

determined by the relative ph~e difference. (J of the interfering light waves scattered from

beams 1 and 2.

Signal visibility has been used as a means for determining the drop size [7J. When

. the drop moves through the intersecting laser beams, the fringe pattern that is produced

appears to move at the Doppler diffe.rence frequency. The DoppJer differe.nce frequency is

a function of the beam intersection angle, light wavelength, and the velocity of the drop.

The spatial frequency of the fringe pattern is dependent upon the angle of observation ..

drop index of refraction. beam intersection angle, laser wavelength and the drop diam€'ter.

Placement of a receiver lens to collect the scattered light will produce a Doppler burst

signal as shown in figure 5. The lens, in effect. acts as a length scale to measure the spatial

frequency of the scattered fringe pattern. Integration of the fringe pattern over the receiver

apert ure produces a Doppler burst signal with a signal visibility that may be related to

the drop size. This relationship is shown in figure 6.

Although the m€'tbod ba..<;:ed on measuring the signal visibility partially eliminated the

problems associated with intensity measurements, provided a means for the simultaneous

measurement of the drop size and velocity, and did this with high spatial resolution. it

bas several shortcomings. .As previously mentioned, the visibility of the scattered fringe

pattern will be equal to unity if the scattered intensities from eacb beam are equal. Since

the measured signal visibility depends on this being true. the incident laser beams must

be o·r equal intensity in addition to having the same linear polarization direction and being

coherent. These requirements may be frustrated by optical imperfections and alternate

attenuations of the beams by lar~~e drops. Because of the Bessels function relationship

between the measured signal visibility and the dimensionless drop size. the error produced

12

I

. . ",. ., .'. .; t" ~

can be telatively large at the small size end of the measurement range. Another difficulty

associated with the instrument response function is the limited size range of 8. factor of 10

or less. Typical drop size distributions extend over a factor of 30 or greater. Because of

the need to have equal intensities from each beam incident upon the drop, the alignment

of the system is also very critical.

2.2 Phase/Doppler Spray Analyzer Technique

As aforementioned, the spatial frequency of the interference fringe pattern produced

by the scattered light is linearly related to the drop size. The mathematical description of

the interrereoce pattern which includes the effects of all of the optical parameters, is then

required before this information may be utilized. With the complete theoretical description

of the s{"attered fringe patterns for the hppropriate parameters, there is no longer any need

to calibrate the system for each measurement task. The drop size measurement can then

be obtained from the accurate measurement of the spatial frequency of the interference

fringE' pattern.

The theoretical description of the fringe pattern was derived and software was generatE'd

to compute the fringe patterns at any selected optical parameters and to plot the resultant

interference fringe pattern. Three scattering regions of practjcal interest were considered:

forward scatter 10° ::s; 0 ::s; 50°, backscatter 1300 ::s; 0 S; liDO, and (j = 90° where 0 is

measured with respect to the transmitted beam direction. Light scattering by a combina­

tion ot refraction and reflection at similar intensity will occur at some angles and under

certain parametric conditions. Where this occurs, tbe spatial fringe pattern is no longer a

pure sinusoidal intensity variation becauseof the multi-component scattering interference.

That is, additional interference between the refracted and. reflected rays will occur and

produ("e significant errors. Such errors can be minimized or eliminated with the proper

selection of detection and processing methods. The computational schemes were able to

accurately represent these phenomena graphically and thus, allowed the deve!opm('ot of

the optics to a\'()id these possible error sources.

An example of the computed interference fringe pattern is given in figure 7. The

sinusoidal fringe pattern was computed (or a plane normal to the beam directions. Only

13

-

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- .

the upper half of the symmetric fringe pattern was reproduced. As observable on the figure, .'

the fringes are hyperbolic curves showing a decreasing spatial frequency with distance from

the beam axis at x = 0, y = O. The pattern is also symmetric about x = O. Thus, the

,- spatial frequency of the fringe pattern is dependent on where the measurement is made.

Measurement of the fringe pattern would be relatively easy if the fringes were of

relatiyely high intensity and or low temporal frequency. Unrortunately, the temporal

frequency of the fringe pattern is essentially the Doppler difference frequency that will

vary according to the speed of the drop. This frequency may be as high as 2 MHz in spray

environments. The scattered light intensities will be low and will vary over several orders

of magnitude. A receiver lens and eitber photomultiplier tubes or solid state detectors are

required to pro-.'ide the necessary sensitivity.

Thescherne used to measure the spatial frequency of the scattered fringe pattern

requires tbe use of two or more detectors separated by fixed spacings. As the drop passes

througb the beam intersection region the fringe pattern appears to move past the receiver

at the Doppler differ~nce frequency. A Doppler burst 'sIgnal similar to that shown in figure

5 will be produced by each detector but with a phase shift between them as illustrated

in figure 8. The signals in .this figure have been l:igh pass filtered to remove the pedestal

component. The phase shift' is then determined by measuring the time between the zero

crossings of the signals from detector 1 and 2 and dividing by the measured Doppler period.

Th'at is.

- 1}_') 0 ~1-2 = _M X 360

1D (25)

where the measurements are averaged over all the cycles in the Doppler burst signal.

Measurements of the phase shift are then related to the droplet size using the linear

relationships shown in figure 9.

In figure g, the effect of changing the optical parameters whicb include tbe laser

beam intersection angle, collection angle, drop index of refraction, laser wavelength. and

scattering component detected is to simply change the slopes of the linear response curves.

That is, only the size scale is changed for the same range of phase angles since tbe curves

all must pass through the origin.

14

, "

'"

ORIGINAL PAGi! IS OF POOR QUALITY

Three detectors are required to ensure that measurement ambiguity does not occur.

When measuring polydisperse sprays without knowing the approximate siz.e a priori, phase

shifts of greater than 3600 could occur. Such occurrences would be measured as being

. less than 3600• The third detector provides a logical test to identiry and eliminate such

uncertainties. Pwper selection of the detector spacings also provides two sensitivity ranges

shown 3,..<; curns 91-2 and 91-3 on figure g. The two phase measurements will allow the

additional testing of the measurements in tbe overlap region and extends the size range

semitivity at on(> optical setting to a factor of approximately 100.

The dynamic range which refers to the ultimate size range measurable at one setting

including the range of the detectors is somewhat less than 100. Because the drops scatter

light approximately in proportion to their diameter squared, the detector would require a

dynamic response over a factor of 104 or greater. A factor of 103 is realistic. However,

th€' det€'ctor gain can be easily set to select the optimum sensitivity without requiring any

optical adjustments and realignment.

Thus. the method has. several potential advantages for obtaining drop size and velocity

data. BeC'ause of the range of validity of the scattering analysis used, the overall size

rang€' is 3 micrometers to 2000 provided the drops remain spherical. Both the drop sizE'

and its velocity are measured simultaneously. This capability is useful in providing a

complete description of the spray drop size distribution (including both the. spatial and

temporal distributions). Measurements of tw(}-phase turbulent flows which requires the'

d(>termination of drop size-velocity correlations and the gas phase velocity based on small

part ides is an important capability of the method. With the use of highly focused beam~

and off-axis scatter detection, these measurements can be made with very high spatial

resolution.

In the following sections, a summary of the experimental evaluations of the method

are discussed along with a presentation or the results.

15

I'·

3.0 EXPERIMENT

The experiment.al ~ffort was directed toward the verification of the theoretical analysis,

evaluation of the phase processing method, and investigation into signal quality effects. The

early stag(> of the experiments was concerned with validating the predicted signal phase

sbifts. Tbis validation was necessary before any effort was expended on signal processor

design and development which would be a major task in itself. As a result, initial phase

measurements were obtained from individual oscilloscope traces of the detector outputs.

Although tedious, this method proved satisfactory in providing the necessary verification

to justify continuation of the experiments. Following the successful validation of the phase

shift theory for a limited number of cases, attention was given to tb~ development of an

electronic signal processing method. A breadboard processor was designed and constructed

allowing the a('cumulation of large data records at a relatively fast rate. The addition

of the signal processor permitted furtber verification of the theory over a wide range of

system parameters. Also investigated was the effect of signal degradation on the measured

phase shift. Sources of signal degradation known to adversely affect similar droplet sizing

schemes were examined to determine their effect on the pbase shift method.

3.1 Description of the Appa.ratus

The basic hardware and equipment needed for tbe experimental phase of the project

included: transmitting optics, receiving optics with detectors, electronic signal amplifiers,

filters, signal processor, oscillos~ope, data management system, and a droplet source.

Although the configuration of the system changed at times and components were added

or omitted as necessary, the basic arrangement remained the same. A photograph of the

apparatus is given in figure 10 and a description of the components follows.

Optics

The optics are divided into a transmitter and receiver package. Each was assembll'd

independentlY on an optical rail which allowed reconfiguration of the light scatter geom(>try

to any desired collection angle. The primary configuration was the arra.ngement shown in

figure 11 whicb was t.he 300 off-axis forward scatter collection geometry. A majority of thl'

data was obtained with this arrangement.

16

ORiGINAL PAGi:: t8 OF POOI~ QW\Llrv"

The transmitter package consisted of a low power He-Ne laser operating at a wavelength

of 0.6328 micrometers, a beamsplitter (BS) which allowed for a variable beam separation,

a.nd transmitter lens (Ll) with (ocal lengths of either 220mm or lOOmm. The beamsplitter

produced two paralle) beams which were caused to intersect by the transmitting lens. The l .

beam crossover is the point at which measurements are made and is referred to as the

'probe volume', Interference between the two incident laser beams occurs at the probe

volume resulting in the formation of planar interference fringes parallel to each other and

separated by a distance known as the fringe spacing gi".'en by:

(26)

where 0 = fring(' spacing, A -. laser wavelength, I = beam intersection angle, J = transmitter lens focal length, and s = beam separation at lens. The number of fringes

within the probe' "olume is given by:

N= du' o (27)

(28)

where S = number of fringes, dw = beam waist diameter, and db = laser beam diameter

at, t ransrnitter lens. Variations in the above values are obtained by adjustment of the beam

separation, S , or transmitter lens rocallength, f.

The receiver package was comprised of a lens assembly (L2, L3) which collected light

scattered by droplets v.ithin the probe volume and focused it onto the pinhole. The pinhole

acted as a spatial filter blocking the light scattered .from other than the probe volume.

Light which passed the pinhole was directed by mirrors Ml and M2 to detectors D1 and D2.

The mirrors Were adjusted so that light from different. well-defined areas of the receiver

lens ,,'a.., directed to either Dl or D2. This n ,>ulted in an effective detector separation

~cross t})e race of the receiver aperture. The separation was variable and referred to as

17

.". ","" " .... ;:~':,'·"·.w·_, ",',' -"'."""'-~"""·'·T -'''--'"P''''''''i''- '''''-,' ,- "~~""'"''''=--''-~ ... --''--''.~ . ,- ' .. ". -' A"J

ORIGINAL pp,C;?' m OF POOI~ QUALITY

the slit s(>paration. The receiver packagp was modular and thus, could be plac(>d at the

sel('c\('d angle of collection, 0, measurE'd from the bisector of tlie int(,l's(>cting laser IH'ams

(transmitter axis).

Electronics and Data .\1anagell1Pnt

Although physically a part of the receiver package, thE' dE'tectors deserve additional

consideration. Solid state silicon photodiodt"s were initially selected to act a.<; detect9rs. The

quantum effidenc), of silicon photocells can approach gOC::O at the He:\'e las(>r wavelength.

The phot.odiodes are rugged, easily manageable, require only nO,minal (-15 VDC) operat­

ing voltag('s, and can be obtained with integral pre-amplifiers. A good deal of time was

sp('nt developing silicon photodiodes to act as ddt'ctors for the phase measurement sys­

t('m: Unfortunately, backgro1Jnd noi~e levels could not be lowere~ ~nough to provide thl'

required signal dynamic range of which the instrument was capahh'. It is felt that silicon

photodiodes or lowN noise silicon a\'alanche photodiodes could be engineered to perform

satisfactorily but such development was beyond ,tllp se~!>,\' of this effort. A decision was

made to employ photomultiplier tubes (P~fT) as signal detectors for the duration of the

experiments. Although more fragile and bulky than solid state devi('es, P~1T's exhil>it

almost negligible background noise levels and provide enough signal gain to overshadow

their laek of quanfumeff.<'iency. They also provided a convenient means of signal len'l

seiection by adjustment of the P~lT high voltag\"

Additional instrumentation utilized in the experiments included variable high pass

filters and logarithmiC' amplifi(,fs for ('ach of the two (·hannels. Th(' high pass filters remond

the pedestal ('omponent. from the burst signals while the log amps provided compression

of the wide range of signal amplitud('s. A dual ehannel oscilloscope \\a5 used to monitor

the signals at various points along the signal conditioning path.

As mentionl'd, initial phase measurl'ments Werl' obtained from individual os('il!o~('op(,

traces which were manually analyz('d (figure 12). As confid('nC'e in tht' theory inC'rl'aspd.

seHral automat.ic processing .,~h('mes were ('onsiciprpd. Among thl'1I1 were analog ph:l-,c'

determination using a signal mixN, <,ross-correlation techniques, signal digitizing using

ana)og-to-digit a) <'onverters, and digital proeessing schemes. Tht' close a.ssociation of t h('

18

i"

,

phase technique with conventional LDV systems provided a basis of existing digital signal

processing technology. The addition of phase determination circuitry to this type or

processing scheme was deemed most suitable and straigbtforward. A prototype processor

was developed which incorporated two parallel channels of Doppler period determination

and a relative phase measurement. Signal validation included amplitude comparison

(threshold and conventional three-level logic) circuitry designed to eliminate erroneous

signal scoring. The processor also included the 'variable N' capability. The number of

fringe crossings needed to produce a valid signal was not fixed as some processing schemes

require. The minimum number of cycles was user selectable, but the processor was designed

to utilize all available signal cycles in its period and phase determination. This capability

guarantees that the most accurate central cycles of each signal burst are included in the

mea.c;urement. Subsequent testing proved this to be a bighly desirable feature of the

processor. Individual signal period and pbase determinations were output by the processor

to the data acquisition system.

An IB:\1 PC micr'oc~mputer was programmed to accept input and manipulate data

from the signal processor. Calc-ulations of droplet size and velocity from the raw phase and

period data were executeG by the computer. A real time display of the size histogram was

also provided by the computer. Upon completion of data acquisition, linear mean diameter

and. when appropriate. Sauter mean diameter were calculated for the accumulated data.

An accompanyinls printer provided a hard copy record of the reduced data.

Drop Sources

Drops necessary to. evaluate tbe system and verify the theory Were generated from

three different de .... ices.

The Berglund-Liu generator 13 device produces a: stream of monodisperse drop:'. ThE'

range of useful drop sizes obtainable was roughly gO to 160 micrometers which often

depended "pon the operating peculiarities oftbe generator. Since the device wa.s operated

off the manufacturers recommended conditions, the drop stream would sometimes becomr

unstablE' producing a scattering of drop sizes instead of the desired monodispersit)'. r'\ot

only could this device produce a known-sized, variable, monodisperse drop field, but the

19

.. ~

v

ORIGI]\J,QL PAC~ m OF POOR QUAUrV

strl'amwise nature of the drops also allowed them to be selecti\'ely directed through specifiC"

parts of the probe volume, The Berglund·Liu was the only drop size standard used for till'

phase m('a~ur<'ment exp<'riments.

Til<' highly controllable nature of the Berglund-Liu device was necessary for verificatioll

of the theory, but did not represent the true environment of most drop fields. Polydisp<.>rsions

of drop siz('s and a certain randomness of drop trajectory, velocity, and arrival at the

prob(> volunw are more characteristic of a spray l'nvironml'nt. Cnfortunat.ely, there are no

'calibrat<,d' drop SOUTces of the type described, due in part, to the lack of instrumentation

capable of p<'rforming such calibrations.

To better simulate the actllal spray environment, several spray nozzles were u~(>d.

TIl(> watN suppl;' Ils(>d in the laboratory had a maximum line pressure of 80 psi. TIl(>

pr('ssur(> was regulated to produce var;:ing spray characteristics. Th£' nozzles used Wl'rt'

low to mociNatt' flow rate (.S to sHl>ralgallcns pt'r hours), solid conl' type. Garden \'ari('ty

nozzles \\('re ~ometimrs used to produc'c larger drops ina hollow cone spray patt~rn. ., "'~":'~ r... "

A spinning disc atomizt>r was utilizPd for onE' !;('t of tests. This dpvice is used for room

humidification and is known ((, g(>n('ratt' very small ("'-' 4-30 JIm) drops in a very n:lrrOW

distrihution. Both of these features wert' used in estimating the performance of the phase

mt'asuremrnt syster:1.

3.~ Experimental Verification of the Theory

Experinwntal data were obtain('d to validate the throretical predictions. Of particubr

importancr was the verification of the linear rrlationship betw('en nwasur(>d signal pha,;e

shift and drop size. Paramrters other than drop size which \\We known to affrc! the

measurement include r~inge spacing. receiving lens g('omet ry, and rt'ceiving angle. Eaeh of

these param('ters was investigated individually. The system response to more realistic spray

environments was tested using a series of spray nozzles and thr spinning dise atomiZ(>r.

Droplet Sizl' Varial ions

TIl(' s),st(>m' was aligned into the 30° forward scatter configuration (figure 11). TIlt.>

monodispt'fse droplet gcnerator \\'as the drop sourcf'. Tlw g('nl'rator ('ould bl' (unrd OY(>f

a range of operating frequencies to produce drop sizes from no to 160 l1licronl('t\'r~ ill

20

J.

,

diameter. This narrow spread in drop diameters was limited by the droplet generator and

represented only a small part of the dynamic J'ange of the instrument, To test the method

at both the upper and lower ends or the size range, the effective detector separation was

changed to double the maximum drop size t.hus lowering the phase shift for the drops

produced by the monodisperse generator. The individual data listings a.re given in figure!>

13 tbru 18. Tbis size data has been converted to phase angle a.nd is compared to the theory

in figure lQ sbowing excellent agreement.

Fringe Sparing Va.riations

Tb(> re5ultant. size range sensitivity for tbe phase measurement instrument is a fUD(,·

tion of fringe spacing, detector separation, receiver collection angle, and drop index or

refraction. The fringe sparing was cbanged with tbe accompanying size range variation in

the next series of tests. Each change in fringe spacing also incurred a proportional cbange

in signal frequency, number of fringes, and signal amplitude. Again, tbe Berglund.Liu

monodisperse generator was used to produce dropsofcODS\sti?nt size and velocity so that' '" .

th(> above changes could be anticipated as the fringe spacing was varied. A cbange in signal

frequency required a similar change in the higb pass filter cutoff while a variation in signal

amplitude required a detector gain adjustment. Compensation for the change in number

of fringes also affected tbe high pass filter setting. Every attempt was made to minimiz(>

the effect of these secondary factors during these tests, The fringe spacing was varied from

10,7 to 44.9 micrometers by adjustment of the beam separation at the beam splitter.. Data

listings for one representative test are given in figures 20 thru 23. These data are converted

to phase angle and plotted against theory in figure 24. Further substantiati,ng data from

similar tests are also shown.

Lens Geometr), Va.riations

Scattered light intercepted by the receiver lens was selectively directed to each of

the two detectors employed in the experiments. This wasac('omplished by masking the

receiver lens into two separate regions such that mirrors could be used to separate and

direct light to respective detectors. Since the receiver masking determined the effective

detector separation, this parameter was very importa.nt in fixing the size sensitivity of the

21

t. I ...

instrumt'nt. In fact, size sensitivity is lineal' with· effective detector spacing. Thus, size

sensitivity could be cODveniently varied by changing the receiver lens masking without

changing other signal characteristics.

To test the response of the instrument to effective detector spacing, a series of receiver

masking geometries was employed. The centroids of the rectangular apertures were taken

as the detector (~enters. The size sensitivity was varied over a factor of six by chang­

ing the receiver lens masking from 12.7mm centroid spacing to 76.2mm centroid spar­

ing. Except for some variation in the signal amplitude due to changes in receiver lens

! /number, no changes other than receiver masking were necessary to vary the size sen­

sitivity. Representative results of this testing using the monodisperse droplet generator

are given in figures 25 thru 28. Receiver masking geometry is shown overlayed on each

data listing with the centroid spacing specified uncler the parameter 'SLIT SEPARATIO:\'.

Measured drop size agrees very well with Berglund-Liu drop size ('B-L SIZE') in all cases.

It was assumed that the centroid of the masked area was tbe appropria.te choice

for detector location. To test this assumption, several masks W('re designed with similar

centroid spacings but dissimilar geometries. typical results from these tests are given in

figures 2Q thru 31. Again, mask geometries are overlayed on tbe data histograms. In

each case the centroid separation was fixed at 50.8mm. Changes in lens aperture resulted

in chariges in signal amplitude which were corrected by defector gain adjustmeLt when

necessary.

Spray Sozzle and Atomizer Tests

This series of tests involved the measurement of polydisperse droplet spray field~

produced by industrial spray nozzles using water. Although the signal processor and

data acquisition system lacked much of the sophistication necessary to makE.> accurate

mea.surement in this type of flow, the data obtained show rema.rkably good agreement with

other measurement sources. In particular, signal rejection based on Doppler frequency and

signal amplification over a dynamic size range greater than 35 were not available with the

breadboard system. Kol' was the third detector required (or additional signal \'~lidatioD

and ambiguity availabJe.

22

OHlGINAL PAG.2 m ,)F POOR QUALITY

The majority of the spray testing utilized spray nozzles from the Delavan Inc with fl()~'

capadties ranging rrom .5 to 1.0 GPH. Typical resu\.s ror two different flowrate nO'z1Ie~

at 60 psig are shown in figures 32 and 33. These particular spray conditions were selected

because of comparative drop size distribution information provided by Deiavan(H). Figure~

34 and 35 show the phase Doppler measurements overlajed with the Delavan data.

Several other spr~y sorr,eswere used including garden variety nozzles and hand or

finger operated pump bv!t1es. Results from these tests were not included as no corroborat·

ing data was available.

A spinning di.'"c atomizer was used to test the small drop size sensitivity of the

instrumpnt. A typical distribution is shown in figure 36. As expectpd the size distribut ion

produced by the atomizer was very nanow with the majority of drops less than 40

minometers dia.meter. Thp distribution of figure 36 can be compared to those of figun·s 37

and 38 in which thp size sensitivity of the instrument was varied in an attempt to shift tnt'

location of the ffip.asurpd phase information. The selr·consistency of tbese measllremc'nb

can bt' seen when they are overlayed with normalized ordinates as in figme 39. The oat a

presented in these tests is in tbe 'raw' form with no probe volume corrections (,\'(,D th()ugb

thi::. type of correction is bt'lieved to be necessary at the small size end of a gwen size rang(·.

The atomizer used in the above tests produces a spray field of moderately high drop](·t

Dumber deosity. Simultaneous droplet probE' cr~iDgs (multiple events) were obseTnd

from timp to time on the oscilloscope. Although this path was not pursu£>d, tbe Syst('Dl

response to multiple events does Dot appear to- preclude its us€' in high droplet number

density environm£>ots.

Backscatter

In many applications, access to a spray field undl'r test can be obtained from only

one side of tbe test rig. At other times it is desirable to locah' tl'ansmittH and re('Pi\ ('r

packag('s nE'xt to each other to provide system rigidity. In some situations the dropkt~

themsel\"f;>s may be opaque to visible light (( •. g. liquid metals) sl,I('h that rt'fractive scatter

does not exist. In all of the above cases, the measurement of backscatteied light could be'

utilized, The backscatter configuration involves arrangement of transmitter anu H'(,(,j\'l'r

23

- .... , "'. !j. ~.

f

! .

su('h tbat light scattered back toward the source (0 = 1500 in ihis case) is collected and

analyzed. For the situation involving opaque drops, this light would be scattered by first

surface reflections. For non·opaque drops,backscaUer may also involve light scattered

from secondary internal refiections which complicate the analysis.

Cal<'ulations of relative scatter intensities were performed prior to experimentation.

The results showed that the light scattering intensity from the droplet surface (first surface

refi(>ctiou) of pure water drops was a.ctually less than the light scattering intensity from

light reCracted into and internally reflected out of the droplets in the same backscatter

direction. It was believed that if the relative intensities of light scattered between the two

modes were sufficiently diff(>rent, onl)' the dominant mode should be detected. To support

this as::umption, phase shift calculations for th(> internal reflection mode (0 = 150°) were

completed and found to be nearly identic .. } to the forward scatter refraction mode (0 = 30°).

A wat er baSed dye was used during these experiments to vary the opacity of the

droplets and suppresstbe internal reflective scatter by absorption or the incident light.

Pure dye used to form droplets eliminated all internal refledions allowing only first surface

reflection. Results involving pure dye are shown in figure 40 to accurately follow the

predktion for first surface backscatter. Results using pure water and the prediction for

internal refi(>ction at (j = 1500 are shown in f:.gurc 41. When the dropiet op2.city rea('hed

a certain value the results fell between the two extremes of figure 40 (essentially 100'[

opaque) and figure 41 (nearly transparent) and often contained data from both modes.

3.3 Signal Quality Effects

Several sources of possible signal degradation were investigated to observe their effect

oIl.the phase measurement scheme. These sources were ideDtified by the investigators based

OIl previous experience with similar light scatter instrumentation. The two ca.tegories into

which these ;>ossible sources fall are optic",l aiignment and incident beam interference.

The probe volume of the phase measurement method is physically limited by the beam

intersection rel!.ion and the receiver ~)inhole. Only light which enters the pinhole will reach

the detectors and this light must origin~teJ!om the beam overlap region. This critical

24

, "

F!i~~'i''iWI'!'"I''~~'''''';''''''"''''''~''i''':=~O''''~''~'~''''l'I''''"'''~''C':''''"''':'-'''""",~,,,,,o"'''''''~:.''~''''''--7,~,"''''I''''wP''''''''''''''''~''''''''''''7'''''''''7~l!!'!':':'~'--"~r;;

, ~

alignment between transmitter and receiver can be affected in ways which could possibly

produce deleterious effects on the size measurements. Perhaps even more crucial to signal

phase measurements is the quality of the incident beams which (olm the probe volume.

Incident beam quality directly affects the quality of the interference fringes. Beam drop-out

or selt'ctiH' attenuation can alter fringe formation and contrast. The phase measurement

technique was promoted because it was potentially less sensitive to loss of signal quality

than existing instrumentation.

Alignment

During thege evaluations, light gathered by the receiver lens was brought to a focus

and passed through a pinbole spatial filter whose size was selected to match the beam waist

at the probe volume. The purpose of the pinhole is two-Cold: first, it prevents light from

other sources (other parts of laser beams or background light) from reaching the detectors

and second, it guarantees signal coincidence at both detectors. Pinhole misalig;nment can

result in 'receiver aperturing' which alters the character of the detector signals. The

monodisperse generator was especially useful when studying' pinhole effects as its droplet

stream could be directed through select parts of the laser beams effectively scattering light

from a single 'point'. When first applied in the receiver package, tests were conducted to

verify that the pinhole did not adversely affect the drop size measurement. The results of

one such test is shown in figures 42 thru 45. A range of pinhole sizes was used including the

smallest which was not expected to transmit all of the focussed light. 1':0 appreciable shift

in measured size could be detected although the smallest pinbole produced a broadened

distribution which may bave been indicative of aperturing problems.

A second series of tests were run to observe induced aperturing effects on size mea..<;ure-

, ments. These tests involved intentional focussing errors at the pinhole plane. Due to the

nature of the focussed light, selective aperturing of light intended for individual detectors

could occur. This apel'turing led to signal distortions with the potential fOT errors and

were, in fact, processed as erroneous drop sizes. A set of data from one such test is given

in figures 46 thru 48. The errors induced in these tests were much more severe than those

expected in practice.

25

The effe('t. of receiver alignment on other than the center of the probe volume was

investigated. At the extremes of the probe volume, a hyperbJlic spreading of the inter­

ference fringes may take place. The monodisperse droplet stream was centered on the beam

crossover and then moved to the detectablf> extremes of the crossover region. The receiver

pinhole was removed for these tests. Figures 49 thru, 54 show the datarrom these tests, A. ('ombination of loss of signal intensity and signal distortion determined the detect(l.bility

limits. No appreciable change in droplet size was observed, although broadening of the

distributions did oceur and was attributed to the decrease in signal·to-noiseratio.

B£>a.m Qua.lity

fringe ('ontTast at the measurement probe volume is dependent upon relative beam

int(>nsity, beam polarization and coheren<.'e. Loss of fringe contrast is disastrous for sizing

techniques based on sign~l visibility since this quantity is directly related to the fringe

contrast or visibility. Although fringe contrast is easily affected, fringe spacing is not. The

phase te('bnique is directly dependent upon fringe spacing but not upon fringe visibility

and thus, should not suffer from losses of contrast. To verify tbis, tests were conducted

in which th£> {ring£> visibility was intentionally lowered. The monodisperse generator was

used as the droplet source. Figure 55 shows results for the case of IOOSC fringe contrast

in which incident beam intensities were nearly equal. A neutral density filter of optical

density 1.0 was then inserted into one of the two beams changing the intensity ratio from

50:50 to {H:9 and reducing fringe visibility to only 20%. The test was repeated with tbt'

results given i:o figure 56. The size measurement was not affected.

Losses of beam intensity and fringe visibility do not occur as cleanly as was sllggestcd

by tbe previous testing. In a spray environment, droplets pass randomly through the

incident beams scattering light away from the incident direction, affecting both beam

intensity and (:oherence. Droplets with sizes comparable to that of the beam diameter can

cause beam drop-.out. The combined effect of many small droplets in the incident beams

can n .. >sult in a severe decrease in signal-t~noise ratio. The oscilloscope trace of figure

57 shows a typical beam attenuation due to droplet interference. SiDce each of the' two

incident beams is affected similarly, but not identically, by droplet passage, the ('haractt'r

of the beams which intersect at the probe volume at any instant cannot be easily predicted.

26

\ ..

""',(.,-.. ' • < .~., •• ,., L ,. ,',

Spray interrereD('e effects on droplet measurements was investigated by introdl,lCing a spr .. y

into tbt' path of the incident beams while measuring the monodisperse droplet stream.

Sin('e droplet sizt' to beam diameter is an important parameter in this test, iwo spray

droplet distributions ot wideJy differing mean drop sizes were used. Spray interference will

also affect light scattered from the probe to the receiver lens. This interference, however,

is not as severe as beam drop-out. The results of a spray interrerence test is given in

figures 58 thru 60. In each case the intertering spray was judged to contain a moderate

to high droplet number density. Test results show that mean siZe measurements did not

('hange although some broadening or the distribution did oc('ur and is attributed to loss of

signal-ta-noise ratio and spray interference directly with the monodisperse droplet stream.

27

;J;'_

, " :, l--~" ."" ', ..

4.0 SUMMARY AND CONCLUSIONS

An innovative approach (or spray drop size and velocity characterizations has been

described. The mt'thod, which obtains the drop size information Crom the interrerence

Cringe pattern produced by the scattered light, bas been shown to have severAl advantages,

Pt'rbaps th(l most significant adva:ntage is the relative insensitivity of the method to un­

certainties in the scattered light intensity and Cringe visibility which occur when the laser

beams are attenuated by the droplets or other optical contamination. The linear relation­

ship between the measured signal phase and the dimensionless drop size simplifies the im~

pl('mentatioD of the method and creates a uniform. size sensitivity over the measurement

range. The dynamic size range of the technique is essentially limited only by the dynamic

range of the dete('tors and the prevailing signal-to..Doise ratio.

~fonodi5pel'st' droplet streams were used as a basic test of the theory and to test sus­

ceptibility of the m('thod to practical effects experienced in tbe application of the method. , , !";" 1

These effects includ<>d optical imperfections, environmental conditions and simple align­

ment errors. In terms of the operating requirements and errors produced, the operation

was analogous to the conventional laser Doppler velocimeter.

~1ethods for size range selection were desctibed and tested. Either the beam inters('C'­

tion angle (fringe spacing) or the detector spacing can be changed to achieve the same

r('sult. The detector or slit spacing was easy to implement, did not require realignment

and did not change the Doppler difference frequency. Also, this method has only a minimal

effect on thE" probe volume. '

Sprays were measured in this preliminary assessm('ut of the technique. Comparisons

of the measurements to other methods were presented and tbe results showed reaSonably

good agreement. However, this cannot be used to make conclusions about the relative

mea.surempnt accuracy. Measurement.s at different size range selections Were also made

to determine theseH-consistency of the metbod, The availability of a "standard spray"

would facilitate the evaluation of these new diagnostic techniques.

Although the tests conducted using the breadboard processor were as extensive as

possible, turther tests are required using the more advanced prototype processor currently

28

under, development. The processor will produce two'simultaneous phase measurements

to prevent ambiguity and toreject spurious signals that may occasionally pass the noise

rejection logic. Testing of the processor's automatic setup runctions and signal qualification

logic will be the first priority in the future work. The processor will be capable of a data

rate greater than 10,000 samples per second so that tests on tbe probe volume effects and

mass flowrate determinations can be considered.

In summary, the recognized potential characteristics of tbe method are:

• linear relationship between tbe measured phase angle and drop size

• size range of 30 or greater at a single optical setting

• overall size range of 3 to 2000 microns

• simultanl?ous size and velocity measurements

• relative insensitivity to beam or light scatter attenuation

• high spatial resolution il.·.

• operation is similar to an LDV

• adaptable to existing LDV systems

• ('an distingui~h between gas phase and droplets

• reduced sensitivity to misalignment

• can perform measurements independent of refractive index.

A patt'nt application has been filed.

29

, . ,

REFERENCES

1. A.;~1. TaweE'l, and J. Landau, "Thrbulence Modulation in Two-Phase Jets," Journal

of Multiphst> FJow,Vo! 3', Hl77, pp. 341-35l.

2. \.;. H. Snyder, and j. L. Lumley, "Some Measuremen.ts of Particle Velocity Autocor­

relation Functions in a. Thrbulent Flow," Journal of Fluid Mecbanics,Vol. 48, Hl71

3. V. Goldschmidt, and S. Eskinazi, "Two-Phase Thrbulent Flow in a Plane Jet," Journal

of Applied .\!ecbanics,Vol. 33, Ser. 3, !\o. 4, 1966.

4. D. Modarress, J. Wuerer, and S. EIgbobashi, "An Experimental Study of a Thrbulent

Round Two-Pha.<;e Jet," St. Louis, Missouri, AIAA Paper !\o. 82-0964, June 1QS2.

5. R. A. Dobbins, L. Crocco, and I. Glassman, "Measurement of Mean Particle Sizes of

Sprays From Diffractively Scattered Light," Am. Inst. Aerollaut. Astronaut. J. Vol. 1,

p. 1882.

6, J. Swithenbank, J. M. Be,';, D. S. Taylor, D. Abbott, and G. C. McCreath, "A laser

Diagnostic for the ~1easuremcnt of Droplet and Particle Size Distrihution," Al'\.A.14th ,

Aerospace Sciences ~feting, Washingtou, D. C., Paper :\0. 76-79, Jan. Hl77.

7. W. ~1. Farnwr, ~~1easurement of Particle Size, !\umber Density, and Velocity Fsing a

Laser Interf€'rometer," Appl. Opt.,rm \'01. 11, lQ72, p. 2603.

8. A J. Yule, .\'. A. Cbigier, S.Atakan, and A. Ungut, "Pa.rticle Size and Velocity

Mea..<;urement by Laser Anemometry," Al·iA 15tb AerospacE' Sciences .\ieeting. Los

Angeles, Paper no. 77·214, 19i7.

9. W. D. Bachalo, ~.\!etbod for .\1easuring tbe Size and Velochy of Spheres by Dual·Beam

Lighl-Scattn Interferometry," AppJ. Opt.," Vol. 19, :0\0.3, IS80.

10. H. C. van de Hulst, Light Scattering by Small Particles, \Vile)', !'\ew 'York, 11)57.

11. J. R. Hodkinson and 1. Greenleaves, "Computations of Light-Scattering and Extinction

by Spberes According to Diffraction and Gl'<)metrical Optics, and Some Comparisons

with the Mie Theory," Journal of the Optical Society of America,Yol. 53, No.5, 19G3 . . .," p. :)/ I.

12. W. J. Glanschnig and S. Chen, "Light Scattering From Water Droplets in the Geo­

metrical Optic's Approximation," Applied Optics," Vol. 20, No. 14, 15 July , Hl81. '.,

30

13. Berglund-Liu MODQdisperse Droplet Gt'nerator, Thermo Systems Inc., Minneapolis,

Minn.

14. Correspondence with Dr. Roger Tate, Delavan Inc, We.<;t Des Moines, Iowa.

31

) ..

. \

~

FIGURF. 1. - Geometrical ray trace of an incident light ray through a sphere.

v

00 "T1;:o .,Q O 2 0:..> ;Or-.0-0 C:~ ):-0 r "I """'; .... ~(t;1

"

-..

· ORIGINAL PAGE (S OF POOR QUALITY

orl' ...... --------------,

I~·

,.. !l ~rn w.,t.f o.opt" -- Gtotl'lt'u". Oph" ---- Mil Tf\tof,

610EGREESl

\~~.---------------.

,::'~--...... - ...... --'--...... -..,.....,__ ........ --....l lIe.O 400

8(OEGREESI

~~r--~-------------~

IC·r-----------------------~

~~O

fJ ItxGllfESI

FIGURE 2. - Comparisons of angular scattering diagrams for 5 micrometer droplets computed from geometrical optics and Mieapproaches.

33

Probe

FocusIng Lens

Beam SpUtter

,.. ;g

Collecting Angle

Laser

Coffectfng System - .

TransmItting System . -.~

,tramr. 3._ ~ch ... m"tic or t.1I ... nntir;d ~y"td;" for an 1.[lV "n,1 (~ropl ... t sizin'1 "ystt"m.

~ ...

OC ..,,~

-0(.) 0:';; 0--;;;:f! re· r.a ~ ~) S S~l

':;1 ~~

. . \."!

ORlGINt\t P.n.GE ,8 OF POOR QUALlT't

'FIGURE 4.' '-. Far field scattered light interference fringe pattern produced by a droplet.

FIGURE 5'. - Laser Doppler burst signal.

35

'J

\

...,;;../

w OJ

>­!:. ~ en (J)

>

1.0

0.8

0.6

0.4

I •• 629 • L ... 12S • ."I.}) IS .. 2!l ~ A .. .5145 IlIl lEFRACTION.

S2 ONLY

0.2L' .

0.0_ r" " 1 "r~ OJ -.

0.0 1.6 3.0 4.6 6.0 7.6 9.0 10.6 12.0 13.6

D/DELTA

FT~URE 6.- Siqnal visibility verRUS dimpnsionlcss drop size; {IS and collection anqlc ]0

.~

00 "';:0 "00 0::; 0 ""­":", ;op .0 " (::';.:' j:> [::. r"" ~n .. ~ _, r

_~ ~.r;~~

'. ;.'C

ORlC:N.ql PP,GiZ iSj

OF POOH QUALITY

DROPLET DI~~ETER. RE:ElvER 'OCAL lE~C~". DIA~ETER P£CElvEft • THE'fA 'R!NCE S~~=!NC • LASER wPvE~E~CTH • IN~E~ O~ RE~A~T!vN •

168,080e '3',B0ee 125,090e .,8009 ,,,,aaee

,6329 1. nee

Droplet

-240 -leu -lie "". e ill X,mm

241 Jae

FIGURE 7.- Computed interference fringe pattern produced by a droplet

37

v , :~

. ,~i­~~~~ ' . . ' . . ,

~ ~ !

P.e , ... <:", ~(1

ORIG\NAL j.\u:' :~. Of POOR QUALII (

Collect1ng Len~tS

:FIGURF. 8. - Schematic of the detector arrangement and the resulting high pass filtered signals.

38

OKtKtors

OJ

w (!)

fid ~

S60

210

< 1$0 x t:l.

90

o 5 10 16 20 25

P/Dl!lTA

FIGURF. 9.- Theoretical prediction showinq the phase variation with the diMcnsiortless drop size

, ,

30

00 ":.:0 -vG3 O 2 0;.:. ;Or-

Du, ep 1>0 r f"~ ~ .. -~ -<! Q..~

,;., ,

~ -~

[1 rJ i~ :~

~-J

,

.t+)

~ '~ '0 .-~

I j

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d ~

ORIGINAL F.c\Ci ;~ OF POOR QUAUi'Y

40

l;...:

c ..r:: c. r.: \..< t"" o

.;.J C

..r:: :::...

..,. I-'-

;OJ

LASER

,

~ BS •

OPTICAL PRonE VOLUHE

DE'l'F.CT()R HJ\SK

DROPLET sou RCI-:

~

~,--~J , ~e-

DETF.CTOR 1

DETECTOR

1·'TGtlI1E 11 - Optical Component l·.:tYo1.lt

COLLECTION ANGLE

M2

00 "":::0 ..,0 0--0 2

=o~ .0.., c :r;. » (.) r ft'l

~ ~c

_ • ...A ;~

··"-· .. ·11 -)j ;!1 ~

j ..<1'

.1 ·f

a;

','

j ~ ,~

1 ,

, (+)

OfHGtNi\L ;;;M'.E \:] OF POOR QUALi"\'l

42

, . ,

I:l C­o U If. o ... ... u

'" C .. N

H )1.

~--~----~I--~--~ i.,

II IIItl III .1 eMU)

m~:~n .. ~.~[: It a, nil un : ~ 1 lUI nOr[ : .~, WfUS : l~

Figure 13 - Drop size variations

""I~ 221fl1 ~I~ m 114,.. ~ S~, 11 2 .1(1'9f6 ~!Ir til ZIl ~lmtl 111M CVCU • Figure 14 - Drop size variations

111 II>4C/'QA1

4

\ll!IT lOO n:c .... IVA S[J 11.4 III ~ ~I'A( H lllltNIt lalH ~IA 4llU\Mi I'iIK.croJ • Figure 15 - Drop size variations

43

'I'l.C~: (1.

OH1GIl\lAL Pl;G2 ;S OF POOR QUt:.lITV

, "

Figure 16 - Drop size variations

H.U: 1'" 11(I'0Il1

i

i .. ~\ '.,r--"i

~;f I ~

' .

j I ~

Fibure 17 - Drop size variations

Figure

UIlf.\ll 1I~1U ! l.l ftl tn'IIi m_1C~ j,

18 - Drop size variations 44

, ... ..,

»18: ua aleNt!

til 0) GJ ~ t1' GJ

"Cl ......

360

300

240

(I) 180 rl Ol

,§ " (I) n Ul

m .c p.. 120

60

0

,!./

0 1

0

e DATA, listings included in report

o DATA, listings not included

THEORETICAL CURVE Phase vs d/delta for f/S=[ 9.741 9=30 deg., m=1. 33

, 19.49J,

2 3 4 5 6 7 8 9 10 d/delta (f/s=9.74)

1 2 3 4 5 d/delta (f/s=19.49)

FIGURE 19 - Plot of Phase Shift vs Non-dimensional Size;

Theoretical Predictions and Experimental Data

11

00 "";0 "tl C) 02 0:>:, AlT'

.0-0 c.: ;:~ -p Iz', r- !':j ...-0; ,.,,-..

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334

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J{ DI~;Il"ltl'!lnsl

"':1 LIl!) m ... B!Q~ m D.· fli~.; '·fQ,: Ii ~ Oleron, 1Jl\l ~!A 2IJ Oimn~ -! Ii :'!': Lf ~

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360

~ 300 ~ 240

180

120

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• DATA, listings included in report

o DATA, listings not included

THEORETICAL CURVE Phase Angle vs d/delta for f/s=9.74, 8=30 deg., m=1.33

00 ." :'J "0 r.) o -... ( ) ;.:; .~ ::Or

.() .... , c.: ;.~"

E r; -1, ~ (,.

I . J

2 3 4 5 6 7 8 9 10 11 12 d/dc1 ta

FIGURE 24 - Plot of 1'11.:1<;e Shift vs Non-dimensionCll Size;

Theoretica1 Predictions and Experimental Data

'\

... ~. --'

.j::. ;xl

v tr

11m·:, rH!-!m

TIl(: lU6:14

H lin: It'.3 Il C11AS

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1i'$Y, m. : 8~U II CPt_! SlQ"ilS : m

.. y COOICT 117

I PlIt\t' C~IC1S

M~iH • C~H1S

Figure 25 - Detector spacing variations; S- 12.7 mm

t~h'lm

n',e HUll: i~,l PleNti

,,'~mt 111' Rl~

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\ l "-

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Figure 27 - Detector spacin~ varlatlon~; s~ 50.B mm

,,~.

ITt: 8HHMJ

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Figure 26 - Detector spacing variations; s- 25.4 mm

I!J;'19U n'~:14

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Flgu~e 2R - Detector spacing variations; 5- 76.2 mrn

00 -n;:tJ

-OC-; O ---" 0 ·­:;.., ?Jr rO"Z1 c: ):.0 ~,-.. [;} ~ r:~, -. .--.. ;! t.~.

~ , :..;.

~

\

---"- , '+ ~ _" .4.~ ., \:!.

l~i'ltn n,.:zt

rJJI~~ IDII U%.t: 118.1 altl'Oll

/, v

/

\ I

'" ~

........... 0.,

I ~ I

,I v " ./

/'

Figure 29 - Detector area variations

ITt" M·ll·I"]

T1~' l?:z;:e

Hmz' IIIU II'*'

'I'C« Ul,t 1lG ~ Illl"Cti

IW.~ r~t:

Figure 30 - Detector area variations

\'II'I li'~li·l~J TIl[' 11:0141

Hun: 1.9.1 rUCPMI

putt' UUIIlC*1

Figure 31 - Detector area variations

49

ORlGlNAl ").'1"",., rey o ' Y"''''l''t'' I" F POOt? Q! ::", 'j __ 1 '

-,,'- i

~,

;/

CJl o

.

DATE: 10-19-1983 \ I I TIME: 13;15:14

l_____ __l_~ ___ J .' r---~-- -~--~---l i I I

..

6 DIAHETER(Microns) 224

XHIT LENS : 22e ~M BEAM SEP: 7.74 M~ FRHG SPAC : 17.9 Microns WAIST DIA : 213 Microns MIH,CYCLE: 4

RCUR LENS : 495 MM COLL.ANGLE: 30 deg SLIT SEPR,: 50,8 MM SLOPE' : .758 SAMPLES : 1~~3e

FIGURE 32 - Drop Size Distribution: Delavan 45B .6 GPH

60 psig Water, 10", below nozzle centerline

HAX.COUNT 583

MEAN SIZE: 47 Micflons

SHD: 102 Micl'ons

00 .,,~

"'05 0-' ::2

~F () ";:I r- "tj)

?~ ~;.; :) ~:'}

~

' ..

-.;~

:,/

C;' J->

MrE: . 10-19-1983

TIME: 17:19:24

6 DIAMETER(Microns)

XHITLENS: 22~ MM BEAN SEP: 7.74 "" FRNG SPAC : 17,9 Microns WAIST DIA : 213 ~icrons HIN.C~CLE : 3 .

RCVR LENS : 495 Hr.. COLL.ANGLE: 30 deg SLIT SEPR~: se.8 MM SLOPE : .758 SAMPLES : le~g0

224

FIGURE 33 - Drop Size Distribution: Delavan 45B 1.0 GPII

60 psig W~tcr, 10" below nozzle centerline

MAX. COutU .. 685

MEAN SIZE: " 48 Mict'ons

SHD: lea Mi Cl'ons

00 ." ~

"On 0::;':: 0:;; ;J;Jr 0,::; c: ;..~,~ :t=- !'j

r:; ~ ... ~~.)

". . ..... -~ . ~ ..

I "'"~~-- _-:,,_,,4,' __ -,'- ,.5'_- '," __ "- '"""'"."..".. a ii, .~, .. >i -em! ,a

f ~

...... ::: .I-l '0 .....

".. ~ z c..:c: c· .... ~.Q ;:.J ~>..

.Q e:: ::.1 '0 c::l ~ ::2N ~,,,,,

fj z~ ":: ::= l-i 0 c -

131n 12 jtt 11 I • 10

9

S

7

6

5 i I 4

3

11 2

1

0

0 40

r·. er w

80 120

n DATA SUPPLiED BY DELAVAN INC 010" 44.8

D32= 99.7

e PHASE/DOPPLER DATA 0 10"'" 47 0 32= 102

160 200 240 280

Droplet Diameter (micrometers)

FIGURE 34 - Drop Size Distribution Overlays

;. Delavan 45B .6 GPH, 60 psig, 10· below CL

oc ":c "00 0

_--:>

o~ ;of-.o-u c :i~ ;t;. r ' c: i'.i _1 ~..! ,~.;;

. ,-1

"

~

c.n CAl

.) ,.,.

--

15

14

13 11

12 1 11

:5 10 'U .,../

E-'~ Z ~~ c U . .-4

r::: ,Q ~1 0...>,

..Q c::; C:G1J ;:J Ci ~t-.; ~.,../

z---,.. ~ ~I

o ~

9

e

7

:j 4

J

2

1

o

(9

•

•

eet 0.

n DATA SUPPLIED BY DELAVAN INC

0 10'" 38.7

0 32= 83.7

o PHASE/DOPPLER DATA

010= 48

032= 105

o 40 80 120 160· 200 Droplet Diameter (micrometers)

FIGURE 35 - Drop Size Distribution Overlays

Delavan 4SB 1.~ CPH, 60 psig, 10· below CL

.':

00 "";0 "0.5 0-0<-~F! .0" C.h ,p. C) r: ftl -'--< i,";2

" I !

I

~

.~

"-f+

1m 'HHHl tlK! .lll.U

• DI..r1[)lftIC,..,.ll m ~:l I.1'IlS : III .. fro ~ : m till JI&I UP: t 1 III 00 tl(ll: )f /II, nx spac : U IIltpttS illl un: Z~ 4 .... It lSI JIA: " alC""'S ,.1 )ia.; m 'lC"~S K\h C'I~ : l Wf:.n : ~ Figure 36 - Drop size distribution:

spinning disk atomizer size range 229 micrometers maximum

SKt 41 "It R"'.'

YJI:l ~ llil NO liOJ UY. : ~l .. 1(U m u... (O~ 1'1;.(: 1i Or, ~ nl: t, •• c,...\ ~~!l \[]i. : ~i I .. .. al ':1 '1'HPOC'S All ":1 : 1I~ 'IC"'.I 1I.l- CYCJ l Sa.w:t.ll : ~

Figure 37 - Drop size distribution: s?inuing disk at0~izer

size range 114 micr~weters maximum

":1 1f·14·HIl

tiKI IUrI'

1m T l.Dfl llli 1111 lOt i.OO: 4'~ "" l't~ m '1.. COu. U(J: » 4fj r~ Sl1: U Illm\ HI! mI.: "Ill> 9111 lil n '11ttAI ~I )it : 14 "it,...\ 1111 mu 2 stem : ~

I'l .... ml tI lott",,1

1IO ta I&CK'l\

Figure 38 - Drop s1ze distributlnn: spinning disk atomiZer

s:!ze' range 76 micrometers maximum

5<1

(Jl U1

~

.9

.8

• 7 (J) 8 %; ':) 6 o . U

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-+- Sizc Range Haximum 76 micrometers 1.1 micrometcr Bin Width

DROP DIAnETER (Micromcters)

FIGURE 39 - Drop ~ize Di~lril)ution Ovcrlays: spinning Disk Atomizer

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B-L SIZE: 123 l'Ii Cl'ons

B-l SIZE: 123 1'I1Cl"Ons

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FIG':F.! 4: - B"ck."ata< Test: 100\ P)'e !Opaque) Drops;

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Ffgure 42 - Plnhol~ effect test: no pinhole, baseline data

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Figure 46 - Pinhole focus test: 400 micrometer plnh'ole in focus

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Figure 47 - Pinhole focus test: probe ima~ed behind pinhole

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Figure 49 - Optical alignment test: probe center. reference data

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59

ORIGINAL PAGE to' OF pom? QU/'.~ ITY

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Figure 52 - Optical alignment test: 6.5 mm upstream of probe center

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ORIGfNAl PAGE rt'4 OF POOR QUALlT'r

. Figl,ire 58 - Spray fnte'rference te~t: monodisperse generator without spray, baseline data

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Figure 60 - Spray interference test: 100 micrometer~ean drop diameter s~ray upstream of probe ,..'>

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End of Document