This paper discusses our proposed experimental and theoretical study of atomization in gas-liquid and liquid-liquid flows. While atomization is a very important process in these flows, the fundamental mechanism is not understood and there is no predictive theory. Previous photographic studies in (turbulent) gas-liquid flows have shown that liquid is atomized when it is removed by the gas flow from the crest of large solitary or roll waves. Our preliminary studies in liquid-liquid laminar flows exhibit the same mechanism. The two-liquid system is easier to study than gas-liquid systems because the time scales are much slower, the length scales much larger, and there is no turbulence. The proposed work is intended to obtain information about the mechanism of formation, rate of occurrence and the evolving shape of solitary waves; and quantitative aspects of the detailed events of the liquid removal process that can be used to verify a general predictive theory

Chang, Hsueh-Chia

Gallagher, Christopher

Leighton, David T.

McCready, Mark J.

English

NASA Technical Reports Server

FUNDAMENTAL PROCESSES OF ATOMIZATION IN FLUID-FLUID
FLOWS
Christopher Gallagher, David T. Leighton, Hsueh-Chia Chang and Mark J. McCready
University of Notre Dame
Department of Chemical Engineering
Notre Dame, IN 46556
/
//
ABSTRACT
This paper discusses our proposed experimental and theoretical study of atomization in gas-liquid and liquid-
liquid flows. While atomization is a very important process in these flows, the fundamental mechanism is not
understood and there is no predictive theory. Previous photographic studies in (turbulent) gas-liquid flows have
shown that liquid is atomized when it is removed by the gas flow from the crest of large solitary or roll waves. Our
preliminary studies in liquid-liquid laminar flows exhibit the same mechanism. The two-liquid system is easier to
study than gas-liquid systems because the time scales are much slower, the length scales much larger, and there is no
turbulence. The proposed work is intended to obtain information about the mechanism of formation, rate of
occurrence and the evolving shape of solitary waves; and quantitative aspects of the detailed events of the liquid
removal process that can be used to verify a general predictive theory.
INTRODUCTION
Atomization of liquids in gas-liquid or liquid-liquid flows is one of the basic processes that determines the
configuration of the phases and the overall behavior of the flows. Here we are defining atomization as the removal
of liquid droplets from a flowing layer of a more viscous phase by shearing action of the (necessarily) faster-moving,
less-viscous phase. For the air-water system, photographic experiments by Woodmansee and Hanratty [1] and
Whalley et al. [2] have directly linked atomization to large waves -- liquid is removed by some mechanism from the
crest of large solitary or roll waves. Preliminary work shown below for a mineral oil - water system exhibits the
same mechanism. The primary questions about atomization to be addressed in our research are: how quantitative
aspects of atomization (e.g. drop size and rate) change as viscosity, density and flow ratios, orientation and the
presence of gravity are varied and which fundamental theory is needed to accurately describe atomization and allow
prediction for situations outside the available range of data. Because atomization occurs at the crests of solitary
waves, the behavior of such waves is of fundamental importance.
Atomization rates are an important issue in the design and operation of many industrial devices. Fore and
Dukler [3] state that up to 20% of the pressure drop is from the atomization and deposition process. A stable oil
film is needed on the walls of hydrocarbon transportation pipelines to protect the pipe from corrosion causcd by CO2
combined with condensed water. For pipelines with large gas-liquid flow ratios oriented close to horizontal sufficient
atomization is needed. Numerous chemical processing operations rely on emulsification or atomization to create the
interfacial area necessary for efficient reactio_n or contacting. Phase mixing is typically effectcd by agitation or
pumping through nozzles. However, both of these processes arc relatively inefficicnt. A fundamental study of the
atomization mechanism can hence allow us to optimize these existing processcs or suggest a more efficient new
process, such as shearing a two layer configuration.
Figure 1 shows a comparison of solitary wavcs for air-water in a 1.27 cm tube taken during B-g aircraft tqights
by Dukler and coworkers. If these are compared to waves in vertical annular Ilow on earth by Schadcl and Hanratty
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https://ntrs.nasa.gov/search.jsp?R=19970000373 2020-06-18T00:07:35+00:00Z
[4] (figure 2) or in our horizontal channel (figure 3) by Peng et al [5], it is seen that the wave amplitude to substrate
ratio is much larger for p,-g. This is evidently because of the lack of uniform gravity that acts to drain liquid from
waves in any flow geometry. Consequently based on the Woodmansee and Hanratty [I] mechanism, it is expected
that atomization will occur more readily under microgravity conditions. As such, it becomes a fundamental
consideration lbr the design of two-phase flow devices in spacecraft, space stations and lunar bases.
The importance of knowing the fraction of liquid entrained in gas-liquid flows has been recognized for many
years. Studies originally focused on measuring the entrained fraction and developing empirical correlations (e.g.
Dallman et al., [6] ; Asali et al., [7];). However as explained by Schadel and Hanratty [4], a better approach is to
measure and correlate the atomization and deposition rates separately. Unfortunately, this still has not produced a
general predictive relationship valid for all tlows. This is perhaps because their correlations do not reflect the
fundamental mechanism.
This paper describes some preliminary results that are intended to lead to verification of the mechanisms of
liquid atomization by a gas stream or an immiscible liquid flow and to provide predictive capabilities for the widest
possible parameter range. Experiments have been done for a two-liquid system in our rotating two-liquid Couette
device and in an oil-water channel flow.
EXPERIMENTAL RESULTS
Figure 4 shows our density-matched, two-liquid rotating concentric cylinder device that can provide wave
behavior that is not limited by the length of contact of the two phases. Some experimental details are given in
Sangalli et al. [8] and an extensive description will be available in a forthcoming publication. The two fluids used
tot the experiments are Dow 710, which is a phenylmethyl polysiloxane fluid and a mixture of ethylene glycol,
water, and Pink Bismuth - an Osco ® brand upset stomach remedy. The Pink Bismuth is used as a source of
refractive particles. The viscosity and density of Dow 710 are 0.555 Ns/m 2 and I 110 kg/m 3 respectively. The
viscosity and density of the ethylene glycol solution are 0.0151 Ns/m 2 and 1108 kg/m 3 respectively. Dow 710 is
loaded on the outer cylinder after the ethylene glycol mixture has been added to the channel.
We use several lighting techniques to probe the experiment -- white light, a vertical plane of laser light, and a
horizontal .laser sheet. The two laser methods provide useful quantitative data that direct visualization with white
light does not allow.
Figure 5 shows a map of wave behavior for conditions where the depth ratio is such that the positive growth
region for wave modes always extends from zero wavenumber up to a cutoff determined by the rotation rate. It is
seen that periodic waves occur at low rotation rates lbr all depth ratios and that waves become more irregular as the
rotation rate is increased. The occurrence of short wavelength, steady periodic waves for a range of conditions where
long waves are unstable is quite surprising and important. In both the falling film problem thai we studied earlier
(Chang, [9]), where the shape of the growth curve is always similar the ones for figure 5, and gas-liquid channel flow
(McCready and Chang, [10]) which also hits the same growth curve at high gas flow, the long distance dominant
disturbance is always a solitary wave or roll wave. Apparently, at low shear rates there is enough nonlinear
stabilization in these two-liquid flows to prevent the long wave modes from gr_wing. This may be caused by direct
interaction between long and short v,'aves similar to what is observed at h_w shear in gas-liquid channel flow (Jurman
etal.,[ll]). At higher rotation rates, solitary waves seem to evolve from periodic waves. Because the numberof
solitary waves in the devicc is always less than the original number of small amplitude periodic waves, solitary wave
formation must also involve some type of energy transfer mechanism hctv, cen short anti hmg waves. These
9O
examplesunderlinetheimportanceofunderstandingi teractionsbetweenlongandshortwaves.Inthecontextofthe
currentinterestoftwo-phaseflowinspace,theysuggestthatevenwithoutgravitystabilization,astratifiedstatethat
doesnotsufferfromatomizationcouldexist.Thus,althoughpreviousstudieshavenotreportedstratifiedflow,it
couldoccur--perhapsinasmallpassageh atexchanged viceataveryinopportunetime.
Figure5indicatesthatatomizationccursatsufficientlyhighrotationrates.Thewavesthatoccurinthisrange
areirregularly-spacedlarge-amplitudewaves.Ourpreliminaryworkhasnotallowedustoclearlydiscernthe
mechanismbehindtheformationofthesewavesandthesubsequentatomizationphenomenonbecauseweneedmore
lightintensitytogetsufficientvideoresolution.Howeverthereisnodoubtthatatomizationccursfromthelarge
solitarywaveswhoseamplitudeandseparationcanbepreciselycontrolledbytheexperiment.
Figure4 showsthecross-sectiona othergeometrythatwehaveconstructedspecificallytodemonstratethe
mechanismof atomization.Thechannelis I cmhighby 14cmwideby2.3m long.Theconcurrentoil-water
channelflowisoperatedsothattheoil-waterinterfaceis imagedirectlyfromthetop.Theoil andwaterhavea
curvedinterfaceandtheoil isconfinedtoaportionofthetopwall.Wehavebeenabletogetsomedramaticvideo f
oildropletsbeingformedbyshearingwater.Whenthewatershearissignificant,largesemi-periodicwavesformand
travelalongtheinterface.Figure7ashowsthatif theshearisstillhigher,thecrestbecomeslongated.Figure7b
showsthelaterstageofthesnap-offprocess.Inthisexperiment,theoil wavesarecompletelysurroundedbywater,
theyarenot touchingthetopwall. Forthisgeometryashearinstabilitymustinitiatethewaves,growthis
probablycausedbyaBernoullieffect,andtheactualpinchoff byacombinedcapillaryandshearinstability.The
sizeofthedropisdeterminedbythesizeofthestreamerfromthecrest.Notethatevenif thegeometryof thebase
flowisparallel,suchasinWoodmansee'sxperiment,dropswillstillbeshearedfromfingersofliquidbecausethe
wavesthatoccuratveryhighshearhavesignificanttransversevariation;therewill neverbewidesheetsof liquid
shearedfromthecrestsof waves.Thusourdemonstrationexperimentisclosetothegeneralsystemthatwill be
studied.
A finalpreliminaryexperimentisintendedtodirectlyprobethebehaviorf wavesthataretheprecursorsof
solitarywaves.Theultimateintentionofourworkistoactivelycontrolthesewaves.Inthetwo-layerCouette cell,
the initial transition from a flat film is supercritical and in agreement with linear and weakly nonlinear theory
(Sangalli et al., [8]). Figure 8 shows a sample experiment where a weak oscillatory component is added to the
steady rotation. The oscillation amplitude is 0.25 cm and the frequency is 6 Hz. It is seen that either with or
without oscillation a supercritical transition occurs. The oscillation in this experiments and others that we have tried
destabilizes the waves. This is consistent with the argument that the oscillation is adding energy to the flow which
causes more wave growth.
DISCUSSION
Figure 5 shows that atomization is a likely consequencc l'or a flow at sufficient shear. However the non
monotonic behavior indicates that onset of atomization is not a simple function of shear rate. Further, no attempt
was made to quantify the extent of atomization. Figure 7 shows that it will be crucial to incorporate the mechanism
into procedures for predicting atomization rates. The atomization process inw)lves instability and growth of waves
on the base state and then a further instability of the large wave structure. The formation of solitary or roll waves,
commonly associated with atomization, and the waves of figure 7 are certainly governed by nonlinear process that arc
presently understood only for very simple model equations (e.g., see [9]).
Development of a complete understanding of the atomization process will take advances on a number of fronts.
Figure 8 shows an example of our efforts in thc weakly- nonlincar region wherc we arc using oscillations to bcttcr
91
understandthewavegrowthandsaturationmechanism.Althoughwehavenotyetfoundaregionwherewavesare
stabilized,theoriesuggestthatcertainrangesof oscillationsshouldepresswaveformation.Wehopethiswill
ultimatelyleadtowavecontrolthatcouldalterpressuredroporatomization.
ACKNOWLEDGMENTS
ThisworkhasbeensupportedbytheNASAMicrogravityScienceandApplicationsDivisionundergrantnumber
NAG3-1398.
REFERENCES
I. D.E.WoodmanseeandT.J.Hanratty,Chem.Eng.Sci.,24(1969)299-307.
2.P.B.Whalley,G.F.HewittandJ.Terry,UKAEA Report, AERE-R9389 (1969).
3. L. B. Fore and A. E Dukler, submitted AIChE J. (1994).
4. S. A. Schadel and T. J. Hanratty, Int. J. Mult. Flow, 15 (1989) 893-900.
5. C. -A. Peng, L. A. Jurman, and M. J. McCready, Int. J. Mult. Flow, 17, (1991) 767-782.
6. J. C. Dallman, J. E. Laurinat and T. J. Hanratty, Int. J. Mult. Flow, 10 (1984) 677-690.
7. J. C. Asali, G. W. Leman and T. J. Hanratty, Physicochemical Hydro. 6, (1985) 207-221.
8. M. Sangalli, C. T. Gallagher, D. T. Leighton, H. -C. Chang and M. J. McCready, Phys. Rev. Let. 75, (1995) 77-80.
9. H. -C Chang, Annual Review of Fluid Mech., 26 (1994) 103-136.
I0. M. J. McCready and H. -C. Chang, Chem. Eng. Comm., 141-142, (1996) 347-358.
II. L. A. Jurman, S. E. Deutsch and M.J. McCready, J. Fluid Mech., 238 (1992) 187-219.
E
E
v
t.-
t.-
1.6'
1.2
0.8
0.4
0.0
4.0
- [us1=.117usg=lO.4
- I
4.5
I
5.0
I
5.5 6.0 6.5
I
Figure 1.
time (s)
Wave tracings for air-water in a 1.27 cm pipe under la-g. Data from NASA Lewis Lear jet flights
supervised by A. E. Dukler.
I
7.0
2.0
1.5
w
..d
0
>
05
O0
O0
(b) ' ' ' '
D=4,20 CM
uc-4o _/s
WL-52 5 G/S
, I I L I
0.2 04 06 08
TIME (S}
Figure 2. Wave tracings in vertical annular flow by Schadel and Hanratty / 1989)
92
0.5
0.4
E 0.3
&
t,-
"_ 0.2
t,--.
0.1
I
I
I RG = 17800 I
RL= 10
_L = 15 cP
0.0 L I I I I I I
0.0 0.5 1.0 1.5 2.0 2.5 3.O
time (s)
Figure 3. Solitary waves in a horizontal gas-liquid flow in earth gravity
Wave Pro_
Video
Coueue Cell
Figure 4. Two-layer Couette experiment with horizontal laser shown.
50
_- 40
"_ 80
o.
• 20
10
0
stable
long waves ]
• No waves
A steady periodic
X unsteady waves
solitary
tong wave stability boundary
X
X I Atomization I
X
A X _ _ X
AX X • _ • X X
AA X • _ • A X
ilia A • _ A X
WHI il • • A A A
li n • • • • A
I I ____I
Figure 5.
0.0 0.2 0.4 0.6 0.8
h
Wave regime map for the two-layer Coucllc cxpcrimcnt. Atomization occurs at high rotation rates.
93
Wavesformandatomization
occurs in thisregion
Oil
,/
Water
Waves form and atomization
occurs
in this region
Figure 6. Cross section of cocurrent oil-water flow that demonstrates the mechanism of atomization.
Figure la.
Large wave in
oil water flow
Figure 7b.
Late stages of
liquid removal
from wave crest
E
"O
*=_
Q.
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>
0.8
06
0.4
0.2 D
0.0 +
20
I
B
E] steady rotation I
© with oscillation I
0
o
[]
0
[]D
[]
0
0 [] [] Ill [] []
[] 0 0 []
I viscosity ratio = 67.7 (outer/inner) ldensity ratio = 1.002
depth ratio = 4.88
[]
[] 1 I0 [] I I I I I
22 24 26 28 30 32 34
rotation speed (cm/s)
Figure 8. Wave amplitude as a function of rotation rate. Weak oscillation slightly destabilizes the waves.
94


Fundamental Processes of Atomization in Fluid-Fluid Flows

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