Transcript Dynamics of slightly buoyant droplets in isotropic turbulence4

BREAK UP OF VISCOUS CRUDE OIL DROPLETS
MIXED WITH DISPERSANTS IN LOCALLY
ISOTROPIC TURBULENCE
Balaji Gopalan & Joseph Katz
What is an Oil Spill ?
 An oil spill is the release of a liquid petroleum hydrocarbon into
the environment
 Crude oils are made up of a wide spectrum of hydrocarbons
ranging from very volatile, light materials such as propane and
benzene to more complex heavy compounds such as bitumens,
asphaltenes, resins and waxes.
 An oil spill may occur due to
 Spillage from a Tanker
 Bursting of pipelines
 Naturally seeping from the ocean floor
Motive:
To understand the effect of addition of dispersants, to the crude
oil spilled in oceans.
How do dispersants work
www.itopf.com
Dispersants are generally a combination
of surfactants with some solvents
Solvents:
 Reduce viscosity
 Help migration towards oil-water
interface
Surfactants:
 Complex molecules with oleophilic
and hydrophylic parts
 Amount of dispersant molecules in
the interface determines the interfacial
tension.
What happens to dispersed oil ?
http://response.restoration.noaa.gov
Breakup of an immiscible fluid
When the disruptive forces in the carrier fluid overcomes the cohesive
forces in the immiscible droplet, it breaks
Capillary Number
Viscosity ratio
Weber Number
Ohnesorge Number
c Gd

d
c
Shear
Dominated
Ref: Grace
(1982)
cu 2 d

d
d d
Pressure
Dominated
Ref: Wiezba
(1990)
Turbulent Breakup
 Turbulence breakup experiments are primarily performed in stirred
tanks and pipelines (Sleicher (), Arai et al. (), Konno et al. (), Calabrese
et al () )
 Recent Breakup experiments have been performed in “simpler”
turbulent flow in an axisymmetric jet by Martinez Bazan et al. (1999) and
Eastwood et al. (2004)
 Breakup time of immiscible viscous droplets scales with the capillary
timescale ( Eastwood et al. (2004))
 Breakup Frequency of Large droplets ( L/D ~ 3-6) scales with the
passage frequency of large scale eddies
Current experiments are performed in a stationary
homogeneous and isotropic turbulence facility, with droplets
injected under “quiescent conditions” and L/D ~ 25-50
Digital Holography

A hologram is a recorded
interference pattern between a wave
field scattered from the object and a
reference wave.
ds
Recorded
Plane (1)
1
r01
0

The images are recorded in digital
format and processed numerically to
obtain the reconstructed image .

There is lower resolution in the
optical direction (depth), compared to
the lateral spatial resolution .
Reconstructed
Plane (0)
Z
Digital inline holography has an extremely
long depth of field (>15cm for our setup) and
requires only a low power coherent light
source
Isotropic turbulence facility with one view in-line
holography setup
Droplets observed in a
17x17x70 mm3 sample
volume
High speed camera (Photron camera with
resolution 1kx1k and frame rate 2000 frames/s)
capturing streaming holograms
Data is recorded at 500
- 1000 fps depending on Spinning
Grids
mixer rpm
Demagnifying
Lens
Section of
Reconstructed
image with infocus droplet
Injector
Spatial Filter
Q – Switched, Diode pumped
Pulsed Laser from Crystalaser
Collimating Lens
y
z
x
Pressurized storage
container
Measuring crude oil properties
Specific Gravity: The specific gravity is obtained
by measuring the extra weight due to addition of 75
ml of crude oil.
Viscosity Measurement: The kinematic viscosity
is measured using glass capillary viscometer
purchased from Canon. We have purchased two
viscometers of different calibrations and the
variation between them is taken as the uncertainty.
Surface Tension Measurement: Surface tension
is measured by measuring the hydrostatic pressure
difference required to transform a flat surface to a
hemisphere.
Oil: Crude oil sample from ANS
Dispersant: COREXIT 9527
DOR: 1:20 and 1:15
Ohnesorge number for a 2 mm
droplet at DOR 1:20 ~ 0.055 (>
0.01 hence droplet viscosity
contribution has to be
included)
Droplet Breakup at DOR 1:20
Weber Number = 0.81
Dissipation
= 19 cm2/s
Recorded at 500 fps
Similarity to breakup in a shear flow
5 mm
Drop size = 2.3 mm
Integral time scale = 1.12 s
Kol length scale = 0.15 mm
Kol time scale
Taylor length scale = 4.1 mm
= 23 ms
Integral length scale = 52 mm
Breakup of an initially extended oil droplet
with DOR 1:20
Weber Number ~ 4
Dissipation
= 256 cm2/s
Recorded at 1000 fps
Breakup of an initially extended oil droplet
with DOR 1:20 (Cont.)
3.5 mm
The stretched portions of
the some daughter
droplets (a, b, c, e) retain
their “tails” instead of
retracting after breakup
Size distribution of
daughter droplets from
25 breakup events
Integral time scale
= 0.34 s
Kol time scale
= 6.3 ms
Kol length scale
= 0.079 mm
Taylor length scale
= 2.28 mm
Integral length scale = 32 mm
Tails pulled from droplets
DOR 1:20
 Tail like structure is pulled from certain droplets
 Very low interfacial tension due to a large dispersant concentration might cause
such instabilities ????
 Breaking up of these threads produce extremely small droplets
Gallery of Tails
After t = 120 ms
After t = 0 ms
0.58 mm
DOR 1:15
After t > 1s
1.4 mm
1 mm
Breaking up of an oil pool by dispersant in
quiescent conditions
Marangoni stresses are
responsible for breaking up of a
pool of oil into “droplets”
Conclusions
 The turbulence is responsible for stretching of droplets while the actual
breaking occurs due to capillary instability
 Breakup of droplets of size L/D >> 1 is governed by inertial and Kolmogorov
timescales
 The size of the daughter droplets is ~ Kolmogorov length scale
 Under certain conditions extended droplets retain their elongated tails after
breakup
 Under certain conditions thread like structures are shed from the droplet
producing very small droplets
 Marangoni stresses cause initial breakup of an oil pool upon spraying of the
dispersants