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.

www.itopf.com

How do dispersants work

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



c

2

u d

 

d

 

d d

Shear Dominated Ref: Grace (1982) 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.



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 .

Recorded Plane (1)

Z

1

ds r 01

Reconstructed Plane (0) 0

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 mm 3 sample volume Data is recorded at 500 - 1000 fps depending on Spinning mixer rpm Grids High speed camera (Photron camera with resolution 1kx1k and frame rate 2000 frames/s) capturing streaming holograms Demagnifying Lens Injector Section of Reconstructed image with in focus droplet Q – Switched, Diode pumped Pulsed Laser from Crystalaser Spatial Filter 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

cm 2 /s Recorded at 500 fps

Similarity to breakup in a shear flow

5 mm

Integral time scale = 1.12 s Kol time scale = 23 ms Drop size = 2.3 mm Kol length scale = 0.15 mm Taylor length scale = 4.1 mm Integral length scale = 52 mm

Breakup of an initially extended oil droplet with DOR 1:20

Weber Number ~

4

Dissipation =

256

cm 2 /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

DOR 1:20

Tails pulled from droplets

 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

After t = 0 ms 0.58 mm

Gallery of Tails

After t = 120 ms After t > 1s 1 mm DOR 1:15 1.4 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