Measurements of Dispersions (turbulent diffusion) Rates and Breaking up of Oil Droplets in Turbulent Flows

Transcription

1 Measurements of Dispersions (turbulent diffusion) Rates and Breaking up of Oil Droplets in Turbulent Flows Balaji Gopalan PI: Dr Joseph Katz

2 Where do we come in? Turbulent diffusion of slightly buoyant oil droplets Effect of dispersants on sample crude oil breakup

3 Digital Holography A hologram is a recorded interference pattern between a wave field scattered from the object and a reference wave. Recorded Plane (1) ds 1 r 01 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. Y 0,Y 1 X 0,X 1 Z Reconstructed Plane (0)

4 Isotropic turbulence facility with one view in-line holography setup Droplets observed in a 50x50x70 mm 3 sample volume Data is recorded at fps depending on mixer rpm Spinning 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 infocus droplet Spatial Filter Q Switched, Diode pumped Pulsed Laser from Crystalaser Collimating Lens y z x Pressurized storage container

5 Reconstructed movie of turbulent diffusion of diesel oil droplets (recorded at 2000 fps)

6 Sample tracked droplets and particles Note : These particles/droplets are tracked over a period of time Data from over (0.5-1 mm Dia) diesel oil droplets and fluid particles (50 µm) are averaged to obtain turbulence statistics Injection Tube Blocked to avoid spurious particles due to injectors fringes and the effect of injector

7 Comparison of droplet diffusion coefficient to fluid diffusion coefficient D U' i /u' i ii = Max( 1 τ t= 0 U ( t') U i i ( t' + t) dt) D dii /.D fii Horizontal Vertical D = UT 2 ii 1.6 i di Horizontal Vertical u i = Fluid velocity rms U i = Droplet velocity rms U q = Droplet Quiscent rise velocity u' i /U 0.2 u' q i /U q u' i /U q T di /T fi Horizontal Vertical The droplet diffusion coefficient is smaller than the fluid diffusion coefficient for smaller values of u i /U q and exceeds it at higher u i /U q The droplet diffusion coefficient in the horizontal direction is greater than the vertical direction at lower u i /U q and become almost similar at higher u i /U q

8 A new empirical relation for droplet diffusion coefficient 8 6 Horizontal Vertical 10 1 Horizontal Vertical Slope = 1.52 Slope = 1.44 D dii /u' i L qi 4 2 D dii /u' i L qi u' i /U q u' i /U q For d << L and St ~ 1-10 D / u T = 0.66( u / U ) dxx x fx x q D / u T = 0.51( u / U ) dyy y fy y q

9 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 c Gd µ σ Viscosity ratio Weber Number Ohnesorge Number µ d µ ρ ( ) c c σ σ c 2 2/3 ud ρ εd d µ d d ρ σ d

10 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 Ohnesorge number for a 2 mm droplet ~ 0.055

11 High speed movies of droplet break up Weber Number = 0.81 Dissipation = 19 cm 2 /s Recorded at 500 fps Weber Number ~ 4 Dissipation = 256 cm 2 /s Recorded at 1000 fps

12 Similarity to breakup in a shear flow 5 mm Drop size = 2.3 mm Integral time scale = 1.12 s Kol time scale = 23 ms Kol length scale = 0.15 mm Taylor length scale = 4.1 mm Integral length scale = 52 mm

13 Shredding of droplets Crude oil droplet (Dia = 1.8 mm) pre-mixed with COREXIT 9527 sheds small droplets when rising in quiescent condition. Extremely thin (becomes < 17 µm) thread like structures are also shredded from the droplet as it rises. Breaking phenomena also observed in turbulent flow Very low interfacial tension due to a large dispersant concentration might cause such instabilities????

14 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

15 Conclusions The diffusion coefficient scaled by turbulence intensity and appropriate length scale is a monotonically increasing function of the turbulence level normalized by the droplet quiescent rise velocity The droplet diffusion coefficient is smaller than the fluid diffusion coefficient for smaller values of u i /U q and exceeds it at higher u i /U q For most of the cases the droplet diffusion coefficient in the horizontal direction exceeds the droplet diffusion coefficient in the vertical direction The turbulence is responsible for stretching of droplets while the actual breaking occurs due to capillary instability 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

16 Thank You!