EFFECT OF SURFACTANT ON THE DYNAMICS OF A CRUDE OIL DROPLET IN WATER COLUMN: EXPERIMENTAL AND NUMERICAL INVESTIGATION

In this study we have investigated the effect of surfactant, sodium dodecyl sulfate (SDS), on the dynamics of a single crude oil droplet, rising in a quiescent water column. Experiments were conducted in a tank, in which an oil droplet was released into a stagnant water column through a nozzle. The droplets ranging from 0.3 to 0.85cm were produced from three different sized nozzles. The shape adopted by the emanating droplets varied from spherical to oblate. SDS concentrations were varied from 0 to 750ppm in water. The adsorption of surfactant reduced the interfacial tension at oil-water interface which resulted in generation of smaller sized droplets at the nozzle and caused the droplet to flatten. Consequently, the rise velocities of droplets decreased. A numerical model based on finite volume method was developed using commercial CFD package ANSYS Fluent (R). The model employed volume of fluid method, suggested by Hirt and Nichols (Journal of Computational Physics1981, 39, 201), with an interface reconstruction technique based on piecewise linear representation for tracking the oil-water interface. The influence of surface tension on the droplet dynamics was captured by including Continuum Surface force (CSF) approach suggested by Brackbill, Kothe, and Zemach (Journal of Computational Physics1992, 100, 335). The shape and rise velocities predicted from model were in good agreement with experimental data. The results from simulations were used, to analyze the wake structure and pressure distribution around the droplet. It was found that the smaller droplets which ascended in rectilinear path were associated with an axisymmetric wakes whereas larger and intermediate sized droplets in high SDS concentration wobbled as they ascended because of asymmetric wakes.