Effects of droplet size on intrusion of sub-surface oil spills

Abstract

This paper explores the effects of droplet size on droplet intrusion and subsequent transport in sub-surface oil spills. In an inverted laboratory set-up, negatively buoyant glass beads were released continuously into a quiescent linearly stratified ambient to simulate buoyant oil droplets in a rising multiphase plume. Settled particles collected from the bottom of the tank exhibited a radial Gaussian distribution, consistent with their having been vertically well mixed in the intrusion layer, and a spatial variance that increased monotonically with decreasing particle size. A new typology was proposed to describe plume structure based on the normalized particle slip velocity UN=us/(BN)1/4, where us is the particle slip velocity, B is the plume’s kinematic buoyancy flux, and N is the ambient stratification frequency. For UN≤1.4 particles detrain from the plume, but only those with smaller slip velocity (UN≤0.3) intrude. An analytical model assuming well-mixed particle distributions within the intrusion layer was derived to predict the standard deviation of the particle distribution, σr=0.9−0.38(UN)0.24π−−−−−−−−−−√B3/8N5/8u1/2s and predictions were found to agree well with experimental values of σr. Experiments with beads of multiple sizes also suggested that the interaction between two particle groups had minimal effect on their radial particle spread. Because chemical dispersants have been used to reduce oil droplet size, this study contributes to one measure of dispersant effectiveness. Results are illustrated using conditions taken from the ‘Deep Spill’ field experiment and the recent Deepwater Horizon oil spill.