Formation of compound droplets during fragmentation of turbulent buoyant oil jet in water

Abstract

Fragmentation of a vertical buoyant silicone oil jet injected into sugar water is elucidated by refractive index matching and planar laser-induced fluorescence. Compound droplets containing multiple water droplets, some with smaller oil droplets, form regularly at jet Reynolds numbers of $Re=1358$ and 2122 and persist for at least up to 30 nozzle diameters. In contrast, they rarely appear at $Re=594$. The origin of some of the encapsulated water droplets can be traced back to the entrained water ligaments during the initial roll-up of Kelvin–Helmholtz vortices. Analysis using random forest-based procedures shows that the fraction of compound droplets does not vary significantly with $Re$, but increases rapidly with droplet diameter, reaching 78 % for 2 mm droplets. Consequently, the size distributions of compound droplets have peaks that increase in magnitude and shift to a lower diameter with increasing $Re$. On average, the interior pockets raise the oil–water interfacial area by 15 %, increasing with diameter and axial location. Also, while the oil droplets are deformed by the jet’s shear field, the interior interfaces remain nearly spherical, consistent with prior studies of the deformation of isolated compound droplets for relevant capillary numbers and viscosity ratio.

A Review on Multiphase Underwater Jets and Plumes: Droplets, Hydrodynamics, and Chemistry

“…The length of the core (also known as the irrotational or potential core) is larger for immiscible fluids, such as oil in water, as observed in the experiments of Xue and Katz (2019), where the core length was around 6 diameters. This behavior seems to be related to the hydrophobicity of the oil to water, and thus, an oil jet tends to have more coherence than a miscible liquid jet.…”

“…Experiments on oil droplets from jets have been conducted from various orifice sizes (Brandvik et al, 2013; Murphy et al, 2016; Xue & Katz, 2019; Zhao et al, 2017). Xue and Katz (2019) provided observations of composite droplets where small droplets exist within a water bath surrounded by the large droplet.…”

“…The symbol “ d ” is the pipe diameter, which was 1.0 cm. Arrows of the same color (red or orange) and shape (solid or dashed) follow the same water ligament in the frames separated by 6 ms. Taken from Xue and Katz (2019).…”

“…The exit velocity was large (around 13 m/s) and the Reynolds number was around 2,000. Xue and Katz (2019) used an index matching method that allowed them to measure the oil DSD in a plane crossing the center axis of plumes emanating from a 10 mm diameter nozzle at Reynolds numbers varying from 600 to 2,400. The nozzle that they used produced a top‐hat (i.e., radially uniform) velocity profile at the orifice.…”

“…The main driving mechanism for the release is usually the high pressure in the reservoir (whether geologic or human made), but upon release in water, buoyancy (due to the lower density of the released fluids) eventually takes over and causes the rise of many hydrocarbons through the water column, potentially to the water surface, and ultimately to the atmosphere. Many of the processes have been poorly understood due to the hydrodynamic complexity of the release of a multiphase plume (liquid hydrocarbons and gas) (Boufadel et al, 2018; Socolofsky et al, 2002), the formation of droplets (Zhao, Shaffer, et al, 2016, Xue & Katz, 2019) and bubbles (Leifer et al, 2015; Wang et al, 2018), and the behavior of hydrocarbon components at depth (Gros et al, 2016).…”