Experimental study of multiphase turbulent buoyant jets and the formation of compound droplets

Motivated by the need to understand the fragmentation, dispersion, and turbulence of the immiscible buoyant jet, experimental techniques capable of yielding simultaneous measurements of both the carrier and dispersed phase is developed. First, the effects of chemical dispersant concentrations on the droplet size distribution within a crude oil plume in crossflow are first investigated using high-speed visualization and submerged in situ digital inline holography imaging technique. Increasing the dispersant concentration dramatically decreases the droplet sizes and increases their number, and accordingly, reduces the rise rates of droplets and the upper boundary of the plume. In the near field, refractive-index matching liquid pairs, simultaneous planar laser-induced fluorescence, and particle image velocimetry are performed to elucidate the fragmentation and characterize the turbulence of a vertical buoyant oil jet. Close to the nozzle, the surrounding water gains momentum when thin layers are entrained into the jet. Also, as oil ligaments begin to extend outward, vortices form around their tips in the water. Further downstream, as the oil breaks up into blobs and then to smaller droplets, some are compound, containing multiple water droplets, some with smaller oil droplets. Such droplets regularly form at moderate and high jet Reynolds numbers and persist for at least up to 30 nozzle diameters. In contrast, they rarely appear at low Re. The origin of some of the encapsulated water droplets can be traced back to the entrained water ligaments during the initial roll-up of KH 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. 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 the 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. The spreading and decay rates of the jet centerline oil fraction are lower than those of the axial momentum. Comparison to a single-phase jet at the same Re shows that the transition from azimuthal shear layers to self-similar profiles of velocity and Reynolds stresses occur earlier in the oil jet. Phase-conditioned statistics in the oil jet reveal significant spatially varying discrepancies between the turbulence level in the oil and water phases. The peripheral turbulence in the water is higher near the jet exit, but lower after six diameters. The latter trend is attributed to the intermittency and lower peripheral shear-dominated turbulence production rate in the entrained water. In contrast, near the jet centerline, the turbulence production rate, hence the turbulent kinetic energy, is higher in the water. Here, while the axial contraction increases the turbulence in both phases, the radial extension in the spreading oil, as opposed to the radial contraction in the entrained water, causes a discrepancy in the production rates