Project 1. Primary (Shear) Breakup in Underwater Jet Flows - Formation of Droplets from Ligaments
Understanding the primary (shear) breakup in jet flows and the formation of droplets from ligaments is important to determine the final droplet size distribution (DSD). The initial droplet size, which affects the final DSD, is considered to be generated by the primary breakup. Large-eddy simulation (LES) was performed to investigate the shear breakup in liquid-liquid jet flows. The explicit Volume of Fluid (VOF) model with the geometric reconstruction scheme was used to capture the oil-water interface. The estimated oil distribution including wave peaks, ligaments, droplets and water streaks were compared to the experiments with a good agreement. The estimated DSD matched with the measurements favorably well. In the simulation, the formation of droplets with a smooth and curved surface from ligaments or sheet-like structures was obtained. Different mechanisms were observed along with the shear layer including the formation of droplets from ligament through the capillary forces, breakage of a droplet into smaller ones and attachment of a droplet to a ligament. The destructive shear forces and resisting surface tension forces were quantified on stretching and retracting ligaments. The influence of internal viscous force was found to be negligible due to low oil viscosity. The critical capillary number was found to be larger than 5.0 for ligaments breaking with the shear breakup. The capillary number was below unity for retracting ligaments. The coalescence of two equal-sized droplets was obtained in the shear breakup region. The shear stress magnitude at the contact region increased more than two folds. The total surface area decreased nearly 20% after the coalescence.
Published paper for details: Daskiran, C., Xue, X., Cui, F., Katz, J. and Boufadel, M.C., 2021. Large eddy simulation and experiment of shear breakup in liquid-liquid jet: Formation of ligaments and droplets. International Journal of Heat and Fluid Flow, 89, p.108810.
Figure 1. Contours of oil volume fraction in the shear breakup region: (a) predicted by numerical simulation at time t=115 ms and (b) observed in the experiments at time t>310 ms. The wave peaks, the ligaments, the droplets and the water streaks obtained due to the water entrainment in both cases are pretty much similar in size and shape. The droplets rounded by yellow circles in (a) are smaller than 400 and are not considered while computing droplet size distribution due to their irregular surface.
Figure 3. Instantaneous iso-surfaces of (a) Q-criterion at 3Ă—103 s-2 and (b) oil volume fraction at 0.5. One notes the vortex rings and wave peaks just above the orifice. These develop into hairpin vortices, elongated ligaments and droplets in the shear breakup region.
Figure 2. Droplet sizes: (a) Number density distribution and (b) the cumulative volume fraction of droplets obtained from many snapshots in the experiment and numerical simulation along the central plane from r/d=2.0 to r/d=7.0 and from z/d=1.0 to z/d=5.0 . The term Da is the apparent droplet size computed based on the cross-section area of droplets and d is the pipe diameter of 10 mm.
Figure 4. (a) Iso-surface of oil holdup at a constant value of 0.5 revealing the coalescence of droplets in the shear breakup region, (b) the temporal profile of shear stress averaged over the surface of oil droplets and (c) the surface area before and after the droplet coalescence. High shear stress inside the region surrounded by dashed lines in (a-1) is not related to the coalescence of droplets.