Breakup of a surface-mounted droplet by an impinging jet flow
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Date
2023-09-18
Authors
Okoye, Kenechukwu
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Droplet removal under wind-forcing arises in many engineering applications, such as surface cleaning. In these applications, the efficiency of the system can be affected by droplet breakup, since smaller droplets are harder to remove. This thesis focused primarily on the breakup process of isolated surface-mounted droplets exposed to an accelerating impinging jet flow. The investigation focused on: (i) the critical flow velocities in the breakup process, and (ii) characterizing the geometry of both the originating droplet prior to breakup and the resulting child droplet.
Impinging jet flows were generated using a custom jet facility in the Fluid Mechanics Research Laboratory at the University of Waterloo. The jet centerline velocity was programmed to ramp up to a target velocity, Uj = 20 m/s at four flow accelerations, dUj/dt = 1.2, 2.2, 3.2, and 4.4 m/s^2. Hot-wire anemometry was used to characterize the background flow field at the jet exit, and various wall-normal distances at the initial stream-wise location of the droplets.
Distilled water droplets of 50, 100, 200, 300, 400, 500, and 600 ul were tested on an anodized aluminium substrate in the impinging jet facility at the aforementioned flow conditions. Droplets were characterized based on side-view and top-view images which were captured simultaneously during the impinging jet ramp-up. The breakup process of surface mounted droplets in an accelerating impinging jet flow comprises three distinctive consecutive stages: depinning, necking, and breakup. The critical Weber number Weh,crit, based on droplet height at depinning, at which droplets in the considered volume range depin is within the range of 3 < Weh,crit < 4 when exposed to impinging jet flow. Shortly after depinning, necking begins as a precursor to the droplet breakup. The Weber number for the onset of necking (Weh,neck) is within the range 4 < Weh,neck < 6. Finally, the necking process culminates in the droplet breaking up into two, or more, smaller child droplets. The Weber number at which breakup occurs (Weh,br) follows a power-law relationship with the Ohnesorge number (Oh). For the Ohnesorge range (8.8 x 10^-4 < Oh < 1.4 x 10^-3 investigated in the present study, Weh,br fell in the range 6 < Weh,br < 7.5 and showed little variation with the tested parameters.
The volume of the largest child droplet resulting from breakup was estimated using a 3D reconstruction of the droplet. Larger originating droplets shed larger percentages of their volumes during the breakup process. This is also the case for droplets exposed to higher flow accelerations. A linear correlation was also found between the lengths of the child droplet and that of the original droplet immediately prior to breakup. Based on this correlation, it was concluded that smaller sessile droplets and droplets exposed to lower flow accelerations likely break up into smaller volume fractions due to the lower elongation they experience. The results of this thesis provide useful guidelines for the optimization of impinging jet configurations in non-touch drying systems.
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Keywords
fluid dynamics, wind forced droplets, droplet depinning, droplet necking, droplet breakup