Dynamics of Drop Release from the Edge of a Spinning Disc

Abstract

The direct formation of drops at the edge of a spinning disc is of fundamental interest. We use high-speed imaging to report here on the process of release of drops from a perfectly wetted disc at low inflow rates. A drop-detachment event begins by forming an incipient bulge on the disc edge, which grows into a series of shapestriangle, inverted U, pear, and finally a nearly spherical bulge connected to the disc with an elongated neck. Neck pinching at the base of the bulge releases a primary drop followed by several secondary drops. The drop shape versus time plots show high variability under fixed conditions, which disappears on a single curve for time scaled with individual drop cycle time. The measurements at a different disc speed show the same scaled time evolution, pointing to a universal drop release process. The rapid stretching of the liquid thread as the bulge moves away before pinch-off follows a parabolic relationship with time but with only half the relative acceleration of a free object released from the disc edge. The mean values of cycle time and necking time follow a power-law decrease with disc speed. All of the drops generated in a single event move with the speed of the disc. There is no slip, which is not the case with the ligament mode of breakup. The intervals of quiescence between the successive release of drops from the entire disc follow the Poisson process.

Spinning Disc–Spinning Bowl Contactor

“…The mean drop size for the operational conditions used is around 300 μm, and the film thickness is of the same order. , The time constant for velocity reduction due to air drag is 1/4 s and the travel time from disc edge to bowl wall about 1 ms based on the measured initial velocity of released drops (Fastcam SA5 775 K-M2 from Photron, operating at 7000 fps and 1 Megapixel image size). Thus, all the kinetic energy of a detached drop (about 50 m 2 s –2 per unit mass) is dissipated on impingement.…”

“…The hydrodynamic considerations suggest that scale-up of the new contactor for high processing rates is possible. The scaling of drop size, drop velocity, release frequency, and spatial distribution with disc size, flow rate, and rotational speed in direct drop mode of atomization is known. , The studies on scaling of film related parametersfilm thickness, residence time, mixing time scale, and axial dispersion with bowl diameter, rotational speed, and flow rateare needed to make the proposed device a scalable option.…”

“…The spinning discs are used to contact liquid streams on them or one liquid stream with the gas phase above. We have used a spinning disc as an atomizer to release drops with significant radial velocity − without an air draft. To our knowledge, this is the first instance of a mixing device relying on droplet impact on a moving liquid film.…”

Characteristics of the Endogenous Rotation-Induced Airflow Field and Its Influence on Droplets Produced by a Rotating Disk

“…Subsequently, the liquid film breaks away from the edge of the rotating disk and disintegrates into droplets, ligaments, or sheets under the action of centrifugal force, surface tension, and viscous force . After numerous investigations on the traditional rotating-disk centrifugal atomizer, − a new gas-assisted hybrid centrifugal technique has garnered increasing attention. Czisch and Fritsching proposed a classical rotating atomizer combined with an auxiliary external gas blast for fragmenting of viscous melt and found that this hybrid technique could considerably reduce particle size compared to a conventional gas atomizer.…”