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Electrodynamic droplet deformation
Electrodynamic droplet deformation is a phenomenon that occurs when liquid droplets suspended in a second immiscible liquid are exposed to an oscillating electric field. Under these conditions, the droplet will periodically deform between prolate and oblate ellipsoidal shapes. The characteristic frequency and magnitude of the deformation is determined by a balance of electrodynamic, hydrodynamic, and capillary stresses acting on the droplet interface. This phenomenon has been studied extensively both mathematically and experimentally because of the complex fluid dynamics that occur. Characterization and modulation of electrodynamic droplet deformation is of particular interest for engineering applications because of the growing need to improve the performance of complex industrial processes(e.g. two-phase cooling,〔Kaji NN, Mori YH, Tochitani YY. Electrically Induced Shape Oscillation of Drops as a Means of Direct-Contact Heat Transfer Enhancement: Part 2—Heat Transfer. J. Heat Transfer. 1988;110(3):700-704.〕 crude oil demulsification). The primary advantage of using oscillatory droplet deformation to improve these engineering processes is that the phenomenon does not require sophisticated machinery or the introduction of heat sources. This effectively means that improving performance via oscillatory droplet deformation is simple and in no way diminishes the effectiveness of the existing engineering system.
== Motivation ==
The heat transfer dynamics in two-phase two component flow systems are governed by the dynamic behavior of droplets/bubbles that are injected into the circulating coolant stream.〔S. Mostafa Ghiaasiaan. Two-Phase Flow, Boiling, and Condensation: In Conventional and Miniature Systems. 2008. Cambridge University Press〕〔Takaaki Mochizuki. Periodic Deformation of Microsize Droplets in a Microchannel Induced by a Transverse Alternating Electric Field. Langmuir 2013 29 (41)〕 The injected bubbles/droplets are typically of a lower density than the coolant and thus experience an upward buoyancy force. They enhance the thermal performance of cooling systems because as they float upwards in heated pipes the coolant is forced to flow around the bubbles/droplets. The secondary flow around the droplets modifies the coolant flow creating a quasi-mixing effect in the bulk fluid that increases the heat transfer from the pipe walls to the coolant. Current two-component, two-phase cooling systems such as nuclear reactors, control the cooling rate by optimizing solely the coolant type, flow rate, and bubble/droplet injection rate. This approach modifies only bulk flow settings and does not provide engineers the option of control of directly modulating the mechanisms that govern the heat transfer dynamics. Inducing oscillations in the bubbles/droplets is a promising approach to improving convective cooling because creates secondary and tertiary flow patterns that could improve heat transfer without introducing significant heat to the system.
Electrodynamic droplet deformation also of particular interest in crude oil processing as a method to improve the separation rate of water and salts from the bulk. In its unprocessed form, crude oil cannot be used directly in industrial processes because the presence of salts can corrode heat exchangers and distillation equipment. To avoid fouling due to these impurities it is necessary to first remove the salt, which is concentrated in suspended water droplets. Exposing batches of crude oil to both DC and AC high-voltage electric fields induces droplet deformation that ultimately causes the water droplets to coalesce into larger droplets. Droplet coalescence improves the separation rate of water from crude oil because the upward velocity of a sphere is proportional to the square of the sphere’s radius. This can be easily shown by considering gravitational force, buoyancy, and Stokes flow drag. It has been reported that increasing both the amplitude and frequency of the applied electric fields can significantly increases water separation up to 90%.〔Byoung-Yun Kim, Jun Hyuk Moon, Tae-Hyun Sung, Seung-Man Yang, Jong-Duk Kim. Demulsification of water-in-crude oil emulsions by a continuous electrostatic dehydrator. Separation Science and Technology. Vol. 37, Iss. 6, 2002〕
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