The effect of applied electric fields on the behavior of liquids and the interaction of liquids with solid surfaces has been a topic of active interest for many decades. This has important implications in phase change heat transfer processes such as evaporation, boiling, and condensation. These processes are critical in industries such as energy generation and transportation, chemical processing, pharmaceutical industry, and desalination. In recent years, electrowetting and droplet heat transfer has been studied extensively with many experimental designs involving the generation of an electric field within the droplet or in an external capacitive field. Although the effect of low to moderate voltages has been studied on sessile and free falling droplets, the transient deformation and thermal impact characteristics of a droplet when subjected to strong electric fields remains less explored. My work involved the design and manufacture of different experimental setups for such studies and the performance of sessile and drop impact experiments at high applied electric fields.
In the first set of experiments, a Steel Film Drop Impact setup was used to study the dynamics of drop impact and the evaporation behavior of the post-impact sessile drop at different impact Weber numbers. This setup allowed for both optical observation of the impacting drop as well as the infrared thermal observation of the substrate from the backside. A wire electrode held close to the substrate was used to generate the electric field. Experiments conducted using the dielectric fluid HFE-7100 generated data consistent with previous studies on the effect of We number on drop impact. Although drop evaporation time was found to vary with applied electric fields, consistent evaporation rate behavior could not be obtained due to the undesired effect of electric field on the size of the initial drop created by the dispensing needle. Further, the applied maximum voltage in this case was not very high (300 V) due to the unavailability of a highvoltage device.
The second set of experiments involved the use of a Hot Plate Drop Impact setup that employed a more sophisticated mechanism for generating the impacting liquid drops and for applying the high electric fields above the hot substrate. Drop impact experiments of distilled water and HFE-7100 dielectric fluid were conducted at a substrate temperature of 150 °C and applied voltages between 0 and 10 kV, with the electric field aligned parallel to the direction of motion of the drops. Experimental results indicate a significant change in the pre- and post-impact behaviors of the drop when the magnitude of the applied electric field is changed. Prior to impact, the applied electric field elongates the drop in the direction of the electric field. The most important effects of the electric field, however, are observed after impact and especially during the recoil phase of the water droplet. This behavior is also different for the two fluids. Whereas the water droplet mostly recoils as a single drop, the dielectric drop recoils as multiple micro droplets into which its breaks up due to the low surface tension for HFE-7100. In case of water, the high electric field elongates the recoiling drop whereas for the dielectric fluid it changes the recoil behavior of the micro droplets. Further, interesting observations are made regarding the effect of droplet charging on droplet elevation, droplet necking, and micro droplet ejection due to boiling. Design and fabrication of an advanced pool boiling rig for conducting future high-electric-field experiments with bubbles is also presented.
|Advisor:||Singh Dhillon, Navdeep|
|Commitee:||Kalman, Joseph, Moghtadernejad, Sara|
|School:||California State University, Long Beach|
|Department:||Mechanical and Aerospace Engineering|
|School Location:||United States -- California|
|Source:||MAI 81/1(E), Masters Abstracts International|
|Subjects:||Fluid mechanics, Thermodynamics, Mechanical engineering|
|Keywords:||Deformation, Drop Impact, Electrohydrodynamics, Fluid Dynamics, High Voltage Electric Field, Optical Imaging|
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