Heat and Mass Transfer Enhancement by Carbon Nanotubes and Supersonically-Blown Nanofibers
Sinha Ray, Sumit
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The present dissertation aims at the development and study of novel nanostructured materials useful for the enhancement of heat and mass transfer at macroscopic scales. For this two novel materials were used- supersonically-blown polymer nanofibers and phase change material encapsulated carbon nanotubes. The supersonic solution blowing was developed to form nanofibers of the order of 50 nm from several polymers. The applicability of the process was first demonstrated with Nylon 6 and then was further introduced to different other polymers to produce 50 nm nanofiber on demand. Supersonically-blown 50 nm Nylon 6 nanofibers were introduced to filtration applications, where such nanofibers were deposited on commercial filters to filter dangerous 100 nm nanoparticles from water. Ultrafine supersonically-blown nanofibers intercepted nanoparticles more than any other nanofibers. Such nanofibers intercept nanoparticles by means of the van der Waals forces and entrap them on the windward or leeward sides. A theoretical model was also developed to study nanoparticle-nanofiber interaction and the theoretical model accurately predicts nanoparticle collection on supersonically-blown nanofibers as seen experimentally. The applicability of supersonically-blown ultrafine PAN nanofibers in thermal management applications was investigated next. A high-power surface mimicking a microelectronic a high-power substrate was coated with supersonically-blown PAN nanofibers. In one case they were metal-plated, whereas in another one pure polymer nanofibers were used. For both cases of metal-plated and non-metal plated nanofibers, it was observed that they facilitate nucleate boiling much more than bare Cu surface and lower surface superheat by several degrees at higher heat flux. Such texturing was also robust. Thermal management of high-power microelectronics was also tackled in the present work using phase change materials (PCM) like wax and meso-erythritol encapsulated in carbon nanotubes. Such CNTs were used to form aqueous suspensions or suspensions in oil and used in through-flow in a microchannel embedded inside a high-power “microelectronics” block. Such nano-encapsulation dramatically shortened the PCM thermal response time and prevented sticking to the wall. With an increase in the CNT-PCM wt%, cooling via PCM melting became more and more pronounced. Finally, a comprehensive quasi-one-dimensional model was developed for multiple polymer jets issued from a die nosepiece into a high-speed air flow and deposited onto a moving screen in solution blowing process. This study is fundamental for the ongoing studies of nanofiber formation in supersonic solution blowing.