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Increasing Critical Heat Flux and Boiling Heat Transfer on Superhydrophilic Nano Porous Surface Using Low Conductive Spots

Najafpour, Sahand | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52090 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Mousavi, Ali
  7. Abstract:
  8. This dissertation argues that bi-conductive textured surfaces increase both Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC) simultaneously. Surface modification is applied to stainless steel specimens by the anodizing method in an electrolyte containing Ammonium Fluoride and DI-Water and Ethylene Glycol as the based solvent. The process of oxidation was under constant DC voltage and constant temperature. The contact angle on self-aligned Nano-porous oxide layer fabricated on the substrate substantially decreases to about 5.7 degree which has a dramatic effect on CHF. Furthermore, the oxide layer augments the boiling efficiency by increasing the number of active sites and nucleation cavities. However capillary wicking action of the superhydrophilic Nano-porous oxide layer enhances the rewetting of dry-spots during boiling and delays the formation of vapor layer through spreading the liquid on the anodized surface, the HTC enhancement at high heat fluxes suppressed. To prevent this matter bi-conductive surface with variation in wetability is employed. Using a 3 mm diameter electrode, disks with pitches between 3 to 6 mm with 1 mm increment were created on the substrate using Electric Discharge Machining technique. All the disks were filled with low conductive two-part epoxy to prevent bubbles coalescence and enhance both CHF and HTC considerably. Since the conductivity of epoxy is low, the temperature falls on the disks which engenders spatially temperature distribution that increases the lateral motion of the liquid toward dry spots that contributes to higher CHF. The simple sample have CHF and HTC equal to 110 W/cm2 and 27363 W/m2.K at 40.2 °C of superheat respectively. The anodized sample with 28.8% increase compared to simple sample had a CHF of 141 W/cm2 at 31.1 °C and the HTC at this point is 45265 W/m2.K which has 65/4% increase. It was found that the maximum CHF is 158 W/cm2 at 27.5 °C of superheat for the specement with 3 mm pitch of the epoxy dots. The maximum enhancement corresponds to the equality of capillary length and the distance between low conductive spots where a resonance like effect occures. The HTC with 109% growth reaches to 57350 W/m2.K
  9. Keywords:
  10. Flux Enhancement ; Augmentation Boiling Heat Transfer ; Superhydrophobic Surfaces ; Nanoporous Oxide Layer ; Bi-Conductive Surfaces ; Anodizing ; Critical Heat Flux

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