چكيده به لاتين
Abstract:
Restriction of energy resources and the need for different industrial equipments to become smaller, necessitates the need for efficient systems for removal of large amounts of heat from a small surface area. Pool boiling is one of the proven ways to remove high heat flux at a low wall superheat, having numerous applications in energy conversion, heat exchanger systems and cooling electronical components with high energy density. Pool boiling heat transfer performance can be improved using various methods such as use of treated surfaces. The foundation of this improvement is based on increasing heat transfer coefficient and critical heat flux. The present thesis emphasizes on the construction of Micro/Nano-structured surfaces using thermal, chemical and mechanical methods to enhance heat transfer in pool boiling process.
In this study, pool boiling performance of a simple copper surface is initially investigated. To improve the thermal performance of this surface, combined methods are utilized. First, to improve critical heat flux and heat transfer coefficient, Nanorods are grown on the entire surface using wet chemical method. Regarding the increase in critical heat flux and the decrease in heat transfer coefficient, resulting from increase in surface wettability, the next step is improving heat transfer coefficient by fabricating mesochannels on the surface. At this stage, nucleation sites increase improves heat transfer coefficient, however, due to the reduction of superhydrophilic surface area, the critical heat flux drops. To enhance this parameter, a superhydrophilic porous layer is fabricated in the mesochannels. By doing so, in addition to improvement of heat transfer coefficient due to increase in nucleation sites, critical heat flux will also improve as a result of increase in surface wetting and capillary properties, as well as increased heat transfer surface area.
At each stage it has been endeavored to explain the thermal performance using images obtained by growth of bubbles on the surface. Moreover, the morphology change effect for every surface on bubble dynamics is investigated. Error sources for a single surface as a sample are examined and the results are presented as graphs. Finally, the optimal surface for the best thermal performance is introduced. In this thesis, effort has been made as much as possible to use simple and low-cost methods. In some cases where these methods were not applicable, attempts were made at using other alternative methods to fabricate surfaces.
Keywords: pool boiling, Mesochannel, wettability, porous coating, Nanowire
