UC San Diego

Interfaces are recognized as being one of the biggest impediments to efficient heat transfer, and interfacial effects tend to dominate heat transfer processes between disparate materials. Despite this, not a great deal of research had been done on exactly how interfaces alter the direction and quality of heat flux on metamaterials, as they had been treated as an effective medium. The aim of the first project was to investigate the variability of the thickness as well as the thermal conductivity of interfaces in composites which may significantly influence thermal transport characteristics and the notion of a metamaterial as an effective medium. The consequent modulations of the heat flux passage are analytically and experimentally examined through a non-contact methodology using radiative imaging, on a model anisotropic thermal metamaterial. It was indicated that a lower Al layer/silver interfacial epoxy ratio of ~25 compared to that of a Al layer/alumina interfacial epoxy (of ~39) contributes to a smaller deviation of the heat flux bending angle. Another major finding was that the role of interfacial conductivity variation was much more significant in altering the heat flux bending angle than that of the interface thickness variation.

Solar irradiation is a valuable source of renewable energy. In fact, the hourly incident solar flux on the surface of the earth is greater than the planet’s annual global energy consumption. In the second project, solar steam generation on carbon foam samples was investigated and solar thermal efficiency of around 60% was achieved. The goal of the project was to examine the effects of three specific parameters known to affect efficiency: pore size and distribution, the relative contribution of conductive and convective heat transfer, and the constitution and characteristics of the materials used in the porous media and related chemistry. One of the chief findings was that evaporation efficiency was boosted by decreased pore size. Furthermore, it was also determined that the chemical modification of surfaces increases capillary pressure. Heat transfer coefficient values through the analysis of the heat transfer mechanism on the carbon foam top surface were also investigated and it was found that heat transfer coefficient values increase with reduced pore size.

Carbon foam based porous media is used to examine water, and ethanol-water mixtures evaporation as well for the third project. A relationship between the consequent rate of mass loss, with respect to the equilibrium vapor pressure, dynamic viscosity, surface tension, and density, was developed to explain experimental observations. The evaporative heat loss was parameterized through two convective heat transfer coefficients – one related to the surface and another related to the vapor external to the surface. The work promotes a better understanding of thermal processes in binary liquid mixtures with applications ranging from phase separation to distillation and desalination.