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This dissertation is focused on studying transport behavior in vapor chambers at ultra-thin form factors so that their use as heat spreaders can be extended to applications with extreme space constraints. Both the steady-state and transient thermal transport behaviors of vapor chambers are studied. The steady-state section presents an experimental characterization technique, methodologies for the design of the vapor chamber wick structure, and a working fluid selection procedure. The transient section develops a low-cost, 3D, transient semi-analytical transport model, which is used to explore the transient thermal behavior of thin vapor chambers: 1) The key mechanisms governing the transient behavior are identified and experimentally validated; 2) the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored and key performance thresholds are identified; and 3) practices are developed for the design of vapor chambers under transient conditions. These analyses have been tailored to ultra-thin vapor chamber geometries, focusing on the application of heat spreading in mobile electronic devices. Compared to the conventional scenarios of use for vapor chambers, this application is uniquely characterized by compact spaces, low and transient heat input, and heat rejection via natural convection.

With the demand of computing power from electronic chips on a constant rise, innovative methods are needed for effective and efficient thermal management. Forced convection cooling through an array of micro pin-fins acts not only as a heat sink, but also allows for the electrical interconnection between stacked layers of integrated circuits. This work performs a multi-objective optimization of three shapes of pin-fins to maximize the efficiency of this cooling system. An inverse design approach that allows for the design of cooling configurations without prior knowledge of thermal mapping was proposed and validated. The optimization study showed that pin-fin configurations are capable of containing heat flux levels of next generation electronic chips. It was also shown that even under these high heat fluxes the structural integrity is not compromised. The inverse approach showed that configurations exist that are capable of cooling heat fluxes beyond those of next generation chips. Thin film heat spreaders made of diamond and graphene nano-platelets were also investigated and showed that further reduction in maximum temperature, increase in temperature uniformity and reduction in thermal stresses are possible.

Two-phase spray cooling has been an emerging thermal management technique offering high heat transfer coefficients and critical heat flux levels, near-uniform surface temperatures, and efficient coolant usage that enables to design of compact and lightweight systems. Due to these capabilities, spray cooling is a promising approach for high heat flux applications in computing, power electronics, and optics. Two-phase spray cooling inherently depends on saturation temperature-pressure relationships of the working fluid to take advantage of high heat transfer rates associated with liquid-vapor phase change. When a certain application requires strict temperature and/or pressure conditions, thermo-physical properties of the working fluid play a critical role in attaining proper efficiency, reliability, or packaging structure. However, some of the commonly used single-component working fluids have relatively poor properties and heat transfer performance. For example, water is the best coolant in terms of properties, yet in certain applications where the system operates at low temperature ambient, it cannot be implemented due to freezing risk. The common solution for this problem is to use the antifreeze mixtures (binary mixtures of water and alcohol) to reduce the freezing point. In such cases, utilizing binary mixtures to tune working fluid properties becomes an alternative approach. This study has two main objectives; (1) to experimentally investigate the two-phase spray cooling performance of water/2-propanol binary mixture, and (2) to numerically investigate the performance of an advanced heat spreader featuring high and directional thermal conductivity materials for high heat flux focal sources. The first part of the study involves experimental characterization of heat transfer performance. Tests are conducted on a small-scale, closed loop spray cooling system featuring a pressure atomized spray nozzle. The test section, made of copper, measures 10 mm x 10 mm x 2 mm with a plain, smooth surface. A cylindrical copper block, with a matching size square protrusion attached onto the back side of the test section, generates heat using cartridge heaters and simulates high heat flux source. Embedded thermocouples are used to determine the spray surface temperature. The working fluid, water/alcohol mixture, has various concentration levels of 2-propanol by mass fraction 0.0 (pure water), 0.25, 0.50, 0.879 (azeotrope) and 1.0 (pure alcohol)), representing both non-azeotropic and azeotropic cases. Spray cooling tests are performed with a constant flow rate of 5.6 ml/cm².s at subcooled temperatures (~20oC) and atmospheric pressure. Experimental procedure involves controlling the heat flux in increasing steps, and recording the corresponding steady-state temperatures to obtain cooling curves in the form of surface superheat vs. heat flux. The second part of the study investigates an advanced heat spreader design for thermal management of a high heat flux focal source. The heat spreader comprises of three layers: a copper layer that interfaces with the heat source, a high and directional thermal conductivity material (such as CVD diamond and Pyrolytic graphite) layer, and another copper layer that is exposed to two-phase spray cooling. The analysis applies various heat fluxes on the heat source side and the experimentally obtained heat transfer coefficients on the spray side of the spreader design to determine the temperature and heat flux distributions, and examine the potential capabilities of this configuration.