SUMMARY
A novel cooling mechanism based on evaporation of thin liquid films is presented for thermal management of confined heat sources, such as microprocessor hotspots. A nanoporous membrane (~5µm) is utilized to maintain a microscopically thin liquid film (~5µm) by capillary action and to provide a pathway for the vapor generated due to evaporation at the liquid-vapor interface. The vapor produced from the heat generated at the hotspot is continuously removed by using a dry sweeping gas, thereby resulting in dissipation of large heat fluxes. This thesis presents a detailed theoretical, computational and experimental investigation of the heat and mass transfer mechanisms that result in dissipating heat in micro and nano-confined geometry. The performance analysis of this cooling mechanism demonstrates heat fluxes over 600W/cm2 for sufficiently thin membrane and liquid-film thickness (~1-5µm). These are obtained by using air jet impingement for advection of vapor from the membrane surface. Based on the results from this performance analysis, a monolithic micro-fluidic device is designed and fabricated incorporating micro and nanoscale features. This MEMS/NEMS device serves multiple functions of hotspot simulation, temperature sensing, and evaporative cooling. Subsequent experimental investigations demonstrate heat fluxes in excess of 600 W/cm2 at 90 C using water as the evaporating coolant. Since the coolant is confined using a nanoporous membrane, a detailed study of evaporation inside a nanoscale cylindrical pore is also carried out. The continuum analysis of water confined within a cylindrical nanopore determines the effect of electrostatic interaction and Van der Waals forces in addition to capillarity on the interfacial transport characteristics during evaporation. The detailed analysis demonstrates that the effective thermal resistance offered by the interface is negligible in comparison to the thermal resistance due to the thin film and vapor advection. In order to further enhance the performance of the MEMS device, a device-level detailed computational analysis of heat and mass transfer is carried out, supported by experimental investigation. The analysis identifies the contributions of various cooling mechanisms for different operating conditions. Identifying vapor advection to be the rate limiting resistance, the analysis proposes operation of this device with on-demand control of coolant flow to minimize the total power required for cooling.