Smart thermal management by which ultra-high heat fluxes (i.e., q''> 100 W/cm²) are dissipated efficiently, is increasingly desirable for many applications in aerospace, electronic packaging, metallurgy, as the existing cooling solutions are highly constrained. For example, the cooling strategy for aircraft must be executed in such a way that will operate independently of orientation while also screen out external heat loads coming from the neighboring electronic boxes and/or external sources. Therefore, it is crucial to develop heat transfer devices which could effectively dump heat away while additionally shield against external heat loads. Thermal diodes, by definition, accomplish this desirable unidirectional heat transfer functionality. Nonetheless, the existing thermal diodes are currently constrained by either a low diodicity (i.e., heat transfer ratio), gravitational dependence, a one-dimensional configuration, or poor durability. Further example for the necessity of smart thermal management would be in firefighting and nuclear reactor safety. Above a critical temperature referred to as the ``Leidenfrost temperature'', the highly effective nucleate boiling is completely replaced by insulating film boiling, causing a dramatic decrease in the essential cooling rate of water pool boiling and spray quenching. In chapter 2, after noting the mechanism and shortcomings of each existing solid-state and phase-change thermal diode, we develop a unique thermal diode, called bridging-droplet thermal diode, which operates independent of orientation, is planar and durable. Our diode is comprised of two opposing copper plates separated by an insulating gasket of micrometric thickness; one plate contains a superhydrophilic wick structure while the other is smooth and hydrophobic. In the forward mode of operation, water evaporates from the heated wicked plate and condenses on the opposing hydrophobic plate. The large contact angle of the dropwise condensate enables bridging across the gap to replenish the wicked evaporator, providing sustained phase-change heat transfer. Conversely, in the reverse mode the heat source is now on the hydrophobic plate, resulting in dryout and excellent thermal insulation across the gap. An orientation-independent heat transfer ratio (i.e. diodicity (η)) of approximately 85 was experimentally measured. In chapter 3, after highlighting that our experimental proof-of-concept discussed in chapter 2, was limited to only a narrow parameter space, we develop a comprehensive thermal circuit model for both the forward and reverse modes of operation to theoretically characterize the bridging-droplet thermal diode over a broad parameter space. Parameters that are varied include the gap height, input heat flux, effective thermal conductivity of the wetted wick structure, height of the wicking micropillars, wettability of the opposing smooth surface, and heat sink temperature. Our findings show that a vapor space height of Hᵥ≈ 250 μm, short and densely packed micropillars, a higher applied heat flux in the forward mode, and a hotter heat sink temperature result in optimal diodicities of η~ 100. In chapter 4, we discuss that the Leidenfrost effect has been a two-phase phenomenon thus far: either an evaporating liquid or a sublimating solid levitates on its vapor. Here, we demonstrate that an ice disk placed on a sufficiently hot surface exhibits a three-phase Leidenfrost effect, where both liquid and vapor films emanate from under the levitating ice. Curiously, the critical Leidenfrost temperature was over three times hotter for ice than for a water drop. As a result, the effective heat flux was an order of magnitude larger when quenching aluminum with ice rather than water over a wide temperature range of 150--550 °C. An analytical model reveals the mechanism for the delayed film boiling: the majority of the surface's heat is conducted across the levitating meltwater film due to its 100 °C temperature differential, leaving little heat for evaporation. In chapter 5, we note that nucleate boiling achieves dissipative heat fluxes as high as q''~ 100 W/cm² and is widely used for power plants, spray quenching metal alloys, desalination, and electronics cooling. However, above a Leidenfrost temperature of about 150 °C for water, an insulating vapor film massively degrades the heat flux by two orders of magnitude. Here, we demonstrate that robust nucleate boiling can be maintained even at temperatures as high as 400 °C by using ice particles in place of water droplets. Ice pellets are periodically released onto a superheated stage and compared to spray quenching at an equivalent mass flow rate. Ice quenching was twice as fast as spray quenching at low superheats, and at large superheats, only ice quenching is successful. Our results demonstrate that ice quenching can maintain groundbreaking heat fluxes of q''~ 100--1,000,W/cm² over a broad range of superheats, far superior than classical spray quenching. / Doctor of Philosophy / Smart thermal management by which enormous heat generated in avionics, electronic packaging, wildfire, etc are removed efficiently, is increasingly desirable as the current cooling solutions are highly constrained. For example, in the context of aircraft, equipment must be cooled down independent of aircraft orientation while also they are shielded from neighboring and/or external heat sources. In firefighting where the temperature of wildfire flames could get beyond 500 °C, dumping large volume of water from aircraft may not be adequate to quench the fire over a reasonable time frame as the liquid water loses its cooling effectiveness above a critical temperature. In chapter 2, after a brief review of existing cooling devices and their corresponding shortcomings commonly used in aircraft and electronic packaging, we develop a unique device for cooling of aircraft which operates independent of aircraft orientation, is durable over time, and can cool down surfaces irrespective of their dimensions. In chapter 3, after highlighting that our proof-of-concept of a new cooling device in chapter 2, was limited to only a finite number of experiments, we theoretically model the operating mechanism of our device to check for the criteria where our device works most efficient. In chapter 4, we discover that by placing an ice disk on a sufficiently hot surface, effective boiling where large amount of heat can be dumped away from the surface to the coolant, is extended to a very large surface temperature. To be specific, liquid water on smooth aluminum loses its cooling efficiency around 150 °C, while cooling the same surface with ice is still effective up to 550 °C. In chapter 5, we report that quenching with ice is twice as fast as quenching with liquid water at low surface temperatures (i.e., 150--300 °C), and at larger surface temperatures (i.e., beyond 300 °C), only ice quenching is successful. Comparing our ice quenching results against current cooling technologies, we note that ice quenching is superior.

Passive two-phase cooling devices such as capillary pump loops, loop heat pipes, and vapor chambers can utilize capillary-fed boiling in the porous evaporator wick to achieve high heat flux dissipation, while maintaining low thermal resistances. These systems typically rely only on passive capillary pumping through the porous wick to transport fluid. This inevitably leads to limits on the maximum heat flux and power dissipation based on the maximum capillary pressure available. To overcome these capillary pumping limitations in these passive devices, a mechanical pump can be added to the system to create a pump-assisted capillary loop (PACL). The pump can actively transport the fluid to overcome the pressure drop in liquid lines, reserving all of the available capillary action to draw liquid from a compensation chamber into the porous evaporator at the location of the heat input.<br>Previous studies on pump-assisted capillary loops have used a porous pathway to draw liquid to the heated evaporator surface from a liquid supply in the compensation chamber. This pathway typically comprises porous posts distributed over the heated surface area to ensure uniform liquid feeding during boiling and to avoid dryout regions. This thesis presents an evaporator design for a pump-assisted capillary loop system featuring a non-porous manifold connection between the compensation chamber and the evaporator wick base where boiling occurs. By using this approach, microscale liquid-feeding features can be implemented without the manufacturing restrictions associated with the use of porous wick pathways (such as sintered powder copper particles).<br>An analytical model for two-phase pressure drop prediction in the base wick is developed and used to define the evaporator geometry and feeding structure dimensions. A parametric analysis of the evaporator geometry is performed with the target of achieving a maximum heat dissipation of 1 kW/cm2 without a capillary limit. A 24 x 24 microtube array configuration with an outside tube diameter of 0.25 mm was identified as a result of this analysis. This manifold delivers liquid the base wick manufactured from sintered copper particles with a mean particle diameter of 90 microns. <br>The resulting evaporator geometry was translated into a manufacturable copper manifold design. A modular test section design consisting of a cover for attachment of fittings, a support structure for holding the manifold, a sintered copper wick base, and a carrier plate was created and manufactured, to accommodate for future testing scheduled to be performed by an external industry partner. The resulting design provides a testing vehicle to investigate the effect of different tubing arrangements and dimensions, as well as multiple base wick configurations. This knowledge can be used to engineer future evaporator architectures for enhanced performance. The improved understanding providing on the effect of liquid feeding distribution into the base wick, the effects of boiling on the base wick pressure drop, and the manufacturing limitations can each improve the performance prediction of evaporators with top feeding. <br>