Two-phase heat and mass transfer in capillary porous media

Ram Ranjan, Purdue University

Abstract

Phase change heat transfer in capillary porous media is of great relevance in diverse industrial applications. Heat pipes, vapor chambers, thermosyphons and cold plates are some of the phase change devices which employ microscopic porous media and are used in the thermal management of high-power electronics. These devices realize high heat transfer rates by exploiting latent heat exchange. The increasing power density of electronic chips requires that the performance of these devices be optimized so that heat can be efficiently removed from the electronic chip while limiting the temperature differential between the chip and ambient. The efficiency of heat spreading in heat pipes and vapor chambers relies on the capillary porous medium (wick structure) used in the device. The wick structure also determines the maximum heat transport capability. The study of phase change heat and mass transfer in wick structures can lead to the optimization of wick design and improved performance of the phase-change cooling devices. In the first part of this thesis, numerical models are developed to study the heat and mass transport in wick structures at the pore scale. The microstructures are characterized on the basis of their wicking and thin-film evaporation performance by modeling the rates of evaporation from the liquid menisci formed in common wick microstructures. Evaporation at the interface is modeled by using appropriate heat and mass transfer rates obtained from kinetic theory. At higher heat inputs, nucleate boiling occurs in the wick structure causing a decrease in the wick thermal resistance and improvement in the device performance. A volume-of-fluid-based model is developed to study the growth of vapor bubbles in wick microstructures. In the second part of this thesis, a transient three-dimensional heat pipe model is developed which is suitable for predicting the hydrodynamic and thermal performance of vapor chambers at high heat flux inputs and small length scales. The influence of the wick microstructure on evaporation and condensation mass fluxes at the liquid-vapor interface is accounted for by integrating a microstructure-level evaporation model (micromodel) with the device-level model (macromodel). The effect of boiling in the wick structure at higher heat inputs on the vapor chamber performance is modeled and the model predictions are validated with experiments performed on custom-fabricated vapor chambers. The model is further utilized to optimize the performance of an ultra-thin vapor chamber. The last part of this work focuses on the design of novel wick micro- and nano-structures for performance improvement of vapor chambers. The thermal and hydrodynamic performance of micro-pillared structures are first modeled and a ten-times improvement in the maximum heat transport capability of vapor chambers is revealed. The viability of utilizing nanostructures such as carbon nanotubes (CNTs) and metallic nanowires as wick structures for heat pipes is also assessed. Using theoretical models, it is concluded that the flow resistance of nanostructures poses a major bottleneck to their use as passive flow-conveying media. An alternative design which combines the micro- and nano-level wicks is proposed which leads to a 14% decrease in the wick thermal resistance.

Degree

Ph.D.

Advisors

Murthy, Purdue University.

Subject Area

Mechanical engineering

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