Multiscale Modeling of Nanostructure-Enhanced Two-Phase Heat Transfer

Han Hu

doi:10.17918/etd-6921

Two-phase heat transfer has been widely used in the thermal management of electronics and energy systems. The critical heat flux and heat transfer coefficient of two-phase heat transfer can be significantly enhanced using nanostructures. The objective of current research is to develop the fundamental understandings regarding how nanostructures affect the two-phase heat transfer in the aspects of disjoining pressure, meniscus shape, Kapitza resistance, and evaporative heat transfer coefficient so as to guide nanostructural design for improving heat transfer performance. A multiscale modeling approach is introduced to examine the effects of nanostructures and electrostatic interactions on the equilibrium meniscus shape and disjoining pressure of a thin liquid film on nanostructured surfaces. A general continuum-level model is developed based on the minimization of free energy, the Derjaguin approximation, and the disjoining pressure theory for a flat surface. Molecular dynamics (MD) simulations are performed for water thin films of varying thickness on gold and alumina surfaces with both triangular and square nanostructures of varying depth. Good agreement is obtained between the continuum-level modeling and MD simulations. The results show that the wave amplitude of the meniscus increases with decreasing thin film thickness and increasing nanostructure depth. The electrostatic interactions are shown to enhance the disjoining pressure and make the meniscus more conformal to the nanostructured surfaces. Furthermore, both van der Waals and electrostatic contributions to the disjoining pressure increase with the nanostructure depth and decrease with the film thickness. The effect of nanostructures on Kapitza resistance of water boiling on a gold surface is examined via molecular dynamics simulations. The results show that Kapitza resistance is reduced with increasing nanostructure depth due to the enhanced solid-liquid interactions, and with decreasing nanostructure spacing due to the reduced mismatch in the vibrational properties between the solid and liquid. A closed-form model for the heat transfer coefficient of thin film evaporation on nanostructured surfaces is derived by integrating the evaporation kinetics, disjoining pressure, and Kapitza resistance. Molecular dynamics simulations are performed for water thin films of varying thickness on square gold nanostructures of varying depth. Good agreement is obtained between the continuum-level models and MD simulations. The results also show that there exists a critical film thickness at which the heat transfer coefficient reaches its maximum value. For a film with thicknesses below the critical film thickness, the evaporation resistance dominates the heat transfer, and the heat transfer coefficient increases with film thickness and decreases with nanostructure depth. For a film with thicknesses above the critical film thickness, the conduction resistance dominates the heat transfer, and heat transfer coefficient decreases with film thickness and increases with nanostructure depth. In addition, the critical film thickness increases with the nanostructure depth. The maximum heat transfer coefficient also increases with the nanostructure depth due to the reduction in Kapitza resistance.

Multiscale Modeling of Nanostructure-Enhanced Two-Phase Heat Transfer

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Multiscale Modeling of Nanostructure-Enhanced Two-Phase Heat Transfer

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