Interfacial phenomena in heat pipes: microscopic and molecular dynamics study

Abstract

Micro-scale transport phenomena have gained prominence in various high heat flux dissipation devices in microelectronics, where the hotspots created require that each chip be cooled individually. Heat pipes have been an attractive choice for high heat flux operations because of their compact, low-maintenance design. Heat pipes are passive heat transfer devices that transport energy from one end to the other based on capillary and Marangoni flows. Temperature variation along the length of the heat pipe together with a gradient in surface tension, cause the fluid to recirculate between the evaporator and condenser ends. Heat pipes have found application in space research, where extremely small devices are needed for an efficient cooling system. Understanding phase-change heat and mass transfer in the contact line region is important for optimizing many industrial and biological processes like nucleate boiling, spreading, coating, self-assembly, evaporation, condensation, and tear films. The three-phase contact line is the micro-region where solid, liquid and vapor phases coexist. The fluid flow and heat transfer occurring in a thin-film depend on the shape of the contact line. Thus, understanding the intermolecular force field, which eventually governs the meniscus shape and curvature is important. Our aim is to study phase-change phenomena in this 3-phase contact line by obtainingthe interfacial characteristics at the meniscus of a thin evaporating film. A finite element model was previously developed that simulates the fluid dynamics and heat transfer in a thin liquid meniscus on a solid substrate by solving a pair of partial differential equations. After reaching a steady-state, the outside solid wall temperature was oscillated with different frequencies and varying amplitude. The effects of oscillation on the liquid film thickness and heat transfer at the solid-liquid boundary were studied. It was observed that the liquid film also oscillates and follows the wall temperature for lower frequencies of oscillation. However, for higher frequencies, oscillations in the film thickness were damped out. The heat flux profiles at the solid-liquid boundary show that there is no optimal oscillation frequency at which the evaporative heat flux is maximum. Knowing what the optimal frequency is will offer better control in designing of a heat transfer device/process, giving the maximum heat transfer efficiency. A heat-transfer cell which houses a 4-inch wafer has also been designed to study the 3-phase contact line experimentally. To validate the theoretical predictions, Hamaker constant was calculated by performing isothermal experiments on silicon surface. By modifying the surface in a simple way and studying the thin-film evaporation on it, we hope to achieve a better understanding of the effect that solid-liquid interactions have on the heat transfer. We discuss the effect that fluid properties have on the behavior of the evaporating liquid and the oscillation of the film. The overall heat flux and amplitude of oscillation of thin film thickness over one cycle were compared across different fluids. The latent heat of vaporization of the liquid and the Hamaker constant of the system were found to control the oscillating thickness of the liquid film on solid. The total heat transfer also depends on the thermal diffusivity of the liquid and solid system. The study could further be extended for a fluid partially wetting a solid surface by modifying the disjoining pressure isotherm. The short-range (polar) contribution to the disjoining pressure alter the interaction between the liquid and solid molecules. Pentane is a commonly used fluid in heat pipes in microgravity. It was reported in our group that using a binary mixture of alkanes (94% pentane and 6% isohexane) in a constrained vapor bubble (CVB) experiment on International Space Station (ISS) improved the performance of the heat pipe as compared to using pure pentane as the working fluid. The change in mixture composition and temperature along the length of the heat pipe caused opposing effects on the surface tension, essentially reducing the Marangoni stress. We studied the liquid-vapor equilibrium of a mixture of n-pentane and 2-methylpentane (isohexane) at a molecular scale using the software GROMACS for running molecular dynamics simulations. Vapor-liquid equilibrium simulations were carried out for two single-component systems (n-pentane and 2-methylpentane) and 3 binary mixture compositions (molar ratio 25:75, 50:50, and 75:25) of pentane and 2-methylpentane. Our objective is to study these mixtures at a molecular level to provide insight into the intermolecular interactions between these alkanes which are important for knowing the properties of the evaporating meniscus. We discuss the structure, thermodynamics and dynamics of the binary mixtures. The surface tension was found to vary linearly with the mixture composition indicating a near-ideal behavior of the binary mixture. The surface tension also decreases with an increase in temperature. Studying the orientations of the molecules revealed that the liquid-vapor interface could be divided into three regions corresponding to different orientations, however, the preference for any orientation is only slight. The isohexane molecule was observed to orient itself such that its bulky end points to the liquid bulk. Molecular dynamics simulations will help in understanding the structure and dynamics of the mixture and aid in better designing the operating parameters of heat transfer devices to improve their effectiveness.