Heat Transfer Enhancement Using Miniaturized Channel Sections With Surface Modifications

Abstract

The objective of this study is to enhance heat transfer process using micro/nano scale channels with surface modifications. An application focus of this study is to design an extremely compact heat-exchanger using single/multi component fluid in miniaturized channels along with surface modifications to achieve higher heat exchange per unit surface area. For the last couple of decades significant progress has been made in characterizing flows in micro channels because of its high surface to volume ratio enhancing heat transfer process. However a centerline question still remains — what should be an optimized size of a miniaturized channel to achieve maximum heat transfer? A lack of theoretical characterization of single or multi component flows in micro to nano scale channels is partially responsible for this setback. Lattice Boltzmann (LB) method, a mesoscopic thermo fluid flow modeling technique, has grown significantly over the last couple of years mainly because of the promise of incorporating mesoscale molecular interaction and also the ability to solve Navier-Stokes equation at the hydrodynamic limit. Moreover, LB method which is based on microscopic models and mesoscopic equations, is considered an attractive numerical alternative for solving multiphase phenomena in a multiscale setup. Also fluid-solid interactions can be implemented conveniently in the LB method without introducing additional complex kernels. At first to address thermo-fluid phenomena over a heated surface Rayleigh-Benard (RB) convection was modeled, and to observe forced flow cavity flow was simulated based on LB method. Some of these results had been compared with literature and documented to have good comparison which showed verification of the current in-house LB simulation codes. After this, effects of surface interaction (hydrophobic and hydrophilic) and miniature cross sections (micro-scale) were calculated using single-component RB convection model. Several results were generated e.g. effects of surface interaction and Knudsen number (Kn) on the average Nusselt number (Nu) in a Rayleigh Benard (RB) convection with hot bottom and cold top surfaces. Results showed higher average heat transfer (Nu) when the bottom surface is hydrophobic and top surface is hydrophilic compared to neutral surface condition in a typical single component RB convection flow over a range of Rayleigh numbers (Ra). Knudsen number (Kn) effect was incorporated to observe the effect of miniature cross section. The average Nu decreased with the increase of Kn i.e. the miniaturization of the channel section from macro-scale to micro-scale also over a range of Rayleigh numbers. However, the number of micro-channels that could be placed in the cross section of a macro-channel increased considerably with increasing Kn. Effect of Kn on the velocity profiles, slip velocities, and maximum velocities were also calculated in a flow between parallel plates. Maximum velocities decreased and slip velocities increased with increasing Kn. Many of these results are in good qualitative comparison with results in literature.