Micro-particles are ubiquitous in microsystems. The effective manipulation of micro-particles is often crucial for achieving the desired functionality of microsystems and requires a fundamental understanding of the particle dynamics. In this dissertation, the dynamics of two types of micro-particles, Janus catalytic micromotors (JCMs) and magnetic clusters, in complex environment are studied using numerical simulations. The self-diffusiophoresis of JCMs in a confined environment is studied first. Overall, the translocation of a JCM through a short pore is slowed down by pore walls, although the slowdown is far weaker than the transport of particles through similar pores driven by other mechanisms. A JCM entering a pore with its axis not aligned with the pore axis can execute a self-alignment process and similar phenomenon is found for JCMs already inside the pore. Both hydrodynamic effect and 'chemical effect', i.e., the modification of the concentration of chemical species around JCMs by walls and other JCMs, play a key role in the observed dynamics of JCMs in confined and crowded environment. The dynamics of bubbles and JCMs in liquid films covering solid substrates are studied next. A simple criterion for the formation of bubbles on isolated JCMs is developed and validated. The anomalous bubble growth law (r~t^0.7) is rationalized by considering the relative motion of growing bubbles and their surrounding JCMs. The experimentally observed ultra-fast collapse of bubbles is attributed to the coalescence of the bubble with the liquid film-air interface. It is shown that the collective motion of JCMs toward a bubble growing on a solid substrate is caused by the evaporation-induced Marangoni flow near the bubble. The actuation of magnetic clusters using non-uniform alternating magnetic fields is studied next. It is discovered that the clusters' clockwise, out-of-plane rotation is a synergistic effect of the magnetophoresis force, the externally imposed magnetic torque and the hydrodynamic interactions between the cluster and the substrate. Such a rotation enables the cluster to move as a surface walker and leads to unique dynamics, e.g., the cluster moves away from the magnetic source and its trajectory exhibits a periodic fluctuation with a frequency twice of the field frequency. / Ph. D.

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京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19994号 / 工博第4238号 / 新制||工||1656(附属図書館) / 33090 / 京都大学大学院工学研究科原子核工学専攻 / (主査)教授 功刀 資彰, 教授 佐々木 隆之, 准教授 横峯 健彦 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

For the application of minichannel heat exchangers, it is necessary to have accurate design tools for predicting heat transfer and pressure drop. Until recently, this type of heat exchangers was not well studied, and in the scientific literature there were large discrepancies between results reported by different investigators. The present thesis aims to add to the knowledge of the fundamentals of single- and two-phase flow heat transfer and pressure drop in narrow channels, thereby aiding in the development of this new, interesting technology with the possibility of decreasing the size of electronics through better cooling, and of increasing the energy efficiency of thermal processes and thermodynamic cycles through enhanced heat transfer. A comprehensive experimental single-phase flow and saturated flow boiling heat transfer and pressure drop study has been carried out on vertical stainless steel tubes with inner diameters of 1.700, 1.224 and 0.826 mm, using R-134a as the test fluid. The heat transfer and pressure drop results were compared both to conventional correlations developed for larger diameter channels and to correlations developed specifically for microscale geometries. Contrary to many previous investigations, this study has shown that the test data agree well with single-phase heat transfer and friction factor correlations known to be accurate for larger channels, thus expanding their ranges to cover mini/microchannel geometries. The main part of the study concerns saturated flow boiling heat transfer and pressure drop. Tests with the same stainless steel tubes showed that the heat transfer is strongly dependent on heat flux, but only weakly dependent on mass flux and vapor fraction (up to the location of dryout). This behavior is usually taken to indicate a dominant influence of nucleate boiling, and indicates that the boiling mechanism is strongly related to that in nucleate boiling. The test data for boiling heat transfer was compared to several correlations from the literature, both for macro- and mini-channels. A new correlation for saturated flow boiling heat transfer of refrigerant R-134a correlation was obtained based on the present experimental data. This correlation predicts the presented data with a mean absolute deviation of 8%. The frictional pressure drop results were compared to both macro- and mini channel correlations available from the literature. The correlation suggested by Qu and Mudawar (2003) gave the best prediction to the frictional two-phase pressure drop within the studied ranges. A unique visualization study of saturated flow boiling characteristics in a vertical 1.332 mm inner diameter quartz tube, coated with a transparent heater has also been conducted. The complete evaporation process in a heated circular mini-channel has been studied visually in detail using high speed CCD camera. The study revealed the developments of the flow patterns and the behavior from bubble nucleation to the dry out of the liquid film. The bubble departure frequency, diameter, growth rate, and velocity were determined by analyzing the images. Finally, a flow pattern map for boiling flow in microchannels has been developed based on the test data. / QC 20100812