Nowadays, the components are made more and more small, the flow import or exports through the components are required frequently, so a set of micro-channel is need. The main aim of this paper is to study the effect of different inlet pressure and inlet flow velocity of micro-channel. By using the proposed numerical simulation, the pressure, velocity and equivalent stress inside the micro-channel corresponding to different inlet pressure and inlet velocity can be obtained. Also, the life of micro-channel was discussed also. Due to the size effect of the micro-channel, the boundary condition is set as . The error between the simulated and experiment results in less than 15%.

Micro-channels are very important in bio-medical and cooler research. Although many research have been conducted with micro-channels, few results have been contributed to discuss the properties of fluids inside a micro-channel. For example, the thermal distribution of the fluid in a micro-channel is only simulated with numerical analysis approach. In our research, we aim to design a device that can measure real thermal data within a micro-channel. We first applied excimer laser lithography on a PMMA substrate to fabricate a micro-channel with 20 mm length, 200£gm width and 200£gm depth. Micro-heater and micro-thermal sensor is than fabricated by sputtering pt thin-thim with PVD process on a glass substrate. Finally, these devices were assembled with UV-curing and than applied for further testing. According to our experiment, lift-off process cannot be easily applied to fabricate micro-heater and micro-thermal sensor since the architecture of these devices were fragile in metal line sidewalls. Wet-lithography is than used to conquer this problem. Our primary test on this micro-thermal sensor shows that its resistance varies with thermal changes. Such mechanism can be applied to measuring thermal field in a micro-channel.

A numerical simulation study for an interdigitated micro-channel PEM fuel cell is presented. Hydrogen gas is supplied to the anode and air is supplied to the cathode. The fuel cell outer surfaces are maintained at a constant temperature. The SIMPLEC algorithm is employed in a control volume numerical scheme. The outflow boundary conditions are specified to all transport equations except that an outlet pressure is specified to the momentum equation. Results are compared and show good agreement with the experimental data. The effects of the mass flow rate, the outlet pressure and the cell surface temperature on the cell performance are studied. The results can provide reference for fuel cell design.

No description available.

Boiling phenomena in micro scale has emerged as an interesting topic due to its complexity and increasing usage in micro electronic and mechanical systems (MEMS). Experimental visualization has discovered five main flow regimes: nucleate boiling, isolated bubbles, confine bubbly flow, elongated bubbly (or slug) flow, and annular flow. Two of these patterns (confine bubbles and slug flow) are rarely found in macro channels and are believed to have very different heat transfer mechanisms to that of nucleate boiling. The development of a phenomenological model demands a deep understanding of each flow regime as well as the transition process between them. While studies in every individual flow pattern are available in literature, the mechanisms of transition processes between them remain mysterious. More specifically, how the isolated bubbles evolve into a confined bubbly flow, and how this further evolves into elongated bubbles and finally an annular flow. The effects of boundary conditions such as wall heat flux, surface tension, and interfacial velocity are unclear, too. The aims of this thesis are to develop and validate a new numerical algorithm, perform a comprehensive numerical study on these transition processes, uncover the transition mechanisms and investigate effects of boundary and operating conditions. Firstly, a sophisticated and robust numerical model is developed by combining a coupled level set method (CLSVOF) and a non-equilibrium phase change model, which enables an accurate capture of the two-phase interface, as well as the interface temperature. Secondly, several flow regime transitions are studied in this thesis: nucleate bubbles to confined bubbly flow, multi confined bubbles moving consecutively in a micro channel, and slug to annular flow transition. Effects of surface tension, heat flux, mass flux, and fluid properties are examined. All these regimes are studied separately, which means an appropriate initial condition is needed for each regime. The author developed a simplified model based on energy balance to set the initial and boundary conditions. / <p>QC 20170403</p>

The main aim of this study is to develop three dimensional models for micro flow-field plate of PEMFC and use numerical simulations to discuss the reliability of micro flow-field plate which works in real. A crack exists in the plate is loaded by the shear force, which is produced by the fuel H2 enter from inlet, and will propagate. The Ag, which is used to collect the electrons, will peel off and the efficiency of fuel cell will decrease. The commercial package software ANSYS was used to simulate the stress state around crack tip. Three modes of stress intensity factors K£L, K£L£L and K£L£L£L, were calculated in order to describe the stressed behavior of crack. Finally, the inlet pressure, geometry of crack and channel size is changed and Taguchi method with ANOVA is used to find the factors which influence the stressed behavior of crack most. The simulation results show that K£L and K£L£L are influenced most by geometry of crack and K£L£L£L is influenced more by geometry of crack and channel size

No description available.

Methanol micro-channel reformer is an important device for generating hydrogen to supply micro fuel-cell needs. In the fuel reforming process, the catalyst is adopted to reduce the activation energy and speed up the reforming reaction. Hydrogen and other chemical substance are produced in the reformer catalytic reaction. The micro-channel structure provides more opportunity for molecules of methanol and steam mixture to collide with catalyst for high reforming reaction to take place. The reforming process of methanol in a micro-channel reformer with Pd/ZnO catalyst is studied numerically in this thesis. The effects of various channel length, channel height, inlet velocity, inlet temperature, and catalyst usage (ratio of wall area covered by catalyst) on the performance of reformer (methanol conversion percentage) are investigated numerically. The results show that the methanol conversion increases with increased channel length until a channel length of about 3000£gm, the conversion approaches 100%. The conversion percentage decreases with increased inlet velocity, however, the production rate of hydrogen depends on flow rate and conversion percentage. Increasing the channel height results in decreased methonal conversion due to less collision opportunity with the catalyst. The methanol conversion percentage increases with the increase of the inlet temperature. However, the production rate of the hydrogen starts to descend when the inlet temperature is higher than about 523 K owing to more methonal preburned in raising the inlet temperature. Methanol conversion increases with the catalyst usage. However, it is worth noting that the increase is only about 15% for catalyst usage from 50% to 100%. The results in this study provide design data for the fuel cell system designer.

No description available.

Heat management in high thermal-density systems such as CPU chips, nuclear reactors and compact heat exchangers is confronting rising challenges due to ever more miniaturized and intensified processes. While searching for heat transfer enhancement, micro-channel flow boiling and the usage of high thermal potential fluids such as nanofluids are found to be efficient heat removal approaches. However, the limited understanding of micro-scale multiphase flows impedes wider applications of these techniques. In this thesis work, liquid-vapour phase change and multiphase flow heat transfer in micro-channels were experimentally investigated. Included are studies on the single phase friction, vapour dynamics, liquid meniscus evaporation, two-phase flow instabilities and heat transfer. An experimental system was built. Rectangular microchannels with different hydraulic diameters (571 μm, 762 μm and 1454 μm) and crosssectional aspect ratios were selected. Transparent heating was utilised by coating the micro-channels with a layer of tantalum on the outer surfaces. FC-72, n-pentane, ethanol, and ethanol-based Al2O3 nanofluids were used as working fluids. Pressures and temperatures at micro-channel inlet and outlet were acquired. Simultaneous visualisation and thermographic profiles were monitored. Single phase friction of pure liquids and nanofluids mostly showed good agreement with the conventional theory. The discrepancies were associated with hydrodynamic developing flow and the early transition to turbulent flow, but nanoparticle concentration showed minor impact. After boiling incipient, the single vapour bubble growth and flow regimes were investigated, exploring the influences of flow and thermal conditions as well as the micro-channel geometry on vapour dynamics. In addition, liquid meniscus evaporation as the main heat transfer approach at thin liquid films in micro-channels was studied particularly. Nanoparticles largely enhanced meniscus stability. Besides, flow instabilities were analyzed based on the pressure drop and channel surface temperature fluctuations as well as the synchronous visualization results. Moreover, study on flow boiling heat transfer was undertaken, the corresponding heat transfer characteristics were presented and the heat transfer mechanisms were elucidated. Furthermore, ten existing heat transfer correlations were assessed. A modified heat transfer correlation for high aspect ratio micro-channel flow boiling was proposed. The crucial role of liquid property and microchannel aspect-ratio on flow boiling heat transfer was highlighted.