This research contributes to worldwide efforts to miniaturize one of the most powerful and versatile analytical tools, gas chromatography (GC). If a rapid, sensitive and selective hand-held GC system is realized, it would have a wide range of applications in many industries and research areas. As a part of developing a hand-held GC system, this research focuses on the separation column, which is the most important component of a GC system. This thesis describes the development of a miniature separation column that has low thermal mass and an embedded heating element for rapid thermal cycling. The worlds first thin polymer film (parylene) GC column has been successfully developed. This thesis includes: first, a study of theoretical column performance of rectangular GC column; second, the design optimization of parylene column and embedded heating element; third, the development of new processes such as parylene micromolding and stationary phase coating technique for parylene column; fourth, the fabrication of parylene GC column with an embedded heating element; and lastly, the testing and evaluation of parylene GC column through GC analysis.

A study of two-phase flow in annular channels with annular gaps of less than 1 mm is useful for the design and safety analysis of high power density systems such as accelerator targets and nuclear reactor cores. Though much work has been done on pressure drop in two-phase flow, designers rely mostly on empirical models and correlations; hence, it is valuable to study their applicability for different channel sizes, geometries, and gas qualities. The pressure drop along a concentric annular test section was measured for cases of either constant quality or variable quality along its length (such as in sub-cooled and flow boiling). A porous tube was used to inject gas along the inner surface of the annular channel, thereby simulating the case of flow boiling along the inner surface. The data were compared to predictions of various models and correlations. Additionally, the effect of wall vibrations on the pressure drop was examined. Experiments were conducted by imposing vibrations of known amplitudes and frequencies on the outer tube of the annulus. Wall vibrations were thought to be important for flow in microchannels where the vibration amplitudes may be significant compared to the channel hydraulic diameter. The results obtained in this investigation indicate that the pressure drop correlation given by Beattie and Whalley provides the best agreement with the data for both porous tube gas injection (i.e. variable quality) and constant quality two-phase flow within the narrow annulus. Furthermore, the results show that there is a minimal effect of vibrations on two-phase pressure drop over the range of frequencies and amplitudes studied.

An experimental and numerical investigation was conducted to study boiling and condensation - the two most important phenomena occurring in a dual-chamber thermosyphon. Boiling experiments were carried out using water at sub-atmospheric pressures of 9.7, 15 and 21 kPa with a three-dimensional porous boiling enhancement structure integrated in the evaporator. Sub-atmospheric pressure boiling achieved heat fluxes in excess of 100 W/cm2 with negligible incipience superheat, for wall temperatures below 85 oC. Reduced pressures resulted in reduction of heat transfer coefficient with decrease in saturation pressure. The boiling enhancement structure showed considerable heat transfer enhancement compared to boiling from plain surface. Increased height of the structure decreased the heat transfer coefficient and suggested the existence of an optimum structure height for a particular saturation pressure. A parametric study showed that a reduction in liquid level of water increased the CHF for boiling with plain surfaces. For boiling with enhanced structures, the liquid level for optimum heat transfer increased with increasing height of the enhanced structure. A numerical model was developed to study condensation of water in horizontal rectangular microchannels of hydraulic diameters 150-375 µm. The model incorporated surface tension, axial pressure gradient, liquid film curvature, liquid film thermal resistance, gravity and interfacial shear stress, and implemented successive solution of mass, momentum and energy balance equations for both liquid and vapor phases. Rectangular microchannels achieved significantly higher heat transfer coefficient compared to a circular channel of similar hydraulic diameter. Increasing the inlet mass flow rate resulted in a higher heat transfer coefficient. Increasing the inlet temperature difference between wall and vapor led to a thicker film and a gradually decreasing heat transfer coefficient. Increasing the channel dimensions led to higher heat transfer coefficient, with a reduction in the vapor pressure drop along the axial direction of the channel. The unique contributions of the study are: extending the knowledge base and contributing unique results on the thermal performance of thermosyphons, and development of a analytical model of condensation in rectangular microchannels, which identified the system parameters that affects the flow and thermal performance during condensation.

The present work aimed at designing a thermally efficient microreactor system coupling methanol steam reforming with methanol combustion for autothermal hydrogen production. A preliminary study was performed by analyzing three prototype reactor configurations to identify the optimal radial distribution pattern upon enhancing the reactor self-insulation. The annular heat integration pattern of Architecture C showed superior performance in providing efficient heat retention to the system with a 50 - 150 degrees C decrease in maximum external-surface temperature. Detailed work was performed using Architecture C configuration to optimize the catalyst placement in the microreactor network, and optimize reforming and combustion flows, using no third coolant line. The optimized combustion and reforming catalyst configuration prevented the hot-spot migration from the reactor midpoint and enabled stable reactor operation at all process flowrates studied. Best results were obtained at high reforming flowrates (1800 sccm) with an increase in combustion flowrate (300 sccm) with the net H2 yield of 53% and thermal efficiency of >80% from methanol with minimal insulation to the heatintegrated microchannel network. The use of the third bank of channels for recuperative heat exchange by four different reactor configurations was explored to further enhance the reactor performance; the maximum overall hydrogen yield was increased to 58% by preheating the reforming stream in the outer 16 heat retention channels. An initial 3-D COMSOL model of the 25-channeled heat-exchanger microreactor was developed to predict the reactor hotspot shape, location, optimum process flowrates and substrate thermal conductivity. This study indicated that low thermal conductivity materials (e.g. ceramics, glass) provides enhanced efficiencies than high conductivity materials (e.g. silicon, stainless steel), by maintaining substantial thermal gradients in the system through minimization of axial heat conduction. Final summary of the study included the determination of system energy density; a gravimetric energy density of 169.34 Wh/kg and a volumetric energy density of 506.02 Wh/l were achieved from brass architectures for 10 hrs operation, which is higher than the energy density of Li-Ion batteries (120 Wh/kg and 350 Wh/l). Overall, this research successfully established the optimal process flowrates and reactor design to enhance the potential of a thermally-efficient heat-exchanger microchannel network for autothermal hydrogen production in portable applications.

Experimental measurements are conducted on uninterrupted and interrupted aluminum microchannel heat sinks of 300, 500, 600 and 900 &mu / m channel widths. Two different versions of interrupted channels are tested / with single interruption and with 7 interruptions. Distilled water is used as the working fluid and tests are conducted at volumetric flow rates in a range of 0.5-1.1 lpm. Thermoelectric foils are used to supply uniformly distributed heat load to the heat sinks such that for all the tests the heat removed by water is kept constant at 40 W. Pressure drop and temperature increase are measured along the channels of different configurations for a number of different flow rates. For the interrupted channels thermal boundary layers re-initialize at the leading edge of each interrupted fin, which decreases the overall boundary layer thickness. Also the flow has been kept as developing, which results in better heat transfer performance. Due to the separation of the flow into branches, secondary flows appear which improves the mixing of the stream. Advanced mixing of the flow also enhances the thermal performance. In the experiments, it is observed that interruption of channels improved the thermal performance over the uninterrupted counterparts up to 20% in average Nusselt number, for 600 micron-wide channels. The improvement of average Nusselt number between the single interrupted and multi interrupted channels reached a maximum value of 56% for 500 micron-wide channels. This improvement did not cause a high pressure drop deviation between the uninterrupted and interrupted microchannels even for the maximum volumetric flow rate of 1.1 lpm. Highest pressure drop through the channels was measured as 0.07 bar, which did not require to change the pump. In the tests, maximum temperature difference between the inlet of the fluid and the base of the channel is observed as 32.8&deg / C, which is an acceptable value for electronic cooling applications.

This project studied the effect of different parameters on the conjugate heat transfer in tube-in-block heat exchangers for various engineering applications. These included magnetic coolers (or heaters) associated with a magnetic refrigeration system, high heat flux coolers for electronic equipment, and hydronic snow melting system embedded in concrete slabs. The results of this research will help in designing the cooling/heating systems and select their appropriate geometrical dimensions and material for specific applications. Types of problems studied in this project are: steady state circular microchannels with heat source in the gadolinium substrate, transient heat transfer in circular microchannels with time varying heat source in a gadolinium substrate, transient heat transfer in composite trapezoidal microchannels of silicon and gadolinium with constant and time varying heat source, steady state heat transfer in microchannels using fluids suspended with nanoparticl es, and analysis of steady state and transient heat transfer in a hydronic snow melting system. For each of these problems a numerical simulation model was developed. The mass, momentum, and energy conservation equations were solved in the fluid region and energy conservation in the solid region of the heat exchanger to arrive at the velocity and temperature distributions. Detailed parametric study was carried out for each problem. Parameters were Reynolds number, heat source value, channel diameter or channel height, solid materials and working fluids. Results are presented in terms of solid-fluid interface temperature, heat flow rate, heat transfer coefficient, and Nusselt number along the length of the channel and with the progression of time. The results showed that an increase in Reynolds number decreases the interface temperature but increases the heat flow rate and Nusselt number. When the heat source varied with time, by applying and removing the magnetic field, the interface temperature, heat flow rate, and Nusselt number attained a periodic variation with time. The decrease in the diameter at constant Reynolds number decreases the interface temperature and increases the heat flow rate at the fluid-solid interface.

This thesis employs computational and experimental methods to explore hotspot cooling and high heat flux removal from a 3-D stacked chip using thermoelectric and microfluidic devices. Stacked chips are expected to improve microelectronics performance, but present severe thermal management challenges. The thesis provides an assessment of both thermoelectric and microfluidic technologies and provides guidance for their implementation in the 3-D stacked chips. A detailed 3-D thermal model of a stacked electronic package with two dies and four ultrathin integrated TECs is developed to investigate the efficacy of TECs in hotspot cooling for 3-D technology. The numerical analysis suggests that TECs can be used for on demand cooling of hotspots in 3-D stacked chip architecture. A strong vertical coupling is observed between the top and bottom TECs and it is found that the bottom TECs can detrimentally heat the top hotspots. As a result, TECs need to be carefully placed inside the package to avoid such undesired heating. Thermal contact resistances between dies, inside the TEC module, and between the TEC and heat spreader are shown to significantly affect TEC performance. TECs are most effective for cooling localized hotspots, but microchannels are advantageous for cooling large background heat fluxes. In the present work, the results of heat transfer and pressure drop experiments in the microchannels with water as the working fluid are presented and compared to the previous microchannel experiments and CFD simulations. Heat removal rates of greater than 100 W/cm2 are demonstrated with these microchannels, with a pressure drop of 75 kPa or less. A novel empirical correlation modeling method is proposed, which uses finite element modeling to model conduction in the channel walls and substrate, coupled with an empirical correlation to determine the convection coefficient. This empirical correlation modeling method is compared to resistor network and CFD modeling. The proposed modeling method produced more accurate results than resistor network modeling, while solving 60% faster than a conjugate heat transfer model using CFD. The results of this work demonstrate that microchannels have the ability to remove high heat fluxes from microelectronic packages using water as a working fluid. Additionally, TECs can locally cool hotspots, but must be carefully placed to avoid undesired heating. Future work should focus on overcoming practical challenges including fabrication, cost, and reliability which are preventing these technologies from being fully leveraged.

Droplet microfluidics is a promising field since it overcomes many of the limitations of single phase microfluidic systems. The improved mixing time scale, the increase of number of samples and the isolation of droplets are some of its virtues. The core of droplet microfluidics is a two-phase flow condition that is subjected to scaling of the confining geometry. With the scaling the complexities of the flow phenomena arise. For that reason both the processes of droplet generation and transport are not fully understood for various flow and fluid conditions. The work in this thesis aims to experimentally examine droplet generation and transport in microchannels for flow and fluid conditions that are experimentally challenging to perform. Examination of droplet generation in a T-junction microchannel design was performed with a quantitative velocity field approach known as micro particle image velocimetry (μPIV). The studies on droplet generation focused on very fast generation regimes, namely transition and dripping that have not been studied for a T-junction design. This achievement was accomplished because of the development of a fast optical detection and triggering system that allowed for acquiring images of different identical droplets at the same position. μPIV results indicate that the quantitative velocity field patterns of different regimes share some similarities. The filling stage in the transition and dripping regimes had some resemblance in their velocity patterns. The velocity patterns for the start of droplet pinch-off were alike for the squeezing and transition regimes. Furthermore, the presence of a surfactant in the droplet phase above the critical micelle concentration (CMC) did not have an effect on the general velocity patterns as long as the capillary number Ca was matched with the no-surfactant condition. The studies of hydrodynamic properties of droplet transport were performed in hard materials to avoid cumulative error sources, such as material pressure compliance and swelling effects. The project had several parts: designing a microchannel network that allowed studying the hydrodynamic properties of small droplets, surface treatments of the channel material for stable droplet generation and examining the hydrodynamics of small liquid droplets with sizes that have not been reported in the literature. The studies examined effects of changing the interfacial tension, viscosity, and flow conditions on the transport of droplets. The experimental results from the hydrodynamic transport studies indicated that for the droplet sizes that were examined the pressure drop of droplets was affected by the capillary number Ca and length of the droplet Ld. Also, the presence of surfactants altered the hydrodynamic properties of droplets. At a high concentration of surfactants the droplets pressure drop was reduced significantly. Moreover, the type of surfactant affected the magnitude of the pressure drop. Experimental results indicate that if the concentration of surfactants was very low (below CMC) it did not have an effect on the droplet excess pressure. These findings are important to consider in designing droplet microfluidic systems with complex channel networks that involve droplet sorting, splitting, and merging for droplets that contain surfactants.

This work describes the results of experimental study of the flow of soft objects (microgels) through microchannels. This work was carried with the intention of building a fundamental biophysical model for the flow of neutrophil cells in microcirculatory system. In Chapter 1 we give a summary of the literature describing the flow of cells and “model cells” in microchannels. Paramount to this we developed methods to modify microchannels fabricated in poly(dimethyl siloxane) (PDMS). Originally, these microchannels could not be used to mimic biological microenvironments because they are hydrophobic and have rectangular cross-sections. We designed a method to create durable protein coatings in PDMS microchannels, as outlined in Chapter 3. Surface modification of the channels was accomplished by a two-step approach which included (i) the site-specific photografting of a layer of poly(acrylamide) (PAAm) to the PDMS surface and (ii) the bioconjugation of PAAm with the desired protein. This method is compatible with different channel geometries and it exhibits excellent longevity under shear stresses up to 1 dyn/cm. The modification was proven to be successful for various proteins of various molecular weights and does not affect protein activity. The microchannels were further modified by modifying the cross-sections in order to replicate cardiovascular flow conditions. In our work, we transformed the rectangular cross-sections into circular corss-sections. Microchannels were modified by polymerizing a liquid silicone oligomer around a gas stream coaxially introduced into the channel, as outlined in Chapter 3. We demonstrated the ability to control the diameter of circular cross-sections of microchannels. The flow behaviour of microgels in microchannels was studied in a series of experiments aimed at studying microgel flow (i) under electrostatic interactions (Chapter 4), (ii) binding of proteins attached to the microgel and the microchannel (Chapter 5) and (iii) under the conditions of varying channel geometry (Chapter 6). This work overall present’s new methods to study the flow of soft objects such as cells, in the confined geometries of microchannels. Using these methods, variables can be independently probed and analyzed.

Microchannel heat exchangers are widely used in air-conditioning and refrigeration systems, high heat flux electronics cooling, and are also being considered for biological devices. Heat transfer and pressure drop in microchannels with single-phase flow have been studied in greater detail compared to two-phase flow. Heat transfer and pressure drop in two-phase flow inside tubes are closely related to the structure of the flow. In convective condensation, the fluid flows in a variety of flow regimes as it changes from vapor to liquid. The flow patterns formed in microchannels differ in type and extent from those seen in conventional tubes. Wavy and stratified flows are virtually absent at microchannel dimension, while intermittent and annular flows predominate. The subject research focuses on understanding the flow physics in microchannels during condensation. The extensive condensation flow-regime database of a previous study is employed for this purpose. This database comprises the flow-regime observations in tubes of hydraulic diameter ranging from 1-4.91 mm during condensation of refrigerant R-134a. The mass flux ranges from 150-750 kg/m2-s over a vapor quality range of 0 to 1. The results from this previous experimental study are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes. Using this understanding and data, criteria for transitions between the different regimes have been developed. These criteria developed in non-dimensional form can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing the applicability of these results. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the choice of the appropriate models for the evaluation of heat transfer and pressure drop, and therefore enable the development of optimum microchannel heat exchangers.