High speed CPUs and electronic chips usually dissipate relatively large quantities of energy in the form of heat. The limited available cooling space requires innovative and efficient thermal management techniques. These techniques must accomplish low operation temperatures along with uniform temperature distribution over the chip surface. Recently, several researches focused on developing different microchannel heat sink (MCHS) designs for this purpose. Among these designs is the double layer microchannel heat sink (DL-MCHS), which can be operated in either parallel flow (PF) or counter flow (CF) operation modes. The thermo-hydraulic characteristics of this heat sink was comprehensively investigated in the literature using numerical methods. However, based on the authors literature survey, all the previous numerical investigations considered the computational domain as a straight section with multi-channels in each layer. This approach assumes a uniform velocity for all the flow channels in each layer and neglect the effect of the inlet and outlet headers. In addition, the heat interaction between the coolant in both layers through the header section was not considered. These assumptions cause a considerable discrepancy between the numerical results in the literature and the realistic conditions. Therefore, in this work, a detailed 3D conjugate heat transfer model is developed. In this 3D model, DL-MCHS is designed with and without headers. The designed heat sinks are operated under PF and CF conditions. For a specific electronic chip with dimensions of 13 mm × 40 mm at a heat flux of 5 kW/m 2 . The model is validated with the experimental results in the literature. It is found that, including the header design in the simulation of the DL-MCHS must be considered in the simulation to predict the accurate thermo-hydraulic performance of the DL-MCHS specially in the CF. Further, using the DL-MCHS in PF operation accomplished a lower average chip temperature compared with the CF. Furthermore, at high coolant flowrate, neglecting the effect of the header in the CFD calculations can be approximated to predict the chip surface temperature. And finally, neglecting the header in the CFD calculations significantly affect the calculated pumping power for both PF and CF operated DL-MCHS.

The scope of this project is to investigate how the geometry of baffles affect heat transfer and pressure loss of a fluid flow at Re = 1000 through a square duct. For this purpose, Large Eddy Simulations are performed to investigate the effect of baffle height and baffle width. Focus is on the fully-developed flow that repeats itself at streamwise stations. The flow field predicted by Computational Fluid Dynamics simulations was validated using Particle Image Velocimetry. The different designs are evaluated in terms of Nusselt number, Nu and a loss coefficient, f, which are normalised using a reference geometry consisting of a square duct without baffles. The two parameters are additionally combined into a performance parameter eta η = (Nu/Nu 0 )/(f / f 0 ) (1/3) . It was found that adding baffles can result in a quadrupling of η. Reducing the height of baffles decreases heat transfer, while significantly reducing pressure loss and ultimately leading to a higher η. Reducing baffle height was also found to increase the temperature gradient at the upper wall and reduce it at the lower wall. Reducing baffle width resulted in the largest temperature gradient, but lead to poor heat transfer within the fluid.

Membrane based energy recovery ventilators (ERV) can be used to recover sensible and latent energy from exhaust-to-supply air in building applications. These typically consist of parallel layers of membrane separating the air streams, across which heat and moisture are exchanged. Reducing equipment cost and size remain a key challenge for continued commercialization and adoption of these devices. As membrane effectiveness improves, the air-side heat resistance can begin to dominate transport. To mitigate this, minichannel flow passages ( D H < 2 mm) can be used to reduce convective heat and mass transfer. Channels can be formed through direct manipulation of membrane (e.g., pleating, corrugating, etc.), or through the use of spacer or other insert. The use of multiple parallel channels can result in large spatial variations in driving temperature and humidity ratio differences in a single layer membrane, impacting overall transport. Furthermore, the local membrane mass transfer resistance is typically a function of the surface temperature and relative humidity and not a constant value throughout the device. Accurate design models are required to appropriately size ERV equipment and maximize performance for a given equipment volume. Thus, the goal of this study is to use simulation tools to understand how the use of parallel mini- and microchannels and non-uniform membrane properties effect the performance of a membrane ERV in a building application. A two dimensional coupled heat and mass transfer resistance network model is developed. The model is compared against existing data from more detailed CFD analysis, and used to parametrically investigate effects different inlet conditions on device performance.

A novel modeling technique for fluid flow and species transport in very large scale microfluidic networks is developed with applications to massively parallelized microreactors. Very large scale integration (VLSI) of microfluidic circuits presents an attractive solution for many biological testing applications such as gene expression, DNA sequencing and drug screening, which require massive parallelization of reactions to increase throughput and decrease time-to-result. However, the design and modeling of VLSI microfluidics remains challenging with conventional 2D or 3D computational fluid dynamic (CFD) techniques due to the large computational resources required. Using simplified models is crucial to reduce simulation time on existing computational resources. Many microfluidic networks can be solved using resistance based networks similar to electrical circuits; however, simplified models for species transport (diffusion plus advection) in microfluidic networks has received much less attention. Here, we introduce a simplified model based on resistance network based modeling for flow dynamics and couple it with a one-dimensional discretization of the advection-diffusion transport equation. The developed model was validated against CFD simulations using ANSYS Fluent for a flow network consisting of a 4 by 4 array of microreactors. It showed good agreement with 2D CFD simulations with less than 6% error in total pressure drop across the network for channels with a length to width ratio of 10. The error was only 3% for a channel length to width ratio of 20. The developed model was then used to optimize the design of a 100-microreactors network used for high purity cyclical loading of reagents. The reactor configuration with a minimum cycle time for reagent loading and unloading and minimum operating pressure were evaluated with the code. In theory, the simulation can be scaled to much larger reactor arrays after further optimizations of the code and utilizing parallel processing.

Understanding trajectories of natural and artificial helical swimmers under confinement is important in biology and for controlled swimming in potential medical applications. Swimmers follow helical or straight trajectories depending on whether the helical tail is pushing or pulling the swimmer. To investigate swimming dynamics of helical swimmers further, we present a Computational Fluid Dynamics (CFD) model for simulation of an artificial microswimmer in cylindrical channels. The microswimmer has a cylindrical head and a left-handed helical tail. The kinematic model solves for the position and rotation of the swimmer based on the linear and angular velocities of the force-free swimmer from a CFD model. Third-order Adams-Bashforth solver is used to obtain the orientation and the position of the swimmer. Viscous, gravitational, magnetic and contact forces and torques are considered in the model. The model is validated with experimental results. 3D trajectories, propulsion and tangential velocities are reported.

The present work represents a 2-D numerical investigation of forced convection heat transfer over three electronic blocks (silicon chip) in an inline arrangement with elliptical shaped vortex generators (VG-copper made) placed on top of the channel, for a range of Reynolds numbers. The block is prescribed with a 1,000 W/m 2 heat flux due to heating of the electronic components installed in the CPU casing. The results show that, vortex generators could effectively enhance the heat transfer in the channel. Subsequently, the effects of Reynolds number (from 500 to 1050), the number of vortex generators (baseline, 1, 2 and 3), aspect ratio of heated block (0.125, 0.15, 0.22), and aspect ratio of vortex generators (0.3125, 0.4, 0.5) on the heat transfer and fluid flow characteristics are examined. The characteristics of the performance parameters are studied numerically with the aid of computational fluid dynamics (CFD). The 3 VG demonstrates nearly 28.35% enhancement of Nusselt number compared to the 1 VG case at Re = 479. The change in pressure drop is less at low Reynolds number compared to higher Reynolds number respective to other parameters. Increasing the aspect ratio of the block increases the convection coefficient while decreasing aspect ratio of VG increases heat transfer coefficient. This enhancement is less significant for the third block as the cooling effect is predominant close to the channel inlet. Increasing consecutive distance between the blocks, enhances the heat transfer coefficient with the penalty of additional pressure drop. However, parametric studies are conducted for the maximum heat transfer enhancement.

Particle-particle interaction is an important phenomenon in the analysis of particle transport in a microfluidic device. This paper presents a computational study to predict the interaction force between particle complexes in a magnetophoretic bio-separation chip. Magnetic flux gradients are simulated in OpenFOAM CFD software and imported to Matlab to obtain the particle trajectories. The interaction force is approximated using a dipole based model and implemented to track the particle motion in a microfluidic device in the presence of an applied magnetic field. The analysis of particle trajectories is performed for cases where the applied magnetic field is parallel or perpendicular to the inter-particle distance of the particle complexes by solving a system of coupled ordinary differential equations.

High utilization of hydrogen is desired in operation of PEM fuel cells. Typically, additional devices for hydrogen recovery at the anode exit are necessary. Alternatively, dead-ended anode (DEA) operation may be considered, however this mode causes severe voltage transients and loss of catalyst support in hydrogen-depleted regions of the active area. Here. ultra-low stoichiometric (ULS) flow conditions that deliver very high hydrogen utilization is considered, however uniform flow distribution is necessary in this case. In order to obtain the flow distribution in the channels and in the inlet and exhaust headers in the anode, a three-dimensional CFD model is developed based on the finite-element method to solve Stokes equations subject to no-slip boundary conditions on the walls, specified pressure at the inlet and specified flow rate at the exit, which is set to a very low value comparable to the typical rate of nitrogen accumulation in the anode side in order to simulate the ULS flow. Uniformity of the flow distribution between the channels is quantified by means of two performance metrics: (i) the root-mean-square (rms) of the channel average velocities; (ii) the ratio of maximum and minimum values of the channel averages. Effects of geometric parameters, such as the widths of the channels and ribs and the position and lengths of the baffles in the inlet and exhaust headers are studied. The final design has less than 5% rms, and less than 1.2 for maximum to minimum average channel velocity ratio.

Characterization of a microchannel solar thermal receiver for a supercritical carbon dioxide (sCO 2 ) is presented. The receiver design is based on conjugate computational fluid dynamics and heat transfer simulations as well as thermo-mechanical stress analysis. Two receivers are fabricated and experimentally characterized — a parallel microchannel design and a microscale pin fin array design. Lab-scale experiments have been used to demonstrate the receiver integrity at the design pressure of 125 bar at 750°C surface temperature. A concentrated solar simulator was designed and assembled to characterize the thermal performance of the lab scale receiver test articles. Results indicate that, for a fixed exit fluid temperature of 650°C, increase in incident heat flux results in an increase in receiver and thermal efficiency. At a fixed heat flux, efficiency decreased with an increase in receiver surface temperature. The ability to absorb flux of up to 100 W/cm 2 at thermal efficiency in excess of 90 percent and exit fluid temperature of 650°C using the microchannel receiver is demonstrated. Pressure drop for the pin array at the maximum flow rate for heat transfer experiments is less than 0.64 percent of line pressure.

With the physical property changing dramatically, the supercritical aviation kerosene obtains unique heat transfer characteristics. In this way, it is difficult to investigate the heat transfer characteristics by normal experiment and therefore we resort to numerical analysis to address the scientific questions in this study. The project is proposed to demystify the heat transfer characteristics of supercritical aviation kerosene with CFD in 4mm inside diameter vertical circular tubes. Under the conditions of different pressures (3.5MPa-5MPa), the physical properties of the fluid are expressed in linear poly-nominal fitting including density, isobaric specific heat, thermal conductivity and viscosity. With the guidance of CFD, we analyze how the heat transfer characteristics can be affected by the value of temperature, pressure, heat flux mass velocity and so on. The result indicates: (1) In primary heating process, convective heat transfer is enhanced significantly. (2) When wall temperature surpasses the critical temperature, heat transfer can be deteriorated. (3) When the temperature continues to go up, the convective heat transfer coefficient will rise greatly again. Furthermore, the project has also compared the numerical analysis result with experimental result, which shows good agreement with each other. Hence, the validation of numerical analysis of supercritical fluid is well recognized.

Metal-organic frameworks (MOFs) have recently attracted enormous interest over the past few years due to their potential applications in energy storage and gas separation. However, there have been few reports on MOFs for adsorption cooling applications. Adsorption cooling technology is an established alternative to mechanical vapor compression refrigeration systems. Adsorption cooling is an excellent alternative in industrial environments where waste heat is available. Applications also include hybrid systems, refrigeration, powerplant dry cooling, cryogenics, vehicular systems and building HVAC. Adsorption based cooling and refrigeration systems have several advantages including few moving parts and negligible power consumption. Key disadvantages include large thermal mass, bulkiness, complex controls, and low COP (0.2–0.5). We explored the use of metal organic frameworks that have very high mass loading and relatively low heats of adsorption, with certain combinations of refrigerants to demonstrate a new type of highly efficient adsorption chiller. An adsorption chiller based on MOFs suggests that a thermally-driven COP>1 may be possible with these materials, which would represent a fundamental breakthrough in performance of adsorption chiller technology. Computational fluid dynamics combined with a system level lumped-parameter model have been used to project size and performance for chillers with a cooling capacity ranging from a few kW to several thousand kW. In addition, a cost model has been developed to project manufactured cost of entire systems. These systems rely on stacked micro/mini-scale architectures to enhance heat and mass transfer. Presented herein are computational and experimental results for hydrophyilic MOFs, fluorophilic MOFs and also flourophilic Covalent-organic frameworks (COFs).

Abundant availability and potential for lower CO 2 emissions are drivers for increased utilization of natural gas in automotive engines for transportation applications. However scarce refueling resources for on-road vehicles impose an infrastructure limited barrier on natural gas use in transportation. A novel ‘bimodal’ engine which can operate in a compressor mode has been developed that allows on-board refueling of natural gas where available without the need for any supplemental device. Engine compression of natural gas however results in considerable heating of the gas which is undesirable from a system stand-point. Micro-channel heat exchangers have been developed to absorb heat from the natural gas using engine coolant and compressed air. This work presents the design and development of the micro-channel heat exchangers as well as a preliminary analysis of system performance. Design methodology for the heat exchanger was based on trade-off studies that correlated system performance with component design. Energy flows through the system are analyzed as a function of engine compression ratio, operating speed, charge flow rate, and ambient air and natural gas conditions. These results are further used to estimate heat transfer co-efficient and effectiveness of the micro-channel heat exchanger. Future work involves developing CFD models of the heat exchanger to obtain a detailed understanding of the conjugate heat transfer and fluid flow processes within the micro-channels.

Carbon nanotubes (CNTs) hold significant promise in the fields of efficient drug delivery and bio-sensing for disease treatment because of their unique properties. In our lab, single and arrayed CNT-tipped devices are manufactured by deposition of carbon on the heated surfaces of templates using chemical vapor deposition (Template-Based Chemical Vapor Deposition, TB-CVD). Experimental results show CNT formation in templates is controlled by TB-CVD process parameters such as flow rate and temperature. However, there is a need for a more comprehensive and low cost way to characterize the flow in the furnace in order to understand how process parameters may affect CNT formation. In this report, 2D and 3D numerical models with Quadrilateral grids were developed using computational fluid dynamic (CFD) commercial codes. Velocity patterns and flow regimes in the tube were compared with experimental data. In addition, statistical techniques were employed to study temperature profiles and velocity patterns in the furnace as a function of flow rate. The outcome of this work will help to elucidate the TB-CVD process and facilitate the efficient manufacture of carbon nanostructures from a variety of templates. The results are broadly applicable to the manufacturing of CNTs and other nanostructured devices used in energy and biomedical fields, including CNT-based devices used in biological applications.

Boiling systems are capable of dissipating high heat fluxes, and as such have potential applications in thermal management of high power microelectronics. Although there are a number of experimental investigations of flow boiling in small flow passages and several empirical correlations have been proposed, the computational fluid dynamics (CFD) modeling of such systems is much less explored. In the present investigation, a phase-change model representing the heat and mass transfer is coupled with the volume of fluid (VOF) model for the transient analysis of flow boiling. The analyzed domain consists of a silicon microchannel with a finite substrate thickness, subjected to non-uniform heat fluxes at localized regions, providing with a more realistic scenario for the case of microelectronics power maps. The results show the strong effect on the two-phase flow characteristics for these configurations and visualization of the induced flow regimes is presented. Furthermore, discussion about the heat transfer mechanisms, challenges and possible solutions are given in order to provide guidelines for effective cooling of these devices.

As Particle Image Velocimetry (PIV) matures, new techniques have been developed for flow analysis that exploit PIV. One such recent use is in determining the underlying mechanisms of energy losses in fluidic systems. Often times, a First Law of Thermodynamics (FLT) approach is taken to determine the energy losses in a given fluidic system. However, a Second Law of Thermodynamics (SLT) approach allows for much greater detail of the energy losses to be determined. This paper will characterize the use of experimental PIV in conjunction with the SLT at the University of Central Oklahoma. A PIV apparatus can be used to observe the flow field in a given region of interest (ROI) of a fluidic system. This paper focuses on the use of PIV in conjunction with the SLT to determine the viscous dissipation rate and entropy generation rate of specified ROIs for dividing laminar flow in square ducts. Computational Fluid Dynamics (CFD) software is employed for simulation and experimental verification. A range of Reynolds numbers in the laminar regime (1–100) are used as a basis for determining volumetric flow rates through the system that are commonly found in micro-scale applications. The ROIs investigated here cover simple flow fields, where the results are compared to known analytical solutions derived for fully-developed flow in square ducts. The ROI is then focused on areas of greater interest that are internal and adjacent to a dividing flow geometry. CFD results have been used to calculate the entropy generation rate in a given dividing flow geometry and then compared to experimental results. Experimentally-based maps of viscous dissipation rate and entropy generation rate of the system are derived for analysis of sources of entropy generation.

Steady state heat transfer through a rarefied gas confined between two parallel plates or two coaxial cylinders maintained at different temperatures is investigated using the nonlinear S-model kinetic equation and the DSMC technique for a large range of gas rarefaction. The profiles of heat flux, density and temperature are reported for different values of gas rarefaction parameter and given values of temperature and aspect ratios. In the slip regime the results of the S-model and DSMC technique are compared to the simulations performed using the Lin and Willis temperature jump boundary conditions at the at the solid surface implemented in ANSYS/Fluent CFD simulations. The analytical expressions for density number, temperature and heat flux in the free molecular regimes are obtained for both parallel plates and coaxial cylinders geometries with hot and cold surfaces having different values of the thermal accommodation coefficient. The solutions of these analytical expressions are compared to the S-model kinetic equation and DSMC technique results in the free molecular regime.

The flow of microbubbles in millichannels with typical dimensions in the range of few millimeters offers a reduced pressure loss with simultaneous large specific contact surface. By flowing through micro orifices, the transformation of pressure into kinetic energy creates a desired secondary flow pattern, which results in continuous dispersion. Differences in velocity and pressure act on the phase boundary of the bubbles and lead to deformations and break-up. In this work, bubble dispersion and bubbly flow in different orifices and channel modules with widths up to 7 mm are studied experimentally and by CFD simulations. The effect of the orifice dimensions on bubble sizes are evaluated for hydraulic diameters of 0.25 to 0.5 mm with different aspect ratios. Several channel structures are analyzed to offer less coalescence and larger residence times. The modules are arranged in a holder and are fixed under a view glass for optical characterization via high-speed camera. Volume flow rates of 10 to 250 mL/min are studied with various phase ratios. Bubble diameters are generated in the range of less than 0.1 to 0.7 mm with narrow size distributions depending on the entire flow rate through the device. The first break-up point is shifted closer to the outlet of the orifices for increasing velocities and smaller hydraulic diameters, but the whole break-up region stays nearly constant for each orifice indicating stronger velocity oscillations acting on the bubble surface. Generally, a linear relation of smaller bubble diameters with larger energy input was identified. Opening angles of the orifices above 6° resulted in flow detachments and recirculation zones around the effluent jet. Independence of the Reynolds number was determined contrary to existing literature models. Flow detachment and coalescence in curves was avoided by an additional bend within the curve based on systematically varied geometrical dimensions.

The application of microchannels is often limited in high-flow-rate processes due to large pressure losses. The flow of microbubbles in millichannels offers a reduced pressure loss with simultaneous large specific contact surface. Hence, a combination of micro- and millistructured channels termed as multi-scale architecture is an alternative scale-up procedure beside numbering-up or equaling-up. Several methods exist generating primary micro dispersions, while bubble coalescence in the channels has a contrary effect. A continuous re-dispersion by flow through micro nozzles prevents this effect. The transformation of pressure into kinetic energy creates an aimed secondary flow. Differences in velocity and pressure act on the phase boundary of the bubbles and lead to deformation and break-up. In this work, a modular redispersion system with different nozzle and channel modules is studied experimentally and by CFD simulations. Each module consists of micro nozzles and channels with a width from 0.1 to 3.0 mm. The design of the modules is flexible to be adjusted to process demands with different nozzles or channel structures. The effect of the nozzle dimensions on bubble sizes are evaluated for hydraulic diameters of 0.1 to 0.5 mm. Different channel structures are analyzed to offer less coalescence and a narrow residence time distribution. The modules are arranged in a holder and are fixed under a glass plate for optical characterization. Volume flow rates from 10 to 500 mL min −1 are studied with various phase ratios. Bubble diameters are generated in the range from 0.1 to 0.7 mm with narrow size distributions dependent on the entire flow rate through the device. Pressure losses account for less than 2 bar. Generally, a linear relation of smaller bubble diameters with larger energy input was identified. Flow detachment in curves resulted in larger coalescence rates, which was avoided by smoother radii.

A microchannel chassis was specifically designed for conduction-cooled 6U CPU boards. Numerical simulations and experiments were performed on an aluminum microchannel chassis with single phase solution of 50% (by volume) ethylene glycol and deionized water. Thermal and CFD analyses were carried out for 440μm and 900μm and 3mm channel widths using commercial software ANSYS Icepak®. Thermal simulation results were seen to agree fairly well with the experimental data. Thermal testing was done on a 440 μm microchannel chassis manufactured with vacuum brazing. CPU heat flux as high as 30 W/cm 2 was tested with coolant flow rates from 0.6 lpm to 1.5 lpm. The heat gained from the chassis was rejected to an air cooled thermal control unit. Experimental results show that CPU junction temperature was 29.6°C higher than the coolant inlet temperature when the flow rate was 0.6 lpm and the chassis pressure drop was 0.22 bar. It was demonstrated that CPU heat flux as high as 30 W/cm 2 could be thermally managed with the developed microchannel chassis at 71°C extreme coolant inlet temperature.

Acoustophoresis which means separation of particles and cells using acoustic waves is becoming an intensive research subject. The method is based on inducing an ultrasonic compression standing wave inside a microchannel. A finite element approach is used to model the acoustic and electro-mechanical behavior of the piezoelectric material, the micro-channel geometry as well as the fluid inside the channel. The choices of silicon and PDMS materials are investigated as the chip materials for the resonator. A separation channel geometry which is commonly used in the literature is implemented in this study and the fluid flow inside the microchannel geometry is simulated using computational fluid dynamics. The acoustic field inside the fluid channel is also be simulated using the finite element method. For the separation process to be successful micro-particles of different diameter groups should end up in different channels of the micro-separator. In order to simulate real life scenarios, each particle size group have a size distribution within themselves. For realistic simulation results the particles will be released into the micro separator from a different starting locations (starting location distribution). The results of this Monte-Carlo based finite element simulation approach will be compared with the reported experimental results.