bluuebunny/crossref_metadata_2025 huggingface dataset:
Modelling Transsonic Flows Through Ring-Shape Thruster Geometries With DSMC
Authors: Groll Rodion , Fastabend Fabian , Rath Hans J.
Published: July 2013

When modeling rarefied gas flows, continuous approximation is limited by the Knudsen regime. The presented cold gas thruster for space applications is investigated for pressure values lying between 10−2 and 103 Pa. It is comprised of a subsonic funnel region, a transsonic region consisting of a ring-shaped nozzle throat and a supersonic diffuser region. Diffusive and specular / mirror reflection is used to describe the behavior of particle/wall collision in the discrete model. Simulation results are compared both with experimental data and with numerical results computed using a finite-volume method.

The transsonic flow through the nozzle throat shows very good agreement with experimental data. Simulation and experimental results emphasize the influence of various geometric factors like size and shape of the nozzle throat. Furthermore, differences in the acceleration behavior of Argon and Xenon are examined.

Results of simulations utilizing the DSMC method [Bird, 1994, Stefanov et al., 2011] with diffusively reflecting boundary conditions present the best agreement with experimental data. Any deviation seen using the finite-volume method with no-slip boundary conditions can be explained by the equilibrium gas-state near the walls [Brenner, 2005, Greenshields et al., 2007]. The non-equilibrium approach produces lower velocity gradients near the wall, especially in wall regions with high levels of surface curvature.
Numerical Simulation of Thermal Transpiration in the Slip Flow Regime With Curved Walls
Authors: Leontidis Vlasios , Baldas Lucien , Colin Stéphane
Published: July 2013

Nowadays, modeling gas flows in the slip flow regime through microchannels can be achieved using commercial Computational Fluid Dynamics codes. In this regime the Navier-Stokes equations with appropriate boundary conditions are still valid. A simulation procedure has been developed for the modeling of thermal creep flow using ANSYS Fluent®. The implementation of the boundary conditions is achieved by developing User Defined Functions (UDFs) by means of C++ routines. The complete first order velocity slip boundary condition, including the thermal creep effects due to an axial temperature gradient and the effect of the wall curvature, and the temperature jump boundary condition are applied. Motivation of the present work is the development of a simulation tool which will help in the pre-calculations and the preliminary design of a Knudsen micropump consisting of successively connected curved and straight channels and in a second step in the numerical optimization of the pump, in terms of geometrical parameters and operating conditions of the system.
Temperature Dependent Viscosity and Thermal Conductivity Effects on the Laminar Forced Convection in Straight Microchannels
Authors: Del Giudice Stefano , Savino Stefano , Nonino Carlo
Published: July 2013

A parametric investigation is carried out on the effects of viscous dissipation and temperature dependent viscosity and thermal conductivity in simultaneously developing laminar flows of liquids in straight microchannels of constant cross-sections. Uniform heat flux boundary conditions are specified at the heated walls. Viscosity is assumed to vary with temperature according to an exponential relation, while a linear dependence of thermal conductivity on temperature is assumed. The other fluid properties are held constant. Two different cross-sectional geometries are considered, corresponding to both axisymmetric (circular) and three-dimensional (square) microchannel geometries. A finite element procedure is employed for the solution of the parabolized momentum and energy equations. Computed axial distributions of the local Nusselt number and of the apparent Fanning friction factor are presented for different values of the Brinkman number and of the viscosity and thermal conductivity Pearson numbers. Moreover, a superposition method is proved to be applicable in order to obtain the correct value of the Nusselt number by considering separately the effects of temperature dependent viscosity and viscous dissipation and those of temperature dependent thermal conductivity. In fact, it is found that the influence of the temperature dependence of thermal conductivity on the value of the Nusselt number is independent of the value of the Brinkman number, i.e., it is the same no matter whether viscous dissipation is negligible or not. Finally, it is demonstrated that, in liquid flows, the main effects on pressure drop of temperature dependent fluid properties can be retained even if only viscosity is allowed to vary with temperature, the other properties being assumed constant.
Computational Modeling of Surfactant-Laden Liquid Plug Propagation in Capillary Tubes
Authors: Muradoglu Metin , Olgac Ufuk
Published: July 2013

Pulmonary surfactant is of essential importance in reducing the surface tension on the liquid film that coats the inner surface of the airways and thus making the lung more compliant. Surfactant-deficiency may result in respiratory distress syndrome (RDS), which is especially common in prematurely born neonates. Surfactant replacement therapy (SRT) is a standard treatment, in which a liquid plug with exogenous surfactant is instilled in the trachea, which subsequently propagates by inspiration and spreads the exogenous surfactant to the airways. The efficacy of the treatment depends on various parameters such as the size of the liquid plug, inspiration frequency and the physical properties of the exogenous surfactant. Unsteady simulations are performed to study surfactant-laden liquid plug propagation using finite difference/front-tracking method in order to shed light on the surfactant replacement therapy.
A Separated-Flow Model for Predicting Flow Boiling Critical Heat Flux and Pressure Drop Characteristics in Microchannels
Authors: Zhang Tie Jun , Chen Siyu , Wang Evelyn N.
Published: July 2013

Two-phase microchannel cooling promises high heat flux removal for high-performance electronics and photonics. However, the heat transfer performance of flow boiling microchannels is limited by the critical heat flux (CHF) conditions. For variable heat inputs and variable fluid flows, it is essential to predict CHFs accurately for effective and efficient two-phase microchannel cooling. To characterize the CHF and pressure drop in flow boiling microchannels, a separated-flow model is proposed in this paper based on fundamental two-phase flow mass, energy, momentum conservation and wall energy conservation laws. With this theoretical framework, the relationship among liquid/vapor interfacial instability, two-phase flow characteristics and CHF is further studied. This mechanistic model also provides insight into the design and operational guidelines for advanced electronics and photonics cooling technologies.
The Lattice Boltzmann Equation Method for Complex Flows
Authors: Schaefer Laura , Ikeda Michael , Bao Jie
Published: July 2013

The lattice Boltzmann equation (LBE) method is a promising technique for simulating fluid flows and modeling complex physics. Because the LBE model is based on microscopic models and mesoscopic kinetic equations, it offers many advantages for the study of multi-component or multiphase flows. However, there are still challenges encountered when dealing with thermal effects and multiphase flows, particularly at small scales or in varying geometries. In this paper, we discuss some techniques to overcome these challenges. First, we present an overview of the LBE method, and show how it can be extended to model multiple phases and thermal effects. Next, we describe our multi-component and multiphase (MCMP) LBE method for high density ratios. While the original formulation of Shan and Chen’s (SC) model can incorporate some multiphase and component scenarios, the density ratio of the different components is restricted (less than approximately 2.0), which limits the applications. Hence, based on the SC model and improvements in the single-component multiphase (SCMP) flow model reported by Yuan and Schaefer, we have developed a new model that can simulate a MCMP system with a high density ratio. An example of that system is shown. Finally, we have developed a parallel computation LBE method based on the Compute Unified Device Architecture for NVIDIA GPUs. Using this method, we are able to efficiently model a number of phases and length scales, examples of which are presented.
3D Numerical Analysis of Heat Transfer in a Low Reynolds Number Microturbine Cascade
Authors: Omri M. , Moreau S. , Fréchette L. G.
Published: July 2013

This paper presents the conjugate heat transfer in a submillimeter scale microturbine characterized by laminar yet highly three-dimensional flows. Such a miniature turbine is part of a MEMS (microelectromechanical system) power plant-on-a-chip currently under development for distributed power generation from waste heat. Adiabatic subsonic flows in the turbine have previously been studied numerically and are characterized by low Reynolds number laminar flow (Re < 2500) but with complex vortical structures. The present work addresses the influence of these flow structures on heat transfer, including the effect of the horseshoe and tip vortices. Calculations were done for tip clearance gaps equal to 0%, 5% and 10% blade height. Three different scenarios were considered: adiabatic walls, the hub and casing temperature of 573K or the hub at 573K and the casing at 450K, for incoming flow at 600K. The heat transfer is more variable in the suction side since dominant vortices are adjacent to this blade side. The heat flux even changes its sign where the vortices begin to separate from the suction side, indicating that gas cooled in the hub and casing boundary layers is transported on the blades by the horseshoe vortices. The tip vortex prevents the top passage vortex from interacting with the suction side, which eliminates the negative heat transfer in this region. Due to the dominant vortices, the Nusselt number is found to be a function of the thermal boundary conditions and cannot be predicted with traditional boundary layer correlations.
A Pore-Network Model for Capillary-Driven Flows Inside Porous Media
Authors: Shahraeeni Mehdi , Hoorfar Mina
Published: July 2013

A pore-network model is adopted to simulate fluid flow in porous media. The model is examined for two different hydrophobic loadings. A new algorithm for displacing the fluid inside the porous medium is developed. The algorithm uses an effective pressure as a combination of capillary pressure and hydrodynamic pressure. The capillary pressure is a function of pore size distribution and the wettability characteristics of the medium while the hydrodynamic pressure is correlated to the local saturation of the pores. Thus, the algorithm automatically switches between the capillary driven and the viscous displacement mechanisms based on the value of the local capillary number. The results are compared with the available experimental and numerical data.
Multiphase Submerged Jet Impingement Cooling Utilizing Nanostructured Plates
Authors: Demir Ebru , Kosar Ali , Izci Turker , Perk Osman Yavuz , Sesen Muhsincan , Khudhayer Wisam , Karabacak Tansel
Published: July 2013

An experimental setup is designed to simulate the heat dissipated by electronic devices and to test the effects of nanostructured plates in enhancing the heat removal performance of jet impingement systems in such cooling applications under boiling conditions. Prior experiments conducted in single phase have shown that such different surface morphologies are effective in enhancing the heat transfer performance of jet impingement cooling applications. In this paper, results of the most recent experiments conducted using multiphase jet impingement cooling system will be presented. Distilled water is propelled into four microtubes of diameter 500 μm that provide the impinging jets to the surface. Simulation of the heat generated by miniature electronic devices is simulated through four aluminum cartridge heaters of 6.25 mm in diameter and 31.75 mm in length placed inside an aluminum base. Nanostructured plates of size 35mm×30mm and different surface morphologies are placed on the surface of the base and two thermocouples are placed to the surface of the heating base and the base is submerged into deionized water. Water jets generated using microtubes as nozzles are targeted to the surface of the nanostructured plate from a nozzle to surface distance of 1.5 mm and heat removal characteristics of the system is studied for a range of flow rates and heat flux, varying between 107.5–181.5 ml/min and 1–400000 W/m2, respectively. The results obtained using nanostructured plates are compared to the ones obtained using a plain surface copper plate as control sample and reported in this paper.
Study on Water Transport Phenomena Through Micro-Porous Layers in PEFC
Authors: Chikahisa Takemi , Tabe Yutaka , Kadowaki Kazumasa
Published: July 2013

Micro-porous layers (MPLs) play an important role in the water management of polymer electrolyte fuel cells (PEFCs), but details of the mechanism that works to suppress water flooding has not been fully understood. In this study, the authors investigated water distribution at the interface between the MPL and the catalyst layer (CL) at the cathode side to clarify the effect of the MPL on the discharge of produced water. A freezing method was applied to observe the distribution of the condensed water, and the ice distribution on the CL surface was quantified by image processing. The effects of operating conditions on the water distribution were examined at normal temperature conditions. The distribution of ice formed on the surface of the CL with and without MPL after −10°C cold start operation was also established. Water transport rate in the vapor phase was analyzed based on temperature and vapor pressure gradient considerations. The experiments and the analysis showed that the MPL functions to prevent accumulation of water on the surface of the CL, resulting in less water flooding.
Pre-Calculation of Evaporation of Water in Parallel Microchannels Using Measurable Fluid Inlet Conditions
Authors: Maikowske Stefan , Brandner Juergen J. , Dittmeyer Roland
Published: July 2013

Evaporation of water in parallel microchannels is examined in the current research study. A simple model approach, based on thermodynamic considerations, enables the prediction in terms of a pre-calculation of a possible full evaporation of water in arrays of parallel microchannels. The model contains easily measureable values of the liquid fluid at the channel inlets such as temperature, pressure and mass flow. The model is verified using water with different microchannel designs and different microstructured inlet distribution geometries. A pressure range of up to ∼106 Pa with liquid fluid flow inlet velocities up to ∼0.8 m · s−1 is examined. Additionally, the model is verified using different heat fluxes and different phase transition lengths, the length of two-phase flow from bubble generation to total evaporation, in the microchannels. An empirical correction function is introduced to fit the idealized model with good agreement of about 5% to the real condition of the measurements and the process conditions. The adjusted model enables the pre-calculation of a possible total evaporation of water in the microchannel arrays by calculating the maximum liquid flow velocity at the channel inlets to achieve saturated steam inside the microchannels.
Flow Through Nano Porous Media: Defining a Benchmark Scenario
Authors: Uth Marc-Florian , Herwig Heinz
Published: July 2013

A benchmark scenario for studying the effect of non-continuum phenomena on the macroscopic properties of nano porous media is introduced. It consists of three geometries typical for the flow situation in porous media and should be representative for complex flows in general. Continuum results are presented as reference cases reaching from no-slip to total slip and different values of slip lengths. The results are compared to Lattice-Boltzmann simulations combined with a modification of the Shan-Chen model to account for slip. The comparison shows deviations between the models that can not be observed in a flow over flat walls. Such continuum results are provided for comparing them with simulations based on molecular dynamics on the nano scale in order to identify the breakdown of the continuum assumptions for small scales.
Numerical Investigation of Rough Surfaces: Coupling Approach
Authors: Rovenskaya Olga , Croce Giulio
Published: July 2013

A numerical analysis of the flow field in rough microchannel is carried out decomposing the computational physical domain into kinetic and continuum sub-domains. Each domain size is determined by the value of a proper threshold parameter, based on the local Knudsen number and local gradients of macro-parameters. This switching parameter is computed from a preliminary Navier–Stokes solution throughout the whole physical domain. The solution is then advanced in time simultaneously in both kinetic and continuum domains: the coupling is achieved by matching half fluxes at the interface of the kinetic and Navier–Stokes domains, taking care of the conservation of momentum, energy and mass through the interface.

The roughness geometry is modeled as a series of triangular obstructions with a relative roughness up to a maximum of 5% of the channel height. A wide range of Mach numbers is considered, from nearly incompressible to chocked flow conditions and a Reynolds number up to 100. Accuracy and discrepancies between full Navier Stokes, kinetic and coupled solutions are discussed, assessing the range of applicability of first order slip condition in rough geometries. The effect of the roughness is discussed via Poiseuille number as a function of local Knudsen and Mach numbers.
Rapid Generation Process of Superheated Steam Using a Water-Containing Porous Material
Authors: Mori Shoji , Kobayashi Ryo , Tanaka Mikako , Okuyama Kunito
Published: July 2013

A simple method for the rapid generation of superheated steam using a water-containing porous material was proposed in a previous paper (Mori and Okuyama, 2007). The start-up and cut-off responses are of the second order, and the maximum energy utilization efficiency for input power is more than approximately 0.9. In the present paper, in order to clarify the mechanism of the rapid generation of superheated steam, the steam temperature, the temperature inside porous materials, and the surface roughness of porous materials have been measured. As a result, it is found that narrow space formed between heater and porous material caused by surface roughness of porous material is one of important factors to generate superheated steam rapidly.
Simulation on the Flow and Heat Transfer Characteristics of Confined Bubbles in Micro-Channels
Authors: Liu Qingming , Palm Björn , Anglart Henryk
Published: July 2013

3D simulations on confined bubbles in micro-channels with diameter of 1.24 mm were conducted. The working fluid is R134a with a mass flux range from 125kg/m2s to 375kg/m2s. The VOF model is chosen to capture the 2 phase interface while the geo-construction method was used to re-construct the 2-phase interface. A heated boundary wall with heat flux varying from 15kW/m2 to 102kW/m2 is supplied. The wall temperature was calculated. The effects of mass flux and heat flux are studied. The shape of the bubble was predicted by the simulation successfully and the results show that they are independent of the initial shape. Both thin film evaporation and micro convection enhance the heat transfer. However, the micro convection which is caused by bubble motion has greater contribution to the total heat transfer at the stage of bubble growth studied.
Fractal Patterns in the Nanofluidic Sessile Droplet Drying
Authors: Crivoi Alexandru , Duan Fei
Published: July 2013

The two-dimensional lattice-gas kinetic Monte Carlo model is used to simulate the process of a nanofluidic sessile droplet drying with a moving contact line in this study. A new modeling approach is implemented by introducing the two-dimensional circular simulation domain in order to operate with the top view of spherical cap of a drying droplet. The non-uniform effective chemical potential function is applied to the model, taking into account the thickness profile of a droplet. Although our simulation is two-dimensional, this modification mimics to some extent the three-dimensional nature of a mesoscopic sessile droplet. Hence we introduce a new, pseudo-3D modification of Monte Carlo simulation model, i.e., a 2D model with additional dependence of the chemical potential on the local droplet thickness. In result, the evaporation-induced nanoparticle self-assembly led to the formation of a fractal-like structure, which is observed in simulation runs. Interestingly, the fractal-shaped patterns obtained in numerous simulations are comparable to the experimentally recorded images of dried nanoparticle structures. The difference in average fractal dimensions of simulated and experimental images is estimated to be less than 5%. It is suggested that the lattice-gas model provides an easy way to reproduce the formation of branched structures from nanoparticles during the drying of spherical sessile droplets.
Analysis of Gaseous Flows in Minichannels by Molecular Tagging Velocimetry
Authors: Samouda Feriel , Barrot Christine , Colin Stéphane , Baldas Lucien , Laurien Nicolas
Published: July 2013

The Molecular Tagging Velocimetry (MTV) technique has been widely used for analyzing velocity fields in liquid mini- and microflows. Concerning gaseous flows, only few works describe the implementation of MTV at millimetric scale, and these studies are limited to the analysis of external flows, such as jet flows. The goal of the present work is to develop this technique for the analysis of internal gas flows in minichannels. It is a first step toward the visualization of velocity profiles in rarefied conditions, and direct measurement of velocity slip at the walls.

A specific experimental setup has been designed. Its features are detailed. Velocity profiles are obtained in a pressure driven steady flow of argon through a long rectangular minichannel of 1.2 × 5 mm2 cross-section and 15 cm length using acetone molecules as tracer. Experiments are carried out at atmospheric pressure, in a laminar continuum flow regime. The accuracy of the method is discussed by comparison between experimental and theoretical velocity profiles.

The potential of the MTV technique for analyzing mini or micro gaseous internal flows is commented on. Perspectives of the work for discussing the validity of boundary conditions in the slip flow regime are presented.
The Effect of the Laser Beam Wavelength and Pulse Width on Micro Grooving: Comparison of Nanosecond and Femtosecond Laser
Authors: Nonaka Shinri , Mori Tastuhiro , Takata Yasuyuki , Kohno Masamichi
Published: July 2013

Processing technique of micro grooves and channels is very important to study the phenomenon of fluids in micro scale. Micro grooves and microchannels play an important role in various devices, such as μ-TAS (Micro-Total Analysis Systems) and micro reactors.

Laser processing is currently widely used for drilling and grooving of various materials including metals, polymers, glasses and composite materials, since laser machining can avoid the problems that conventional machining methods have. For example wear of a working tool, lowering of processing accuracy, and wear debris becoming contaminants are some of the problems of the conventional method. Additionally, compared to other non-contact machining processes such as electron beam machining (EBM) and focused ion beam (FIB), machining a vacuum is not required. Therefore, applicability is wider and setup costs can be more economical.
Transport and Adsorption Phenomena in Mesoporous Silica
Authors: Daiguji Hirofumi
Published: July 2013

The analysis and control of transport phenomena in fluidic nanopores and nanochannels is important in applications such as biochemical analysis, power generation and environmental protection. A unique aspect of nanofluidics is that the relevant length scale is comparable to the range of various surface and interfacial forces in liquids (such as electrostatic, van der Waals and steric interactions). Thus, to obtain an adequate description of transport phenomena in nanospace, it is necessary to understand the discreteness of molecules, especially when the size decreases to 2 nm. Micelle-templated mesoporous silicas (MPSs) possess highly ordered structures such as 2D hexagonal and 3D cubic structures and pores within the 2–50 nm range. In particular, 2D hexagonal films that generally have pore channels parallel to the surface plane have been widely synthesized by using various types of template molecules. If the pore channels of such materials are aligned in a certain direction, these materials can be employed for various purposes such as the fabrication of oriented nanowires, optoelectronic devices, recording media, selective separations, and nanofluidic systems. 3D cubic structures give large surface areas and become good candidates for highly efficient catalysts and sensors. Advances in the synthesis, measurement and analysis of nanotubes and nanochannels have allowed ion and liquid transport to be routinely examined and controlled in spaces with dimensions that range from 10 to 100 nm. The ability to now explore transport and adsorption phenomena in confined spaces of around 2 nm offers a range of possibilities. We have investigated several unique transport and adsorption phenomena in mesopores measuring a few nanometers in diameter, including nonlinear I–V curves of ionic current passing through MPS thin films filled with aqueous solutions, humidity-dependent adsorption rate of water into MPS, and the reduction of melting and freezing temperature of water in MPS.
DSMC Simulations of Squeeze Film Between a Micro Beam Undergoing Large Amplitude Oscillations and a Substrate
Authors: Diab Nadim A. , Lakkis Issam A.
Published: July 2013

This paper investigates the behavior of a gas film in a micro RF switch. A Two-dimensional numerical study of the flow field is performed as the micro-beam oscillates harmonically between its equilibrium position and the fixed substrate underneath. Unlike previous work in literature, the beam undergoes large displacements throughout the film gap thickness and the behavior of the gas film along with its impact on the moving RF switch (force exerted by gas on the beam’s front and back faces) are discussed. Since the gas film thickness is of the order of few microns (i.e. 0.01<Kn<1), the rarefied gas exists in the non-continuum regime and, as such, the Direct Simulation Monte Carlo (DSMC) method is used to simulate the fluid behavior. The impact of the squeeze film on the beam is investigated over a range of frequencies, velocity amplitudes, and for different film gases, corresponding to ranges of dimensionless flow parameters such as the Reynolds (Re), Strouhal (St) and Mach (Ma) numbers on the gas film behavior.
A Multi-Scale Method for Modeling Nanochannel Flows
Authors: Borg Matthew K. , Lockerby Duncan A. , Reese Jason M.
Published: July 2013

A multiscale numerical scheme is presented for modelling nano flow systems in which length-scale separation exists, but only in one direction; for example, in long nanochannel geometries. The scheme is a hybrid atomistic/continuum method, coupling a conventional fluid model with more-costly, but accurate, molecular dynamics (MD). The hybrid approach presented here is different to the common domain-decomposition style of approach, and is more akin to distributed methods, such as the Heterogeneous Multiscale Method (HMM). In our method, the molecular dynamics solver is applied only at a few physical slices of the full macro-scale domain. The greater the scale separation, the greater the computational speed-up (when compared to a full MD simulation of the same geometry). The new method is successfully tested on a converging/diverging nanochannel case containing flow of a simple Lennard-Jones liquid. Very good agreement is found when comparing the multiscale simulation results to those of a full MD simulation.
In-Line Characterization of Micro-Droplets Based on Partial Light Reflection at the Solid-Liquid Interface
Authors: Weber Emanuel , Puchberger-Enengl Dietmar , Vellekoop Michael J.
Published: July 2013

In this paper a novel optofluidic setup, fabricated on a single layer device for in-line droplet characterization yielding droplet-size, droplet-frequency, and optical properties with compatibility for full on-chip integration is presented. Chips were fabricated using a simple, fast, and cost effective technology. A T-junction arrangement on the device is used for droplet generation. The optical part of the setup consists of an external light source, external silicon photodetectors, integrated air micro-lenses, and an integrated waveguide. The design makes use of partial light reflection/transmission at the solid-liquid interface to count, size, and discriminate droplets based on their optical properties. When passing the interrogation point, droplets having a lower refractive index as the continuous phase result in light deflections. Both, reflected and transmitted light, are detected simultaneously. A relation of those two signals is then used for the analysis resulting in a continuously stable signal. The generated pattern is unique for different droplets and can be exploited for droplet characterization.

Using this arrangement, droplets of de-ionized water (DI) were counted at frequencies of up to 320 droplets per second. In addition, information about the droplet sizes and their variations could be obtained. Finally, 5 mol/L CaCl2 and DI droplets, having different indices of refraction were examined and could clearly be discriminated based on their unique reflected and transmitted light signals. This principle can be applied for the detection of dissolved molecules in droplets as long as they influence the index of refraction. Examples could be the determination of DNA or protein content in the droplet.
Microfluidic Combinatorial Screening Platform
Authors: Zec Helena , Rane Tushar D. , Chu Wen-Chy , Wang Tza-Huei
Published: July 2013

We propose a microfluidic droplet-based platform that accepts an unlimited number of sample plugs from a multi-well plate, performs splitting of these sample droplets into smaller daughter droplets and subsequent synchronization-free, reliable fusion of sample daughter droplets with multiple reagents simultaneously. This system consists of two components: 1) a custom autosampler which generates a linear array of sub-microliter plugs in a microcapillary from a multi-well plate and 2) A microfluidic chip with channels for sample plug introduction, reagent merging and droplet incubation. This novel system generates large arrays of heterogeneous droplets from hundreds to thousands of samples while concurrently screening these arrays against a large array of reagents. This high throughput system minimizes sample and reagent consumption and can be applied to a gamut of biological assays, ranging from SNP detection to forensic screening.
Performance Improvements by Embedded Thin Film Current Collectors for Microfluidic Fuel Cells With Porous Electrodes
Authors: Lee Jin Wook , Kjeang Erik
Published: July 2013

In the present work, a chip embedded thin film current collector for vanadium fueled microfluidic fuel cells is proposed. Embedded current collectors are expected to increase the number of contact points with the porous electrodes and thereby reduce the overall contact resistance and ohmic losses of the cell. The micromachining based thin film process is compatible with the overall cell fabrication scheme, which is based on soft lithography, and does not require a substantial modification of the original cell design. The flow-through microfluidic fuel cell architecture with porous carbon paper electrodes is used as a proof-of-concept in this study. Our preliminary study shows that the peak power density of the cell with the current collector is increased by 79% at 300 μL min−1 compared to an otherwise identical cell without current collector, indicating that the contact resistance is significantly reduced. Electrochemical impedance spectroscopy analysis is carried out to estimate the overall cell ohmic resistance and confirms a 32% reduction using the current collectors.
Inertial Focusing and Ordering in Low Aspect Ratio Microchannels Using Reference Frame Tracking and Rotation Measurements
Authors: Lagus Todd P. , Edd Jon F.
Published: July 2013

Inertial focusing and ordering in microchannel flows refers to the tendency of finite-sized particles to migrate across streamlines and to form linear, equally spaced trains in the direction of flow. This study utilizes a motorized microscope stage moving along the length of a low aspect ratio microchannel at up to 10 cm/s and provides a Lagrangian view of particles to obtain more complete time dependent trajectory and rotation histories from the channel inlet to outlet. We observe monodisperse particle dynamics, rotations, and interactions over time scales significantly longer (exceeding a 30-fold increase) than static reference frames. The results present new insight into particle interactions which show quasi-steady state equilibrium spacing which oscillates at a constant frequency at a fixed flow rate, which is different from the damped oscillatory interactions suggested in the literature. The average spacing shows little dependence on flow rate, but the oscillation frequency is dependent both on flow rate and particle size.
A Method to Measure Viscosity of Fluids in Microchannels
Authors: Pan Lichao , Arratia Paulo E.
Published: July 2013

In this paper, we present a microfluidic-based rheometer that uses in-situ pressure signals to measure the shear viscosity of liquids. The shear-viscosity is measured using a PDMS-based microfluidic device that consists of a channel layer and a sensing membrane integrated with an array of piezoresistive pressure sensors via plasma surface-treatment. The micro-pressure sensors are fabricated using conducting particles-PDMS composites, synthesized by mixing 82 wt% micrometer-sized silver particles (1–3 μm in diameter) with PDMS gels. The simple design and fabrication enable nearly perfect bonding of a 120 μm-thick sensing membrane to different channel geometries that map the pressure differences at various locations within the channel in order to measure the fluid shear stress in-situ and to estimate the fluid viscosity. Experimental validation of this microfluidic-based rheometer is performed using calibration oil of known viscosity as well as macroscopic rheometric data, both of which achieve good consistency. The present work allows in-situ measurements of rheological properties of fluids at low Re.
Monte Carlo Simulations to Determine Effective Gas Diffusivity of an Actual Porous Medium Image Obtained From Focused Ion Beam Scanning Electron Microscopy
Authors: Joseph Jerry , Gunda Naga Siva Kumar , Mitra Sushanta K.
Published: July 2013

Computer simulation methods have been widely used to study the effective transport properties of porous media. Simulation methods such as Monte Carlo (MC) have been used to characterize the randomness associated with such media. Most of these simulations have been carried out in artificially generated porous media represented as random pack of spheres or randomly arranged cylindrical fibers. In the present work, we apply MC simulation to images of an actual porous medium, obtained by using Focus Ion Beam Scanning Electron Microscopy (FIB SEM). The porous medium under consideration is the porous electrode of Solid Oxide Fuel Cell (SOFC). The effective gas diffusivity of the SOFC electrode is calculated based on mean square displacement method. The simulations are performed initially on a 2D image and are further carried out on a 3D image. For the validation of the numerical method, the porosity of the electrode is calculated and is used for the computation of the effective diffusivity by Bruggeman equation. In such case, the porosity is calculated by Hit and Miss MC approach and is further validated with the porosity values available in the literature. This work also addresses the effect of tessellation of solid surfaces in discrete geometries on the calculation of effective transport properties. Such computation of effective properties from the actual porous media geometry can be further used in the cell modeling and optimization of different components of the SOFC.
Fabrication of a Microstructure-Enhanced Surface Area Solar Thermal Collector
Authors: Ogbonnaya E. , Chukwu S. , Wood D. , Weiss L.
Published: July 2013

Solar energy is a renewable and sustainable energy source that has a promising potential for the rapidly growing energy demands across the world. Large scale power generation from the energy of the sun is well established utilizing both direct thermal energy conversion and conversion to electricity via photovoltaic processes. Solar thermal systems have been limited to macro systems, even though they operate at higher efficiency compared to photovoltaic systems. Solar energy harvesting requires the use of collector plates to capture incident radiation. The surface area exposed to incident radiation is critical in solar thermal energy harvesting. In this work, we have integrated micro technology processes and solar thermal energy to design and fabricate a micro solar thermal system for power generation. This work specifically examined surface area enhancement using MEMS-based techniques to maximize solar thermal absorption. Selective absorber coating and enhanced surface areas due to the incorporation of micro structures on the collector substrates were utilized. In this manner, an important component to an autonomous micro power supply is investigated.

Advanced microfabrication and electrochemical deposition techniques were employed to generate a selective absorber surface with enhanced surface area on a silicon substrate. Microchannels were used to enhance the surface area on the substrate. The selective absorber coating consists of a bimetallic structure consisting of tin and nickel.
A Preliminary Study for 3D Numerical Simulation of a Through-Plane Temperature Profile in a PEMFC Incorporating Coolant Channels
Authors: Koz Mustafa , Kandlikar Satish G.
Published: July 2013

In a proton exchange membrane fuel cell (PEMFC), the water removal from the cathode to reactant flow channels is a critical aspect of cell operation. This is an active area of research to understand the transport mechanisms of water. In the available literature, it has been shown that a significant portion of the product water is removed in vapor form by the heat pipe effect through the gas diffusion layer (GDL). The intensity of the heat pipe effect is dependent on the local mean temperature and the through-plane temperature gradient across the GDL. This gradient is spatially affected by the reactant channel-land patterns of the bipolar plate (BPP) and the coolant plate operation. Therefore, the heat pipe effect can have spatial variances depending on the BPP design and cooling method.

In order to show the local temperature and through-plane temperature gradient distribution in a GDL, a numerical approach was taken in this work using a commercially available software package, COMSOL Multiphysics® 4.2a. A repetitive cathode section of the PEMFC was modeled in 3D with domains of a GDL and BPP. In-plane thermal conductivity of the GDL was incorporated by using experimentally obtained values from the available literature. By changing the design and operating conditions of the coolant system, the thermal profile and so, the vapor flux across the GDL were investigated. It was found that the increasing temperature non-uniformity on coolant plates leads to less uniform distribution of vapor flux. This is expected to lead to more condensation of water vapor under the lands.
Characterization of Ion Transport and -Sorption in a Carbon Based Porous Electrode for Desalination Purposes
Authors: Clifton Rebecca L. , Perez Carlos A. Rios , Naylor Rachel , Hidrovo Carlos
Published: July 2013

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although much of the attention and efforts have focused on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high-surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses electric fields to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization (EDI) and electro-dialysis reversal (EDR) where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow-rates. Voltage and electrical current measurements are conducted during the desalination and porous electrode regeneration processes and used to evaluate the percentage of energy recovery.. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption/desorption efficiencies. Correlation of these measurements with an analytical species transport model provides information about the net ionic adsorption/desorption rates in non-saturated-carbon-electrode scenarios. The results show a strong dependence of the net electrical energy requirements with the number of carbon electrodes regeneration cycles. Finally, a non-dimensional number that compares the convective and electro-kinetic transport times is presented. The energy requirements and adsorption/desorption rates analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the re-process of spent nuclear fuels in a near future.
Analytical and Hybrid Solutions for Heat Transfer in Combined Electroosmotic and Pressure-Driven Flows
Authors: Sphaier L. A.
Published: July 2013

This paper presents solutions to heat transfer problems that occur in flow micro-channels driven by the combined effect of electroosmosis and a pressure gradient. Fully developed velocity profiles are considered, and the thermal developing region is analyzed. The solution methodology is based on the Generalized Integral Transform Technique, which leads to fully analytical solution for all presented cases. With the solution of the temperature fields, the behavior of the Nusselt number is investigated for different test-cases. The effects of the flow driving mechanisms, viscous dissipation and Joule heating, as well as axial diffusion are analyzed. The approximated solution with thin Electric Double Layers (EDL) is considered, but cases without this restriction are also analyzed. The cases including axial heat conduction are analyzed for a simplified case of purely electroosmotic-driven flow with a thin-EDL, which leads to a simple analytical solution.
A Heat Transfer in Iso-Thermal Triangular Channel Filled With Porous Media
Authors: Mortazavi S. Negin , Hassanipour Fatemeh
Published: July 2013

This study presents an analysis of forced convection in a porous triangular channel. The flow is laminar, fully developed and assumed to have constant properties. The porous channel has an isotropic matrix and the boundary conditions are fixed with a constant temperature. In this paper, accurate analytical solutions are presented to determine the effects of apex angle and porous media properties on the temperature distribution in a triangular channel along with the Nusselt number NuT.
A Novel Thermo-Hydraulic Test Platform for Micropillared Array Thermal Wick Optimization
Authors: Miers Collier , Wehmeyer Geoff , Hidrovo Carlos H.
Published: July 2013

Heat pipes have immense potential in the future of thermal management in electronic devices. As a passive device, they rely solely upon capillary forces to recirculate the coolant from the condenser to the evaporator via a wicking structure. In intermediate temperature heat pipes the limiting factor for heat removal is the capillary limit, which indicates the maximum recirculation rate that the capillary forces can induce. This capillary limit must be increased to allow heat pipes to remain a viable option for heat management within electronic devices. The aim of this work is to characterize and optimize the capillary limit of micropillared thermal wicks for heat pipe application in micro-electronics cooling.

Towards this goal, an analytical model, and a novel thermo-hydraulic experimental setup was developed. The analytical model of the micropillared array wicking structure provides a theoretical basis from which the pillar geometry and arrangement can be optimized. A capillary limit model was used to determine the geometric relationship between the pillar arrays and the maximum capillary flow rate through the wick. This model considers the effects of gravity and mass transfer due to evaporation.

Finally, the thermo-hydraulic characterization setup, designed to minimize environmental losses, was used to experimentally determine the capillary limits of different silicon based micropillared wick samples. The heater and wicking structure were enclosed in a temperature and humidity controlled vacuum chamber. The results obtained from this setup were used to validate the analytical model shown in this paper.
CFD Modeling of Droplet Flows in Microchannels in Slip Flow Regime
Authors: García Janneth M. , Pineda Saira F. , Blanco Armando J.
Published: July 2013

Flow in microchannels is an important area of research in engineering because its importance in areas such as micro-machinery. In addition, hydrophobic materials have become increasingly attractive for use in fabrication of microfluidic devices. However, even when in macroscopic flows the non-slip boundary condition on solid wall has been well accepted, a number of recent studies have found evidences of slip velocities for liquid flow on hydrophobic surface.

In this study, numerical simulations of two-phase flow of micro-droplet in a continuous base fluid in a 2D microchannel were performed using a commercial software package. Both slip and non-slip flows were considered. Continuous phase slip over wall channels was considered modeling all range of possible Knudsen numbers. Three drops of different sizes were modeled. Results were expressed in term of pressure drop and Reynolds numbers.

Droplet interface deformation and velocity fields inside both droplets and continuous phase were determined. Results show that for Knudsen numbers between 0.01 and 0.1, Reynolds number increases in a proportion in the order of 20%. In addition pressure drop notably increase when large drops are considered.
Statistical Analysis of Velocity Fields Obtained From Experimental Study of Micro-Porous Media
Authors: Sen Debjyoti , Nobes David S. , Mitra Sushanta K.
Published: July 2013

An experimental technique based on scanning a thin field of view through a volume of interest and micro-PIV is used to determine velocity field within a micro porous media. Statistical methods are used to investigate the characteristic distributions of velocity for the micro scale flow. The similarity in velocity trends between macro porous media and micro porous media is established. The behavior of these characteristic velocity distributions in a single pore and multipore systems is discussed.