bluuebunny/crossref_metadata_2025 huggingface dataset:
Mathematical Modeling of Rivulet Flow Driven by Variable Gravity and Gas Flow in a Minichannel
Authors: Bartashevich M. V. , Kuznetsov V. V. , Kabov O. A.
Published: June 2009

Rivulet flows are a special type of thin film flows with a bounded width. The use of rivulet flows is very perspective in various types of process equipment such as evaporators. The flow dynamics in rivulets has some peculiarities, the investigation of which allows understanding the gist of possible mechanism of enhancement of heat transfer. In the present work a mathematical model for the rivulet flow in conditions of a variable gravity has been elaborated. The liquid flow takes place in a slot between two plates and is caused by a co-current gas flow. The numerical calculations of the flow parameters depending from the gravity forces have been made. The analytical formula for connection of the main rivulet parameters (width, contact angle, liquid flow rate ...) in a linearized approach has been derived. The comparison of numerical results with experimental data obtained during 44 Parabolic Flights campaign of the European Space Agency has been carried out. Liquid film of FC-72 driven by the Nitrogen gas has been studied in experiments. Force balance is changed during a parabolic flight and due to surface tension effect the liquid film in a horizontal minichannel 40 mm width became a flattened rivulet 9 mm width at microgravity.
Three-Dimensional Hydrodynamic Focusing With a Circular Microchannel
Authors: Sheen Jyh-Jong , Ho Kuan-Yi , Lee Yao-Min
Published: June 2009

We propose a simple method to implement three-dimensional (3D) hydrodynamic focusing with a circular micro-channel for the sample flow. We utilized soft lithography technique and a cylindrical object such as an optical fiber to construct a 3D hydrodynamic focusing master mold made of photoresist and the fiber. The 3D microfluidic focusing device can be duplicated by polydimethylsiloxance (PDMS) casting. When peeling the PDMS cast from the master, there is a crack along the circular channel. However, this crack can be recovered from the thermal bonding of the PDMS cast and a PDMS substrate. We applied this technique to fabricate 3D flow-focusing devices for a microfluidic fluorescence detection chip and a micro-emulsion chip. In the case of microfluidic fluorescence detection, the sample flow can form a small and stable cylindrical flow by controlling the flow rates of sheath flow and sample flow. This allows fluorescent particles to be focused at the center of the channel and to be excited uniformly by laser. Experimental results show that the coefficients of variation for 3D hydrodynamic focusing is 4 to 10 times lower than those of 2D hydrodynamic focusing. As a consequence, the 3D focusing chip provides advantages of stable sample flow and highly uniform detection signals. The micro emulsion chip with 3D hydrodynamic focusing can avoid the wetting problem of two immiscible fluids and successfully produce emulsion droplets.
A Systematic Approach to Wall Roughness Effects in Laminar Channel Flows: Experiments and Modelling
Authors: Gloss D. , Dittmer J. , Herwig H.
Published: June 2009

In this study, the influence of wall roughness on laminar gas flow in small channels is investigated experimentally, comparing results to CFD solutions including the calculation of entropy production. In the experimental set-up, air flows between two parallel disks that create a channel of continuously adjustable height, ranging from macro to micro size. The roughness is created by a processed silicon wafer with homogeneous surface elements. The experimental results after a careful error analysis are compared to numerical models in order to validate these models. In rough micro channels, a crucial problem is the actual location of the wall. This location strongly influences the corresponding theoretically determined pressure drop of the flow. This influence, as well as entropy production considerations are discussed. The effects found in the experiments are in good agreement with the corresponding CFD-models and hence strongly indicate an influence of wall roughness on laminar flow.
A Scaling Analysis Inquiring Into Two-Phase Flow Reversal
Authors: Rops C. M. , Lindken R. , Geers L. F. G. , Westerweel J.
Published: June 2009

Physical processes limit the maximum achievable heat flux when miniaturising heat transfer equipment. In case of boiling heat transfer literature reports large pressure fluctuations, flow instabilities, and possible vapour backflow. The occurrence of the flow instabilities during boiling in small channels (defined by the Confinement Number, Co > 0.5) are explained by the formation of slug bubbles blocking the entire channel. These particular bubbles are likely to emerge during nucleate flow boiling in small diameter channels. Slug bubble blockage during flow boiling is investigated experimentally by creating a single hotspot in a small-diameter channel (Co∼5). For different liquid flow rates the detachment length of such a blocking slug bubble is determined. A scaling analysis offers to insight into the physical phenomena causing the flow instabilities. The position of the bubble caps as a function of time is identified as an important parameter.
Simulation of Flow in Continuum-Transition Regime in Stepped-Microchannels Using Burnett Equations
Authors: Kamali Reza , Ravesh Saleh Rezaei , Movahed Saeid
Published: June 2009

In present study, the Navier-Stokes equations and the Burnett equations with Maxwell-Smoluchowski slip conditions for some values of Knudsen number are used to resolve the viscous compressible fluid flow of air in the stepped microchannel. An explicit finite difference scheme is employed to develop a two-dimensionl numerical Burnett solver for microfluidic flows and the second order stresses and heat fluxes in the Burnett equations are implemented into the code. Velocity slip/temperature jump conditions on the wall of the channel and on the step within duct are also used. Results are compared with those obtained by using the Navier-Stokes equations with and without the slip-wall conditions using flow in a microchannel. The effects of Knudsen number on the flow and the heat transfer characteristics of the microchannel are also investigated.
Computation of Rarefied Gaseous Flows in Micro to Nano Scale Channels With Slip to Transient Regimes Using General Second-Order Quadratic Elements
Authors: Darbandi M. , Daghighi Y.
Published: June 2009

A new finite-volume-based finite-element method using the quadratic elements is developed in the present study, to analyze the flow in micro and nano sizes with higher-order slip boundary conditions. The method is applied to gaseous flow in micro and nanoscale-channels. The developed method is carried out over a wide range of Knudsen numbers, which cover not only the continuum slip flow regime with 0≤Kn≤0.1 but also it entire the range of transient regime with 0.1<Kn≤10. To make the present computational model capable of simulating micro and nano sizes with the help of the Navier-Stokes equations, the modified high-order slip boundary conditions are applied which need utilizing the advantages of general quadratic second order elements in the computational domain. In other words, this paper introduces a new developed method, which is applied on higher-order elements, and employing reliable boundary conditions that all of these issues are used for the first time in the Micro/Nano study as well. The results reveal excellent agreement with those represented by analytical, DSMC, and Boltzmann calculations. The proposed method (using finite-volume-element strategy which benefits from the advantages of general quadratic second-order elements) is proved to be an efficient, practical, and accurate tool, which robustly extends the capability of our primitive large scale Navier-Stokes solver to micro and nano-scale flow predictions in slip and transient regimes. It can be regarded as a super alternative to classical molecular dynamics-based methods.
Effects of Membrane Structural Parameters on Heat Mass Transfer Property of a Composite Supported Liquid Membrane
Authors: Zhang Li-Zhi
Published: June 2009

Supported liquid membranes have been used in many industries due to their inherent high mass transfer rates. In this study, the heat and moisture transfer through the composite supported liquid membrane is numerically investigated. The composite membrane comprises two hydrophobic skin layers and a hydrophilic porous layer in which a layer of LiCl solution is supported. Focus of this study is on the effects of membrane structural parameters such as pore diameters, porosity, tortuosity, and membrane thickness on the effective moisture diffusivity and the effective heat conductivity of the whole membrane. The established effective diffusivity and heat conductivity are combined with the heat and moisture conservation equations for air streams to calculate the temperature and humidity profiles in the unit. The variations of the effective diffusivity with membrane parameters are estimated.
System-Level Modeling of Microflows in Circular and Rectangular Channels
Authors: Lakkis Issam A.
Published: June 2009

System-level models for unsteady, incompressible, low Reynolds number flow in channels of slightly varying cross sections of different geometries are presented. The models are based on approximate solution of the unsteady Navier-Stokes equation subject to no-slip and first-order slip boundary conditions. The proposed model, relating the volume rate to the average pressure drop across the channel, is cast into an electric circuit model that consists essentially of an infinite number of parallel branches in series with a resistor and a nonlinear component. The resistor and the nonlinear element in series account respectively for viscous dissipation in the direction of the flow and for the convective part of the inertia term. The set of parallel branches captures the unsteady behavior as well as viscous dissipation normal to flow direction. Previous channel models available in the literature proved to be a special case of the models proposed in this paper at steady state or in the limit of vanishing Reynolds number. The proposed models offer superior accuracy when transient behavior and associated dynamic characteristics are of interest. The models also become more accurate for flows in slightly divergent channels, flows at larger Reynolds number, and in flows that experience sudden changes or that are subjected to forced oscillations with large values of the Strouhal number. The proposed models are flexible in the sense that accuracy and cost can be easily traded by increasing or decreasing the number of branches included in the model. The derived models are compared with previous models and with numerical solutions of the full Navier-Stokes equations.
Dynamics of Two-Layer Film Under the Joint Action of Thermocapillary Effect and Intermolecular Forces
Authors: Nepomnyashchy Alexander , Simanovskii Ilya
Published: June 2009

The development of instabilities under the joint action of the van der Waals forces and the Marangoni stresses in a two-layer film, is investigated. The problem is solved by means of nonlinear simulations. Different types of nonlinear evolution have been observed.
A Novel Liquid Metal and Microfluidic Based Temperature Sensor
Authors: Sai Sumanth Pottigari , Kwon Jae W.
Published: June 2009

We present a novel liquid metal based temperature sensor. This sensor utilizes thermal expansion property when temperature changes. The liquid metal (mercury) is used for electrical interconnect between capacitors as it expands through a microfluidic channel. Among various temperature sensing methods, platinum resistive thin film devices [1] and acoustic wave devices [2] are commonly being used. The platinum thin film based device utilizes a unique property that the resistant of the thin film changes with respect to temperature variation. A complete sensor can be realized in a balanced wheat stone bridge. When the temperature changes, the resistance of the thin film changes correspondingly and the bridge is no longer balanced. There is a current flow through the bridge upon the amount of temperature variation. However, the current brings a self-heating problem, which negatively affects the performance. On the other hand, the acoustic wave device utilizes a property that a resonance frequency shifts per temperature variation. There are many difficulties in calibrating this device and, in liquid environment, there is a significant drop of q factor [3], which decreases the sensitivity of the sensor. Our new device can overcome many difficulties described above.
Numerical Modelling Based on Lattice Boltzmann Method of the Behaviour of Bubbles Flow and Coalescence in Microchannels
Authors: Yan Y. Y. , Zu Y. Q.
Published: June 2009

This paper introduces a newly developed model of lattice Boltzmann method (LBM) for simulating bubbles flow and coalescence in a microchannel. The model is developed based on the work by Inamuro et al. (2004) aiming to test the suitability of LBM for studying the dynamics of bubbles flow and coalescence in a surface dominated rectangular microchannel and to reduce computational cost. A simple linear function is applied to the order parameter to approximate the density within the interface of gas-liquid and meanwhile employing a new form of the free energy density (rather than the van der Waals free energy) to extend the suitability of the model for simulating two-phase flow on wetted solid boundaries. The evolution of two isothermal air bubbles flowing through a water-filled microchannel and the interactions between the flow fields and the interface of gas-liquid are obtained and investigated numerically.
Reducing Undesired Effects of Turns on Electroosmotic Flow Field in Nanochannels
Authors: Kamali Reza , Movahed Saeid , Ravesh Saleh Rezaei
Published: June 2009

There has been a growing interest in developing the miniaturized fluidic devices for transportation, separation and detection of bio-samples. In many of these phenomena electrokinetic effects play an important role. Studies show that in fabricating these devices, if long channels are used, separation efficiency can be improved. In these devices, turns must be induced to integrate a long channel within the small area of a single chip. One of the ensuing results of these turns is serious distortion in flow field that named dispersion. Dispersion can result in degraded separation quality. The present study shows that how dispersion after the turns can be reduced by minimum changing in geometry. It is also shown that how using an optimized distribution for ζ-potential near the turns can reduced the magnitude of dispersion without changing the geometry. The optimized distribution of ζ-potential has been found by genetic algorithm technique. Computer code based on the genetic algorithm has been developed to study the effectiveness of purposed methods.
Adjustable Nanofluidic Channels by Tunnel Cracking of a Constrained Brittle Layer
Authors: Mills K. L. , Huh Dongeun , Takayama Shuichi , Thouless M. D.
Published: June 2009

There is widespread and rapidly increasing interest in adopting nanofluidics as a strategy for developing new capabilities in various scientific areas. A fast, easy, and inexpensive method for creating nanochannels may address technical limitations associated with the limited availability of conventional nanofabrication technologies and contribute to expanding the application of nanofluidics to areas of biological or chemical interest. For example, there is considerable interest in the controlled confinement and manipulation of single polymeric or bio-molecules (e.g., DNA) for analysis. Motivated by this, we present here a cracking-based method for fabrication of adjustable nanofluidic channels.
On the Characteristics of a Two-Dimensional Two-Fluid Model
Authors: Chung Moon-Sun , Lee Sung-Jae , Kim Jong-Won
Published: June 2009

In this study, we will suggest a two-dimensional two-fluid model considering the effect of mass and momentum interactions to simulate more realistic two-phase flow than the conventional model did. A hyperbolic two-fluid model had been developed for one-dimensional two-phase flow by Chung et al. [1] and it has been improved and applied to analyze one-dimensional two-phase flow problem including surface tension effect for either ordinary pipe system or minichannels. However, in order to simulate the two-dimensional two-phase flow problem efficiently in the future, the above one-dimensional model has need to be extended to two-dimensional equations and adopted to an upwind numerical method.
The Effect of the Contact Angle on a Flow With Condensation and Evaporation Through a Microchannel
Authors: Loimer Thomas
Published: June 2009

The flow of a fluid that is in a state of saturated vapor at the upstream side of a channel is considered. The flow is described by applying the balances of mass, momentum and energy. One-dimensional flow and heat conduction is assumed. Account is taken of the Joule-Thomson effect and, for contact angles smaller than 90°, of the capillary pressure given by the Young-Laplace equation and of the vapor pressure reduction at a curved meniscus due to Kelvin’s equation. For a diameter of the channel below a certain threshold the vapor condenses fully, liquid flows through a part of the channel, the fluid evaporates at a location within the channel and vapor flows through the remaining part of the channel. The threshold diameter below of which the fluid condenses fully is a function of the fluid properties and of the effective thermal conductivity of the fluid-filled channel. The threshold diameter lies in the range from a few tens of nanometers for bad heat conductors to a few hundreds of nanometers for very good heat conductors such as multi-walled carbon nanotubes. Solutions are given for the flow problem with a contact angle of 90° and with contact angles smaller than 90°. The mass flux is considerably larger for contact angles smaller than 90°. However, as long as the contact angle is smaller than approx. 70°, the mass flux is nearly independent of the contact angle.
Thermal Contact Behaviour in Micro-Injection Moulding
Authors: Nguyen-Chung Tham , Lo¨ser Cindy , Ku¨hnert Ines , Ju¨ttner Ga´bor
Published: June 2009

Micro-injection moulding is one of the most efficient technologies for large-scale manufacturing of thermoplastic polymer micro parts. Growing demands for high precision require new concepts for moulding machines and micro moulds on the one hand and a better understanding of the process characteristics on the other hand. Especially, moulding micro features of high aspect ratios still remains a challenge due to the high surface-to-volume ratio of the part which makes the melt freeze-in much faster than in conventional injection moulding.
Study on Nanoimprint Lithography of the Polymer Chains (CH2)n With Anti-Adhesion Layer on Stamp by Molecular Dynamics Method
Authors: Hsu Quang-Cherng , Lin Chien-Liang , Fang Te-Hua
Published: June 2009

This paper aims at the study on nanoimprint lithography (NIL) of the polymer material in (CH2)n Chains. The simulation codes were built based on molecular dynamics (MD) method for observing material deformation behaviors in atomic scale. The deformation mechanism of NIL of polymer material (CH2)n pressed by silicon stamp was first studied, by which the effects of critical punch tip width, imprint depth, temperature, and adhesion effect were studied. Next, the nanoimprint processes with stamp tips covered by anti-adhesion material, which is a self-assembled monolayer (SAM), were studied to compare to those processes without having anti-adhesion layer. When deforming polymer material at or above room temperature, adhesion problems occur between stamp and polymer. Polymer materials adhere to stamp more severe than they adhere to each others because potential energies between long chains of polymers are smaller than those between polymer and stamp. From the relation between system energy and stamp translation based on the MD simulations, the system energy increases when stamp moves gradually. When unloading, the system energy will return to its minimum energy status and remains stable. However, when punch leaves polymer materials, energy fluctuation occurs due to some polymer materials adhere to the stamp. Finally, the analysis of stamp with and without SAM based on the MD method was conducted and discussed.
Analytical Model of Single Microjet Cooling of Electronic Equipment
Authors: Mikielewicz Dariusz , Mikielewicz Jarosław
Published: June 2009

In the paper presented is a solution of a conjugate problem of convective heat transfer on a plate combined with conduction within the plate, where uniform generation of heat is present. The equations governing the problem have been solved both numerically and analytically showing a good consistency. The principal achievement presented in the paper is a simple analytical model describing radial temperature distributions of the cooling fluid and a plate wall. Acquired relations enable examination of the influence of various parameters such as flow rate of cooling fluid, its inlet temperature and kind of the plate material on the temperature response of a cooled plate, representing the microprocessor. The presented model has been confronted against some experimental data from literature and the numerical solution of energy balance equations for fluid and a plate shows a satisfactory consistency.
Study on IC Probe Card Simulation by Molecular Dynamics and Finite Element Method
Authors: Hsu Quang-Cherng , Chen Chao-An , Tu Chien-An
Published: June 2009

Probe card detection plays important role during IC packaging and testing processes. Through the contact and penetration of probe tips into pad surfaces of IC, the functionality can be verified and the yielding ratio can be obtained. During the testing process, the probe card tips may deform, wear and more often attach some pad materials. These will reduce the life of probe card. In the proposed paper, a computer code based on molecular dynamics method was developed to study the deformation and penetration mechanism between probe tip and pad surface. The adhesion problem of Aluminum pad material on probe tip and the bulge profiles of pad surface after probing were studied. The simulation results of molecular dynamics show that the probing force increases when the inclination angle becomes small. The trend of probing force related with probe inclination is the same according to the simulation results of molecular dynamics and the finite element method. However, experimental study should be conducted for the future work to verify the simulation results.
Modelling Immunomagnetic Cell Capture in CFD
Authors: Baier Tobias , Mohanty Swaty , Drese Klaus Stefan , Rampf Federica , Kim Jungtae , Scho¨nfeld Friedhelm
Published: June 2009

The separation of cells from a complex sample by immunomagnetic capture has become a standard technique in the last decade and has also obtained increased attention for microfluidic applications. We present a model that incorporates binding kinetics for the formation of cell-bead complexes, which can easily be integrated into a computational fluid dynamics (CFD) code. The model relies on the three equation types: Navier-Stokes equations governing the fluid dynamics, convection-diffusion equations for non-magnetic cells and a Nernst-Planck type equation governing the temporal evolution of cell-bead complex concentrations. The latter two equations are augmented by appropriate ‘reaction’ terms governing the binding kinetics which is formulated as a population rate balance between creation and annihilation of cell-bead complexes. First, the simulation results show, that by means of the developed approach appropriate parameter sets can be identified which allow for a continuous separation of tagged cells (cell/bead complexes) from non-magnetic particles such as non-target cells entering with the target cells. Moreover tagged cells can be, to a certain extend, separated from unbound beads. Second, the computed concentrations at the outlet show a drastic drop for higher cell/bead complexes beyond a certain number of beads per cell. We show that a critical number of beads per cells exists where the binding is considerably reduced or the reaction cascade ceases completely. This occurs when cell/bead complex have a similar magnetic mobility as the free magnetic beads. The presented CFD model has been applied to the simulation of a generic continuous cell separation system showing that the method facilitates the design of magnetophoretic systems.
Mass Flow Rate Measurements: From Hydrodynamic to Free Molecular Regime
Authors: Ewart Timothe´e , Graour Irina A. , Perrier Pierre , Me´olans J. Gilbert
Published: June 2009

An experimental investigation in a single silica microtube in isothermal stationary flow for various gases is made from the hydrodynamic to the near free molecular regime to study the reflection/accommodation process at the wall. This kind of investigation requires, more than other Micro-Electro-Mechanical-Systems (MEMS) experiments, a powerful experimental platform to measure very small mass flow rate. A global analytic expression, based on the Navier-Stokes (NS) equations with second order boundary conditions, is used to yield the Tangential Momentum Accommodation Coefficient (TMAC) in 0.003–0.3 Knudsen number range. Otherwise, the experimental results of the mass flow rate is compared with theoretical values calculated from kinetic approaches using variable TMAC as fitting parameter over the 0.3–30 Knudsen number range. Finally, whatever the theoretical approach the TMAC values obtained from the different gas-surface pairs are rather close one to other, but the TMAC values seem decreasing when the molecular mass increases.
Boundary Conditions for Multi-Component Slip-Flows Based on the Kinetic Theory of Gases
Authors: Qazi Zade Azad , Renksizbulut Metin , Friedman Jacob
Published: June 2009

General temperature-jump, velocity-slip, and concentration-jump conditions on solid surfaces in rarefied multi-component gas flows are developed using the kinetic theory of gases. The presented model provides general boundary conditions which can be simplified according to the problem under consideration. In some limiting cases, the results of the current work are compared to the previously available and widely used boundary conditions reported in the literature. The details of the mathematical procedure are also provided to give a better insight about the physical importance of each term in the slip/jump boundary conditions. Also the disagreements between previously reported results are investigated to arrive at the most proper expressions for the slip/jump boundary conditions. The temperature-jump boundary condition is also modified to handle polyatomic gas flows unlike previously reported studies which were mostly concerned with monatomic gases.
Heat Transfer Predictions Using Accommodation Coefficients for a Dense Gas in a Micro/Nano-Channel
Authors: Nedea S. V. , Markvoort A. J. , Spijker P. , van Steenhoven A. A.
Published: June 2009

The influence of gas-gas and gas-wall interactions on the heat flux predictions for a dense gas confined between two parallel walls of a micro/nano-channel is realized using combined Monte Carlo (MC) and Molecular Dynamics (MD) techniques. The accommodation coefficients are computed from explicit MD simulations. These MD coefficients are then used as effective accommodation coefficients in Maxwell-like boundary conditions in MC simulations. We find that heat flux predictions from MC based on these coefficients compare good with the results of explicit simulations except the case when there are hydrophobic gas-wall/gas-gas interactions. For this case an artificial wall was introduced in order to measure these MD accommodation coefficients at this artificial border. Good agreement is found then for both hydrophilic and hydrophobic gas-wall interactions and we show this by confronting the heat fluxes from explicit MD simulations with the the MC heat flux predictions for all the generic accommodation coefficients.
Embedding of Carbon Nanotubes on p-Type Silicon for Use in Solar Cells and PV Devices
Authors: Kumar Abhishek , Dhawan Nikhil
Published: June 2009

Carbon nanotube bundles were precisely grown atop a p-type silicon wafer that had been treated with catalysts to produce geometries that resemble three-dimensional nano-models to extract more power from the sun. The embedded carbon nanotubes bundles on silicon wafer promise more opportunity for each photon of sunlight to interact with resulting solar cell, as a result of increase of surface area available to produce electricity. The paper discusses morphology of grown nanotubes on silicon wafer along with future prospects of Si-CNTs fabricated solar cells.
Laminar Fully Developed Flow in Periodically Converging-Diverging Microtubes
Authors: Bahrami M. , Akbari M. , Sinton D.
Published: June 2009

Laminar fully developed flow and pressure drop in linearly varying cross -section converging-diverging microtubes have been investigated in this work. These microtubes are formed from a series of converging-diverging modules. An analytical model is developed for frictional flow resistance assuming parabolic axial velocity profile in diverging and converging sections. Frictional flow resistance is found to be only a function of the geometrical parameters. To validate the model, a numerical study is carried out for the Reynolds number ranging from 0.01 to 100, for various taper angles and maximum-minimum radius ratios ranging from 0.5 to 1. Comparisons between the model and numerical results show that the proposed model predicts the axial velocity and the flow resistance accurately. As expected, the flow resistance is found to be effectively independent of the Reynolds number from numerical results. Parametric study shows that the effect of radius ratio is more significant than the taper angle. It is also observed that for small taper angles, flow resistance can be determined accurately by applying the local Poiseuille flow approximation.
On Two Phase Flow Regimes in a Microscale Direct Methanol Fuel Cell
Authors: Weinmu¨ller Christian , Bieri Nicole R. , Poulikakos Dimos
Published: June 2009

The area of microfluidics has experienced a tremendous increase in research activities in recent years with a wide range of applications, such as micro heat exchangers and energy conversion devices, microreactors, lab-on-chip devices, micro total chemical analysis systems (μTAS) etc. The occurrence of two phase flow can lead to several mechanisms enhancing or extending the performance of single phase microfluidic devices [1]. On the other hand, in a micro fuel cell the second, non-immiscible phase is considered to hamper the performance of the fuel cell [2]. Regardless of its effect, two phase flows in microfluidics deserve special research attention.
Estimation of Nusselt Number in Microchannels of Arbitrary Cross-Section With Constant Axial Heat Flux
Authors: Sadeghi Ehsan , Bahrami Majid , Djilali Ned
Published: June 2009

In many practical instances such as basic design, parametric study, and optimization analysis of thermal systems, it is often very convenient to have closed form relations to obtain the trends and a reasonable estimate of the Nusselt number. However, finding exact solutions for many practical singly-connected cross-sections, such as trapezoidal microchannels, is complex. In the present study, the square root of cross-sectional area is proposed as the characteristic length scale for Nusselt number. Using analytical solutions of rectangular, elliptical, and triangular ducts, a compact model for estimation of Nusselt number of fully-developed, laminar flow in microchannels of arbitrary cross-sections with “H1” boundary condition (constant axial wall heat flux with constant peripheral wall temperature) is developed. The proposed model is only a function of geometrical parameters of the cross-section, i.e., area, perimeter, and polar moment of inertia. The present model is verified against analytical and numerical solutions for a wide variety of cross-sections with a maximum difference on the order of 9%.
Slip-Flow Pressure Drop in Microchannels of General Cross-Section
Authors: Tamayol A. , Bahrami M. , Taheri P.
Published: June 2009

In the present study, a compact analytical model is developed to determine the pressure drop of fully-developed, incompressible, and constant properties slip-flow through arbitrary cross-section microchannels. An averaged first-order Maxwell slip boundary condition is considered. Introducing a relative velocity, the difference between the bulk flow and the boundary velocities, the axial momentum reduces to the Poisson’s equation with homogeneous boundary condition. Square root of area is selected as the characteristic length scale. Bahrami et al.’s model, which was developed no-slip boundary condition, is extended to cover the slip-flow regime in this study. The proposed model is a function of geometrical parameters of the channel: cross-sectional area, perimeter, polar moment of inertia and the Knudsen number. The model is successfully validated against existing numerical and experimental data from different sources in the literature for several shapes, including: circular, rectangular, trapezoidal, and double-trapezoidal cross-sections and a variety of gases such as: nitrogen, argon, and helium.
Investigation of Water Transport in an Ex-Situ Experimental Facility Modelled on an Actual DOE Automotive Target Complient Fuel Cell
Authors: Lu Z. , White A. D. , Pelaez J. , Hardbarger M. , Domigan W. , Sergi J. , Kandlikar S. G.
Published: June 2009

This work utilizes the channel design of a real fuel cell to study cathode side water transport in the gas channels of a proton exchange membrane fuel cell (PEMFC). All experimentation was performed under controlled water and air flow conditions aimed to meet the DOE targets [1] for the automotive fuel cells. The experimental facility provides independent control for water flow along the length of the channels to reduce the effects of channel pressure drop on the water flow. Details of channel design and instrumentation are described, as well as some initial results.
A Critical Review of Water Transport Models in Gas Diffusion Media of PEM Fuel Cell
Authors: LaManna Jacob , Kandlikar Satish G.
Published: June 2009

Proton Exchange Membrane (PEM) fuel cells are gaining popularity as a replacement to the internal combustion engine in automobiles. This application will demand high levels of performance from the fuel cell making it critical that proper water management is maintained. One of the areas of interest in water management is the transport of water through the Gas Diffusion Medium (GDM) on the cathode side of the cell. Research is currently being conducted to understand how water moves through the porous structure of the GDM. Due to the small scale of the GDM, most work done is analytical modeling. This paper will focus on reviewing current models for water transport within the GDM of a PEM fuel cell to address state of the art and provide recommendations for future work to extend current models.
Dynamic and Thermal Evolution of Twin Inclined Jets Among a Cool Crossflow: Jets’ Inclination Impact
Authors: Radhouane Amina , Mahjoub Sai¨d Nejla , Mhiri Hatem , Lepalec George , Bournot Philippe
Published: June 2009

This paper treats the complex and very interesting “twin inclined jets in crossflow” configuration. The complexity of the problem originates from the double interaction occurring between the jets on one hand and between the jets and the crossflow on the other hand. The interest of the configuration is essentially due to its presence in more than an application (VSTOL aircrafts, chimney stacks, etc..) and in more than a field (industrial, academic, etc..). For the matter an experimental study and a numerical simulation were conducted. The experiments were carried out in a wind tunnel. The results were tracked by means of the PIV technique. That could provide us with the velocity and the turbulence cartographies. The numerical processing was based upon the resolution of the Navier-Stokes equations by means of two different turbulent models: the standard k-ε model and the RSM (Reynolds Stress Model) second order turbulent model. The confrontation of their corresponding results together with the experimental data decided on the better efficiency of the RSM model to represent well the different processes occurring within the resulting flowfield. After that, a non uniform grid particularly tightened near the injection nozzles was adopted. Once the validation reached, we could generalize our study to introduce a non reacting fume through the twin jet nozzles and to test the impact of two major factors on the mixing process; we want to mean the initial transverse inclination of the injection nozzles.
Effect of Operation Mode on Static and Dynamic Characteristics of Thermal Micro Flow Sensor
Authors: Nagayo Hidetomo , Kamiunten Shoji , Motosuke Masahiro , Honami Shinji
Published: June 2009

This paper reports the effect of the operation mode of a thermal micro flow sensor on the response characteristics. Micro Flow Sensor (MFS) is a thermal type fluid mass flow sensor fabricated by utilizing the MEMS technology. The performance of MFS greatly depends on the operation mode. The constant temperature mode provides wider measuring range and faster dynamic characteristics than the others. In the constant temperature mode, it is typically classified into two types of the control circuit, individual or integrated mode. The purpose of the present study is to clarify the effect of the operation mode on static and dynamic characteristics of MFS. Two arrangements of MFS with the different distance between the heaters are tested. In the static characteristic test, the output signal level of the individual mode is much larger than that of the integrated mode. It depends on the distance between the heaters. The dynamic characteristics are evaluated in the frequency range up to 500 Hz under the oscillating flow condition generated by an acoustic speaker in an air flow channel. As a result, the individual mode indicates faster response than the integrated mode, and it is found that the frequency response of the individual mode depends on the distance between the heaters.
Local Microflow Control Using Photothermal Viscosity Distribution
Authors: Shimakawa Jun , Motosuke Masahiro , Honami Shinji
Published: June 2009

A novel method of microflow control by locally heated liquid using an optical technique is described in this paper. Microflow control in the present study utilizes temperature dependence of the fluid property which becomes dominant in microscale. Since it is known that viscosity has strong temperature dependence, a local viscosity distribution has a potential to change microflow behavior. The purpose of the present study is to validate this concept of microflow control by using the local distribution of viscosity. In order to induce the temperature variation, photothermal effect is utilized. Absorption of a laser beam causes the local heating spot. This method has attractive features such as non-intrusive and high time- and spatial- resolution, and also has a possibility for a flexible flow control. We have developed an experimental system to irradiate focused laser beam on a flow in a microfluidic device and to measure velocity and temperature field of the microflow. The local temperature rise in microchannel flow is generated by a focused laser spot. During the laser irradiation, the velocity profile of the liquid flow in microchannel with 500 μm in width and 50 μm in height was measured by micro-PIV (Particle Image Velocimetry). The temperature measurement of the flow field was performed by micro-LIF (Laser-Induced Fluorescence). Around the heated area, the local increase of the flow velocity can be observed. It is found that the effect depends on the laser intensity and is independent of the bulk velocity. In addition, numerical simulation was conducted to identify the dominant factor causing the velocity change. As a simulation result, the cause of the velocity variation is the viscosity decrease corresponding to the temperature rise in fluid. Possibility of microflow control using the photothermal viscosity distribution is confirmed.
A Micromotor Concept
Authors: Asfar Khaled R.
Published: June 2009

A novel concept of a micromotor is presented. The idea of this micromotor is to transform the vibrational energy into rotational mechanical energy in a simple way. The principle of operation of this motor is not the same as the ultrasonic motor. The vibrational energy can be a direct mechanical or acoustical energy. It can be also used as a sensor of vibrational motion. The device consists of a pivoted fork with a straight handle. The fork ends touch the teeth of a sprocket (ratchet). The ratchet is connected to a microshaft at its center, with appropriate bearings, which delivers the rotational energy as the output of the micromotor. When the vibrational (mechanical or acoustic) energy excites the handle of the fork causing it to vibrate, which in turn exerts intermittent forces on the sprocket’s teeth causing it and the connected shaft to rotate in one direction only. It is not sensitive to the initial conditions nor to the direction of flow of vibrational energy. A macrolevel working model is constructed to demonstrate the feasibility of this micromotor. Early testing showed successful transformation of vibrational energy to rotational energy in the macromotor.
Thermal Transport Properties of Nanofluids Containing Carbon Nanotubes
Authors: Xie Huaqing , Chen Lifei , Li Yang , Yu Wei
Published: June 2009

Multiwalled carbon nanotubes (CNTs) have been treated by using a mechanochemical reaction method to enhance their dispersibility for producing CNT nanofluids. The thermal conductivity was measured by a short hot wire technique and the viscosity was measured by a rotary viscometer. The thermal conductivity enhancement reaches up to 17.5% at a volume fraction of 0.01 for an ethylene glycol based nanofluid. Temperature variation was shown to have no obvious effects on the thermal conductivity enhancement for the as prepared nanofluids. With an increase in the thermal conductivity of the base fluid, the thermal conductivity enhancement of a nanofluid decreases. At low volume fractions (<0.4 Vol%), nanofluids have lower viscosity than the corresponding base fluid due to lubricative effect of nanoparticles. When the volume fraction is higher than 0.4 Vol%, the viscosity increases with nanoparticle loadings. The prepared nanofluids, with no contamination to medium, good fluidity, stability, and high thermal conductivity, would have potential applications as coolants in advanced thermal systems.
Numerical Study of Pressure Drop in the Separator Channel and Gas Diffusion Layer of Polymer Electrolyte Fuel Cell and Deformation Effect of Porous Media
Authors: Saha Litan Kumar , Kurihara Eru , Shi Wanyuan , Oshima Nobuyuki
Published: June 2009

The flow behavior in the separator channel and gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) has been investigated by using a transient, isothermal and three-dimensional numerical model. Gas channel and gas diffusion layers are considered as the important parts of PEFC as they transport reactant gases to the catalyst layer and also byproduct from the catalyst layer. The deformation of GDL plays an important role on the performance of polymer electrolyte fuel cell since the physical properties such as porosity and permeability of the GDL and the cross sectional area of the gas channel are affected by the structural deformation of GDL. In this present investigation, non-uniform deformations shape of GDL are taken into consideration and chosen as in the experimental data. Numerical simulations are performed for a wide range of porosity and permeability values. Further, the effects of these parameters on the pressure distribution are measured. It is revealed that the increase of porosity and permeability parameter caused the decrease of pressure drop (difference of pressure from inlet and outlet) but the decreasing rate is not uniform. It is also found that there is an effective range of porosity and permeability values for which these parameters have a very strong effect on the pressure drop. The results obtained by numerical simulation are also compared with the experimental as well as theoretical solution.
Experimental Investigation of Heat Transfer to Nanofluids
Authors: Chmiel-Kurowska Klaudia , Dzido Grzegorz , Gierczycki Andrzej , Jarze˛bski Andrzej B.
Published: June 2009

Experimental investigations of convective heat transfer in nanofluid based on the Cu (approx. 0.15% and 0.25% vol.) nanoparticles synthesized in polyol process were conducted at constant heat flux conditions. A 30% increase in average heat transfer coefficient was found against the results obtained for a pure host liquid (ethylene glycol). Even more significant increase was in the entrance region.
Modelling the Influence of Wetting Properties on the Solid Liquid Impact
Authors: Do-Quang Minh , Amberg Gustav
Published: June 2009

The impact of a solid object on a free liquid surface is quite complex. This problem has challenged researchers for centuries and remains of interest today. Recently Duez et al. [1] published experimental results on the splash when a solid sphere enters a liquid. Surprisingly, a small change in the surface chemistry of the object can turn a big splash into an inconspicuous disappearance and vice versa. We study this problem by solving the Navier-Stokes together with the Cahn-Hilliard equations, [2, 3], which allows us to simulate the motion of a free air-water surface in detail, in the presence of surface tension and dynamic wetting. Quantitative computational modeling of dynamic wetting is difficult in itself, but here the use of this tool allows us to study in detail how the wetting properties determine whether a splash appears or not. Our simulated results are compared with the experiments of Duez et al.
Microfluidic Separation of Cells by Rolling on Patterned Receptors
Authors: Karnik Rohit , Hong Seungpyo , Bose Suman , Zhang Huanan , Mei Ying , Anderson Daniel G. , Karp Jeffrey M. , Langer Robert
Published: June 2009

Cell separation based on markers present on the cell surface has extensive biological applications. However, current separation methods involve labeling and label removal steps which are often slow and intrusive. We envisioned that the ability to control the direction of transport of cells based on specific receptors on the cell surface without labeling and label removal steps would enable simple continuous-flow microfluidic cell separation systems with minimal processing steps and active components. We therefore explored whether receptor patterning could be used to direct the transport of cells in a label-free manner through the formation of transient receptor-ligand bonds that result in cell rolling.
Microfluidic Synthesis of Polymeric Nanoparticles
Authors: Karnik Rohit , Gu Frank X. , Bose Suman , Basto Pamela , Cannizzaro Christopher , Langer Robert , Farokhzad Omid C.
Published: June 2009

A central challenge in the development of drug-encapsulated polymeric nanoparticles is the inability to control the nanoparticle physicochemical properties that affect their biodistribution, drug release, and efficacy. Nanoparticles may be developed by mixing and nanoprecipitation of polymers and drugs dissolved in organic solvents with non-solvents. Inadequate control over this mixing process is a source of variability in the synthesis of these nanoparticles by nanoprecipitation. We demonstrate that rapid and tunable mixing through hydrodynamic flow focusing in a microfluidic device can be used to control nanoprecipitation of poly(lactic-co-glycolic acid)-bpoly(ethylene glycol) (PLGA-PEG) diblock copolymers as a model polymeric biomaterial for drug delivery.