bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Wall Roughness and Heterogeneity of Potential on Electroosmotic Flow Within a Nano-Channel
Authors: Bhattacharyya S. , Nayak A. K.
Published: June 2009

Electroosmotic flow in micro- and nano-channels is characterized by low Reynolds number, so the inertial effects are negligible. The motivation of the present study is to generate strong convection by introducing channel wall roughness in the form of a block which has a step jump in ζ–potential on the upper face of the wall mounted block. The width and length of the channel is considered to be high compare to the height of the channel, thus a two dimensional EOF is considered in the present analysis. The present model is based on the Nernst-Plank model including convective effects. A numerical method based on the pressure correction iterative algorithm (SIMPLE) is adopted to compute the flow field and mole fraction of the ions. Potential field is computed through the SOR technique. The EOF is found to be predominantly two-dimensional with strong convectional effect near the wall roughness. The usual plug-like profile for velocity is absent near the wall roughness. The potential patch induces a strong recirculation vortex which inturn generates strong pressure gradient. The streamlines follow a tortuous path near the wall roughness. The volume flow rate enhanced significantly. We found that the ionic distribution follow the equilibrium Boltzmann distribution away from the wall roughness. But the solutions due to Boltzmann distribution and Nernst Plank model are different when inertial effects are significant. The strength of the vortex, which appears adjacent to the potential patch, increases almost linearly with the increase of overpotential above the patch.
An Extension to the Navier-Stokes-Fourier Equations by Considering Molecular Collisions With Boundaries
Authors: Arlemark Erik J. , Dadzie S. Kokou , Reese Jason M.
Published: June 2009

In this paper we propose a model for micro gas flows consisting of the Navier-Stokes-Fourier equations (NSF) extended by a description of molecular collisions with solid boundaries and discontinuous velocity slip and temperature jump boundary conditions. By considering the molecular collisions with the solid boundaries in gas flows we capture some of the near wall effects that the conventional NSF with linear stress/strain-rate and heat-flux/temperature-gradient relationships seem to be unable to describe. The model that we propose incorporates the molecular collisions with solid boundaries as an extension to the conventional definition of the average travelling distance of molecules before experiencing intermolecular collisions (the mean free path). By considering both of these types of collisions we obtain an effective mean free path expression, which varies with distance to surfaces. The effective mean free path is proposed to be used to obtain new definitions of effective viscosity and effective thermal conductivity, which will extend the applicability of NSF equations to higher Knudsen numbers. We show results of simple flow cases that are solved using this extended NSF model and discuss limitations to the model due to various assumptions. We also mention interesting ideas for further development of the model based on a more detailed gas description.
Scale Effects on Liquid Emission Phenomena During Boiling Nucleation in Mini/Micro Tubes
Authors: Wu D. , Peng X. F.
Published: June 2009

A series of experiments was conducted to visually observe the fundamental features of the scale effect on boiling nucleation in mini/micro tubes. Boiling nucleation and/or bubble generation was observed in mini tubes of 1.5 to 2.5 mm and micro tubes of 50 to 200 μm in inner diameter. As heat flux was increased, bubbles were normally observed generating and growing inside the tubes of 2.5 mm. As tube diameter decreased from 1.5 to 0.05 mm, or in mini and micro tubes, the liquid was emitted instantaneously rather than generating bubbles and bubble growth, which is referred as liquid exploding emission in this paper. In mini tubes of Di = 1.5 mm, violent jet flows with macro bubbles were formed, and the liquid was almost totally emitted out of the tube. While in micro tubes of Di = 50–200 μm, the emission was much stronger, and only fog-like jet flow was observed consisted of extremely tiny bubbles, which indicates the mechanism of nucleation is different from that in mini tubes. It is expected that the scale effects intensely influence on not only the onset of emission, but also the strength and nature of emissions.
Impinging Flow as a Model for Atherosclerosis
Authors: Gentile John P. , King Michael R.
Published: June 2009

Impinging flow in the vascular system is found at bifurcations and long curved vessels, and also in the pathological state of stenosed vessels. These are also the most prevalent regions of the vascular system for the progression of atherosclerosis. To model the in vivo vascular geometry we modified a parallel-plate flow chamber for in vitro experiments. This new impinging flow chamber recreates the local fluid streamlines of flow separation near a stagnation point in Stokes flow. We have mapped out the near wall x- and y- velocity components of this impinging flow chamber using 1.9 μm diameter fluorescent tracer beads. The near-wall flow velocity shows good agreement with the theoretical analysis of Heimenz’s solution for 2-D plane impinging flow, and with a 3-D computational fluid dynamics simulation of the model impinging flow chamber.
Development of Rapid Manufacturing Technology of Polymer Microfluidic Devices by Micro Moulding Using Silicon Mould Inserts
Authors: Zhao J. , Ong R. , Chen G. , Juay Y. K. , Ng F. L. , Lee M. W. , Kua C. H.
Published: June 2009

Plastics are the choice materials for fabrication of many microfluidic devices as mass production methods such as the injection moulding and hot embossing processes can be used for disposable polymer device fabrications. However, fabrication of mould inserts with micro features is a very critical step towards successful moulding of polymer micro components. Due to technical limitations many micro features cannot be fabricated directly onto metal mould inserts. Silicon based micro machining technologies, on the other hand, are well developed and are capable of producing very fine micro structures. In the study, silicon insert moulding method is developed for fabrication of polymeric biochips. Although it has been reported to use silicon dies for hot embossing applications, careful consideration of mould design is required to use fragile silicon inserts in moulding applications where higher mould clamping forces are encountered. Silicon inserts instead of traditional metal inserts, are successfully manufactured and used for polymer biochip moulding. It has been found that the silicon inserts can last for tens of shots if properly installed in an injection mould.
An Experimental and Numerical Study of a Micro-Synthetic Jet in a Shallow Cavity
Authors: Sinclair Alexander , Timchenko Victoria , Reizes John , Rosengarten Gary , Leonardi Eddie
Published: June 2009

By disrupting laminar flow, micro-scale synthetic jets have the potential to significantly increase mixing and heat transfer rates in micro-devices. Due to the difficulty involved in performing measurements on the micro-scale, few experimental studies of micro-synthetic jets exist. In this paper we describe instantaneous velocity fields obtained by μPIV measurements in the vicinity of a synthetic jet orifice 24 μm in diameter issuing into a confined geometry. Numerical results for a synthetic jet operating under similar conditions have been used to help validate and clarify the experimental results. Comparisons between the experimental and numerical results during the expulsion phase of the actuator cycle for a synthetic jet with a Reynolds number (based on maximum velocity), Re = 239 and Stokes number, S = 9, indicate there is good agreement, thereby demonstrating that the μPIV technique can be used successfully for future studies. Experimental difficulties encountered are presented and methods of overcoming them discussed.
Modeling and Characterization of an Industrial Inkjet Head for Micro Patterning on Printed Circuit Boards
Authors: Kim Changsung Sean , Sim Wonchul , Kim Young-Jae , Yoo Youngseuck
Published: June 2009

A conceptual design using computational fluid dynamics (CFD) and micro-electro-mechanical systems (MEMS) fabrication has been performed to develop an industrial inkjet head for micro-patterning on printed circuit boards. The printhead has been fabricated with silicon and silicon on insulator (SOI) wafers by MEMS process and silicon to silicon bonding method. The measured displacement waveform from piezoelectric actuator by Laser Doppler Vibrometer (LDV) was used as input data for the three-dimensional flow solver to simulate the droplet formation. The mechanism of droplet ejection from piezoelectric-type inkjet heads was investigated by simulating two-phase flows of the air and metal inks. Parametric studies are followed by the design optimization process to deduce key factors to inkjet head performance. The effects of nozzle geometry, pulse amplitude, ink viscosity, and micro bubble formation were also investigated based on numerical simulations. The present design tool based on two-phase flow solver and experimental measurements has shown its promising applicability to various concept designs of industrial inkjet system for micro-patterning on electronic chips and boards.
Thermal Rectification in Silicon Nanowires With Mass Gradients
Authors: Wang Shuai-Chuang , Liang Xin-Gang , Xu Xiang-Hua
Published: June 2009

Thermal rectification as a new phenomenon is attracting great attention. Thermal rectification in silicon nanowires with axial mass gradient is investigated by molecular dynamics simulation. The results of the simulations show that the thermal conductivities are different for the heat flux with opposite directions. The rectification efficiency becomes larger when the mass gradient increases. The effect of temperature gradient on the thermal rectification is also considered. The phonon density of states is calculated to explain the phenomenon. It is found that the interface is responsible to the thermal rectification.
Pool Boiling Heat Transfer on Vertical Micro Porous Coated Surface in Confined Space
Authors: Yang Chien-Yuh , Liu Chien-Fu
Published: June 2009

Attributed to its high heat transfer coefficient, evaporating cooling involving the use of micro heat exchangers is considered a possible thermal management solution for cooling of high heat flux electronic devices. The present work desires to develop high-performance micro heat exchangers operating in the evaporation regime. The pool boiling heat transfer performance on one plain plate and one micro porous coated plate were tested in a vertical open and a 1-mm confined spaces. The test results show that the heat transfer was enhanced by the confined space at low and moderate heat fluxes but degraded at high flux condition on plain surface. The micro porous coating may significantly enhance the pool boiling performance. However, the heat transfer characteristic in confined space is not exactly the same as that on open surfaces. Owing to the interaction of forced removal of the superheated liquid due to the bubble departure and retard the departure of bubbles by the confined plate, there is no much difference for pool boiling heat transfer on micro porous coated surface in confined and unconfined spaces at low and moderate heat fluxes. At high heat flux, large amount of bubbles were confined by the cover plate. This caused the partial dry out and significant degrade on heat transfer performance.
Numerical Study of Marangoni: Thermocapillary Convection Influence During Boiling Heat Transfer in Minichannels
Authors: Radulescu Cristina , Robinson Anthony J.
Published: June 2009

Marangoni thermocapillary convection and its contribution to heat transfer during boiling has been the subject of some debate in the open literature. Currently, for certain conditions, such as microgravity boiling, is being shown that has a significant contribution to heat transfer [1]. Typically, this phenomenon is investigated for the idealized case of an isolated and stationary bubble resting atop a heated solid which is immersed in a semi-infinite quiescent fluid or within a two-dimensional cavity. However, little information is available with regard to Marangoni heat transfer in miniature confined channels in the presence of a cross flow. As a result, this paper presents a numerical study that investigates the influence of steady thermal Marangoni convection on the fluid dynamics and heat transfer around a bubble during laminar flow of water in a minichannel with the view of developing a refined understanding of boiling heat transfer for such a configuration. This mixed convection problem is investigated for channel Reynolds numbers in the range of 0 ≤Re ≤500 and Marangoni numbers in the range of 0 ≤ Ma ≤ 17114. The influence of the thermocapillary flow is most pronounced for low Re and high Ma numbers showing an average of 40% increase in heat transfer. For low Ma and high Re inertial effects dominate and the thermocapillary effect is not as noticeable. However, the disruption of the fully developed flow does tend to enhance the heat transfer at the expense of additional pressure drop.
Numerical, Experimental and Robust Design Investigations of a Grooved Heat Pipe in Microgravity-Like Conditions
Authors: Goffaux Ce´cile , Van Oost Ste´phane , Barremaecker Laurent
Published: June 2009

In this paper, we describe a hydraulic model able to predict the maximum heat transport capacity of grooved heat pipes for microgravity and gravity assisted applications. We compare numerical results and experimental measurements obtained on a heat pipe in microgravity-like conditions. To reproduce the 0-g environment, a setup based on the rotation of the heat pipe was built up. A good agreement has been found between the code and the experiment. We discuss the influence of the manufacturing uncertainties on the global performance of the heat pipe. By a genetic algorithm technique, we perform a robust design analysis to estimate the maximum and the minimum heat pipe performances in the geometric range imposed by the manufacturing process.
Optimization of Transitional Nano-Channel Cooling Heat Sink Using a Genetic Algorithm
Authors: Kamali Reza , Movahed Saeid
Published: June 2009

Fluid flow in micro/nano-scales channels has received much research interest due to its seemingly endless applications. By entering technology to the area of micro and nano scales, the design and fabrication of miniaturized instruments, such as microelectronics devices, MEMS, NEMS and ..., become very desirable. In such small sizes, the performance of the devices can be affected by some undesired small scale world phenomena. One of them is the generated heat in miniaturized devices that has suicidal effect on the performance of the devices and must be removed. The traditional cooling methods, such as free convection of air, are not enough efficient for cooling of these devices, therefore, other effective methods must be considered. Using nano- and micro-channels as heat sinks is one of the most effective methods for cooling the miniaturized devices. Increasing cooling performance and decreasing pressure drop along the channels are two important factors in designing nanochannels as a heat sink. Optimized design of cooling systems has significant effect on improving cooling performance and energy management of the devices. In this study, using a Genetic Algorithm technique, it is desired to find the optimized design for the cooling performance of transitional nano-channel heat sink.
Natural Convection in a Micro Size Horizontal Cylindrical Annulus With Constant Volumetric Heat Flux
Authors: Mansour Kamyar
Published: June 2009

We consider the two-dimensional problem of steady natural convection in a narrow (Micro size) horizontal cylindrical annulus filled with viscous fluid and constant volumetric heat flux. The solution is expanded in powers of a single combined similarity parameter, which is the product of the gap ratio to the power of four, and Grashof number and the series extended by means of symbolic calculation up to 14 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is almost zero but Pade approximation lead our result to be good even for much higher value of the similarity parameter.
Transient Analysis of a Liquid Flow in Microchannel of Varying Cross-Section
Authors: Dituba Ngoma Guyh , Sadiki Amsini , Erchiqui Fouad
Published: June 2009

In this study the transient analysis of a laminar liquid flow in a three-dimensional microchannel of varying rectangular cross-section was numerically investigated. The imposed heat fluxes on the microchannel upper and bottom wall, the Joule heating due to an applied electric potential at the microchannel inlet and outlet, the electroosmosis, the pressure-driven flow, and the liquid temperature-dependent thermophysical properties were accounted for. The time-dependent liquid flow velocity profile and the time-dependent liquid temperature distribution were obtained respectively solving the Navier-Stokes equations and the energy equations for a laminar incompressible liquid flow using a computational fluid dynamics code. The effects of the microchannel divergence angle and the microchannel dimensions on the liquid flow and the heat transfer in the microchannel were analyzed. Results obtained show significant influences of the pressure difference between the microchannel inlet and outlet, the imposed heat flux, and the microchannel inlet width on the transient and steady states of the liquid flow velocity and the liquid temperature distribution. A comparison of the results of the developed model with those achieved considering the liquid constant thermophysical properties and those obtained from a microchannel of a constant cross-section was made.
Effects of Interfacial Position on Drag Reduction in a Superhydrophobic Microchannel
Authors: Enright Ryan , Dalton Tara , Krupenkin Tom N. , Kolodner Paul , Hodes Marc , Salamon Todd R.
Published: June 2009

The use of superhydrophobic surfaces in confined flows is of particular interest as these surfaces have been shown to exhibit a drag reduction effect that is orders of magnitude larger than those due to molecular slip. In this paper we present experimental results of the pressure-driven flow of water in a parallel-plate microchannel having a no-slip upper wall and a superhydrophobic lower wall. Pressure-drop versus flow-rate measurements characterize the apparent slip behavior of the superhydrophobic surfaces with varying pillar-to-pillar pitch spacing and pillar diameter. The superhydrophobic surface consists of a square array of cylindrical pillars that are fabricated by deep reactive ion etching on silicon and coated with a hydrophobic fluoropolymer. A major challenge, in correlating our experimental results with existing theoretical predictions, is uncertainty in the location of the gas/liquid interface and the associated gas/liquid/solid contact line within the pillar features comprising the superhydrophobic surface. We present experimental results, from laser-scanning confocal microscopy, that measure the location of the gas-liquid interface and associated contact line for fluid flowing through a parallel-plate microchannel. Knowledge of the contact line location is then used to correlate experimental pressure-drop versus flow-rate data with a theoretical model based on porous-flow theory that takes into account partial penetration of liquid into a superhydrophobic surface.
The Effects of Water Injections in Wet Steam Flow in Different Regions of a Mini Laval Nozzle
Authors: Mahpeykar M. R. , Amirirad E. , Lakzian E.
Published: June 2009

Progress in the development of the steam turbines brings about a renewal of interest in wetness associated problems. In turbine steam expansion, the vapour first supercools and then condenses spontaneously to become a two phase mixture. The flow initially is single phase but after Wilson point water droplets are developed and there is a non equilibrium two phase flow. The formation and behavior of the liquid create problems that lower the performance of the turbine wet stage and the mechanisms underlying this are insufficiently understood. This growing droplets release their latent heat to the flow and this heat addition to the supersonic flow cause a pressure rise called condensation shock. Because of irreversible heat transfer in this region the entropy will increase tremendously. Removal of condensates from wet steam flow in the last stage of steam turbines significantly promotes stage efficiency and prevents erosion of rotors. The following study investigates the spraying water droplets at inlet and at throat of mini Laval nozzle and their effects on nucleation rate and condensation shock. According to the results, the nucleation rate is considerably suppressed and therefore the condensation shock nearly disappeared. In other words the injecting droplets would decrease the thermodynamic losses or improve the turbine efficiency.
Wettability Effect on Flow Boiling in an MEMS-Based Single Glass Microchannel
Authors: Choi Chiwoong , Huh Cheol , Kim Dongeok , Kim Moohwan
Published: June 2009

Many studies have investigated boiling heat transfer in microchannels; however, such phenomena are not yet fully understood, and conflicting results have been reported. There are many important parameters that govern behavior in microsystems, including channel shape, roughness, and choice of material. One potentially important parameter is hydrophobicity of the microchannel surface, as interfacial forces play an increasing role at microscales. We developed a new method of fabricating a single glass rectangular microchannel and microheater using the MEMS (Micro-Electro-Mechanical System) fabrication technique. The glass was used as a hydrophilic surface, and a self-assembled monolayer was coated with OTS (Octadecyl-Trichloro-Silane) to obtain a hydrophobic surface. We conducted an experiment of boiling heat transfer in a microchannel using two surfaces of contrasting hydrophobicity. The resulting flow was observed using a long-distance microscope and a high-speed camera. This approach enables a qualitative analysis of the hydrophobicity effect on the flow regime.
Evaporating Interface Oscillation of Subcooled Flow Boiling in Locally Heated Microchannels
Authors: Liu Jiang-Tao , Peng Xiao-Feng
Published: June 2009

An investigation was conducted to understand flow boiling of de-ionized water in locally heated parallel microchannels. High speed visualization technology was employed to visually observe the transient phase change process in an individual microchannel. The process at locally heated condition was different from those at entirely heated condition where vapor column(s) stayed quasi-stable for a long time without venting out. Intensive interface oscillation occurred during phase change. Thin film evaporation appeared to be the main evaporation mode at the upstream vapor column cap, whereas nucleation happened occasionally at favorable flow, heat transfer and local wall conditions. Continuous condensation at the downstream vapor column cap led the flow back to single liquid phase regime out of the microchannel. A signal analysis method was introduced to study the interfacial oscillation behavior at the evaporating cap, which showed marked impact of the evaporation mode on the characteristic frequency.
Numerical Simulation of Heat and Mass Transfer Enhancement in Absorption Process With Double-Side Film-Inversion
Authors: Chen Ya-Ping , Shi Chen-Jie , Shi Ming-Heng , Ling Chen-Min
Published: June 2009

Film-inversion is an effective way recently developed to enhance heat and mass transfer in absorbers. However, only one-side of round or rectangular tube i.e. half of the total heat transfer area is used to form film-inverting configuration in the published literature. The paper presents a double-side film-inverting scheme, which consists of two plate bundles and a set of comb shaped conjunction guiders between them for leading solution film from both-sides of each couple of the upper plate bundle to the opposite sides of the bottom ones. A two-scale crosswise corrugation plate bundle, which has vertical large corrugations and horizontal small ones, is suggested instead of the plane plate bundle. The horizontal small corrugation can make the film turbulent and film distribution uniform before and after inversion with surface tension effect, thus increasing the heat and mass transfer coefficients of the absorption process. A mathematic model for heat and mass transfer in absorption process with aqueous Li-Br solution falling film-inverting on two sequential vertical plane plates was established and solved numerically. The distributions of dimensionless velocity, temperature and concentration of liquid film profile before and after film-inverting were obtained. The influence of the number of inversion on heat and mass transfer characteristics was analyzed. The calculation results show that the heat and mass transfer coefficients of the once-film-inverting scheme have about 58% and 73% increment respectively over these of the none film-inverting scheme.
Non-Equilibrium Studies of Molecular Flow Through Carbon Nanotubes
Authors: Cannon James , Hess Ortwin
Published: June 2009

Carbon nanotubes are likely to form an integral part of future nano-fluidic devices. In order to realise such devices, an understanding of the dynamics of molecular flow through nanotubes is crucial. We have conducted continuous-flow non-equilibrium molecular dynamics simulations of argon and hydrogen flow through nanotubes, in order to decipher the fundamental driving forces behind the flow dynamics, upon which the motions of more complex molecules are based. We detail the fundamental mechanisms of flow in the nano-confined space of a carbon nanotube, and demonstrate how this knowledge can be utilised to control the flow-rate and even convert from smooth to pulsed flow.
Hydrodynamic and Thermal Performance of a Vapor-Venting Microchannel Copper Heat Exchanger
Authors: David Milnes P. , Marconnet Amy , Goodson Kenneth E.
Published: June 2009

Two-phase microfluidic cooling has the potential to achieve low thermal resistances with relatively small pumping power requirements compared to single-phase heat exchanger technology. Two-phase cooling systems face practical challenges however, due to the instabilities, large pressure drop, and dry-out potential associated with the vapor phase. Our past work demonstrated that a novel vapor-venting membrane attached to a silicon microchannel heat exchanger can reduce the pressure drop for two-phase convection. This work develops two different types of vapor-venting copper heat exchangers with integrated hydrophobic PTFE membranes and attached thermocouples to quantify the thermal resistance and pressure-drop improvement over a non-venting control. The first type of heat exchanger, consisting of a PTFE phase separation membrane and a 170 micron thick carbon-fiber support membrane, shows no improvement in the thermal resistance and pressure drop. The results suggest that condensation and leakage into the carbon-fiber membrane suppresses venting and results in poor device performance. The second type of heat exchanger, which evacuates any liquid water on the vapor side of the PTFE membrane using 200 ml/min of air, reduces the thermal resistance by almost 35% in the single-phase regime in comparison. This work shows that water management, mechanical and surface properties of the membrane as well as its attachment and support within the heat exchanger are all key elements of the design of vapor-venting heat exchangers.
Numerical Study of the Flow and Mass Transfer in Micromixers
Authors: Ait Mouheb Nassim , Solliec Camille , Montillet Agnes , Comiti Jacques , Legentilhomme Patrick , Havlica Jaromir
Published: June 2009

Computational fluid dynamic simulations are used to characterize the flow and the liquid mixing quality in a micromixer as a function of the Reynolds number. Two micromixers are studied in steady flow conditions; they are based on two geometries, respectively T-shaped and cross-type (⊤ and + shapes). Simulations allow, in the case of ⊤ micromixers, to chart the topology of the flow and to describe the evolution of species concentration downstream the intersection. The streamline layout and the mixing quality curves reveal the three characteristic types of flow, depending on Reynolds number: stratified, vortex and engulfment flows. Vortices appear after impingement, in the exit channel. They become asymmetrical and gain in length with an increase in Re making the flow unsteady, which induces an enhancement of the mass transfer by advection between the two liquids. In the case of cross-type micromixers, the structure of the flow is strongly three-dimensional. It is characterized by symmetrical vortices in both output channels. In the zone close to the impingement, a back flow is observed which induces strong shear stresses. The results show that the + shaped system can improve the mixing process in comparison with the micromixers having ⊤ geometry. The numerical study also allows to select the locations of the most relevant zones of study, from an experimental point of view. It will allow to choose the location of PIV planes and local non intrusive sensors, such as electrochemical microprobes, in order to experimentally investigate the flow.
Velocity Correlations Between Impinging and Reflecting Particles Using MD Simulations and Different Wall Models
Authors: Spijker Peter , Markvoort Albert J. , Nedea Silvia V. , Hilbers Peter A. J.
Published: June 2009

The replacement of the the explicit wall in Molecular Dynamics (MD) simulations of nanochannels with an appropriate wall model is important to model the behavior of the gas close to the channel wall correctly. In this paper a comparison of four different wall models with explicit wall MD simulations is reported in the case of a gas confined between two plates of different temperature. Accommodation coefficients are computed from the explicit wall MD simulations using different techniques. Examples are given that for the same system different methods may produce different accommodation coefficients. The computed accommodation coefficients are subsequently used as input to some of the wall models. From comparison of simulations with explicitly modelled walls or using wall models it is shown that not only is it important to look at the distribution of outgoing velocities, but also at the correlation of incoming and outgoing velocities.
Molecular Dynamics for Fluid Mechanics in Arbitrary Geometries
Authors: Macpherson Graham B. , Reese Jason M.
Published: June 2009

Simulations of nanoscale systems where fluid mechanics plays an important role are required to help design and understand nano-devices and biological systems. A simulation method which hybridises molecular dynamics (MD) and continuum computational fluid dynamics (CFD) models is able to accurately represent the relevant physical phenomena and be computationally tractable. An MD code has been written to perform MD simulations in systems where the geometry is described by a mesh of unstructured arbitrary polyhedral cells that have been spatially decomposed into irregular portions for parallel processing. The MD code that has been developed may be used for simulations on its own, or may serve as the MD component of a hybrid method. The code has been implemented using OpenFOAM, an open source C++ CFD toolbox (www.openfoam.org). The requirements for two key enabling components are described. 1) Parallel generation of initial configurations of molecules in arbitrary geometries. 2) Calculation of intermolecular pair forces, including between molecules that lie on mesh portions assigned to different, and possibly non-neighbouring processors. A case study of flow in a realistic nanoscale mixing channel, where the geometry is drawn and meshed in engineering CAD tools is simulated to demonstrate the capabilities of the code.
Mesoscopic Simulations of Flow in Microchannel and Comparison With Continuum Model
Authors: Kumar Anurag , Asako Yutaka , Faghri Mohammad
Published: June 2009

Dissipative particle dynamics (DPD) is a mesoscopic particle-based simulation method, where each particle represents a group or packet of actual molecules of the flow field. Despite its usefulness, a fundamental problem of the DPD is that there is no straightforward procedure to relate the soft-particle model to a realistic continuum or molecular model. In the present work, we studied the dynamics of a simple fluid flow in a microchannel based on the coarse-graining of the molecules and expressed DPD units in terms of real units. To compare DPD methodology with continuum theory, a relation between the viscosity of the fluid and dissipative coefficient of DPD was established. DPD equations and parameters were expressed in non-dimensional forms and related to the known hydrodynamic parameters such as Reynolds number and Peclet number. Poiseuille flow of water in microchannel of height 35 μm was modeled for different parameters. Coarse-graining for water was ranged from 5×109 to 5×1010 for each DPD particle. Re and Pe of the flow were varied from 0.7 to 60 and 5 to 140, respectively. Simulated results were compared to the continuum results with a good agreement.
An Experimental Study of Friction Factor in the Transition Region for Single Phase Flow in Mini- and Micro-Tubes
Authors: Ghajar Afshin J. , Rao Rahul P. , Cook Wendell L. , Tang Clement C.
Published: June 2009

A systematic and accurate experimental investigation of friction factor in the transition region for single phase flow in mini- and micro-tubes has been performed for eight stainless steel tubes with diameters ranging from 2083 μm to 667 μm. The pressure drop measurements were carefully performed by paying particular attention to the sensitivity of the pressure-sensing diaphragms used in the pressure transducer. Experimental results indicated that the start and end of the transition region was influenced by varying the tube diameter. The friction factor profile was not significantly affected for the tube diameters between 2083 μm and 1372 μm. However, the influence of the tube diameter on the friction factor profile became noticeable as the diameter decreased from 1372 μm to 667 μm.
DSMC Solution of Supersonic Scale to Choked Subsonic Flow in Micro to Nano Channels
Authors: Roohi Ehsan , Darbandi Masoud , Mirjalili Vahid
Published: June 2009

In this study, the supersonic and choked subsonic flows through micro/nano channels are investigated using direct simulation Monte Carlo (DSMC) method. The supersonic case is simulated at different Knudsen numbers covering slip to transition flow regimes, while the effects of inlet Mach and back pressure are studied in details. The inlet/outlet pressure boundary conditions are suitably implemented benefiting from the basics of characteristics theory. A behavior similar to the one predicted by the Fanno theory is observed here; i.e., the supersonic flow velocity decelerates up to a choking condition where any further increase in Knudsen number is impossible unless strong normal/oblique shocks appear at the inlet and the inlet conditions change to the subsonic ones. However, a subsonic flow appears near the outlet section if one imposes a back pressure lower than the ordinary exit pressure at the outlet. Our investigation showed that applying the back pressure boundary condition right at the real channel exit would overwhelm the solution. A more realistic behavior can be achieved by inserting suitable buffer zone beyond the real channel exit, where the back pressure is applied there. This strategy results in capturing a more realistic physics of flow at the channel outlet and enforces choking condition at the outlet.
Heat Transfer Analysis of Force Fed Evaporation on Microgrooved Surfaces
Authors: Cetegen Edvin , Dessiatoun Serguei , Ohadi Michael
Published: June 2009

Force Fed Evaporation (FFE) technique was parametrically analyzed using refrigerant R245fa over a 1cm2 microgrooved surface. The heat transfer tests were performed so as to isolate the effects of heat flux, mass flux, and saturation pressure and to identify the dominant heat transfer mechanism in FFE. In particular, results were obtained for selected values of mass flux with various heat flux levels, as well as for selected values of heat flux with various saturation pressure levels. Tests were performed for heat fluxes up to 562W/cm2 and qualities up to 29%. For these conditions, it was found that heat transfer is mainly dependent on heat flux, and not on mass flux or quality. This further implies that the dominant heat transfer mechanism is nucleate boiling.
Development of a Closed-Loop Heat-Pipe for Heat Recovery With Mini-Channels Enhancing Evaporation
Authors: Duarte Rocha Bruno , Reyes Mazzoco Rene´
Published: June 2009

A vast number of industrial processes require the addition and removal of heat at different stages of the same process. To accomplish heat recovery there are several technological options with specific characteristics. Thermal integration has been the focus of recent industrial developments to reduce variable costs through new investments when using fossil fuels for thermal energy generation. The conventional equipment for heat recovery uses conduction and convection heat transfer mechanisms and high surface area of a metal wall (normally subjected to corrosion and fouling) for the contact of the cold and hot fluids. Heat pipes are an option for heat recovery with less metal surface area (and therefore lower tendency to corrosion and fouling) because of their higher thermal conductivity, but have been used mostly for heat removal in electronic equipment. This study develops a closed-loop heat pipe configuration for heat recovery in industrial operations starting with a configuration for desalination. The variables of the heat pipe operation that were considered include the mini-channels structures associated to the evaporator section, the effect of the inclination of condenser to evaporator, the working fluids (ethanol-water mixtures, acetone, methanol, and isopropanol), and the internal pressure of the heat pipe (atmospheric and vacuum). The highest heat flux values (3.2 W/cm2) were obtained with a single layer mini-channels structure, 90° of inclination between the condenser and the evaporator, and a mixture of 16% ethanol in water as working fluid in the loop-heat-pipe because this mixture had the highest heat transfer parameter (that describes the capacity of the fluid to perform in a heat pipe) among the fluids tested and corresponded to the highest heat transfer rate measured.
Modeling and Preliminary Experiment for Rarefied Gas Flows in Constricted Microchannels
Authors: Guo Xiaohui , Huang Chihyung , Alexeenko Alina , Sullivan John P.
Published: June 2009

Gaseous slip flows in 3D rectangular microchannels with constrictions have been study numerically, and the experiment using pressure-sensitive-paints (PSP) for polymer microchannel pressure measurements are proposed. Constrictions inside microchannels, either being manufacturing defects or functional design features such as micro-orifices or micro-nozzles, will change the flow pattern because of additional frictional resistance and flow separation. In current research, mass-flowrate reduction due to constrictions has been investigated numerically for air flows in the slip regime, where Knudsen number ranges from 0.003 to 0.07. The results have been compared with both straight microchannels and with 3D analytical solutions. Similar to nozzle cases at macroscale, chocked flows has been observed at the critical pressure ratio of about 1.89. A numerical model including finite inlet and outlet chambers has been used in simulations to evaluate effects of reflection waves. Slip effects have been studied for different accommodation coefficients in presence of constrictions. By implementing multi-species numerical models, thermal induced mass transport has also been studied. Preliminary experiment based on PSP measurement for polymer microchannels has able to generate high spatial resolution pressure data, which are comparable with numerical simulations. Finally, further improvement of experimental setup is discussed.
A Hybrid Particle-Continuum Framework
Authors: Borg Matthew K. , Reese Jason M.
Published: June 2009

A new hybrid particle-continuum numerical code is currently being developed as an engineering tool for accurate and fast computational modelling of nanoflows. Molecular Dynamics (MD) and Computational Fluid Dynamics (CFD) are the components/solvers used within the particle and continuum zones respectively. In this paper the development of a two-component hybrid framework, based on domain-decomposition, is described. The main objective of the framework is to facilitate hybrid MD-CFD simulations within complex geometries, using a mesh of structured/unstructured arbitrary polyhedral cells, identical to that used in engineering CFD. This requires complex three-dimensional (3D) interfaces and overlap regions (comprising user-defined sub-regions) to be constructed between adjacent zones. The individual sub-regions serve as an appropriate means of exchanging information between components (i.e. coupling or boundary condition imposition), in 3D, during the hybrid simulation. The global domain is decomposed appropriately into MD and CFD sub-domains such that internal boundaries within the overlap regions become the external boundaries on the separate meshes, prior to commencing the hybrid simulations. The hybrid framework is implemented in OpenFOAM [1], an open source C++ CFD toolbox, using a general, case-independent approach and is parallelised. Two nanochannel test cases are investigated to show that the hybrid environment is flexible and well-suited for engineering design applications as well for the development of new hybrid codes and coupling models.
Experimental Investigation of Critical Heat Flux Enhancement by Micro/Nanoscale Surface Modification in Pool Boiling
Authors: Kim Seontae , Kim Hyungmo , Kim Hyung Dae , Ahn Ho Seon , Kim Moo Hwan , Kim Joonwon , Park Goon-Cherl
Published: June 2009

Nanofluids, which contain uniformly and stably dispersed nanoparticles, exhibit an abnormal enhancement of the critical heat flux (CHF) when used as a working fluid in pool boiling. It has recently been demonstrated that optimal CHF enhancement in nanofluids is attained by the significant deposition of nanoparticles on the heater surface during pool boiling. The surface deposition of oxidized metal nanoparticles significantly enhances the wettability, and fractal micro/nanostructures formed by nanoparticle deposition induce liquid suction due to capillary wicking. It is supposed that the superior wettability and capillary wicking of the nanoparticle-fouled surface enhances CHF by promoting the dry patches to be effectively rewetted during the boiling process. In this regard, the excellent CHF performance of the nanoparticle-deposited surface can be reproduced using artificial structures via innovative surface-modification methods that yield good wettability and capillarity. To accomplish this goal, we plan to design and fabricate various artificial micro/nano-structured surfaces with good surface wettability and capillarity, and investigate their CHF performance. In the present study, we examined experimentally the CHF performances of a series of surface-modified samples (plane, micro-structured, nano-structured, and micro/nano structured surfaces). Pool boiling heat transfer of pure water on sample surfaces was investigated under atmospheric conditions. The CHF increase due to artificial surface modification is discussed based on solid-liquid interfacial parameters (static contact angle, roughness) that are closely related to CHF phenomenon in pool boiling.
Time Optimization of Heterogeneous Immunoassay Using PDMS Microfluidic Chip for Quantitative Detection of a Model Analyte
Authors: Li Peng , Abolmaaty Assem , Barek Thomas , Anagnostopoulos Constantine , Faghri Mohammad
Published: June 2009

Heterogeneous immunoassay (HI) has been recently used for on-chip biomolecule detection by many researchers due to its high specificity and precision. However, the protocols that have been published are too time-consuming. For HI based immunosensor to be attractive they should have short analysis times. In our work, a polydimethylsiloxane (PDMS) microfluidic device was fabricated to quantify a model analyte (bovine IgG) and used for optimizing the time required for each process. The results show that the incubation time of one single process could be reduced from typical 1 hour to 5 minutes without loss of performance. Our studies reveal that microfluidic devices are only effective in reducing incubation times for diffusion limited processes but not for interaction or adsorption limited process.
Investigation of a Steam Reforming System of Methane Integrated With a Combustion Burner
Authors: Park Joonguen , Lee Shinku , Bae Joongmyeon , Kim Myungjun
Published: June 2009

The objective of this study is to analyze the steam reforming system using numerical method. The system consists of a cylindrical-type steam reformer and a combustion burner. Heat is supplied to the endothermic steam reformer by the combustion gases which flow around the reformer. Eddy Break-Up (EBU) model is incorporated for the combustion reaction, and pseudo-homogeneous model is used for the steam reforming reaction. The temperature at the reformer center and the concentration of species at the outlet are compared with the measured data for code validation. The correlation between the performances and the shapes of the system has been studied by using two different configurations. One has the flame guide between the combustion burner and the steam reformer, and the other does not. The flame guide makes the flow of the combustion gas changed. The operating parameters are reactant flow rates which are supplied to the steam reformer and the combustion burner. Reactor temperature profiles, heat transfer rates, fuel conversion, and the hydrogen yields are calculated as the numerical results. Moreover, fuel feed ratio between the burner and the reformer is also manipulated as an operating parameter to discuss about efficiency.
Sheath Free Three-Dimensional Particle Focusing Using Electro-Hydrodynamics
Authors: Kim Young Won , Yoo Jung Yul
Published: June 2009

A conventional flow cytometry is a crucial and versatile instrument that measures the fluorescence and light scattering of individual cell and other particulate or molecular analytes in biomedical research. Recently, there have been many efforts to achieve this conventional flow cytometry on lab-on-a-chip devices powered by microfluidics. One of the key techniques of this instrument is to hydrodynamically focus samples within the sensing area of optical systems in order to optimize detection signals. Recent three-dimensional focusing methods using MEMS technology contain sheath flows with very complex channel geometries. In the present investigation, we demonstrated 3-D particle focusing in a single straight micro-capillary without sheath flows using electro-hydrodynamics. Particle beam was successfully made with a beam diameter of about 10 μm for rigid particles. Proposed device provides critical solutions for simple and innovative 3-D particle focusing method for the applications to the MEMS-based micro-flow cytometry.