bluuebunny/crossref_metadata_2025 huggingface dataset:
Self‐Determination: A Buffer Against Suicide Ideation
Authors: Bureau Julien S. , Mageau Geneviève A. , Vallerand Robert J. , Rousseau François L. , Otis Joanne
Published: May 2012

Self‐determination was examined as a protective factor against the detrimental impact of negative life events on suicide ideation in adolescents. It is postulated that for highly self‐determined adolescents, negative life events have a weaker impact on both hopelessness and suicide ideation than for non‐self‐determined adolescents. In turn, hopelessness is hypothesized to generate less suicide ideation for highly self‐determined individuals. Results from multigroup analyses confirm that both the direct and indirect links between negative life events and suicide ideation were significantly weaker among participants high in self‐determination. The protective role of self‐determination against negative life events is discussed.
Condensation of homomorphism spaces
Authors: Lux Klaus , Neunhöffer Max , Noeske Felix
Published: May 2012

We present an efficient algorithm for the condensation of homomorphism spaces. This provides an improvement over the known tensor condensation method which is essentially due to a better choice of bases. We explain the theory behind this approach and describe the implementation in detail. Finally, we give timings to compare with previous methods.
Sequential B-Spline Surface Construction Using Multiresolution Data Clouds
Authors: Yuan Yuan , Zhou Shiyu
Published: May 2012

B-spline surfaces are widely used in engineering practices as a flexible and efficient mathematical model for product design, analysis, and assessment. In this paper, we propose a new sequential B-spline surface construction procedure using multiresolution measurements. At each iterative step of the proposed procedure, we first update knots vectors based on bias and variance decomposition of the fitting error and then incorporate new data into the current surface approximation to fit the control points using Kalman filtering technique. The asymptotical convergence property of the proposed procedure is proved under the framework of sieves method. Using numerical case studies, the effectiveness of the method under finite sample is tested and demonstrated.
Assessment of Infant Movement With a Compact Wireless Accelerometer System
Authors: Gravem D. , Singh M. , Chen C. , Rich J. , Vaughan J. , Goldberg K. , Waffarn F. , Chou P. , Cooper D. , Reinkensmeyer D. , Patterson D.
Published: May 2012

There is emerging data that patterns of motor activity early in neonatal life can predict impairments in neuromotor development. However, current techniques to monitor infant movement mainly rely on observer scoring, a technique limited by skill, fatigue, and inter-rater reliability. Consequently, we tested the use of a lightweight, wireless, accelerometer system that measures movement and can be worn by premature babies without interfering with routine care. We hypothesized that this system would be useful in assessing motor activity, in identifying abnormal movement, and in reducing the amount of video that a clinician would need to review for abnormal movements. Ten preterm infants in the NICU were monitored for 1 h using both the accelerometer system and video. A physical therapist trained to recognize cramped-synchronized general movements scored all of the video data by labeling each abnormal movement observed. The parameters of three different computer models were then optimized based on correlating features computed from accelerometer data and the observer’s annotations. The annotations were compared to the model’s prediction on unseen data. The trained observer identified cramped-synchronized general movements in 6 of the 10 infants. The computer models attained between 70% and 90% accuracy when predicting the same observer label for each data point. Our study suggests that mini-accelerometers may prove useful as a clinical tool assessing patterns of movement in preterm infants.
Numerical Simulation of a Novel Jet-Impingement Micro Heat Sink With Cross Flow Under Non Uniform Heating Condition
Authors: Fan Yanfeng , Hassan Ibrahim
Published: May 2012

A novel micro heat sink applying the jet-impingement and cross flow is proposed to dissipate the heat from the electrical devices. Six hotspots of 2 mm × 2 mm are positioned on a flat plate of 25.4 mm × 25.4 mm. The area of flat plate except the hotspots is provided a constant heat flux of 20 W/cm2 as background heating source among cases. Four heat fluxes from 40 to 100 W/cm2 on the hotspots are tested to simulate the different operation conditions. The cross flow is used to remove the background heat flux and jet flow is supplied into the swirl microchannel, located at the right top of hotspot, to dissipate the large heat flux from hotspots. The channel depth is 0.5 mm and the width of swirl microchannel is 0.38 mm. The cross flow and jet flow velocity vary from 0.1 m/s to 0.5 m/s and from 0.5 m/s to 2 m/s, respectively. The effects of cross flow and jet flow on the cooling performance are investigated by numerical simulation. The local heat transfer coefficient and Nusselt number are calculated to evaluate the cooling performance of proposed micro heat sink for the targets of low maximum temperature, temperature gradient and pressure drop. The results show that the maximum temperature of the proposed design occurred at the outlet is approximately 65 °C among tested cases. The corresponding pressure drop is 5.5 kPa. The overall thermal resistance reaches as small as 0.23 K/W.
Simulations of Micro Gas Flows by the DS-BGK Method
Authors: Li Jun
Published: May 2012

For gas flows in micro devices, the molecular mean free path is of the same order as the characteristic scale making the Navier-Stokes equation invalid. Recently, some micro gas flows are simulated by the DS-BGK method, which is convergent to the BGK equation and very efficient for low-velocity cases. As the molecular reflection on the boundary is the dominant effect compared to the intermolecular collisions in micro gas flows, the more realistic boundary condition, namely the CLL reflection model, is employed in the DS-BGK simulation and the influence of the accommodation coefficients used in the molecular reflection model on the results are discussed. The simulation results are verified by comparison with those of the DSMC method as criteria.
Thermal Transport Characteristics of Non-Newtonian Electroosmotic Flow in a Slit Microchannel
Authors: Babaie Ashkan , Sadeghi Arman , Saidi Mohammad Hassan
Published: May 2012

Electroosmosis has many applications in fluid delivery at microscale, sample collection, detection, mixing and separation of various biological and chemical species. In biological applications, most fluids are known to be non-Newtonian. Therefore, the study of thermal features of non-Newtonian electroosmotic flow is of great importance for scientific communities. In the present work, the fully developed electroosmotic flow of power-law fluids in a slit microchannel is investigated. The related equations are transformed into non-dimensional forms and necessary changes are made to adapt them for non-Newtonian fluids of power-law model. Results show that depending on different flow parameters like Debye-Hu¨ckel or related viscous dissipation and Joule heating parameters, non-Newtonian characteristics of the flow may lead to significant deviations from Newtonian flow behaviors.
Micromixer: Alternative Passive Solution With Injection Amplificator System
Authors: Dennai Brahim , Khelfaoui Rachid , Bentaleb A. hak , Maazouzi A. hak
Published: May 2012

Mixing rate is characterized by the diffusion flux given by the Fick’s law. A passive mixing strategy is proposed to enhance mixing of two fluids through perturbed jet flow. A numerical study of passive mixers has been presented. This paper is focused on the modeling of a micro-injection systems composed of passive amplifier without mechanical part. The microsystem modeling is based on geometrical oscillators form. An asymmetric micro-oscillator design based on a monostable fluidic amplifier is proposed [2,7]. The characteristic size of the channels is generally about a few hundred of microns. The numerical results indicate that the mixing performance can be as high as 92% within a typical mixing chamber of 2.25 mm diameter and 0.20 mm length when the Reynolds number is Re = 490. In addition, the results confirm that self-rotation in the circular mixer significantly enhances the mixing performance. The novel micro mixing method presented in this study provides a simple solution to mixing problems in micro system.
Geometrical Dimension Optimizations for “V”-Shaped Micro-Grooves for Enhancing Heat Transfer Purpose in Micro-Channels
Authors: Liu Yang , Li Weizhong , Cui Jing
Published: May 2012

Heat transfer performance of water flowing over “V”-shaped micro-channel with different dimensions is studied using two three-dimensional numerical methods, i.e. traditional CFD (computational fluids dynamics) and LBM (lattice Boltzmann method) in this paper. The inlet flow is considered to be laminar. We altered the inclined angle and the height of the “V”-shaped micro-groove and simulated the corresponding water flow in it. The CFD code is based on the platform of Fluent and the LB method is self coded. Simulation results show that the two approaches can produce reasonable and agreed results. It also suggests that in our geometrical dimension varying range, micro-channel heat transfer performance increases with the micro-groove height monotonically. However, with the increase of inclined angle, the heat transfer performance decreases firstly and increases secondly. This can be intuitively explained as the result of changed surface area. We introduced the novel concept of field synergy principle to have an insight view of the convective flow field and temperature result. It is found that it is the synergy level between velocity and temperature gradient that finally results in the different heat transfer performance for different sized “V”-shaped micro-grooves. The pressure loss is also investigated using the two methods proposed in this paper. It is found that the pressure loss varies for different sized micro-grooves, and the pressure loss results exhibit a similar trend with that of the temperature field. This implies that to gain better heat transfer performance by altering the geometrical dimensions, the corresponding cost is quite considerable pressure loss. The widely studied perpendicularity “V”-shaped micro-channel has the lowest heat transfer performance compared with the inclined ones depicted in this study, but its pressure loss is the lowest. In contrast, the structure with the best heat transfer performance in this research possesses the largest pressure loss.
Molecular Dynamics Simulation of Water Transporting Through Nanotube Driven by Concentration Difference
Authors: Zhang Ning , Li Weizhong , Cui Jing
Published: May 2012

A flowing water chain in a (6, 6) carbon nanotube (CNT) driven by concentration difference is studied by molecular dynamics (MD) simulation. Water molecules in the CNT form a continuous water chain, which occupy the space of a 2 Å radius along the axis of channel. By computing the trajactory from the simulation run, the water density profile along the CNT is obtained and the flow behavior of water in the CNT is studied. The simulated results show that the density distribution in the CNT is lower than that in the bulk water and the solution, but the free energy distribution appears a contrary tendency. In addition, the quantity of hydrogen-bonds (H-bonds) forming in the CNT appears a fluctuation along with time, by analyzing which, it is found that the formation of H-bonds in the CNT is related to flow rate of water.
Computational Investigation of Microdroplet Formation in a Crossflow Membrane Emulsification Process
Authors: Pathak Manabendra
Published: May 2012

Monodisperse microdroplets are formed, when a liquid is injected through a micropore into another immiscible liquid. Depending on the relative flow between the two phases, droplets may form in quiescent, coflowing and crossflowing environment. The dispersions of one phase liquid in another crossflowing liquid are observed in liquid emulsification process and the system has been used extensively in microfluidic devices to produce monodisperse microdroplets with controllable size. Liquid emulsions are widely used in food, cosmetics, pharmaceutics and polymer industries. In the present work, microdroplet formation in a crossflow membrane emulsification process has been investigated computationally using VOF/finite volume method. The full transient simulation has been carried out starting from the injection of dispersed phase to breakup into drops for different values of dispersed phase and continuous phase flow rate, surface tension and viscosity ratio of both the phases. Depending upon the values of the both phases, the droplet formation process shows the dripping and jetting behavior. The qualitative features of the two regimes and their transition have been correlated with different non-dimensional numbers such as Capillary number, Weber number and viscosity ratio of the two phase liquids. Some interesting nonlinear behavior such as period doubling been observed near the transition between the dripping and jetting regimes has. The topological characteristics of dripping, jetting and transition regimes in membrane emulsification have been observed different than in the cases of T-junction emulsification and flow focusing emulsification. Two ways of dripping to jetting transition have been observed, one with the increasing dispersed phase flow rate at constant continuous phase flow rate and other way is reducing the surface tension at constant dispersed phase flow rate. The effect of inertia force has been observed negligible for high value of surface tension and significant for lower surface tension value.
Local Friction Factor of Compressible Laminar or Turbulent Flow in Micro-Tubes
Authors: Murakami Shintaro , Asako Yutaka
Published: May 2012

Laminar/turbulent flows of compressible fluid in microtubes were simulated numerically to obtain the effect of compressibility on the local pipe friction factors. For gaseous flows, the effect of compressibility had not been clarified except for laminar flow whose Mach number is less than 0.45, so the present work extended this to handle higher speed flows including choked ones and turbulent flows. The numerical procedure based on arbitrary-Lagrangian-Eulerian method solves two-dimensional compressible momentum and energy equations. The Lam-Bremhorst Low-Reynolds number turbulence model was adopted to calculate eddy viscosity coefficient and turbulence energy. The physical domain of simulation with the back region downstream from the outlet of the micro-tube was used to be able to calculate the case of under-expansion flow in the tube. The orthogonal curvilinear grid was used for the computational mesh to obtain accurate results. The computations were performed for a wide range of Reynolds number and Mach number including laminar/turbulent choked flows. It was found that in laminar regimes the ratio of the Darcy friction factor to its conventional (incompressible flow’s) value is a function of Mach number and the same goes for the Fanning friction factor. On the other hand, in turbulent regimes, the ratio is still a function of Mach number for the Darcy friction factor but is equal to about unity for the Fanning friction factor. Namely, the Fanning friction factor of gaseous flow in micro-tubes coincides with Blasius formula, even when Mach number is not small and compressibility effect appears. This fact can be seen in choked flow.
Investigation of Cooling Performance of a Swirl Microchannel Heat Sink by Numerical Simulation
Authors: Fan Yanfeng , Hassan Ibrahim
Published: May 2012

High heat fluxes have been created by the semiconductor devices due to the high power generation and shrank size. The large heat flux causes the circuit to exceed its allowable temperature and may experience both working efficiency loss and irreversible damage due to excess in their temperatures. In this paper, a swirl microchannel heat sink is designed to dissipate the large heat flux from the devices. The numerical simulation is carried out to investigate the cooling performance. Uniform heating boundary condition is applied and single phase water is selected as coolant. The present micro heat sink applies multiple swirl microchannels positioned in a circular flat plate to enhance the heat convection by creating the secondary flow at high Reynolds numbers. Copper is selected as the material of heat sink. The channel depth and width are fixed as 0.5 mm and 0.4 mm, respectively. The heat is injected into the system from the bottom of heat sink at the heat fluxes from 10 to 60 W/cm2. Flow is supplied from the top of micro heat sink through a jet hole with a diameter of 2 mm and enters swirl microchannels at the volume flow rates varying from 47 to 188 ml/min. The cooling performances of swirl microchannel heat sinks with different curvatures and channel numbers are evaluated based on the targets of low maximum temperature, temperature gradient and pressure drop.
Molecular Dynamics Simulation of Nano Channel as Nanopumps
Authors: Darbandi Masoud , Abbasi Hossein Reza , Khaledi-Alidusti Rasoul , Sabouri Moslem , Schneider Gerry E.
Published: May 2012

We use three-dimensional molecular dynamics simulation to investigate the driven flow between two parallel plates separated by argon atoms. Our simulations show that fluids in such channels can be continuously driven. Difference in surface wettability can cause a difference in fluid density along the nano channel. To control the nanochannel temperature walls, we use the thermal wall idea, which models the walls using atoms connected to their original positions by enforcing linear spring forces. In this study, we propose a nanochannel system in which, half of the channel has a low surface wettability, while the other half has a higher surface wettability and that the middle part of channel wall has a high temperature. In another test case, we study a channel with a high temperature at one side of channel and impose a low temperature at the other side. Imposing a high temperature at the middle of nanochannel breaks the molecular force balances and a driven flow is formed in the channel due to the difference in fluid density. Also, imposing a temperature gradient at the walls causes a momentum difference between the atoms on the opposite sides of channel, which is a reason for driving flow through the channel. We use these molecular dynamics tools to achieve better volumetric results in the nanochannel.
Influence of Deposits by Crystallization Fouling in Microchannels on the Heat Transfer Performance of Micro Heat Exchangers
Authors: Bucko Ju¨rgen , Mayer Moriz , Benzinger Walther , Augustin Wolfgang , Dittmeyer Roland , Scholl Stephan
Published: May 2012

In the last few years many new microstructured devices were developed and manufactured for different applications in chemical industry and for process engineering. However, the use of micro structured devices in the chemical industry is still rare in spite of the potential benefits these devices offer. One essential reason for the limited use of micro structured devices could be the intrinsic advantage of these structures: the small dimension. In applications of micro structured devices it might be possible that the microchannels get blocked by contaminations in the fluid or that deposition (fouling) could induce flow restrictions in the microchannels. Fouling is defined as the deposition of material on surfaces, e.g. by chemical reactions or phase transition due to the crystallization of inversely soluble ions. This can yield in a reduction of the cross section of the microchannels up to total blocking. Furthermore, fouling can cause a degradation of the heat transfer performance, an increase of the pressure drop in the device, a change of the fluid distribution in the micro structure and a shorter residence time of the fluid. Experimental investigations concerning crystallization fouling in micro structures were carried out at the Institute for Micro Process Engineering (IMVT) of the Karlsruhe Institute of Technology (KIT) and at the Institute for Chemical and Thermal Process Engineering (ICTV) of the Technische Universita¨t Braunschweig. The intention of these investigations is the development of experimentally verified and generally applicable correlations to describe the crystallization fouling in micro heat exchangers.
Microfabrication Technologies Based on Electro-Dispensing
Authors: Nichols Jacqueline , Born Brandon , Landry Emily L. , Holzman Jonathan F.
Published: May 2012

A microfabrication technology with real-time polymer processing control is introduced in this work. The technique is titled electro-dispensing, as it employs a metal micro-dispensing tip that is biased with a user-defined voltage. The in-situ voltage directs an electric field through the underlying dispensed polymer structure. Polymer droplets are dispensed directly onto a chip with precise (pL) volumes, and the in-situ micro-dispensing tip voltage is varied to adjust the polymer droplet morphology during the microfabrication process. The technique is carried out within a glycerol ambient filler solution, to create an initial high contact angle (160°) on the polymer microdroplets, and voltage tunability is applied to control the microdroplet shapes. Ultraviolet curing is subsequently employed to solidify the micro-spheroid structures on the desired locations across the chip. The electro-dispensing process is demonstrated in this work for numerous microdroplets, with a variety of polymer morphologies and diameters down to 150 μm. The capabilities of the electro-dispensing process are also demonstrated in this work for a specific application relating to integrated photonic circuitry. Polymer microdroplets in the past have been limited to use as lenses for vertical beam focusing (through the plane of the chip), because of their exceedingly low contact angles on solid surfaces. In this work, polymer micro-droplets are introduced for lateral beam focusing and retroreflection (above and parallel to the plane of the chip). These new technologies for on-chip optical beam dispersion management are brought about by the capabilities of electro-dispensing: the use of an ambient filler allows the dispensing process to create high-contact-angle near-spherical microdroplets; the electro-dispensing process then allows this droplet to be tuned for its specific role within the integrated photonic chip (e.g. as a spherical element for in-plane focusing or an elliptical element for in-plane retroreflection). Ray-based analyses and electromagnetic models are used to characterize the optical responses of the micro-spheroid structures, and the results are compared to experimental measurements with on-chip laser beam control.
Simulation of Two Designs of Micro-Mixers With Enhanced Advection Mechanisms
Authors: Kazemi S. A. , Passandideh-Fard M. , Esmaeelpanah J.
Published: May 2012

In this paper, a numerical study of two new designs of passive micro-mixers based on chaotic advection is presented. The advection phenomenon in a T-shaped micro-mixer is enhanced using a segmented gas-liquid flow; and a peripheral/axial mixing mechanism. The simulations are performed for two non-reactive miscible gases: oxygen and methanol. The numerical model employed for this study is based on the solution of the physical governing equations namely the continuity, momentum, species transport and an equation to track the free surface development. The equations are discretized using a control volume numerical technique. The distribution of the species concentration within the domain is calculated based on which a mixing intensity factor is introduced. This factor is then used as a criterion for the mixing length. In the first micro-mixer design with a drop injection mechanism for a typical condition, the mixing length is reduced by nearly 15%. Compared to that of a simple T-shaped micro-mixer with the same flow rates, the two gases interface area is increased in axisymmetric micro-mixer leading to an increase of the mixing efficiency and a reduction of the mixing length. Also, the effects of the baffles height and span on the mixing efficiency and length in axisymmetric micro-mixer are studied. Having baffles in the channel can substantially decrease the mixing length.
Optical Measurement of the Relative Motion of a Spherical Particle in a Micro-Capillary
Authors: Sen Debjyoti , Nobes David S. , Bhattacharjee Subir , Mitra Sushanta K.
Published: May 2012

The motion of a neutrally buoyant spherical micro particle suspended in a liquid inside a ‘U’ shaped micro channel is detected using particle tracking velocimetry. The ratio of the particle diameter to the hydraulic diameter of the channel is found to be 0.1. Poiseuille velocity profile is observed inside the micro channel and the spherical particle is convected with the Poiseuille velocity field. The magnitude of the lag factor is determined experimentally and is compared with the values reported in existing literature. The effect of other forces and interactions on the particle is quantitatively studied. It was found that effects of Brownian, Van der Waals and electrostatic forces do not significantly contribute to the velocity of the particle.
On the Validity of Two-Dimensional Heat Transfer Simulation of Moving Droplets Between Parallel Plates
Authors: Talimi V. , Muzychka Y. S. , Kocabiyik S.
Published: May 2012

Use of moving droplets between two parallel plates has been investigated widely in recent years for cooling purposes. While the real shape of the droplets is a cylinder with curved side (convex or concave) i.e. a three dimensional shape, most of the researchers assumed a two-dimensional computational domain including vertical mid plane of the droplet, which is applicable for not realistic long droplets. In this paper, the differences between these two approaches are investigated numerically, using ANSYS Fluent package.
Thermal Interaction of Laser Beam With Particulate Flow in Mini-Channels
Authors: Zabetian Mohammad , Saidi Mohammad Said , Saidi Mohammad Hassan , Shafii Mohammad Behshad
Published: May 2012

Optical propulsion via laser source is a relatively new and non-contact tool for manipulation of microscopic objects. The method is based on the radiation pressure of light photons on the micron sized particles. Applications of the technique mainly cover microscopic separation, purification and cellular studies. Due to high power intensity of laser beams, absorption of light may result in heating and damage of objects to be manipulated. In addition, the difference between heated and cold zones can lead to a naturally driven flow around the objects. So precisely controlled conditions should be set up to avoid thermal effects. In this work, a theoretical study is conducted to investigate the thermal effects on a particulate flow in a mini-channel. Thermal effects are studied as a result of light absorption either in carrier fluid or dispersed phase. A continuous laser beam is considered as a radiation heat source and wave propagation and interaction with suspension of microspheres in water is simulated. The main objective of mentioned simulation is to determine the conditions of pure hydrodynamic laser-particle interaction. Especially the criteria for properties of particles and beam parameters are presented. Derivation of limits is based on the maximum allowable temperature increase and the magnitude of thermal induced flow. The results are crucial for design and selection of elements in microscopic separation by laser beam.
The Effects of Geometry and Knudsen Numbers on Micro- and Nanochannel Flows
Authors: Kim J. H. , Frijns A. J. H. , Nedea S. V. , van Steenhoven A. A.
Published: May 2012

In this work we use a three dimensional Molecular Dynamics simulation method to study the effect of different geometries and Knudsen number regimes on the gas flow in micro-nanochannels. Argon molecules have been used for the simulations. Thermal wall and diffusive-specular wall types were used for the boundaries of the channels. The velocity profiles in the channel were obtained and analyzed with three different channel geometries that are commonly used in the industry: circular, rectangular (square), and slit channel. We found that when using the same driving force, the maximum velocity of the flow increases when the geometry changes in the order from circular geometry to rectangular geometry to slit geometry, where the latter becomes 1.2∼1.5 times as large compared with either the rectangular or circular channel. While the absolute values of the velocity profiles show a distinct difference according to the different geometries, geometry effect on the shape of the velocity profile also shows interesting features. Rectangular tube shows much flatter profile compared with the other two channels. Also the effect of the size of the channels and different Knudsen numbers on the velocity profiles is investigated. Two different sizes were used here: 100nm and 10nm corresponding to typical sizes of a nano channel and carbon nanotubes. We found that the Knudsen number has an effect on the slip and maximum flow velocity for the slit geometry even for higher Knudsen number. For the Kn higher than approximately 3, it was found that the Knudsen number has a small influence on the slip flow velocity for the circular channel and rectangular channel than for lower Knudsen number.
Effects of Channel Length on Gas-Liquid Two-Phase Flow Phenomena in a Microchannel
Authors: Ide Hideo , Kimura Ryuji , Kawaji Masahiro
Published: May 2012

An optical measurement system was used to investigate the effect of microchannel length on the characteristics of adiabatic gas-liquid two-phase flow. Experiments were conducted with a 1,676 mm long, circular microchannel with an inner diameter of 100 μm. Two-phase flow patterns, void fraction and velocities of gas plug/slug and liquid slugs were measured at different locations between the gas-liquid mixer and channel exit. The experimental values of the mean void fraction and the mean velocity of liquid slug agreed well with the homogeneous flow model predictions when the liquid flow rate was constant and the mass velocity of the gas was low. The flow pattern transition from slug flow to ring film flow was observed when the mass velocity of the gas was increased while the liquid flow rate was kept constant.
Homogeneous Oxidation of Hydrogen in Catalytic Mini/Microchannels
Authors: Zade Azad Qazi , Renksizbulut Metin , Friedman Jacob
Published: May 2012

Gas phase reaction effects in the catalytic oxidation of hydrogen on platinum-coated minichannels and microchannels are investigated numerically in planar geometry. The main objective of this work is to identify the relative importance of the gas phase and surface reactions under different operating conditions. A collocated finite-volume method is used to solve the governing equations. Detailed gas phase and surface reaction mechanisms along with a multi-component diffusion model are used. As the channel size is reduced, heat and radical losses to the walls can significantly alter the combustion behavior. While catalytic walls help in sustaining the gas phase reactions at very small length scales by reducing the heat losses to the walls owing to heat release associated with the surface reactions, they may inhibit homogeneous reactions by extracting radicals due to typically high absorption rates of such species at the walls. Thus, the radical chain mechanisms can be significantly altered by the presence of wall reactions, and the build-up of radical pools in the gas phase, which lead to homogeneous ignition, can be suppressed as a consequence. In the present study, the effects of two key parameters, i.e. channel height and the inlet mass flux on the interaction of gas phase and surface reactions will be explored. In each case, the limiting values beyond which the gas-phase reactions become relatively negligible compared to surface reactions will be identified for hydrogen/air mixtures.
Modeling of Electroosmotic Nanoflows With Overlapped Double Layer
Authors: Nosrati Reza , Raisee Mehrdad , Nourbakhsh Ahmad
Published: May 2012

In the present paper a new model is proposed for electric double layer (EDL) overlapped in nanochannels. The model aimed to obtain a deeper insight of transport phenomena in nanoscale. Two-dimensional Nernst and ionic conservation equations are used to obtain electroosmotic potential distribution in flow field. In the proposed study, transport equations for flow, ionic concentration and electroosmotic potential are solved numerically via finite volume method. Moreover, Debye-Hu¨ckle (DH) approximation and symmetry condition, which limit the application, are avoided. Thus, the present model is suitable for prediction of electroosmotic flows through nanochannels as well as complicated asymmetric geometries with large nonuniform zeta potential distribution. For homogeneous zeta potential distribution, it has been shown that by reduction of channel height to values comparable with EDL thickness, Poisson-Boltzmann model produces inaccurate results and must be avoided. Furthermore, for overlapped electric double layer in nanochannels with heterogeneous zeta potential distribution it has been found that the present model returns modified ionic concentration and electroosmotic potential distribution compare to previous EDL overlapped models due to 2D solution of ionic concentration distribution. Finally, velocity profiles in EDL overlapped nanochannels are investigated and it has been showed that for pure electroosmotic flow the velocity profile deviates from the expected plug-like profile towards a parabolic profile.
Parametric Analysis of Microdroplet Formation in Co-Axial Co-Flowing Immiscible Liquids in Microtubes
Authors: Yang In-Hwan , El-Genk Mohamed S.
Published: May 2012

This paper presents numerical results of disperse liquid droplets forming in the dripping regime at the tip of a microtube into another co-flowing immiscible liquid in a coaxial microtube of larger diameter. Investigated are the effects of the interfacial surface tension, velocities and viscosities of the liquids and the diameters of the coaxial microtubes on the forming dynamics and the size of the droplet. The 2-D, transient Navier-Stockes equations, in conjunction with the momentum jump condition across the interface between the co-flowing liquids are solved using a finite element method. The solution tracks the interface and the growth of the droplet and predicts droplet size and forming frequency. The droplet’s dimensionless radius (rd*) is correlated within ± 10% in terms of the continuous liquid capillary number (Cac) and ratios of Reynolds numbers (Red/Rec) and microtube radii (Rc/Rd) of the co-flowing liquids as: rd*=0.225R*0.466/(Cac0.5)(Red/Rec).0.05
Coupling Kinetic and Continuum Equations for Micro Scale Flow Computations
Authors: Croce Giulio , Rovenskaya Olga
Published: May 2012

An hybrid method, coupling the direct numerical solution of the Bhatnagar-Gross-Krook (BGK) kinetic equation and a Navier-Stokes model is presented. The computational physical domain is decomposed into kinetic and continuum sub-domains using an appropriate criteria based on the local Knudsen number and proper gradients of macro-parameters, computed via a preliminary Navier-Stokes solution throughout the whole physical domain. The coupling is achieved by matching half fluxes at the interface of the kinetic and Navier-Stokes domains, thus taking care of the conservation of momentum, energy and mass through the interface. The proposed method is used for the simulation of the flow through a micro-slit. Outlet to inlet pressure ratio of 0.1, 0.5 and 0.9 are considered, for a wide range of Knudsen number. The local parameters (density, velocity and temperature) along symmetry axis show satisfactory agreement with those computed by the continuum model.
Study on the Characteristics of Contact Line and Liquid Film in Rectangular Microgrooves Under Vibration Conditions
Authors: Cao Wei , Hu Xuegong , Guo Chaohong , Tang Dawei
Published: May 2012

With the help of a high-speed camera (30000 Frames/second) and a wide-field stereo-microscope, the effects of mechanical vibration on the meniscus film and triple-phase contact line in rectangular microgrooves were experimentally investigated. Distilled water was used as working liquid. The images of the oscillated meniscus film in an oscillation period were captured through the high speed camera and they were analyzed using a MATLAB program. The results show that as the vibration table moves upward, the length of contact line increases; as the vibration table moves downward, the length of contact-line decreases. During the oscillation, the axial liquid film spreads upward further along the microgrooves and the deformation of the contact line becomes more obvious. The increase of the triple-phase contact line length caused by the external mechanical vibration is helpful for contact line heat transfer enhancement. Besides, deformation curve of the contact line with and without heat input under different vibration conditions is similar, while the contact line with heat input is shorter.
Nanostructures Length Effect on Phase Transition Phenomena of Ultra-Thin Liquid Film From a Nanostructured Surface: A Molecular Dynamics Study
Authors: Morshed A. K. M. M. , Paul T. C. , Khan Jamil A.
Published: May 2012

A molecular dynamics simulation has been employed to investigate the boiling phenomena of few molecular-layer thin liquid-film adsorbed on a nanoscale roughened solid surface. The molecular system comprises of three phase system: solid platinum wall, liquid argon and argon vapor. A few layer of liquid argon has been placed on the nanoposts decorated solid surface where nanoposts ensemble surface roughness. Nanoposts height has been varied keeping liquid film thickness constant to capture three scenario: (i) Liquid-film thickness is higher than the height of the nanoposts (ii) Liquid-film and nanoposts are of same height (iii) Liquid-film thickness is less than the height of the nanoposts. Rest of the simulation box space has been filled with argon vapor. The simulation starts from the equilibrium three phase system and then suddenly the wall is heated to a higher temperature which resembles an ultra fast laser heating. Two different jump temperatures has been selected: one is a few degrees above the boiling point to initiate normal evaporation and the other one is far above the critical point temperature to initiate explosive boiling. Simulation results indicate nanostructures play significant role in both the cases. Argon responds very quickly in the nanoposts decorated surface and evaporation rate increases with the nanoposts height. Different boiling behavior has been observed for the nanoposts decorated surface.
An Experimental Investigation on Heat Transfer Characteristics of Air and CO2 in Microtubes
Authors: Chen Chia-Wei , Lin Ting-Yu , Yang Chien-Yuh , Kandlikar Satish G.
Published: May 2012

Several researches dealing with the single-phase forced convection heat transfer inside micro channels have been published in the past decades. The performance of liquid flow has been proved that agree with the conventional correlations very well (Yang and Lin [2007]). However, owing to the low heat transfer coefficient of gaseous flow, it is more difficult to eliminate the effects of thermal shunt and heat loss than water flow while measuring its heat transfer performance. This study provides an experimental investigation on forced convective heat transfer performance of air and gaseous carbon dioxide flowing through two microtube with inner diameter of 920 μm. A non-contacted liquid crystal thermography (LCT) temperature measurement method that proposed by Lin and Yang [2007] was used in this study to measure the surface temperature of microtube. The test results show that the conventional heat transfer correlations for laminar and turbulent flow can be well applied for predicting the fully developed heat transfer performance in microtubes while taking account of the compressibility effect of high pressure gaseous flow in micro tubes. There is no significant difference between CO2 and air in both heat transfer and friction.
CFD Analysis on Taylor Slug Flow Through 3D Vertical Mini-Channel
Authors: Mehta Hemant B. , Banerjee Jyotirmay , Bambhania Mehul P. , Desai Jay B.
Published: May 2012

The hydrodynamic behavior of a gas-liquid flow through circular mini-channel is studied. The gas-liquid flow in Taylor slug flow regime for a mini-channel is modeled using volume of fluids method. The Taylor slug length has direct influence on radial heat transfer and heat transfer rate. So it is important to study the characteristic of such flow regimes. The simulation is carried out first with 2D geometry. The results are then obtained in order to appreciate the effect of third dimension on hydrodynamics of Taylor’s slug flow. The effect of different inlet geometries is studied by varying the inlet angle to the main channel which includes 60°, 90°, 120° and 180° angular orientations. The effect of different gas and liquid velocities (0.01m/s, 0.025m/s, 0.05m/s, 0.1m/s), the effect of channel size (0.5mm, 1mm, 2mm, 4mm), and the effect of diverse contact angles (0°, 30°, 60°, 90°, 120°, 135°) are simulated and studied. Simulation results show that surface tension plays a dominant role in determining the slug properties. Liquid wall contact angle is important parameter in determining the slug shape and size. This study will be useful in proper selection and configuration of mini-channel for gas-liquid mass transfer and reaction.
Investigation of Wall Effect on Flow Characteristics in Supersonic Rarefied Gas Flow Past a Confined Square Cylinder Using the DSMC Method
Authors: Joneidipour Mohamad M. , Kamali Reza
Published: May 2012

In the present study, the effect of wall in supersonic rarefied gas flow past a square cylinder is numerically studied. Therefore, a supersonic rarefied gas flow over a square cylinder is simulated first. Then, the simulations are repeated for a square cylinder confined between two parallel plates. In both cases, the Mach number distribution in the flow field of supersonic rarefied gas over the square cylinder is obtained using the direct simulation Monte Carlo method. Close inspection of contour lines of Mach number over the square cylinder shows that a discontinuity in the flow field occurs across the shock wave at the slip regime while there is no discontinuity at the transition flow regime. In the present paper, the effect of blockage ratio (defined as the distance between two parallel plates divided by the cylinder length) on the Mach number distribution in the flow field of supersonic rarefied gas over the square cylinder is also studied. Meanwhile, the obtained results from both mentioned cases are compared to each other. It is found that the deviation of two sets of data diminishes gradually as the blockage ratio increases.
DSMC Simulation of Supersonic Gas Flow in Microchannel
Authors: Joneidipour Mohamad M. , Kamali Reza
Published: May 2012

The present study is concerned with the flow characteristics of a microchannel supersonic gas flow. The direct simulation Monte Carlo (DSMC) method is employed for predicting the density, velocity and temperature distributions. For gas flows in micro systems, the continuum hypothesis, which underpins the Navier-Stokes equations, may be inappropriate. This is because the mean free path of the gas molecules may be comparable to the characteristic length scale of the device. The Knudsen number, Kn, which is the ratio of the mean free path of the gas molecules to the characteristic length scale of the device, is a convenient measure of the degree of rarefaction of the flow. In this paper, the effect of Knudsen number on supersonic microchannel flow characteristics is studied by varying the incoming flow pressure or the microchannel height. In addition, the microchannel height and the incoming flow pressure are varied simultaneously to investigate their effects on the flow characteristics. Meanwhile, the results show that until the diffuse reflection model is used throughout the microchannel, the temperature and the Mach number in the microchannel entrance may not be equal to free-stream values and therefore a discontinuity appear in the flow field.
Investigation of Argon Flow Induced by Temperature Difference Using the Molecular Dynamics Simulation
Authors: Radmehr Payam , Kamali Reza
Published: May 2012

In the present work, Molecular Dynamics technique is performed to study free convection in the square cavity. Simulated system is a 3-dimensional box containing fluid atoms. The walls of box are solid and formed in FCC. Fluid and solid atoms are chosen as argon and platinum respectively. The pair potential between fluid-fluid and fluid-wall atoms is governed by truncated Lennard-Jones (6–12) potential as these molecules have well-established molecular potential interaction. The periodic boundary condition (PBC) is applied in only one direction being perpendicular to the simulation surface. It helps ignore the effects of wall in this dimension. Half of the box is remained in a constant temperature (T1) and the temperature of opposite half is set to different value (T2). The velocity field is obtained and its dependency to the temperature ratio (T1/T2) is investigated. Also, we are eager to obtain the effect of gravity force on the velocity fields.
Computational Simulation of Magnetic Drug Targeting in Human Body
Authors: Kamali Reza , Keshavarzi Gholamreza
Published: May 2012

Development of novel particle carrier methods has led to enhanced advances in targeted drug delivery. This paper has aimed the investigation of targeting drugs via attached magnetic particles into human body. This goal was approached by inducing a magnetic field near a specific part of the human body to target the drug or as it is called magnetic drug targeting (MDT). Blood flow and magnetic particles are simulated under the presence of the specified properties of a magnetic field. In order to demonstrate a more realistic simulation, the flow was considered pulsatile. Finally, the results provided show valuable information on magnetic drug targeting in human body.
Viscous Dissipation and Variable Properties Effect on Two Dimensional Conjugate Heat Transfer of Nanofluids in Microchannels
Authors: Ramiar A. , Ranjbar A. A.
Published: May 2012

Laminar two dimensional forced convective heat transfer of Al2O3–water nanofluid in a horizontal microchannel has been studied numerically, considering axial conduction, viscous dissipation and variable properties effects. The existing criteria in the literature for considering viscous dissipation in energy equation are compared for different cases and the most proper one is applied for the rest of the paper. The results showed that nanoparticles enhance heat transfer characteristics of the channel and inversely, viscous dissipation causes the Nusselt number and friction factor to decrease. The viscous dissipation effect may be emphasized by increasing Reynolds number and decreased by raising the exerted heat flux. Also, it was found that there is a critical Reynolds number below which the average Nusselt number of the nanofluid changes abnormally with Reynolds number as a result of variable properties effect.