bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Triangular Roughness Elements on Pressure Drop and Laminar-Turbulent Transition in Microchannels and Minichannels
Authors: Brackbill Timothy P. , Kandlikar Satish G.
Published: September 2008

Roughness elements affect internal flows in different ways. One effect is a transition from laminar to turbulent flow at a lower Reynolds number than the predicted Re = 2300. Initial work at RIT in the subject area was performed by Schmitt and Kandlikar (2005) and Kandlikar et al. (2005), and this study is an extension of these efforts. The channel used in this study is rectangular, with varying separation between walls that have machined roughness elements. The roughness elements are saw-tooth in structure, with element heights of 107 and 117 μm for two pitches of 405 μm and 815 μm respectively. The resulting hydraulic diameters and Reynolds numbers based on the constricted flow area range from 424 μm to 2016 μm and 210 to 2400 respectively. Pressure measurements are taken at sixteen locations along the flow length of 88.9 mm to determine the local pressure gradients. The results for friction factors and transition to turbulent flow are obtained and compared with the data reported by Schmitt and Kandlikar (2005). The roughness elements cause an early transition to turbulent flow, and the friction factors in the laminar region are predicted accurately using the hydraulic diameter based on the constricted flow area.
3D Simulation and Experimental Validation of an Electrically-Excited Flow in a Folded Microchannel
Authors: Barz Dominik P. J. , Farangis Zadeh Hamid , Ehrhard Peter
Published: September 2008

We investigate the flow field in a folded microchannel, which serves as an essential part of an electrically-excited micromixer. A mathematical model that allows for the numerical treatment of the channel core is developed. The boundary conditions at the walls are implemented by means of an asymptotic matching procedure. Three-dimensional finite-elements simulations are performed and verified against experiments. The comparison shows good agreement.
Scaled Navier-Stokes-Fourier Equations for Gas Flow and Heat Transfer Phenomena in Micro- and Nanosystems
Authors: Zheng Yingsong , Reese Jason M. , Scanlon Thomas J. , Lockerby Duncan A.
Published: September 2008

A new hydrodynamic model is proposed in order to model critical phenomena in gas flows at the micro- and nanoscale. A scaling is applied to the conventional Navier-Stokes-Fourier equations, mathematically equivalent to using an “effective” viscosity and an “effective” thermal conductivity in the original linear constitutive relations. Expressions for this “effective” viscosity and this “effective” thermal conductivity are obtained from two ideal half-space flow problems: Kramer’s problem, and the temperature jump problem. Our model ensures the correct viscous stress is maintained in the region of the wall in isothermal flow (or the correct heat flux in the pure heat transfer situation); it is only the relationships between stress and the corresponding near-wall strain-rate, and between heat flux and the near-wall temperature gradient, that are altered. The advantage of our model over the traditional linear hydrodynamic model is that the non-equilibrium flow in the Knudsen layer is described. Its advantage over higher-order hydrodynamic models for rarefied gas flows is that no additional boundary conditions are required (although there are minor changes in the slip/jump coefficients), so modifications of current CFD codes to incorporate this new model would be minimal. As an application example, we solve for the velocity profiles and drag force on a micro-sphere moving in a gas at different Knudsen numbers (Kn). For this problem, our model gives excellent results for Kn<0.1 and accptable results up to Kn = 0.25: this is considerably better that the tradition Navier-Stokes model with non-scaled constitutive relations.
Numerical Investigation of EDL Effects on the Flow Characters of Polar Fluids in Rectangular Microchannels
Authors: Li Zhuo , Tang Gui-Hua , He Ya-Ling , Tao Wen-Quan
Published: September 2008

Recently, a large number of experiments have been conducted to examine the applicability of Navier-Stokes equations to predict the friction factor for the laminar flow of polar fluids in microchannels. However, significant discrepancies still exist between various results. In order to investigate the effect of electric double layer on the pressure-driven flow of polar fluid in smooth rectangular microchannels and to reveal whether continuum model can still be applied, numerical investigations are conducted in this paper. The simulated microchannels are made of silicon engraved substrate with Pyrex cover, and the measured relative roughness of channels is less than 0.5%. Deionized water and tap water with different ion concentration and electrical conductivity are used as the working fluid. The governing equations include the two-dimensional, non-linear Poisson-Boltzmann equation, the modified N-S equation and the electric field Ez equation. The steady state electric field Ez equation is coupled with the momentum equation. The FVM (finite volume method) is adopted to discretize the governing equations. The non-uniform grid systems 152×102 is applied and the Reynolds number ranges from 0.1 to 300. Good agreements are achieved between the numerical results and the experimental data available in the literature. The effects of electrical potential, fluid ion concentration, electrical conductivity and the channel dimensions on the EDL profile, and the electroviscous effect and the friction coefficient are presented in detail. The simulation results reveal that the Debye thickness depends on ion concentration greatly. Only when the ratio of Dh/δ (the ratio of channel hydraulic diameter to the Debye thickness) is low, the electroviscous effect should be considered. According to the experimental conditions, the numerical results show that if the ratio of Dh/δ is greater than 15, the predicted friction factors agree well with the macroscale classical law, and the disparities are less than 4%.
Numerical Simulation of Heat and Flow of Liquid Through Minichannels
Authors: Yun Heming , Cheng Lin , Wang Liqiu , Zhang Shusheng
Published: September 2008

In this paper the heat transfer and flow in minichannels was investigated by using CFD methods. The numerical simulation results show that the equivalent diameter has little influence on resistance coefficient in the laminar region. In the turbulent flow region, the resistance coefficient decreases with the increasing of the equivalent diameter. In all computation region, the friction factors increases with increasing of the aspect ratio, and the friction factors decreases obviously with increasing of Reynolds number. The numerical simulation results show that the equivalent diameter has little influence on heat transfer Nusselt number in laminar flow region. In turbulent region, the Nusselt numbers are larger than those in macro channels. The Nusselt numbers increase with decreasing of equivalent diameter and the aspect ratio for a given Reynolds number.
A 2D Electric Double Layer Model for Biological Nanochannels
Authors: Lu Fuzhi , Kwok Daniel Y.
Published: September 2008

We developed a 2D electric double layer model for biological nanochannels based on the linearlized Poisson-Boltzmann equation with arbitrary surface potential. Time dependent adsorption kinetics was used in the model to examine the variation of electric double layer distribution and compare with that from the classical model. Based on the 2D model, EDL interaction for heavily patched arbitray surface potential was found to be much weaker in such biological nanochannels. Channel end effects are also found to be significant and not negligible.
Development of a Silicon-Based Passive Gas-Liquid Separation System for Microscale Direct Methanol Fuel Cells
Authors: Hsieh C. C. , Alyousef Yousef , Yao S. C.
Published: September 2008

The design, fabrication, and performance characterization of a passive gas-liquid separation system is presented in this paper. The gas-liquid separation system is silicon-based and its fabrication is compatible with the existing CMU design of the microscale direct methanol fuel cell (DMFC). Both gas and liquid separators consist of staggered arrays of etched-through holes fabricated by deep reactive ion etching (DRIE). The gas separator is coated with a thin layer of hydrophobic polymer to substantiate the gas-liquid separation. To visually characterize the system performance, the gas-liquid separation system is made on a single wafer with a glass plate bonded on the top to form a separation chamber with a narrow gap in between. Benzocyclobutene (BCB) is applied for the low-temperature bonding. The maximum pressure for the liquid leakage of the gas separators is experimentally determined and compared with the values predicted theoretically. Several successful gas-liquid separations are observed at liquid pressures between 14.2 and 22.7 cmH2O, liquid flow rates between 0.705 and 1.786 cc/min, and CO2 flow rates between 0.15160 to 0.20435 cc/min.
Investigation of the Three-Dimensional Flow in a T-Shaped Micromixer by Means of Scanning Stereoscopic Micro Particle Image Velocimetry (Stereo µPIV)
Authors: Lindken Ralph , Westerweel Jerry
Published: September 2008

The flow in a T-shaped micromixer at Reynolds numbers larger than Re = 100 becomes three-dimensional and intensive mixing occurs. To investigate the laminar, stationary three-dimensional flow in the T-mixer a stereo-μ-PIV system for the simultaneous measurement of all three components of the velocity vector field in a measurement plane (2D-3C) in a closed microchannel has been developed. Due to the very small confinement, standard calibration procedure by means of a calibration target is not possible, and therefore stereo-μ-PIV measurements in closed microchannels require a calibration based on the self-calibration of the tracer particle images. In order to include the effects of different refractive indices (of the fluid in the microchannel, the entrance window and the surrounding air) a three-media-model is included in the triangulation procedure of the self-calibration. Measurements in the mixing zone of a T-shaped micromixer at Re = 120 show that three-dimensional flow in a microchannel with dimensions of 800 × 200 μm can be measured with a resolution of 44 × 44 × 15 μm. The stationary flow in the 200 μm deep channel has been scanned in 22 μm wide steps, providing in a full 3D measurement of the averaged velocity distribution in the mixing zone of the T-mixer. The detailed full 3D measurement of the flow in the mixing region shows that the two inflows merge and bend over 90° in the clockwise/counter clockwise direction. When merging, the two inflows interact. The distribution of the liquid originating from the two inflows can be explained with the three-dimensional motion in the mixing zone. The liquid coming from the left inflow is deflected upwards, while the liquid coming from the right inflow is deflected downwards. Further downstream the two streams role up, causing the inflow from the right to move upwards on the lift side of the channel and vice versa for the inflow from the left. The complex three-dimensional flow structure increases the surface area between the two inflows, which leads to an increase of diffusive mixing of the laminar flow. In the region, where the two streams merge, the largest measured velocity gradient in the z direction is ∂νinplane/∂z = 2 · 104 l/s.
A Computational Study of Drop Formation in an Axisymmetric Flow-Focusing Device
Authors: Filiz Ismail , Muradoglu Metin
Published: September 2008

We investigate the formation and dynamics of drops computationally in an axisymetric geometry using a Front-Tracking/Finite-Difference (FT/FD) method. The effects of viscosity ratio between inner and outer liquids on the drop creation process and drop size distribution are examined. It is found that the viscosity ratio critically influences the drop formation process and the final drop distribution. We found that, for small viscosity ratios, i.e., 0.1 < λ < 0.5 drop size is about the size of the orifice and drop distribution is highly monodisperse. When viscosity ratio is increased, i.e., 0.5 < λ < 1 a smaller drop is created just after the main drop. For even higher viscosity ratios, the drop distribution is usually monodisperse but a satellite drop is created in some cases. The effect of the flow rates in the inner jet and the co flowing annulus are also studied. It is found that the drop size gets smaller as Qin / Qout is reduced while keeping the outer flow rate constant.
Turbulent Flow in a Micro-Channel
Authors: Kowalewski Tomasz A. , Blonski Slawomir , Korczyk Piotr M.
Published: September 2008

Turbulent flow of water in a narrow gap of an emulsifier is investigated experimentally using micro-PIV (micro Particle Image Velocimetry) technique and compared with numerical predictions performed using the commercial code Fluent. The purpose of the investigations is to develop a procedure for well-controlled generation of mono-disperse suspension of micro droplets. These droplets will form a matrix for collection of nano-particles into well-structured configuration [1]. The micro-flow measurements are based on epi-fluorescence illumination and high-speed imaging. The experimental data are compared with the numerical results obtained using both turbulent and laminar flow models. It was found that, due to small channel dimensions and very small flow development length, the turbulent energy dissipation takes place mainly in the gap and shortly behind it. Very low amount of oil-phase fraction in investigated emulsions justifies us to use mean energy dissipation estimated for pure water to predict mean diameter of oil droplets. These predictions are validated using experimental data for the emulsion.
Numerical Simulation of Micro-Droplet Formation in a Coflowing Liquid Using Front Tracking Method
Authors: Hua Jinsong , Lou Jing , Zhang Baili
Published: September 2008

Micro-droplets can be formed when a disperse liquid is injected via a needle nozzle into another immiscible co-flowing fluid. The mode of droplet formation depends on many factors such as liquid flow rates of the inner disperse phase and outer continuous phase, liquid viscosity, nozzle dimensions, interface tension force, etc. In this paper, the drop formation in a co-flowing system is simulated numerically using front tracking method to investigate the drop formation mechanism, which is very critical in the design of micro-fluidic devices for generating micro-droplets in a controllable manner. One set of Navier-Stokes equations for both liquid phases are solved numerically on a fixed Eulerian two-dimensional cylindrical coordinate mesh to account for the flow dynamics, and the front tracking method is applied to track the movement of the interface between the two immiscible liquids as well as the surface tension force. In this set of governing equations for modeling, the effects of flow inertial, capillary, viscous, and gravitational forces are all accounted to explore the droplet formation modes and dynamics in co-flowing system. The simulations reasonably predict the process of droplet formation in the co-flowing liquid. In addition, the effects of the continuous phase flow speed, viscosity and the interface tension force on droplet formation are investigated.
Experimental Study on Fundamental Phenomena of Boiling by Using Heat Transfer Surface With Well-Defined Cavities Created by MEMS: The Effect of Spacing Between Cavities
Authors: Sato Takato , Koizumi Yasuo , Ohtake Hiroyasu
Published: September 2008

Pool nucleate boiling heat transfer experiments were performed for water by using the well-controlled and -defined heat transfer surfaces. Artificial cavity(ies) was (were) created on the mirror-finished silicon plate of 0.525 mm thickness by utilizing the Micro-Electro Mechanical System (MEMS) technology. Each cavity had cylindrical shape. The diameter and the depth of the cavity were 10μm and 40μm, respectively. Experiments were performed in a range of a heat flux ∼6.0 × 104 W/m2 for distilled water. When the cavity interval was close, the horizontal and declining coalescence of bubble on the cavities were dominant. This vigorous bubble coalescence created strong convection. The heat carried by this convection took a main part in the heat transfer when cavities were close. As the cavity interval became wide, the horizontal and declining coalescence did not take place anymore. The coalescence was limited only to the vertical lift or no coalescence. In this situation, bubbles grew large on the cavities and absorbed latent heat sufficiently. Bubbles themselves took the main role of carrying heat away from the heat transfer surface when cavities were further apart.
Simulation of Dielectrophoretic Motion of Microparticles Using a Molecular Dynamics Approach
Authors: Lin Yuan , Amberg Gustav , Aldaeus Fredrik , Roeraade Johan
Published: September 2008

We model and simulate dielectrophoresis of micro-scale particles using the finite element method. A soft sphere system molecular dynamics model is presented, which solves a set of equations for the motion of every particle. The model couples most of the significant forces, i.e. the dielectrophoresis (DEP) forces, the particle-particle electrostatic forces, particle-particle interfacial repulsive forces, particle-wall repulsive forces and the hydrodynamic forces in Stokes flow. Since the system of equations is stiff, an implicit scheme is used. To obtain the particle trajectories, a constant time-step is applied. We present some numerical tests computing hydrodynamic force, electrostatic force and DEP force using our model, including simulated trapping of particles in a micro channel by dielectrophoresis. The results are in agreement with the theories and the experimental observations.
Boiling Heat Transfer for Freon R21 in Rectangular Minichannel
Authors: Kuznetsov V. V. , Shamirzaev A. S.
Published: September 2008

In this paper we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with size 1.6×6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m2s and G = 50 kg/m2s. The temperature dispersions over channel perimeter and in time were calculated. Different heat transfer mechanisms were revealed for different flow patterns. We distinguish the dominant nucleation boiling and the joint mode of nucleation boiling and convective evaporation. We also found the modes when the evaporation of thin liquid films makes the main contribution to heat transfer. The modified model of Liu and Winterton describes the experimental data for the flow patterns when the nucleation boiling and convective transfer make the most contribution to the heat transfer.
Measurement and Modelling of the Residence Time Distribution in Microfluidic Devices
Authors: Bosˇkovic´ Dusˇan , Lo¨bbecke Stefan
Published: September 2008

An experimental method has been developed to measure the residence time distribution (RTD) in continuously operated microfluidic devices. RTD measurements are performed by a stimulus-response technique using a special dye as a tracer. The tracer concentration is spectroscopically measured at two points of the setup, one close to the inlet and the other close to the outlet. The experimental setup provides sufficient flexibility since microfluidic devices can be easily exchanged and thus individually analysed. The method was optimized in order to achieve high repeatability and to eliminate errors, such as low measuring frequencies and tracer adsorption on the microchannel walls. The measured concentration profiles were used for modeling the RTD in the microfluidic devices investigated. Since the measured overall signal is influenced by the RTD behaviour of other setup components (e.g. capillaries) has been it must be deconvoluted to obtain the RTD of the microfluidic device solely. In this work, RTD measurement and modeling was performed exemplarily for one split-and-recombine micromixer. It turned out that conventional models that are used to describe the RTD of macroscopic reactors are not suitable to describe the RTD on the micro scale adequately. The asymmetric distribution caused by the laminar flow in microchannels is better approximated by empirical models.
Optical Measurement of Void Fraction and Bubble Size Distributions in a Microchannel
Authors: Ide Hideo , Kimura Ryuji , Kawaji Masahiro
Published: September 2008

An optical measurement system was developed to investigate gas-liquid two-phase flow characteristics in a circular microchannel of 100 μm diameter. By using multiple optical fibers and infrared photodiodes, void fraction and gas plug and liquid plug lengths, and their velocities were measured successfully. The probes responded to the passage of gas and liquid phases through the microchannel adequately so that the time-average void fraction could be obtained from the time fraction for each phase. Also, by cross-correlating the signals from two neighboring probes, the interface velocity representing gas plug velocity or ring-film propagation velocity depending on the flow pattern could be computed. Within the ranges of superficial gas and liquid velocities covered in the experiments (jL = 0.2∼0.4 m/s and jG = 0 ∼ 5 m/s), the gas plug length was found to increase with the increasing superficial gas velocity, but the liquid plug length was found to decrease sharply as the superficial gas velocity was increased, so that the total length of the gas-liquid plug unit decreased with the superficial gas velocity.
Experimental Investigation of the Compressibility Effects on the Friction Factor of Gas Flows in Microtubes
Authors: Morini Gian Luca , Lorenzini Marco , Colin Ste´phane , Geoffroy Sandrine
Published: September 2008

The numerical and experimental works dealing with the behaviour of gas flow through microchannels have shown results which are by no means univocal, sometimes agreeing with the classical correlations, and other times contradicting them. It is now agreed upon that the effects due to both rarefaction and compressibility must be accounted for. In addition, the experimental works have demonstrated that sometimes compressibility and rarefaction effects can be coupled in microchannels: since these two actions contrast each other, the scatter of the friction factor data for gaseous flows is remarkably large. This paper is aimed at determining the friction factor for commercial short and long Peek microtubes with nominal internal diameters between 300 and 100 μm and different values of the length-to-diameter ratio L/D, ranging between 167 and 5000. Nitrogen flows inside the microtubes, with a maximum value of the supply pressure equal to 10 bar. Very low Knudsen numbers (Kn<0.001) are considered in order to uncouple the rarefaction effects from the compressibility effects. The role of compressibility on the friction factor is analyzed and discussed in order to draw its limit of significance.
Experiments on Liquid Pressure-Drop in Rectangular Microchannels, Subject to Non-Unity Aspect Ratio and Finite Roughness
Authors: Wibel Wolf , Ehrhard Peter
Published: September 2008

We concentrate in this contribution onto the pressure-drop in rectangular stainless steel microchannels with a hydraulic diameter of dh ≈ 133 μm. Three aspect ratios are engaged, namely 1:1, 1:2, 1:5, whereas the hydraulic diameter is maintained constant. The roughness of the channel walls is around r ≈ 1.3 μm, the Reynolds number is up to Re ≈ 4000. We investigate all microchannels in two different lengths to infer highly-accurate correlations for fully-developed flow conditions from pressure difference measurements between the inlet- and outlet plenum. We find with this new technique pressure drop correlations for the fully-developed flow, which agree, both in the laminar and the turbulent regime, reasonably with conventional (macroscopic) correlations. In all cases the laminar/turbulent transition is in the range Rec ≈ 1800–2300, consistent with findings in macroscopic channels. The influence of roughness is found to be particularly strong for the microchannel of aspect ratio 1:5. This raises the question, whether the dimensionless group (r/dh) remains the responsible parameter for roughness at extreme aspect ratios.
Boundary Conditions and Integrated Sensors in Microchannel Convective Heat Transfer
Authors: Lee Man , Cheung Luthur Siu Lin , Lee Yi-Kuen , Zohar Yitshak
Published: September 2008

A microchannel heat sink, integrated with pressure and temperature microsensors, is fabricated to study convective boiling under uniform heat flux boundary condition. Utilizing a wafer bond and etch back technology, the heat source, temperature and pressure sensors are separated from the fluid flow by a membrane only 1.5μm in thickness; thus, allowing good control of the thermal boundary conditions. Temperature and pressure distributions for various power levels and flow rates are measured while, simultaneously, the flow patterns are recorded. Single-phase flow results, compared with numerical simulations, confirm that the heat flux boundary condition is indeed nearly uniform. The sensor arrays, particularly for two-phase flow, provide the spatial and temporal dependence of both the temperature and pressures fields.
Mass Flow Rate Measurements in Nitrogen Flows
Authors: Perrier Pierre , Ewart Timothe´e , Me´olans J. Gilbert , Graur Irina A.
Published: September 2008

The main objective of this experimental investigation on the gas flow slip regime is to measure the mass flow rate in isothermal steady flows through cylindrical micro tubes. Two technical procedures devoted to mass flow rate measurements are compared, and the measured values are also compared with the results yielded by different approximated analytical solution of the gas dynamics continuum equations. Satisfactory results are obtained and the way is clearly open to measuring mass flow rates for higher Knudsen numbers, over all the micro flow transitional regime.
Acceleration Methods for Coarse-Grained Numerical Solution of the Boltzmann Equation
Authors: Al-Mohssen Husain A. , Hadjiconstantinou Nicolas G. , Kevrekidis Ioannis G.
Published: September 2008

We present a coarse-grained steady state solution framework for the Boltzmann kinetic equation based on a Newton-Broyden iteration. This approach is an extension of the equation-free framework proposed by Kevrekidis and coworkers, whose objective is the use of fine-scale simulation tools to directly extract coarse-grained, macroscopic information. Our current objective is the development of efficient simulation tools for modeling complex micro/nanoscale flows. The iterative method proposed and used here consists of a short Boltzmann transient evolution step and a Newton-Broyden contraction mapping step based on the Boltzmann solution; the latter step only solves for the macroscopic field of interest (e.g. flow velocity). The predicted macroscopic field is then used as an initial condition for the Boltzmann solver for the next iteration. We have validated this approach for isothermal, one-dimensional flows in the low Knudsen number regime. We find that the Newton-Broyden iteration converges in O(10) iterations, starting from arbitrary guess solutions and a Navier-Stokes based initial Jacobian. This results in computational savings compared to time-explicit integration to steady states when the time to steady state is longer than O(40) mean collision times.
A Review of Manufacturing Processes for Microchannel Heat Exchanger Fabrication
Authors: Ashman Sean , Kandlikar Satish G.
Published: September 2008

This paper provides a summary of the types of manufacturing processes currently being used in the fabrication of micro heat exchangers with the main focus on passages with hydraulic diameter of less than 200 micrometers. The following manufacturing processes are reviewed: LIGA, Chemical Etching, Stereolithography, and micro-machining. A comparison of different techniques related to tolerances, material compatibility, and ease of manufacturing is given. Processes capable of high volume fabrication are highlighted.
A Study on Characteristics of Fluid Flow and Mixing in a Microchannel With a Controlled Local Electric Field
Authors: Heo Hyeung Seok , Suh Yong Kweon , Kang Sang Mo
Published: September 2008

In this study a newly designed microchannel is proposed. This design is comprised of a channel and a series of metal electrodes periodically attached on the both side’s surfaces of the channel. In this configuration, the mixing effect is greatly enhanced at a certain parameter values. To characterize the flow field and the mixing effect, both numerical and experimental methods were employed. To obtain the potential field, electric field, and velocity vectors three-dimensional numerical computation was performed by using a commercial code, CFD-ACE+. The fluid-flow solutions were then cast into studying the characteristics of stirring with the aid of mixing index. In this study the mixing index were computed manually because the commercial code does not provide the corresponding tool. In the experiment, flow visualization was performed by using water with fluorescent particles. The numerical results show that the velocity pattern in the microchannel heavily depends on the metal electrodes arrangement on both sides. It was shown that the rotational flow is significantly enhanced by a higher voltage of metal electrode.
Characteristics of Micro-Liquid-Layer Behavior in a Mini-Channel Vaporizer for Water and Ethanol
Authors: Utaka Yoshio , Tasaki Yutaka , Ichii Kousuke
Published: September 2008

With growing demands for a cleaner global environment, there is a need for automotive powertrains that can provide higher efficiency and contribute to lower C02 emissions. From this perspective, fuel cell vehicles are expected to gain a share of the automotive market in the 21st century. The reformer type fuel cell vehicle requires a heat exchanger with high efficiency and a low heat capacity to meet powertrain requirements for quick response and compactness. The use of a mini-channel vaporizer for the reformer is one possible way of meeting these requirements. In a mini-channel, the bulk liquid, bubbles and thin film that form between the heating surface and the bubbles affect the heat transfer characteristics in complex ways, just as in the case of pool boiling. However, as reported in the literature, the characteristics of evaporation in a mini-channel are completely different from those of pool boiling, because of the important role played by the superheated thin film that forms in the process of bubble expansion. For example, although deterioration of heat transfer characteristics has been reported for an extremely small gap, the mechanism involved has not yet been analyzed. In order to research and develop a mini-channel vaporizer efficiently, it is necessary to elucidate the mechanism of these phenomena, and clarify measures for improving heat transfer characteristics. Then a simple method may be devised, based on the related findings, to predict the heat transfer characteristics of a mini-channel vaporizer with sufficient accuracy at the development stage. In this study, by using ethanol instead of water as used in past research, the thickness of the thin film that plays an important role in the heat transfer characteristics was measured by application of the laser extinction method for channel gap sizes of 0.3 and 0.15 mm. The process of bubble growth was simultaneously recorded with a high-speed camera. These results were then compared with those obtained for water. Similar trends as those obtained for water were observed for ethanol. That is, observations show that the thin film thickness is essentially influenced by the growth speed of the interface between the bubble and the bulk liquid and that it approaches a certain value above a certain speed. At the same time, the thickness of the thin film decreases with a decrease in gap size.
Design and Fabrication of a Metallic Micro Heat Pipe Based on High-Aspect-Ratio Microchannels
Authors: Oh K. H. , Lee M. K. , Jeong S. H.
Published: September 2008

The cooling capacity of a micro heat pipe is mainly governed by the magnitude of capillary pressure induced in the wick structure. For microchannel wicks, a higher capillary pressure is achievable for narrower and deeper channels. In this study, a metallic micro heat pipe adopting high-aspect-ratio microchannel wicks is fabricated. The micromachining of high-aspect-ratio microchannels is done using the laser-induced wet etching technique in which a focused laser beam irradiates the workpiece placed in a liquid etchant along a desired channel pattern. Because of the direct writing characteristic of the laser-induced wet etching method, no mask is necessary and the fabrication procedure is relatively simple. Deep microchannels of an aspect ratio close to 10 can be readily fabricated with little heat damage of the workpiece. The laser-induced wet etching process for the fabrication of high-aspect-ratio microchannels in 0.5mm thick stainless steel foil is presented in detail. The shape and size variations of microchannels with respect to the process variables, such as laser power, scanning speed, number of scans, and etchant concentration are closely examined. Also, the fabrication of a flat micro heat pipe based on the high-aspect-ratio microchannels is demonstrated.
Submicron Polymer Flow Cells
Authors: Mandisloh Kristin , Achenbach Sven , Mappes Timo , Rogge Tilmann , Truckenmueller Roman
Published: September 2008

This paper investigates the fabrication processes of polymer micro channels integrated into flow cells. The cross sectional dimensions of these flow cell channels are in the range of microns containing structures or structure details in the submicron range. Single-component and double-component cells are presented. Single-component cells are entirely made of one polymer. They are composed of a micro structured substrate and a cover plate to hermetically seal the subjacent microfluidic structures. Flexible fluidic ports are added to facilitate interfacing. Polymethylmethacrylate (PMMA) is used as an industrially prefabricated foil (HESA®Glas VOS; HESA®Glas HESAlite). Double-component cells are made of spincoated PMMA (MicroChem 950k PMMA A11; AllResist GmbH 950k PMMA A9) that is micro structured on glass substrates prior to sealing with a cover plate. PMMA enables high resolution direct lithographic patterning of the fluidic structures. We apply Deep UV Lithography (DUV) to photochemically degrade PMMA and subsequently dissolve the degraded areas in an organic solvent. This process had previously primarily been utilized to pattern polymer waveguides. For minimum feature size devices, initial samples have been fabricated applying Deep X-Ray Lithography (DXRL) instead of DUV. Final sealing with PMMA cover plates is performed using thermal and UV bonding or solvent welding.
Simulation and Analysis of Metal Microfluidic Heatsinks
Authors: Motieifar Alireza , Shafai C. , Soliman H. M.
Published: September 2008

Microfluidic heatsinks are usually fabricated in silicon substrates using micromachining methods. Fabrication of microfluidic heatsinks in metals is captivating due to the potential of higher heat conductance of many metals compared to silicon. In this work, metallic microfluidic structures fabricated of copper, silicon, and nickel are simulated and analyzed using the FEM method. The parametric dependence of the overall thermal resistance on the heatsink material, fluid velocity, and channel geometry is examined. Their performance is compared based on the shape and size of the microchannels, their separating distance from each other, and the flow rate. Simulations were done using FEMLAB v.3.2 (Comsol Multyphysics). The length of the microchannels considered is 1cm. Their cross section is rectangular. Simulated heatsinks are covered with 50 μm of the same material as the body and the total area of the heat sink is 1cm × 1cm. Water is used as the coolant with an input temperature of 300 K. These simulations show that most of the thermal resistance in the heatsink occurs in the fluid region. For the constant and uniform input heating power of 100 W/cm2 the performance of the structure is studied at steady state with the fluid velocity varied from 1 to 14m/s. Finally, experimental fabrication of an electroplated copper micro-channel system with rectangular channels is shown.
Study of Slug Flow Inside Vertical Capillaries With Phase Change
Authors: Kumagai Kenji , Iwai Hiroshi , Yoshida Hideo , Kuno Masashi
Published: September 2008

Numerical simulations of a slug flow in a vertical capillary tube undergoing phase change are conducted to obtain detailed information on flow and thermal fields of such flows. The VOF (Volume of Fluid) method is adapted to handle two-phase flow. The PLIC (Piecewise Linear Interface Calculation) scheme is used for the advection of the gas-liquid surface, while the CSF (Continuum Surface Force) model is employed for the calculation of surface tension. The growth of a single vapor bubble is numerically predicted under evaporation or condensation conditions. A qualitative comparison of the numerical results with the preliminary experiment is also carried out. Results of the numerical prediction agree fairly well with experimental counterparts. It is found that the evaporation or condensation process of a bubble can be classified depending on if the bubble has a spherical shape or a shell shape. Phase change is enhanced when a bubble has a shell shape reflecting the thin liquid part formed near the solid wall.
Nanofluidic Devices for Sensing and Flow Control
Authors: Karnik Rohit , Castelino Kenneth , Duan Chuanhua , Fan Rong , Yang Peidong , Majumdar Arun
Published: September 2008

Nanofluidics is concerned with fluidic channels that are typically 1–100 nm in size. We have fabricated nanofluidic devices using both 1-D silica nanotubes and 2-D nanochannels to explore transport phenomena at the nanoscale. Here we review our work on 2-D nanochannels that provide confinement in one dimension. Our work mainly deals with two aspects of nanofluidics (a) effects related to electrostatic interactions and (b) effects related to biomolecule size. Surface charge plays an important role in nanofluidic channels, when the channel size is comparable to the Debye length. Using both electrical conductance measurements and fluorescence imaging, we studied the effects of surface charge in our nanofluidic devices, and demonstrated that the environment in nanochannels is governed by surface charge. We modified the nanochannel surface and showed that these modifications can be sensed by measuring ionic conductance of the nanochannels. Further, binding reactions involving biomolecules can be sensed at both low and high ionic concentrations. Our results showed that at low concentrations, conductance is governed by biomolecule charge, while at high concentrations it is governed by biomolecule size. Based on electrostatic effects in nanochannels, we also developed a nanofluidic transistor for flow control. This metal-oxide-solution field effect transistor was fabricated by patterning a metal gate electrode over nanochannels, similar to a MOSFET. Just as the gate voltage of a MOSFET controls carrier concentration in the semiconductor, we demonstrated that the gate voltage in a nanofluidic transistor controls the concentration of ions and biomolecules in the nanochannel, and hence controls their transport. Our fabrication process uses standard lithography, and is amenable to making networks of nanochannels. It suggests that rationally designed nanofluidic networks could be developed using this process for applications in sample preparation, sensing and switching. We are currently studying flow control and switching using field-effect, as well as ionic transport using patterned surface charge in nanofluidic devices.
Design of a Fractal Tree-Like Microchannel Net Heat Sink for Microelectronic Cooling
Authors: Hong F. J. , Cheng P. , Ge H. , Goh Teck Joo
Published: September 2008

In this paper, a numerical simulation is carried to study pressure drop and heat transfer in a fractal tree-like microchannel net heat sink of 10mm×12.5mm×0.5mm in dimensions. The numerical result is obtained by solving three-dimensional Navier-Stokes equations and energy equation, taking into consideration conjugate heat transfer in the microchannel walls. A comparison of fractal tree-like microchannel net heat sink with 6 branch levels to parallel microchannels heat sink, with respect to the pressure drop, thermal resistance and temperature uniformity, was also performed under the condition of the same heat sink dimensions. The results indicates that for a mass flow rate of water less than 0.00175kg/s, the fractal tree-like microchannel is much better than parallel channel heat sink with respect to all of three aspects. Therefore, the fractal tree-like microchannels net heat sink using water as the coolant is promising to be used in the future electronic cooling industry.
Capillary Pumped Loop (CPL) With Cone Shaped Capillary Structure for Cooling Electronic Device
Authors: Jung Jung-Yeul , Oh Hoo-Suk , Lee Dae Keun , Ko Chang-Bok , Kwak Ho-Young
Published: September 2008

A capillary pumped loop (CPL) which can be used for cooling electronic devices such as a CPU of a personal computer or note book was developed. An evaporator and a condenser with the same size of 30 mm × 30 mm × 1.55 mm were designed and fabricated using 3 layer of silicon wafer for the CPL. In the evaporator, 525 μm thickness silicon wafer where an array of 40 × 40 cone shaped micro holes were fabricated was inserted above the compensation cavity for liquid transportation instead of porous wick. The same cone shaped microstructure was used in the condenser to create a stable interface between the liquid and the vapor phases. The CPL fabricated was tested under various conditions such as different relative heights, fill ratios and heat fluxes. The operation conditions of CPL were varied depending on relative height and fill ratio. With an allowable temperature of 82 °C on the evaporator surfaces, the CPL can handle a heat flux of about 3.7 W/cm2 with the air cooled condenser. Steady state operation condition was achieved within 10 minutes.
Heat Transfer Characteristics of Gaseous Flows in Micro-Channels With Negative Heat Flux
Authors: Hong Chungpyo , Asako Yutaka
Published: September 2008

Two-dimensional compressible momentum and energy equations are solved to obtain the heat transfer characteristics of gaseous flows in micro-channels with CHF (constant heat flux) whose value is negative. The combined effect of viscous dissipation and compressibility is also investigated. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The computations are performed for channels with constant heat flux with range from −104 to −102 Wm−2. The channel height ranges from 10 to 100 μm and the aspect ratio of the channel height and length is 200. The stagnation pressure varies from 120 to 500 kPa. The outlet pressure is fixed at the atmosphere. The wall and bulk temperatures in micro-channels are compared with those of the case of positive heat flux and also compared with those of the incompressible flow in a conventional sized channel. In the case of negative heat flux, temperature profiles normalized by heat flux have different trends in the case of positive heat flux, when flow is fast. A gas temperature falls down due to the energy conversion. A correlation for the prediction of the wall temperature of the gaseous flow in the micro-channel is proposed.
Development and Analysis of Multi-Laminar Chemical Delivery Platform Toward Single Molecular Application
Authors: Lee Sangwook , Yamamoto Takatoki , Segawa Shigeki , Hiroshi Kimura , Noji Hiroyuki , Fujii Teruo
Published: September 2008

The fluidic operations in microfluidic system such as chemical delivery, solution exchange, and chemical reactions by mixing are one of the most important technical issues to realize high-throughput and automated chemical / biochemical analysis with extremely small amount of species down to single molecular level. In order to realize such fluidic operations in micro and nano scale, we have been trying to develop a chemical delivery system to transport reagents to an arbitrary location in microchannel. In this work, we have developed and demonstrated a chemical delivery system in microchannel using precisely controlled multilaminar flow created by our previously developed multi-channel micropump embedded on the system. As a demonstration of the chemical delivery system, a single-molecule analysis of ATP dependent rotation of F1- ATPase was investigated. As a result, ATP dependent rotation of F1-ATPase was successfully investigated by addressing one of the streams, which are included or not included ATP, to the immobilized single-molecule of F1-ATPase.
Experimental Investigation on Flow and Thermal Characteristics of a Micro Phase-Change Cooling System With a Microgroove Evaporator
Authors: Hu Xuegong , Tang Dawei
Published: September 2008

In this paper, a natural convection micro cooling system with a capillary microgroove evaporator is proposed. An experimental study on the characteristics of thermal resistance, pressure drop and heat transfer of the cooling system was carried out. Experimental results indicate that the liquid fill ratio has a significant influence on thermal resistance and heat transfer in the cooling system. Increasing system’s cooling capacity at higher input power depends on decreasing the thermal resistance between the outer surfaces of the condenser and ambient environment. Compared with a flat miniature heat pipe (FMHP) and a current fan-cooled radiator for CPU chip of Pentium IV, the present micro cooling system has a stronger heat dissipation capacity. Its best cooling performance at a surface temperature of heat source below 373K reaches 1.68×106W/m2 and the maximum heat transportation capacity is 131.8W. The novel kind of cooling system is suitable for remote cooling of those electronic parts with micro size, high power and thermal sensitivity.
Compressibility and Rarefaction Effects on Pressure Drop in Rough Microchannels
Authors: Croce Giulio , D’Agaro Paola , Filippo Alessandro
Published: September 2008

A numerical analysis of the flow field in rough microchannel is carried out with a finite volume compressible solver, including generalized Maxwell slip flow boundary conditions suitable for arbitrary geometries. Roughness geometry is modeled as a series of triangular shaped obstructions. Relative roughness from 0% to 2.65% were considered. Since for truly compressible flow we have no fully developed flow condition, the simulation is performed over the whole length of the channel. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 200 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to 0.0249. Performance charts are presented in terms of both average and local Poiseuille number as a function of local Kn, Ma and Re. In particular, it appears that roughness strongly decreases the reduction in pressure loss due to rarefaction. Thus, roughness effect is stronger at high Kn. Furthermore, compressibility effect has a major effect on pressure drop, as soon as local Mach number exceed 0.3.