bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Mixing and Flow Structure in Different Passive Micromixers
Authors: Hossain Shakhawat , Ansari Mubashshir Ahmad , Kim Kwang-Yong
Published: September 2010

This work presents a numerical investigation on mixing and flow structures in microchannels with different geometries: zig-zag; square-wave; and curved. To conduct the investigation, geometric parameters, such as the area of the cross-section of channel, height of the channel, axial length of the channel, and number of pitches, are kept constant for all three cases. Analyses of mixing and flow fields have been carried out for a wide range 0.267 to 267 of the Reynolds number. Mixing in the channels has been analyzed by using Navier-Stokes equations with two working fluids, water and ethanol. The results show that the square-wave microchannel yields the best mixing performance, and the curved and the zig-zag microchannels show nearly the same performance for most Reynolds numbers. For all three cases, the pressure drop has been calculated for channels with equal streamwise-lengths. The curved channel exhibits the smallest pressure drop among the microchannels, while the pressure drops in the square-wave and zigzag channels are approximately the same.
Numerical Simulation and Experimental Observations of Confined Bubble Growth During Flow Boiling in a Microchannel With Rectangular Cross-Section of High Aspect Ratio
Authors: Zu Y. Q. , Gedupudi S. , Yan Y. Y. , Karayiannis T. G. , Kenning D. B. R.
Published: September 2010

Bubble nucleation and growth to confinement during flow boiling in microchannels lead to high heat transfer coefficients. They may also create pressure fluctuations that change the superheat driving evaporation and cause flow reversals that promote transient dry-out and uneven distribution of flow between parallel channels. The work described in this paper is part of a programme to develop models for these processes that will aid the design of evaporative cooling systems for devices operating at high heat fluxes. Video observations of water boiling in a single copper channel of rectangular cross-section, 0.38 × 1.6 mm and a heated length 40 mm, were performed. The top side of the channel was a glass window. Results are presented for a heat flux, averaged over the area of the three metal sides, of 210 and 173 W/m2K for incompressible and compressible inlet flow conditions. The inlet pressure was about 1.12 bar and the mass flux was 747.5 kg/m2s for both conditions examined. The results demonstrated the strong influence of compressibility on the mode of bubble detachment and growth and therefore on flow patterns, pressure fluctuations and heat transfer rates. The fluid mechanics of boiling in this size channel were also successfully investigated by 3-D numerical simulation for bubbles growing at a defined rate with a fixed inlet flow rate using the 3-D CFD code FLUENT 6 (no upstream compressibility). The study examined the fluid mechanics of bubble motion with heat transfer, but the mass transfer across the bubble-liquid interface was not simulated in the present work. A small vapour bubble was injected at the wall to ensure the bubble generation is under a quasi nucleation condition. Its growth was driven by an internal source of vapour, at a rate derived by analysis of the experimental measurements of growth. The simulation reproduced well the observed motion and shape of the bubble. The simulation was then extended to model bubbles generated and growing randomly in a 2-D channel.
Enhanced Multi-Objective Optimization of a Microchannel Heat Sink Using Multiple Surrogates Modeling
Authors: Husain Afzal , Kim Kwang-Yong
Published: September 2010

A liquid flow microchannel heat sink has been studied and optimized with the help of three-dimensional numerical analysis and multiple surrogate methods. Two objective functions, thermal resistance and pumping power have been selected to assess the performance of the microchannel heat sink. The design variables related to the microchannel top and bottom widths, depth and fin width, which contribute to objective functions, have been identified and design space has been explored through some preliminary calculations. Design of experiments was performed and a three-level full factorial design was selected to exploit the design space. The numerical solutions obtained at these design points were utilized to construct surrogate models namely Response Surface Approximations and Kriging. A hybrid multi-objective evolutionary algorithm coupled with surrogate models and a gradient-based search algorithm is applied to find global Pareto-optimal solutions. Since, the surrogate models are highly problem-dependent, the accuracy of the two surrogate models has been discussed in view of their predictions at on- and off-Pareto-optimal front. The trade-off analysis was performed in view of the two competing objectives. The Pareto-optimal sensitivity (change in value along the Pareto-optimal front) of the design variables has been found out to economically compromise with the design variables contributing relatively less to the objective functions. The application of the multiple surrogate methods not only improves quality of multi-objective optimization but also gives the feedback of the fidelity of the model near the optimum region.
Microvascular Structure and Function in Vitro
Authors: Stroock Abraham D. , Choi Nak Won , Wheeler Tobias D. , Cross Valerie , Verbridge Scott , Fischbach Claudia , Bonassar Lawrence J.
Published: September 2010

Vascular structure — a network of convective paths — is a ubiquitous element in multicellular, living systems. The key function of vascular structure in animals and plants is mediation of convective mass transfer over macroscopic distances; this transfer allows an organism to monitor and control the chemical state of its tissues. In our laboratory, we are developing methods to embed and operate microfluidic systems within tissue-like materials in order to capture this function for both biological and non-biological applications. I will present two examples to illustrate our efforts: 1) Capillary beds for the culture of mammalian cells in three-dimensions. In this section, I will discuss the development of methods both to fabricate synthetic capillary beds and to grow them directly out of endothelial cells. I will highlight how simple ideas from continuum mechanics and material science have guided our efforts. 2) Synthetic xylem networks that allow for the transpiration of water at large negative pressures. I will point out the unusual thermodynamic and transport phenomena that are involved in the transpiration process in plants. I will then present our perspectives on the design criteria for systems — synthetic and biological — that mediate this process. Finally, I will describe our experiments with “synthetic trees” in which we have reproduced the main features of transpiration. I will conclude with perspectives on applications and generalizations of both these classes of vascularized materials.
Nanocarpets Decorating the Walls of Microchannels
Authors: Bayazitoglu Yildiz , Tullius Jami F.
Published: September 2010

Without the appropriate cooling, the operating temperature of electronic devices and micro systems could reach values where the components lose their physical integrity, and the proper functioning would cease. In response to this demand, many techniques have been studied and developed such as laser drilled cavities, heat sinks, micro fins, etc. but have still not been able to reach an adequate cooling performance necessary for the components to operate properly. Because of the large heat transfer surface area to volume ratio, microchannels cooled with gas or liquid coolant have been shown to be strong prospects. By lining the walls for the microchannel with nanocarpets — fins, pins, etc. that varies the roughness of the surface — it can alter the effect of the microchannel performance. Carbon nanotubes have extremely high thermal conductivities and they also can be made to have an additional increase in the wall heat transfer surface area. The use of approximated Boltzmann equations, Molecular Dynamics, and Computational Fluid Dynamics to model the heat flow in nanostructured surface microchannels, and also the nanotstructure-fluid interface will be reviewed. The explanation of important interactions, heat transport phenomena, the numerical experimentations related to effect of nanotube length, the nanotube type, the spacing of the nanotubes, and their staggered pattern, and their other physical and material properties in relation to the fluid flow properties within the microchannels will be discussed.
Frictional Characteristics of Liquid Flows in Narrow Confinements
Authors: Chakraborty Suman , Das Tamal
Published: September 2010

Probing the interfacial friction in narrow confinements is becoming increasingly important in various applications encompassing fluid dynamics over micrometer and nanometer length scales, including applications pertinent to molecular biology, biochemistry, and energy conversion. The present article aims to present a broad overview on the progresses made towards understanding the fundamental mechanisms dictating such interfacial phenomenon, from empirical, semi-empirical to more rigorous physical and mathematical considerations. Futuristic outlook on the related research activities are also outlined, both from fundamental considerations as well as from application viewpoints.
Flooding Visualization and Enhanced Water Management in PEM Fuel Cell
Authors: Cho Hyung Hee , Lee Sanghoon , Rhee Dong-Ho
Published: September 2010

Internal water management in proton exchange membrane (PEM) fuel cell has been considered as one of most significant key factors for its performance enhancement. It is because relative humidity of hydrogen and air is strongly related to the performance of PEM fuel cell in terms of H+ movement within the membrane. In addition, production of H2O by chemical reactions can bring several problems during concentration loss region since combination of vapor in supplying air and byproduct of chemical reaction should lead to excess H2O remaining in PEM fuel cell, resulting flooding phenomena which may block air flow channels. Therefore, in order to understand and manage such phenomena to enhance the performance of PEM fuel cell, especially under concentration loss region, this paper focuses on the visualization of the flooding phenomena and application of the modified flow path on the cathode separator for flooding reduction.
Flow-Based Isolation and Neutralization of Circulating Tumor Cells
Authors: Western Laura T. , Rana Kuldeepsinh , King Michael R.
Published: September 2010

Circulating tumor cells (CTC) have the potential to be used clinically as a diagnostic tool and a treatment tool in the field of oncology. As a diagnostic tool, CTC may be used to indicate the presence of a tumor before the tumor is large enough to cause noticeable symptoms. As a treatment tool, CTC isolated from patients may be used to test the efficacy of chemotherapy options to personalize patient treatment. One way for tumors to spread is through metastasis via the circulatory system. CTC are able to exploit the natural leukocyte recruitment process that is initially mediated by rolling on transient selectin bonds. Our capture devices take advantage of this naturally occurring recruitment step to isolate CTC from whole blood by flowing samples through selectin and antibody-coated microtubes. Whole blood was spiked with a known concentration of labeled cancer cells and then perfused through pre-coated microtubes. Microtubes were then rinsed to remove unbound cells and the number of labeled cells captured on the lumen was assessed. CTC were successfully captured from whole blood at a clinically relevant level on the order of 10 cells per mL. Combination tubes with selectin and antibody coated surface exhibited higher capture rate than tubes coated with selectin alone or antibody alone. Additionally, CTC capture was demonstrated with the KG1a hematopoietic cell line and the Du145 epithelial cell line. Thus, the in vivo process of selectin-mediated CTC recruitment to distant vessel walls can be used in vitro to target CTC to a tube lumen. The microtube device can also be used to capture CTC of hematopoietic and epithelial tumor origin and is demonstrated sensitivity down to the order of 10 CTC per mL. In a related study aimed at reducing the blood borne metastatic cancer load, we have shown that cells captured to a surface can be neutralized by a receptor-mediated biochemical signal (Rana et al. 2008). In the proposed method we have shown that using a combined selecting and TRAIL (TNF Related Apoptosis Inducing Ligand or Apo 2L) functionalized surface we are able to kill about 30% of the captured cells in a short duration of 1 hour whereas it took about 4 hours to kill the same proportion of cells without flow on a similarly functionalized. Here we have taken the approach a step further by showing that with very small doses of chemotherapeutic agents like Bortezomib, we can increase the kill rate of CTCs., thus allowing the device to function in senarios where the patient is undergoing treatment. We show here with leukemic cells that are treated with Bortezomib that we are able kill about 41% of the captured cells.
Water Management in PEMFC Stack of Fuel Cell Vehicle
Authors: Lee Sungho , Jeong Heeseok , Whang Inchul , Lim Taewon
Published: September 2010

The PEMFC (Polymer Electrolyte Membrane Fuel Cell) requires well hydration for acceptable protonic conductivity, but liquid water in the catalyst layers and gas diffusion layers can cause performance loss due to blockage of reactants to the catalysts. Many activities have been done on the water management in PEMFC stack to guaranty better performance and its longevity. Some approaches for PEMFC stack in Hyundai-motor will be shown in this presentation based on analytic modeling, CFD, and experiment, then some challenges for better understanding of water behavior in PEMFC will be shown at the end of this paper.
High Temperature Microsystems: Recent Advances in Microcombustion
Authors: Prakash Shaurya , Phang Yin Fee
Published: September 2010

Recent times have seen a growing interest in developing next generation energy systems and devices for building very small engines, power plants, and high temperature microchemical reactors, all running on the combustion of hydrocarbon fuels due to their inherently high energy densities. In particular, much interest lies in creating small-scale fuel reformers to produce hydrogen and/or syngas for fuel cells. Over the past decade, most microscale combustion systems that have been developed employ catalytic and heterogeneous combustion processes. In this paper, discussion towards the development of sub-millimeter or microscale homogeneous combustion systems operating at high temperatures, which can approach adiabatic flame temperatures, to achieve potentially high power densities (∼ 103 W/cm3) will be presented. Results from previous work are summarized to discuss the role and importance of surfaces in creating and sustaining homogeneous flames in narrow, confined structures with channel dimensions as small as 100 μm. At these length scales, some unusual flame structures and flame dynamics have been observed that vary strongly with changes in boundary conditions. This paper reviews recent experimental and computational data, observations of flame structure and dynamics, and discusses several open questions that remain to be answered.
Micro-Structured Reactors and Catalysts for the Intensification of Chemical Processes
Authors: Renken Albert
Published: September 2010

Process intensification is the term which describes an innovative design approach in chemical engineering aiming on a significant increase of the specific performance of chemical reactors and plants miniaturization, of at least an order of magnitude. In addition, the running costs should be reduced and the process should be more efficient, safer, and less polluting than the existing ones. Micro process technology is considered as means of process intensification leading to better use of raw materials and energy. Chemical micro-structured reactors (MSR) are devices containing open paths for fluids with dimensions in the sub-millimeter range. Mostly they consist of multiple parallel channels with diameters between ten and several hundred micrometers where the chemical transformations occur. This results in a high specific surface area in the range of 10,000 to 50,000 m2m−3 and allows a more efficient mass and heat transfer compared to traditional chemical reactors having usually ∼100 m2m−3. Another important feature of micro-structured reactors is that the heat exchange and the reaction are mostly performed in the same gadget. Intensification of heterogeneous catalytic processes involves besides of innovative engineering of micro-structured reactors, the proper design of the catalyst. This requires the simultaneous development of the catalyst and the reactor. The catalyst design should be closely integrated with the reactor design taking into consideration the reaction mechanism, mass/heat transfer and the energy supply / evacuation resulting in high selectivity and yield of the target products. Besides general criteria for the choice and proper design of micro-structured reactors for process intensification, particular needs for homogeneous and multiphase reactions will be discussed.
Micro- and Nanofluidics for Cell Biology, Cell Therapy, and Cell-Based Drug Testing
Authors: Takayama Shuichi , Huh Dongeun , Song Jonathan , Cha Wansik , Heo Yunseok
Published: September 2010

Many biological studies, drug screening methods, and cellular therapies require culture and manipulation of living cells outside of their natural environment in the body. The gap between the cellular microenvironment in vivo and in vitro, however, poses challenges for obtaining physiologically relevant responses from cells used in basic biological studies or drug screens and for drawing out the maximum functional potential from cells used therapeutically. One of the reasons for this gap is because the fluidic environment of mammalian cells in vivo is microscale and dynamic whereas typical in vitro cultures are macroscopic and static. This presentation will give an overview of efforts in our laboratory to develop microfluidic systems that enable spatio-temporal control of both the chemical and fluid mechanical environment of cells. The technologies and methods close the physiology gap to provide biological information otherwise unobtainable and to enhance cellular performance in therapeutic applications. Specific biomedical topics that will be discussed include, in vitro fertilization on a chip, microfluidic tissue engineering of small airway injuries, breast cancer metastasis on a chip, electrochemical biosensors, and development of tuneable nanofluidic systems towards applications in single molecule DNA analysis.
Velocity Fields in Opto-Electrically Induced Fluid Flows
Authors: Wereley Steven T. , Judokusumo Edward , Kumar Aloke , Williams Stuart
Published: September 2010

In this paper we demonstrate opto-electrothermal pumping in a very simple setup consisting primarily of parallel electrodes and explore the characteristics of such flows with different optical intensity patterns. For our parallel electrode configuration setup, ITO-coated electrodes were used to generate electric fields. The optical illumination system uses a focused laser beam 1,064nm, continuous mode. The experiments are analyzed in terms of existing analytical models for electrothermal flows. Microvortices created using the electric field and optical illumination resembles a sink/source type flow with the laser spot as the center of the sink/source. The flow velocity is characterized as a function of the AC signal frequency and the strength of electric field. At larger frequencies (f > 1 MHz), the velocity of the vortices decreases and around f > 5 MHz, Brownian motion is more dominant than the vortices. The line illumination is created by holographically stretching the point illumination. The line is about 28 μm in length. Result of this experiment is determined by means of visualization only. The creation of these vortices can not only be used to create microfluidic pumps but also also show immense promise as microfluidic mixers without utilizaing any invasive components.
Energy Transport Mechanisms in Nanofluids and Its Applications
Authors: Xuan Yimin , Li Qiang
Published: September 2010

Nanofluid is a solid-liquid mixture consisting of solid nanoparticles or nanofibers with sizes typically of 1–100 nm suspended in liquid. Thermal conductivity and heat transfer performance of nanofluids is superior to those of the original pure carrier fluids because the suspended nanoparticles remarkably improve energy exchange capability of the suspensions. In the present paper, the investigations efforts cover microscopic and mesoscaled approachs for the heat transfer enhancement mechanism of the nanofluid, flow and heat transfer mechanism and the relevant control methods of the magnetic fluid by suspending magnetic nanoparticles in base fluids, and some applications of nanofluid on a variety of thermal systems in order to understand energy transfer mechanism of nanofluids and guide future applications of nanofluids to thermal engineering.
Development of a Mini Liquid Cooling System for High Heat Flux Electronic Devices
Authors: Yang Chien-Yuh , Yeh Chun-Ta , Huang Kou-Chung , Tsai Shao-Nong
Published: September 2010

The size of the most of the current commercialized liquid cooling systems is apparently too large to be easily adapted in a notebook or a mini size desk top computer. This study incorporated the authors’ previous micro heat exchanger design with an extra slim pump concept proposed by a local manufacturer to develop a high performance miniature liquid cooling system. An integrated pump and cold plate assembly was also developed for further reducing the overall size of the system. In comparing to the commercial products, the test results show that the micro pump provides a higher maximum pressure head and maximum flow rate performance. The cold plate has the lowest thermal resistance at moderate and high flow rate region. And the performed of the entire liquid system is similar to that of the recently announced product. It is emphasized that the size of the present developed cold plate, pump and liquid cooling system is much smaller than that of all commercial products.
Flash Chemistry - Fast Chemical Synthesis in Micro Flow Systems
Authors: Yoshida Jun-ichi , Nagaki Aiichiro
Published: September 2010

Flash chemistry is a field of chemical synthesis where extremely fast reactions are conducted in a highly controlled manner. A key element of flash chemistry is the control of extremely fast reactions to obtain the desired products selectively. For extremely fast reactions, kinetics often cannot be used because of the lack of homogeneity of the reaction environment when they are conducted in conventional reactors such as flasks. Fast micromixing by virtue of short diffusion path solves such problems. Fast reactions are usually highly exothermic, and heat removal is an important factor in controlling such reactions. Heat transfer occurs very rapidly in micro flow systems by virtue of a large surface area per unit volume, making precise temperature control possible. Another important point is that fast reactions often involve highly unstable intermediates, which decompose very quickly, making reaction control difficult. The residence time can be greatly reduced in micro flow systems, and this feature is quite effective in controlling such reactions. The concept of flash chemistry has been successfully applied to various organic reactions for synthesis including (a) reactions in which undesired byproducts are produced in the subsequent reactions in conventional reactors, (b) highly exothermic reactions that are difficult to control in conventional reactors, and (c) reactions in which a reactive intermediate easily decomposes in conventional reactors. The concept of flash chemistry can be also applied to polymer synthesis. Cationic polymerization can be conducted with an excellent level of molecular-weight control and molecular-weight distribution control. Radical polymerization in micro flow systems leads to better molecular weight distribution control than macro batch systems. Anionic polymerization can also be carried out micro flow systems at higher temperatures than macro batch systems with high degree of molecular weight distribution control.
Formation of Droplets in a T-Junction Microfluidic Device Using the Lattice Boltzmann Method
Authors: Gupta Amit , Murshed S. M. Sohel , Kumar Ranganathan
Published: September 2010

In recent years, microfluidic devices that generate micron sized droplets/bubbles have found widespread applications in drug delivery, microanalysis, tumor destruction, as ultrasound agents and in chemical reactions at the micron level. In the current work, simulations results are being presented for a T-junction device for formation of micron-sized droplets using the lattice Boltzmann method. In this work, the key parameters of interest for estimating the frequency and volume of the generated droplets are the flow rates and viscosities of the two fluids, and the geometry of the flow channel. Simulations at low Capillary number indicate that droplets formed occupy the whole volume of the main channel and undergo a squeezing regime. At higher Ca, droplets of size smaller than the width of the channel are formed, and the frequency is dependent on the flow rates of the two liquids. The effect of the width of the dispersed and continuous phase channels is also investigated.
Noncontact Bubble Manipulation in Microchannel by Using Photothermal Marangoni Effect
Authors: Takeuchi Hiroyuki , Motosuke Masahiro , Honami Shinji
Published: September 2010

A novel method of noncontact bubble manipulation by optically-induced local surface tension gradient is described in this paper. In microfluidic devices, the effects of interfacial phenomena become dominant with decreasing of a length scale. An unexpected adhesion of a bubble on the channel wall is a serious problem which can cause the large pressure loss and the deterioration of the device. Thus, the removal or manipulation technique of the bubble is strongly required. In this study we controlled the thermocapillary force around the bubble by means of optical technique. The purpose of this study is the verification of the optical manipulation method of bubble. Particularly, the detail of migration process including the effect of bubble size, fluid viscosity and optical power is discussed. The manipulation experiments were conducted for the bubble with the diameter of 40 to 140 μm in a microchannel filled with silicone oil. An FEP (fluorinated ethylene propylene copolymer) tube with the inner diameter of 200 μm was used as the microchannel. The optical system for the heating is composed of a scanning setup and a compact laser diode. In this technique, two types of motion for the bubble transport are possible. One motion is the detachment of the bubble from the channel wall. When a laser beam is irradiated into the liquid in the vicinity of the bubble attached to the wall, the Marangoni convection is induced and the difference of pressure is generated around the bubble. The bubble is detached from the wall when the pressure difference overcomes the anchoring force between the bubble and the wall. Then, the bubble detached from the wall is suspended in the liquid at the balanced position between the thermocapillary and buoyancy force. This position can be controlled by adjusting the laser power. The other motion is the manipulation of the bubble along the channel. When the focal spot is scanned along the channel, it is possible to manipulate the bubble as if the bubble follows the light. In addition, the minimum optical power necessary to transport the bubble along the microchannel was measured. The minimum optical power strongly depends on bubble size, liquid viscosity, and scanning speed. These results show the relationship between the driving force induced by photothermal Marangoni effect and the resistance force related with the viscosity and the scanning speed.
A Novel Method to Determine the Zeta Potential of Porous Substrates by Measuring the Deflection of Two Coupled Droplets
Authors: Barz Dominik P. J. , Vogel Michael J. , Steen Paul H.
Published: September 2010

We developed a novel method to determine the zeta potential of porous substrates saturated with a liquid. When two droplets are connected by their internal pressure, a bistable system in terms of volume distribution is established. Switching between the stable volume distributions may be realized by electroosmosis, arising near the solid/liquid boundary within the substrate when an electric potential difference is applied. Measuring the deflection of the liquid/gas interface of the droplets gives the strength of the induced electroosmotic flow. A model relates the induced electroosmotic flow to the zeta potential resulting from the combination of substrate and liquid. Measurements of a borosilicate substrate in contact with different aqueous electrolytes are performed. The results of the measurements clarify the influence of the ionic strength and pH value on the zeta potential. The results are also used to derive a new empirical correlation, reducing the number of parameters which have to be determined.
Convective Heat Transfer Studies in Mini-Channels Using Digital Interferometry
Authors: Sajith V. , Haridas Divya , Sobhan C. B. , Reddy G. R. C.
Published: September 2010

Convective heat transfer in micro and mini channels has been recommended as an effective heat removal method for various electronic packages and systems. Experimental and theoretical investigations on the thermal performance of micro and mini channels have gained immense attention and hence, heat transfer studies in mini channels are of great importance. Some of the experimental results found in the literature on heat transfer in small-dimension channels are of contradicting nature even though some generally agreeing results are also found. One of the probable reasons for such deviations is the intrusive nature of the measurement techniques used. The traditional method of temperature measurement in channels uses the thermocouple probe, and for obtaining temperature distribution across the channel either a number of probes or a moving probe technique is required, both of which disturb the flow field and cause measurement errors. Hence a non intrusive measurement technique, such as an optical method is preferable for temperature measurement in small channels. In the present work, convective heat transfer studies have been performed on water flowing through a mini channel of hydraulic diameter 4 mm, using the non-intrusive technique of laser interferometry, coupled with digital image processing. The channel is fabricated using high quality optical glass and aluminum blocks. Mach Zehnder Interferometry is used for obtaining the temperature distribution in the channel. The experimental arrangement consists of two identical channels, one placed in the test section and the other in the reference section of the interferometric set up. As the test section is heated, a density variation is produced in the medium, which causes a refractive index variation, deforming interference fringes. This enables the calculation of the temperature distribution inside the channel. The interferograms are grabbed using a CCD camera and an AVT Fire package software. Digital image processing technique, using MATLAB software is used for locating the fringe-centers, and calculating the temperature distribution. The temperature profiles are obtained at different sections of the channel for various values of the average Reynolds number and various heating levels. The local and average heat flux values are obtained from the constructed temperature distributions. Variations of the local and average heat transfer coefficients and Nusselt number are determined and discussed. Results of parametric studies are compared and contrasted with relevant entry length solutions from the literature.
Impact and Spreading of a Microdroplet on a Solid Wall
Authors: Muradoglu Metin , Tasoglu Savas
Published: September 2010

Impact and spreading of a viscous micro droplet on dry solid surfaces are studied computationally using a finite-difference/front-tracking method. The problem is motivated by single cell epitaxy developed for printing biological cells on a solid substrate using ink-jet printer technology. In this study, we consider impact and spreading of a simple droplet on a partially wetting substrate as a first step in developing a complete compound droplet model for the single cell epitaxi. The numerical method is general and can treat non-wetting, partially wetting and fully wetting cases but the focus here is placed on partially wetting substrates. The contact angle is specified dynamically using the empirical correlation given by Kistler (1993). In addition, a precursive film model is also used especially for the highly wettable cases, i.e., the static contact angle is smaller than 30° due to numerical difficulty of resolving thin liquid later penetrating into surrounding gas near the solid surface. The numerical method is first applied to simple droplet spreading and the results are compared with experimental data of Sikalo et al. (2005). Then the effects of governing non-dimensional numbers on the spreading rate, apparent contact angle and deformation of the droplet are investigated. Finally a few preliminary results are presented for the impact and spreading of a compound microdroplet on a partially wetting surface.
Asymmetrical End Effect on Boiling Explosive Emission in Micro Capillary Tubes
Authors: Wu Di , Peng Xiao-Feng
Published: September 2010

In present investigation, the critical emitting heat flux was measured and analyzed corresponding to liquid explosive emission phenomenon occurring in a capillary tube, as well as the temperature outside tube wall. The asymmetrical distribution of two non-heated side sections of the tube had significant effect on the explosive emission. Two dimensionless parameters, geometrical and thermal asymmetry, and the asymmetrical function were introduced to describe the general asymmetrical effect based on the experimental results of tubes with different tube inner diameters, heating length and heating systems. For a larger thermal asymmetry, the critical emitting heat flux decreased significantly as geometrical asymmetry became larger. For a smaller thermal asymmetry, geometrical asymmetry has a little effect on the critical emitting heat flux.
Transport Phenomena Within the Porous Cathode for PEM Fuel Cell
Authors: Liu Juanfang , Oshima Nubuyuki , Kurihara Eru , Saha LitanKumar
Published: September 2010

In the paper, a two-phase, one-dimensional steady model of a proton exchange membrane fuel cell is developed for the porous cathode electrode. The interacted phenomena of cathode flooding and mass-transport limiting are investigated. In the model, the catalyst layer is treated as a separate computational domain with finite thickness and structure. Furthermore, through the Tafel kinetics, the mass transport processes of oxygen and liquid water are coupled. The transport of liquid water across the porous electrode is driven by the capillary force based on the Darcy’s law. However, the transport of gas is driven by the concentration gradient based on the Fick’s law. From the simulated results, it is found that the thin catalyst layer is very detriment for better understanding of the concurrent phenomena inside the electrode of a fuel cell, particularly, the flooding phenomena. More importantly, the saturation-level jump at the gas diffusion layer and catalyst layer interface is attained in order to satisfy the continuity of the capillary pressure on both sides of the GDL and CL interface. Meanwhile, the results show that the flooding phenomenon within the CL is more serious than that in the GDL, which has a significant influence on the mass transport of oxygen. Finally, the effect of some important parameters, such as the boundary value of saturation, absolute permeability, the cathode surface overpotential, on the saturation distribution inside the electrode is also obtained.
Three-Dimensional Molecular Dynamic Study on Accommodation Coefficients in Rough Nanochannels
Authors: Sun Jun , Li Zhixin
Published: September 2010

To understand the wall effects in nanochannels, the accommodation coefficients for argon gas in platinum walls were calculated according to a proper statistical algorithm in three-dimensional MD method in isothermal flows and thermal conductions. In smooth channels, the wall lattice arrangement may induce atomic roughness on the surface so that the accommodation coefficients were different for different lattice configurations. While in nanochannels with hemispherical nanoscale rough cells, the roughness increased the possibility of multiple gas-wall interactions. The NMAC in rough channels was less than unity and decreased with larger roughness. Meanwhile, the TMAC in the isothermal flows and the EAC in the thermal conductions in rough channels were larger than those in smooth channels, and they both increased with larger roughness.
Theoretical Discussions on Oscillating Heat Transfer Performance in Small Ducts
Authors: Peng Xiao-Feng , Zou Jiang , Yan Wei-Mon
Published: September 2010

A theoretical investigation was conducted to understand the temperature fluctuation corresponding to a typical perturbation velocity of sinusoidal oscillation in small ducts or small staggered ducts, and an emphasis was addressed on the characteristics of temperature fluctuation in a two-dimensional system. For a very small temperature fluctuation or flow oscillation at very large frequency, the local temperature fluctuates with the same frequency as the flow oscillation, and can be divided into two parts, each one caused by the flow oscillation in one flow direction. Normally the temperature fluctuation is directly proportional to the flow oscillation amplitude and the time average temperature gradient, and inversely proportional to the flow oscillation frequency. And the temperature fluctuation has nearly a phase angular difference close to π/2 with respect to velocity oscillation.
Molecular Dynamic Simulation of Surface Roughness Effects on Nanoscale Flows
Authors: Kharazmi Ali , Kamali Reza
Published: September 2010

In the present study, a molecular based scheme has been developed for simulating flows in nano- and micro-channels with roughness. In micro channel flow, there is some difference on the flow friction between roughness and cavitations which is not well studied. The presented approach is based on the molecular dynamics (namely MD) in which different ensemble has been used. For modeling the simulation the classical Newtonian particles are allowed to obey Newtonian mechanics and intermolecular forces are founded by integrating intermolecular potential. Lennard-Jones potential is used to model the interactions between particles. Particles equation of motion is integrated using fifth order Gear predictor-corrector. To ensure rapid sampling of phase space, the time step is made as large as possible. Periodic boundary condition is implemented via minimum image convention. Each atom of the solid wall is anchored at its lattice site by a harmonic restoring force and its temperature has been controlled by utilizing Nose-Hoover thermostat. The roughness is implemented on the lower channel wall. To make a comparison between the effect of roughness and cavitation, the same dimension is used for both for different aspect ratio. To allow comparison with previous results the same fluid density has been used. The effects of surface roughness and cavitation on velocity distribution of hydrophobic and hydrophilic wall undergoing Poiseuille flow are presented.
Ion Transport in 2-NM Nanochannels
Authors: Duan Chuanhua , Majumdar Arun
Published: September 2010

In this paper, we report ion transport in 2-nm-deep nanochannels. These nanochannels are formed by controlled dry etching in silicon wafer and following anodic bonding with pyrex substrate. Our results show that surface charge dominates ion transport in these channels at concentration up to 100 mM. Due to geometry confinement effect, these nanochannel take ultra long time to reach a steady state, 3 or 4 magnitude longer than bulk diffusion. Conductance data at different ionic concentrations are compared with simulation results from a one dimensional model that accounts for salt & pH-dependent surface charge. Difference between model and experimental data indicates that proton mobility at low concentration is one magnitude higher than bulk mobility while Na+/K+ mobility doens’t change at this length scale.
Two-Dimensional Unstructured Direct Simulation Monte Carlo Method for Micro/Nanochannel Gas Flows
Authors: Raisee Mehrdad , Mohammadi Shad Mahmood , Hosseinalipoor Seyyed Mostafa , Farokhirad Samaneh
Published: September 2010

In this paper, the fluid flow and heat transfer characteristics of two-dimensional micro/nanochannel flows are examined. The Direct Simulation Monte Carlo (DSMC) method for molecular gas dynamics is utilized to simulate the gas flows through two-dimensional micro/nanochannel. The collision process has been treated in a statistical way using the no-time-counter (NTC) scheme and the variable hard sphere (VHS) model has been employed to simulate the collision from the kinetic viewpoint. Results revealed that the temperature and velocity distributions have an unequalled behavior and may have a significant effect on the advancement and the design of MEMS and NEMS.
An Experimental Study of Surface Tension-Dependent Pool Boiling Characteristics of Carbon Nanotubes-Nanofluids
Authors: Murshed S. M. Sohel , Milanova Denitsa , Kumar Ranganathan
Published: September 2010

This paper reports an experimental investigation of the pool boiling heat transfer characteristics of single-walled carbon nanotubes (SWCNTs)-nanofluids. Two main characteristics were studied to identify their influence on boiling heat transfer: one is the surface tension through the addition of surfactant and the other is the chemical treatment of nanotubes sidewalls (i.e. oxidized and untreated sidewalls). A Transmission Electron Microscope was used to study the morphology of the functionalized nanotubes and their deposition on heater wire. The maximum enhancement of both the critical and burnout heat fluxes of this nanofluid over those of the pure deionized water are found to be 492% and 265%, respectively at a surfactant to carbon nanotubes concentration ratio of 1:5. This indicates that high enhancement of heat flux is possible and would depend on the concentration of the surfactants. Present results also demonstrate that CNT-nanofluids in a pool boiling environment can extend the saturated boiling regime and the burnout of the heated surface. The burnout heat flux is found to be a strong function of the relaxation of nanofluid surface tension with the base fluid. Based on the best fit of experimental data, an empirical correlation between the burnout heat flux of nanofluid and its relaxation of surface tension is introduced.
On the Road for a Mechanism of Thermal Conductance Enhancement in Nano-Fluids: Part I—General Introduction to the Chemical Actuator Mechanism
Authors: Reda M. R.
Published: September 2010

It was concluded after reviewing the literature that the existing medium theory (Maxwell model) cannot explain this anomalous increase of thermal conductivity of nano-particles that are suspended in fluid (nano-fluids). This new mechanism restricts the definition of nano-particles as particle that perfectly obeys the Gibbs rule of thermodynamics and act as a chemical actuator during heat transfer. This driving force for the chemical actuator is the microstructure phase change of the passive layer during heat transfer. The new mechanism require the presence of nano-convection (Brownian motion) in the vicinity of nano-particles surface, explain temperature and particles size dependency, require the presence of a shell or a sheath where chemical interaction occurs between the surface and the fluid and many others. The high rate of heat transfer of the nano-fluid is due to first, the bubbles formations inside the passive layer and their departure from grain boundaries and second due to nano-convection generated in the vicinity of the nano-particles of the nano-particles as result of the chemical actuation of the nano-particles. This nano-convection produces Brownian motion which increase rate of heat transfer.
Power Generation From Concentration Gradient by Reverse Electrodialysis in Ion Selective Nanochannel
Authors: Kim Dong-Kwon , Duan Chuanhua , Chen Yu-Feng , Majumdar Arun
Published: September 2010

In this article, ion selective nanochannels are studied to generate electric power from concentration gradient by reverse electrodialysis. When nanochannels bring into contact with aqueous solution, the surface of nanochannels acquires charges from ionization, ion adsorption, and ion dissolution. These surface charges draw counter-ions toward the surface and repel co-ions away. Therefore, when an electrolyte concentration gradient is applied to nanochannels, counter-ions are transported through nanochannels much more easily than co-ions, which results in a net charge migration of ions. Gibbs free energy of mixing, which forces ion diffusion, thus can be converted into electrical energy by using ion-selective nanochannels. Silica nanochannels with heights of 26 nm and 80 nm fabricated by glass-silicon anodic bonding were used in this study. We experimentally investigated the power generation from these nanochannels placed between two potassium chloride solutions with various combinations of concentrations. The power generation per unit channel volume increases when the concentration gradient increases, while it decreases as channel height decreases. The highest power density measured is 26 kW/m3. Our data also indicates that the efficiency of energy conversion and the ion selectivity increase with a decrease of concentrations and channel height. The best efficiency obtained is 24%. Compared with ion-selective membranes, nanochannels promise more reliable operation since they are readily compatible with standard CMOS process and do not shrink and swell in response to their environment. Power generation from concentration gradient in ion selective nanochannels could be used in a variety of applications, including micro batteries and micro power generators.
Experimental Investigation of Critical Heat Flux in Microchannels for Flow-Field Probes
Authors: Kos¸ar Ali , Peles Yoav , Bergles Arthur E. , Cole Gregory S.
Published: September 2010

Critical heat flux (CHF) of water in circular stainless steel microchannels with inner diameters ranging from ∼127μm to ∼254 μm was investigated. Forty-five CHF data points were acquired over mass velocities ranging from 1,200 kg/m2s to 53,000 kg/m2s, heated lengths from 2 cm to 8 cm, and exit qualities from −0.2 to 0.15. Most of the exit qualities fell below 0.1. It was found that CHF conditions were more dependent on mass velocity and heated length than on exit thermal condition. The results were also compared to six CHF correlations, with a mean average error ranging from 22% to 261.8%. A new correlation was proposed to better predict the critical heat flux data under the thermal-hydraulic conditions studied in this investigation. In developing the correlation, 319 data points were added from two previous studies.