bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Nanoparticle Deposition on Rewetting Temperature and Quench Velocity in Experiments With Stainless Steel Rodlets and Nanofluids
Authors: Kim H. , Buongiorno J. , Hu L. W. , McKrell T.
Published: September 2010

Quenching of small stainless steel rods in pure water and nanofluids with alumina and diamond nanoparticles at low concentrations (0.1 vol%) was investigated experimentally. The rods were heated to an initial temperature of ∼1000 °C and then plunged into the test fluid. The results show that the quenching behavior of the nanofluids is nearly identical to that of pure water. However, due to nanofluids boiling during the quenching process, some nanoparticles deposit on the surface of the rod, which results in much higher quenching rate in subsequent tests with the same rod. It is likely that particle deposition destabilizes the film-boiling vapor film at high temperature, thus causing the quenching process to accelerate, as evident from the values of the quench front speed measured by means of a high-speed camera. The acceleration strongly depends on the nanoparticle material used, i.e., the alumina nanoparticles on the surface significantly improve the quenching, while the diamond nanoparticles do not. The possible mechanisms responsible for the quench front acceleration are discussed. It is found that the traditional concept of conduction-controlled quenching cannot explain the acceleration provided by the nanoparticle layer on the surface.
Heat Transfer Characteristics of Gaseous Slip Flow in Concentric Micro Annular Tubes
Authors: Hong Chungpyo , Asako Yutaka , Suzuki Koichi
Published: September 2010

A concentric micro annular passage is a basic and important micro-geometry of micro-fluidic-systems from simple heat exchanger to the most complicated nuclear reactors. Therefore, heat transfer characteristics of gaseous flows in concentric micro annular tubes with constant heat flux whose value was positive or negative were numerically investigated. The slip velocity, temperature jump and shear stress work were considered on the slip boundary. The numerical methodology was based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The computations were performed for two thermal cases. This is, the heat flux was constant at the inner wall and outer wall was adiabatic (Case 1) and the heat flux was constant at the outer wall and the inner wall was adiabatic (Case 2). Each constant heat flux of 104 Wm−2 for the positive value and −104 Wm−2 for the negative value was chosen. The outer tube radius ranged from 20 to 150 μm with the radius ratio 0.02, 0.05, 0.1, 0.25 and 0.5 and the ratio of length to hydraulic diameter was 100. The stagnation pressure was chosen in such a way that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure was fixed at the atmospheric pressure. The heat transfer characteristics in concentric micro annular tubes were obtained. The wall and bulk temperatures with positive heat flux are compared with those of negative heat flux cases and also compared with those of the simultaneously developing incompressible flow. The results show that the compressible slip flow Nusselt number is different from that of incompressible flow. And, the temperatures normalized by heat flux have different trends whether heat flux value is positive or negative. A correlation for the prediction of the heat transfer characteristics of gas slip flow in concentric micro annular tubes is proposed.
Friction Factor Correlations of Gas Slip Flow in Concentric Micro Annular Tubes
Authors: Hong Chungpyo , Asako Yutaka , Suzuki Koichi
Published: September 2010

A concentric micro annular passage is a basic and important micro-geometry of micro-fluidic-systems from simple heat exchangers to the most complicated nuclear reactors. Therefore, the product of friction factor and Reynolds number (f·Re) for quasi-fully developed high speed and slip flow in a concentric micro annular tubes of Re<1000 and Ma<1.0 was obtained numerically. The numerical methodology was based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The compressible momentum and energy equations with slip boundary conditions were solved for both isothermal flow and no-heat conduction flow conditions. The outer tube radius ranged from 5 to 40 μm with the radius ratios of 0.2, 0.5 and 0.8. The ratio of length to hydraulic diameter was 100. The stagnation pressure was chosen in such a way that the exit Mach number ranged from 0.1 to 1.0. The outlet pressure was fixed at the atmospheric pressure. The value of f·Re for compressible slip flow in concentric micro annular tubes were obtained. The detail of the incompressible slip f·Re is documented and its value defined as a function of r* and Kn is represented. The results show that in the case of fast flow, the values of f·Re for compressible slip flow is higher than those for incompressible slip flow due to compressibility effects. Also, the f·Re correlation for compressible slip flow is obtained from that of compressible no-slip flow and incompressible slip flow as a function of Mach and Knudsen numbers and radius ratio. In addition, a f·Re correlation from the values of f·Re obtained for micro-channel, micro-tube, and micro annular tube referred in author’s previous works that is applicable to micro-geometries whose cross-sections are parallel, rectangular, circular and annular, is proposed.
Molecular-Dynamic Simulations on Aqueous Solutions Confined Between Uniformly Charged Hydrophobic Plates
Authors: Hai Hoang , Kang Sangmo , Suh Yong Kweon
Published: September 2010

In this paper, we present the numerical study on the properties of aqueous solution confined between uniformly charged hydrophobic plates using the molecular-dynamic simulations. Two kinds of aqueous solutions used in our study are aqueous sodium and aqueous chloride. The SPC/E model and combination of quaternion coordinates with Euler angles are used to model the water molecules and constraint the structure of water molecules, respectively. The hydrophobic plates are separated by 2.6 nm and the space contains 680 water molecules and 20 ions. The charge density on each surface of the plate is the same with each other and satisfies the charge-neutralization criterion over the whole domain. We employed the modified Ewald summation for the uniformly charged surface PPPM method (particle-particle-particle-mesh) to compute the long-range interactions. The static and dynamic properties of solvent and ions are computed. We also compare our results with those provided by solving the Poisson-Boltzmann equation.
Compact Micro-Mixer Using the Switching Electroosmosis Flow
Authors: Higashi Hideaki , Matsumoto Ryosuke
Published: September 2010

Rapid mixing is essential in the micro-fluidic systems. The purpose of this study is to develop the compact micro-mixer by switching the electroosmosis flow on the simple cross channel. Micro-mixers have been developed in a variety of ways; by the parallel and serial lamination, and the chaotic advection on the passive micro-mixers. On the active micro-mixers, the external disturbance effects such as pressure, electrohydrodynamics and acoustic are applied to enhance the mixing. However, the mixing process occurs at the mixing channel located at the downstream of junction or the mixing chamber, thus the mixing length is required to achieve a homogeneous mixing. In this study, the new concept of the mixing process, a batch processing mixing, is applied to the micro-mixer. The mixing and the transport processes are separated by switching the electroosmosis flow. On the mixing phase, the oscillating reversed flow induces the mixing at the junction of the cross channel. After the mixing process, the liquid mixture flows out through the outlet channel. The mixing is completed at the junction, thus the outlet mixing-channel is not necessary for the batch processing mixing. Experiments are carried out on the PDMS micro cross-channel chip. Channel is 200μm in width and 50μm in depth. The concentration field is measured by the fluorescence technique. The electrical field in the micro channel is generated by the high voltage power supply and is switched by the relay circuit controlled by the D/A converter. The mixing degree reaches about 90% within 0.9mm length of the outlet channel in the case of 500V applied voltage with the frequency of 1.41Hz. On the batch processing mixing, however, the reversed flow occurred at the mixing phase, the flow rate of the liquid mixture becomes small than that in the steady state voltage pattern. To increase the flow rate while keeping the same flow pattern, the electroosmotic velocity and the switching frequency are increased to keep the same Strouhal number. In the case of 900V applied voltage with 2.55Hz, the flow rate becomes 1.8 times keeping the same mixing degree. The batch processing mixing is effective for the compacting the micro-mixer.
Bubble Dynamics and Pressure Drop of a Single Bubble in a Rectangular Microchannel
Authors: Choi Chiwoong , Yu DongIn , Kim Moohwan
Published: September 2010

Most correlations of pressure drop in microchannel have been developed based on modifying of C-value in Lockhart and Martinelli’s correlation for various flow regimes. However, the pressure drop is highly related with flow pattern. And the elongated bubble flow is major flow regime in microchannel. To study behaviors and pressure drop of bubble flow regime in microchannel, a glass rectangular microchannel was fabricated using a photosensitive glass. Experiments of adiabatic two-phase flow in rectangular microchannels were conducted with hydraulic diameters of 503 and 322 μm. Moreover, the visualization of bubble flow patterns was achieved with a high-speed camera and a long-distance microscope. The bubble velocity, lengths of liquid slug and bubble and void fraction were evaluated based on a unit cell model using image processing. The pressure drop of single bubble in a rectangular microchannel was extracted from measured total pressure drop. Especially, pressure drop correlation of a single elongated bubble was achieved for Capillary number and diameter.
Subcooled Flow Boiling in a Minichannel
Authors: Oshima Akira , Suzuki Koichi , Hong Chungpyo , Mochizuki Masataka
Published: September 2010

It has been considered that the dry-out is easy to occur in boiling heat transfer for a small channel, a mini or microchannel because the channel was easily filled with coalescing vapor bubbles. In the present study, the experiments of subcooled flow boiling of water were performed under atmospheric condition for a horizontal rectangular channel of which size is 1mm in height and 1mm in width with a flat heating surface of 10mm in length and 1mm in width placed on the bottom of the channel. The heating surface is a top of copper heating block and heated by ceramics heaters. In the high heat flux region of nucleate boiling, about 70 ∼ 80 percent of heating surface was covered with a large coalescing bubble and the boiling reached critical heat flux (CHF) by a high speed video observation. In the beginning of transition boiling, coalescing bubbles were collapsed to many fine bubbles and microbubble emission boiling was observed at higher liquid subcooling than 30K. The maximum heat flux obtained was 8MW/m2 (800W/cm2) at liquid subcooling of higher than 40K and the liquid velocity of 0.5m/s. However, the surface temperature was extremely higher than that of centimeter scale channel. The high speed video photographs indicated that microbubble emission boiling occurs in the deep transition boiling region.
Subcooled Boiling in the Ultrasonic Field (on Cause of MEB Generation)
Authors: Inagaki Fumio , Suzuki Koichi , Hong Chungpyo
Published: September 2010

Subcooled quasi-pool boiling for water, ethanol aqueous solutions of 10wt% and 50wt% and ethanol in ultrasonic field is performed for the upward flat heating surface of copper block with 10mm in diameter under the atmospheric condition. Tested liquid subcooling is 15K, 20K and 25K for water and aqueous solutions of ethanol and 20K, 30K and 40K for 100wt% ethanol. At 20K of liquid subcooling for water and ethanol aqueous solutions, no microbubble emission boiling (MEB) has been observed in quasi-pool boiling. Even if MEB occurred, the heat flux does not increase and it turns easily to film boiling. In ultrasonic field, MEB occurs remarkably and the heat flux increases higher than the ordinary critical heat flux as observed in highly subcooled boiling. The experimental results show that the ultrasonic vibration introduces the instability of interface of liquid and vapor and accelerate MEB at 20K of liquid subcooling for water and aqueous solutions of ethanol. At 15K of liquid subcooling for water and aqueous solutions, no effect of ultrasonic vibration is observed. At 25K of liquid subcooling, the ultrasonic vibration extends MEB region to higher superheat of heating surface for aqueous solutions of ethanol. The maximum heat flux in MEB decreases with increasing of ethanol concentration and becomes CHF for 100wt% ethanol. No effect of ultrasonic vibration on boiling is observed for the 100wt% ethanol in the present experiments.
A Study of Critical Heat Flux of Butanol Aqueous Solution
Authors: Nishiguchi Shotaro , Shoji Masahiro
Published: September 2010

Some alcohol aqueous solution such as butanol shows nonlinear surface tension dependence. Namely, contrary to ordinary liquid or solution, surface tension increases with temperature at the range of high liquid temperature. At the triple-phase point on a heated surface, the thermo-capillary force acts for the liquid to wet the heated surface, so the solutions are sometimes called as “self-rewetting liquid”. Self-rewetting liquids may prohibit the dry-out of a heated surface so that the heat transfer performance would be enhanced. For this reason, applications of self-rewetting liquids to heat transfer devices such as heat pipes are actively studied in recent years. However, the heat transfer characteristics of boiling of self-rewetting liquids are not fully understood. In the present research, a boiling experiment of butanol aqueous solution was performed on a heated wire in order to make clear the fundamental heat transfer characteristics, especially Critical Heat Flux (CHF), by changing solution concentration density and liquid temperature in a wide range. Bubbling aspects were observed by high-speed video camera with the rate of 1000 frames per second. It is found from the experiment that CHF is generally enhanced when compared to the case of pure water. CHF increases with concentration density at any temperatures. CHF generally increases with subcooling but at low subcooling region, it once decreases and then increases after taking a minimum. It is also found that peculiar boiling takes place where many tiny bubbles generate and bubbles are unlikely to coalesce. At high subcoolings, the mode of boiling similar to the so-called MEB (Micro-bubble Emission Boiling) was observed. These results of the present experiment indicate a possible application of butanol aqueous solution to high-performance-cooling-devices utilizing micro-channels because generating bubbles are small enough so that the pressure loss of the coolant may be small and the heat transfer rate is sufficiently high even at the saturated condition.
The Critical Heat Flux in Nucleate Boiling Heat Transfer: Part I—The Chemical Actuator Mechanism
Authors: Reda M. R.
Published: September 2010

Nucleate boiling heat transfer is first introduced and the literature is reviewed. It was concluded that the passive layer and the grain boundaries are responsible for the transfer to the nucleate boiling regime. Based on the recent work of Biener and his collaborators (Nature Material 2008) and the Gibbs rule of thermodynamics, a possible mechanism was outlined. The mechanism assumes that each grain in the passive layer act as a chemical actuator which is driven by microstructure phase change. The new mechanism agrees well with the experimental results, in good agreement with previous models and can explain why and how CHF occurs.
Long-Wave and Integral Boundary Layer Analysis of Falling Film Flow on Walls With Three-Dimensional Periodic Structures
Authors: Gambaryan-Roisman Tatiana , Yu Hongyi , Lo¨ffler Karsten , Stephan Peter
Published: September 2010

Falling films exhibit very complex wavy patterns, which depend on the properties of the liquid, the Reynolds number, the wall inclination angle, and the distance from the film inlet. The film hydrodynamics and the surface patterns have a high impact on heat and mass transfer. Our aim is to control and enhance heat and mass transport by using walls with specific micro topographies that influence the falling film flow, stability and wavy pattern. In the present work long-wave theory and integral boundary layer (IBL) approximation are used for modelling the falling film flow on walls with three-dimensional periodic microstructures. The wall topography is periodic both in the main flow direction and in the transverse direction. Examples of such microstructures are longitudinal grooves with sinusoidal path (or meandering grooves) and herringbone structures. The effects of the Reynolds number, the wall inclination angle and the longitudinal and transverse periods of the structure on the shape of liquid-gas interface are investigated. It is shown that, as opposed to straight grooves in longitudinal direction, grooves with meandering path may lead to significant interface deformations.
1-D Modelling and 3-D Simulation of Confined Bubble Formation and Pressure Fluctuations During Flow Boiling in a Microchannel With a Rectangular Cross-Section of High Aspect Ratio
Authors: Gedupudi S. , Zu Y. Q. , Karayiannis T. G. , Kenning D. B. R. , Yan Y. Y.
Published: September 2010

A simple 1-D model with low requirements for computing time is required to investigate parametric influences on the potentially adverse effects of pressure fluctuations driven by confined vapour bubble growth in microchannel evaporative cooling systems operating at high heat fluxes. A model is developed in this paper for the particular conditions of a channel of rectangular cross-section with high aspect ratio with a constant inlet flow rate (zero upstream compressibility). (The model will later be extended to the conditions of finite upstream compressibility that lead to transient flow reversal). Some parametric trends predicted by the model are presented. The simplifying assumptions in the model are examined in the light of a 3-D simulation by a commercial CFD code, described in an accompanying paper by the same authors. The predictions of pressure changes are in reasonable agreement. It is suggested that the 1-D model will be a useful design tool.
Experimental Investigation of Non-Uniform Heating on Flow Boiling Instabilities in a Microchannels Based Heat Sink
Authors: Bogojevic D. , Sefiane K. , Walton A. J. , Lin H. , Cummins G. , Kenning D. B. R. , Karayiannis T. G.
Published: September 2010

Two-phase flow boiling in microchannels is one of the most promising cooling technologies able to cope with high heat fluxes generated by the next generation of central processor units (CPU). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as a cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated, with hotspots located in different positions along the heat sink. It was observed that boiling inside microchannels with non-uniform heating led to high temperature non-uniformity in transverse direction.
Electrokinetics of Nanochannels: The Next Nanotechnology for Ion/Molecule/Nanocolloid Sensing, Concentration and Filtration
Authors: Chang Hsueh-Chia
Published: September 2010

Because nanochannels act as lenses that can focus electric fields on a chip, they can concentrate and filter ions, molecules and nanocolloids by ion selectivity, electrodeless dielectrophoresis and surface hybridization. However, before fabricated nanochannels can lead to the next-generation of chip-scale dialysis membranes, preconcentrators for mass spectrometers, ion sieves, molecular sensors, solar/fuel cell membranes etc, the anomalous dc I-V characteristics and ac impedance of such nanochannels must be understood. Curious phenomena such as limiting and over-limiting currents, rectification, hysteretic cyclic voltametry, Warburg impedance, channel cross-talk etc will be studied with fabricated nanochannels, confocal microscopy and fluorescence correlation spectroscopy and analyzed mathematically with fundamental Nernst-Planck ion transport theories, Guoy-Chapman equilibrium electrolyte theories and new non-equilibrium extended polarization theories.
Fuel Cells for Aircraft Application
Authors: Friedrich Kaspar Andreas , Kallo Josef , Schirmer Johannes
Published: September 2010

Although air transport is responsible for only about 2% of all anthropogenic CO2 emissions, the rapidly increasing volume of air traffic leads to a general concern about the environmental impact of aviation. Future aircraft generations have to face enhanced requirements concerning productivity, environmental compatibility and higher operational availability, thus effecting technical, operational and economical aspects of in-flight and on-ground power generation systems. Today’s development in aircraft architecture undergoes a trend to a “more electric aircraft” which is characterised by a higher proportion of electrical systems substituting hydraulically or pneumatically driven components, and, thus, increasing the amount of electrical power. Fuel cell systems in this context represent a promising solution regarding the enhancement of the energy efficiency for both cruise and ground operations. For several years the Institute of Technical Thermodynamics of the German Aerospace Center (Deutsches Zentrum fu¨r Luft- und Raumfahrt, DLR) in Stuttgart is engaged in the development of fuel cell systems for aircraft applications. The activities of DLR focus on: • Identification of fuel cell applications in aircraft in which the properties of fuel cell systems, namely high electric efficiency, low emissions and silent operation, are capitalized for the aircraft application. • Design and modeling of possible system designs. • Experimental investigations regarding specific aircraft relevant operating conditions. • Qualification of airworthy fuel cell systems. • Set up and full scale testing of fuel cell systems for application in research aircraft. In cooperation with Airbus several fuel cell applications within the aircraft for both ground and cruise operation could be identified. In consequence, fuel cell systems capable to support or even replace existing systems have been derived. In this context, the provision of inert gas for the jet fuel (kerosene) tank and electrical cabin power supply including water regeneration represent the most promising application fields. The contribution will present the state of development discussing the following points: • Modeling of different system architectures and evaluation of promising fuel cell technologies (PEFC vs. SOFC). • Experimental evaluation of fuel cell systems under relevant conditions (low-pressure, vibrations, reformate operation, etc.). • Fuel cell system demonstrator Hyfish (hydrogen powered model aircraft). • Fuel cell test in DLR’s research aircraft ATRA (A320) including the test of an emergency system based on hydrogen and oxygen with 20 kW of electrical power. The fuel cell system was integrated into an A320 aircraft and tested up to a flight altitude of 25 000 feet under several acceleration and inclination conditions.
In-Situ Measurement in Through-Plane Direction in PEMFC
Authors: Ito Kohei , Lee Sangkun , Yamamoto Atsushi , Hirano Masaaki , Muramatsu Hidetaka , Sasaki Kazunari , Ogawa Kuniyasu
Published: September 2010

Water management is a large issue for putting PEMFC to practical use. Appropriate water management enables us to suppress the drying in PEM (Polymer Electrolyte Membrane) and the flooding in GDL (Gas Diffusion Layer), which degrade the performance of PEMFC. Against the background of importance for the water management, we challenged to develop the measurement method to grasp the water behavior in PEMFC. Especially, we focused on through-plane direction measurement, because the through-plane direction in cell has major role for the transport of mass, heat and electric charge in the cell. We developed the three methods to measure the water in cell directly or indirectly: cross sectional cell: micro NMR-sensor array: micro thermocouple array. These three methods successively captured the distribution of the liquid water in GDL, the water content in PEM and the temperature in cell. The data obtained help us to give the possible mechanism of how the water in cell impacted the cell voltage.
Thermal Characterization of Interlayer Microfluidic Cooling of Three-Dimensional IC With Non-Uniform Heat Flux
Authors: Kim Yoon Jo , Joshi Yogendra K. , Fedorov Andrei G. , Lee Young-Joon , Lim Sung Kyu
Published: September 2010

It is now widely recognized that three-dimensional (3D) system integration is a key enabling technology to achieve the processing speeds and performance needs of future microprocessor integrated circuits (ICs). To provide modular thermal management in 3D stacked ICs, interlayer microfluidic cooling scheme is adopted and analyzed in this study. The effects of cooling scheme and essential geometry variations on the routing completion and congestion of electrical interconnect are quantitatively analyzed. Also, the thermal and hydraulic performance of several two-phase refrigerants is discussed in comparison with single-phase cooling. The results show that refrigerants in two-phase flow are thermally preferred due to the higher heat transfer coefficients, and relatively constant fluid temperature throughout the microchannel. However, the large internal pressure and pressure drop act as significant limiting factors in realizing the merits of two-phase cooling. It is also concluded that integration of high performance hot-spot thermal management is a key to addressing a challenge of mass flow rate mal-distribution.
Nucleate Pool Boiling: The Dominant Bubble Heat Transfer Mechanisms
Authors: Kim Jungho
Published: September 2010

Enhanced convection, transient conduction, microlayer evaporation, and contact line heat transfer have all been proposed as mechanisms by which bubbles transfer energy during boiling. Models based on these mechanisms contain fitting parameters that are used to fit them to the data, resulting a proliferation of “validated” models. A review of the recent experimental, analytical, and numerical work into single bubble heat transfer is presented to determine the contribution of each of the above mechanisms to the overall heat transfer. Transient conduction and microconvection are found to be the dominant heat transfer mechanisms.
Transitional Flow and Related Transport Phenomena in Complex Microchannels
Authors: Kockmann Norbert , Holvey Craig , Roberge Dominique M.
Published: September 2010

In microchannels with typical dimensions from 10 μm to few hundreds μm, the flow is dominated by viscous forces, leading often to laminar flow conditions. At the entrance or in bends and curves, where the flow is accelerated or changes its direction, inertial forces generate transverse flow velocities. Due to continuity, compensating transverse velocity components generate vortex pairs, such as Dean flow in circular bends. The flow is still laminar, steady, and shows no statistically distributed fluctuations typical for turbulent flow. This deviation from straight laminar conditions, often in larger channels (100 μm to few mm) or for higher flow rates, is called transitional flow. That embraces the first occurrence of pulsating vortices, period doubling of vortex pairs, flow bifurcation, and regularly fluctuating wake flow or vortex shedding. With increased flow velocity, this process leads to chaotic flow phenomena being first evidence of turbulence. This paper describes the transitional flow characteristics in single channel elements such as bends and T-junction as well as around fins and posts in channels. These elements are used to augment the transport characteristics in microchannels for enhanced heat and mass transfer and for performing chemical reactions in microreactors. The profound understanding of the flow characteristics is fundamental for the understanding of transport phenomena. Additionally, this knowledge can be used to design successful microstructured devices for various applications by knowing how to generate and control vortices in microchannels. Concepts from chaotic advection are presented here to describe vortex flow and related transport characteristics. Though recent advances has shed new light on transport phenomena in complex channel structures, many issues are still unknown and huge potential is hidden in optimized channel devices.
In Situ Characterization of Two-Phase Flow in Cathode Channels of an Operating PEM Fuel Cell With Visual Access
Authors: Sergi Jacqueline M. , Lu Zijie , Kandlikar Satish G.
Published: September 2010

Water management is a critical factor in the performance and durability of a proton exchange membrane (PEM) fuel cell. In situ experiments are needed to gain a better understanding of water transport within the channels of the cell during operation. In this work a 50 cm2 fuel cell with optical access is designed and tested in an in situ experimental facility. Two-phase flow in the cathode channels of the cell is observed, and flow patterns are characterized. Three primary two-phase structures are identified — slug flow, film flow, and mist flow — and a flow pattern map is developed. A comparison between in situ and ex-situ flow pattern maps shows that ex-situ experimentation can be used to predict some in situ flow characteristics, but cannot capture the effects of reaction kinetics or relative humidity. The total pressure drop signature is seen to be a useful parameter for predicting two-phase flow dynamics in the gas channels. In addition, channel to channel flow variation caused by the presence of liquid water in the cathode channels is investigated using entrance region pressure drop measurements.
An Aptamer-Functionalized Microfluidic Platform for Biomolecular Purification and Sensing
Authors: Nguyen Thai Huu , Lin Qiao
Published: September 2010

Aptamers are oligonucleotides (DNA or RNA) that bind to chemical and biological analyte targets via affinity interactions. Through an in vitro synthetic process, aptamers can be developed for an extremely broad spectrum of analytes, such as small molecules, proteins, cells, viruses, and bacteria. Target recognition by aptamers is highly selective, as affinity interactions result in secondary aptamer conformational structures that specifically fit the target. The aptamer-target binding is also reversible and depends strongly on external stimuli such as pH and temperature. The specificity and stimuli-responsiveness of aptamers are highly attractive to biological purification and sensing, which generally involve isolating minute quantities of targets from complex samples with non-specific molecules and impurities present at orders-of-magnitude higher concentrations. We present an aptamer-functionalized microfluidic platform that by design exploits the specificity and temperature-dependent reversibility of aptamers to enable biomolecular purification and sensing. Using the specificity of aptamers, we demonstrate highly selective capture and enrichment of biomolecules. Employing thermally induced, reversible disruption of aptamer-target binding, we accomplish isocratic elution of the captured analytes and regeneration of the aptamer surfaces, thereby eliminating the use of potentially harsh reagents. Using integrated microfluidic control, the eluted analytes are detected in a label-free fashion by mass spectrometric methods.
Cryogenic and Fluidic Ways Lead to Low Cost Micro/Nano Devices
Authors: Liu Jing , Yang Yang
Published: September 2010

Building systems as compactly as possible has been a major theme in modern science and engineering practices. However, such enthusiastic endeavor often encounters big troubles due to high cost and complexity of the process it involves. Part of the reasons comes from the methodology itself, the fabrication, designing and characterization procedure etc. Among various disciplines to making micro/nano object, those enabled from the thermal and hydrodynamic science plays a rather important role. In this article, we will illustrate a cryogenic way for realizing a group of different micro/nano devices which can be implemented as mechanical, hydraulic, electrical, or optical functional units. The basic principle of the method lies in the formation of ice crystals in small area, from which micro/nano aqueous objects or signals transmitting across them can be blocked, manipulated and analyzed. In this way, a series of micro/nano devices such as freeze tweezer, ice valve, freeze-thaw pump, electrical or optical signal switch and micro thermal analyzer etc. can be developed via a rather simple and low cost way. As examples, some latest advancement made in the authors’ lab will be reviewed. Their innovative applications in a wide variety of micro/nano engineering fields will be discussed. Further, to illustrate the low cost way to directly manufacture micro/nano objects, we will explain a bubble fabrication method whose basic principle lies in the chemical reaction occurring at the fluidic interfaces between two or more soap adjacent bubbles. A unique virtue of the bubble is that it can have a rather huge diameter however an extremely small membrane thickness, whose smallest size can even reach nano scale. Therefore, the administrated chemical reaction in the common interface of the contacting bubbles would lead to products with extremely small size. Particularly, all these results were achieved via a rather straightforward way. The bubble builds up a bridge between the macroscopic manipulation/observation and the fabrication in small world. Several typical micro structures as fabricated in the lab will be illustrated. As a flexible, easily controllable, and low cost method, the bubble fabrication can possibly be developed as a routine strategy for making micro/nano structures in the near future.
Capillarity, Wettability and Interfacial Dynamics in Polymer Electrolyte Fuel Cells
Authors: Mukherjee Partha P.
Published: September 2010

In the present scenario of a global initiative toward a sustainable energy future, the polymer electrolyte fuel cell (PEFC) has emerged as one of the most promising alternative energy conversion devices for different applications. Despite tremendous progress in recent years, a pivotal performance/durability limitation in the PEFC arises from liquid water transport, perceived as the Holy Grail in PEFC operation. The porous catalyst layer (CL), fibrous gas diffusion layer (GDL) and flow channels play a crucial role in the overall PEFC performance due to the transport limitation in the presence of liquid water and flooding phenomena. Although significant research, both theoretical and experimental, has been performed, there is serious paucity of fundamental understanding regarding the underlying structure-transport-performance interplay in the PEFC. The inherent complex morphologies, micro-scale transport physics involving coupled multiphase, multicomponent, electrochemically reactive phenomena and interfacial interactions in the constituent components pose a formidable challenge. In this paper, the impact of capillary transport, wetting characteristics and interfacial dynamics on liquid water transport is presented based on a comprehensive mesoscopic modeling framework with the objective to gain insight into the underlying electrodics, two-phase dynamics and the intricate structure-transport-interface interactions in the PEFC.
Can We Learn From Nature to Design Membranes? The Intricate Pore Structure of the Diatom
Authors: Rosengarten Gary
Published: September 2010

Membranes are ubiquitous functional elements used in separation processes. An ideal membrane will stop certain species penetrating it while having excellent transport properties for others. Membranes are used in synthetic systems such as fuel cells and desalination plants, but are also formed naturally in biological systems. For example all cells use a membrane to contain the cellular contents, while allowing transport of nutrients though the cell wall. I will present our recent work on examining diatoms, which are unicellular algae that grow in water. They have a self assembled silica membrane wall with a regular array of nanopores whose function is very poorly understood. I will outline the unique structure of the pores and our experimental work on understanding their structure to help develop membranes with better performance.
Wetting and Spreading of Liquid Metals Through Open Micro Grooves and Surface Alterations
Authors: Sekulic Dusan P.
Published: September 2010

Surface tension driven flows of micro layers of complex liquids over substrates under reactive wetting conditions are greatly influenced by interface interactions and topography of surface alterations. For example, understanding of spreading of molten metals over metal substrates with complex topography may be interpreted as spreading over multiple connected networks of open micro channels. Hence, understanding of the kinetics of wetting and spreading of such reactive systems through micro channels is of a key interest. This keynote lecture will provide an overview of wetting/spreading phenomena related to migration of the molten metal micro layer over smooth, rough, and/or well-organized-topography surfaces, such as micro channels. Systems involving a liquid metal medium temperature range (Al-Si over Al), and a low temperature range (Pb-Sn and Ag-Sn over Cu and Cu-Sn) have been considered. Kinetics data involving triple line movement and its modeling will be supported by real-time in situ visualizations. Targeted applications of these fundamental studies involve art of brazing of compact aluminum heat exchangers for HVAC&R, thermal management for aerospace, and soldering processes (in particular lead-free) for electronics industries.
Liquid Film Thickness in Micro Channel Slug Flow
Authors: Shikazono Naoki , Han Youngbae
Published: September 2010

Slug flow is the representative flow regime of two-phase flow in micro channels. It is well known that the thin liquid film formed around the confined vapor bubble plays an important role in micro channel heat transfer. In the present study, experiments are carried out to clarify the effects of parameters that affect the formation of the thin liquid film in micro channel slug flow. Laser focus displacement meter is used to measure the thickness of the thin liquid film. Air, ethanol, water and FC-40 are used as working fluids. Circular tubes with five different diameters, D = 0.3, 0.5, 0.7, 1.0, 1.3 mm, and square channels with two different sizes, 0.3 × 0.3 and 0.5 × 0.5 mm, are used. It is confirmed that the liquid film thickness is determined only by capillary number at small capillary numbers. However, the effect of inertial force and flow acceleration cannot be neglected as capillary number increases. The effect of cross sectional shape is also investigated. Experimental correlation for the adiabatic liquid film thickness in circular tubes based on capillary number, Reynolds number and Weber number is proposed. When viscous boundary layer is thin, liquid film thickness is limited by the viscous boundary layer thickness. Thus, in order to develop precise flow boiling models in micro tubes, it is important to consider the effects of inertial force and boundary layer thickness on the liquid film thickness.
Model-Based Shape Design and Fault Diagnosis for Microreactors
Authors: Tonomura Osamu
Published: September 2010

In micro-spaces, viscous force, surface tension, conduction heat transfer, and molecular diffusion become dominant. These features achievable in micro-spaces make it possible to handle highly exothermic and rapid reactions and to produce fine particles with narrow size distribution. The goal of our research is to develop a fundamental approach to design, operation, and control of microreactors. In the first half of this paper, three types of systematic design methods are explained. In the second half of this paper, the development of process monitoring system, which is applicable for effectively diagnosing faults such as blockage and catalyst deterioration in microreactors, is explained.
Micro/Meso Scale Modelling of Two-Phase Flow on Functional Surfaces: A Numerical Simulation of Water Droplets on Natural Hydrophobic Surfaces With Micro Roughness
Authors: Yan Y. Y.
Published: September 2010

A micro/meso scale modelling of two-phase droplets move on hydrophilic/hydrophobic surfaces with micro roughness is reported. The physical model is basically of two-phase flow interacting with the surfaces of different hydrophobicity or wettability. Numerical modelling based on the lattice Boltzmann method (LBM) is developed and applied to the computational calculation and simulation. The LBM modelling deals with surface tension dominated behaviour of water droplets in air spreading on a hydrophilic surface with hydrophobic strips of different sizes and contact angles under different physical and interfacial conditions, and aims to find quantitative data and physical conditions of the biomimetic approaches. The current LBM can be applied to simulate two-phase fluids with large density ratio (up to 1000), and meanwhile deal with interactions between a fluid-fluid interface and a partial wetting wall. In the simulation, the interactions between the fluid-fluid interface and the partial wetting wall with different hydrophobic strips such as single strip, intersecting stripes, and alternating & parallel stripes, of different sizes and contact angles are considered and tested numerically; the phenomena of droplets spreading and breaking up, and the effect of hydrophobic strips on the surface wettability or self-cleaning characteristics are simulated, reported and discussed.
Numerical Simulation of Thermocapillary-Buoyancy Convection in Encapsulated Liquid Column With Deformable Interfaces
Authors: Peng Lan , Liu Juan-Fang , Li You-Rong , Wang Yan
Published: September 2010

The physical and mathematical models of thermocapillary-buoyancy convection in two immiscible liquid layers are established. In order to capture the free surface and the liquid-liquid interface deformation, Runge-Kutta method is adopted to solve the Young-Laplace equation. Numerical simulation of thermocapillary-buoyancy convection in encapsulated liquid bridge is performed by employing primitive valuable method and finite volume method. The fluids of liquid bridge and liquid encapsulation are FC-70 and KF-96, respectively. Furthermore, the profiles of temperature and velocity in encapsulated liquid bridge are obtained. Meanwhile, the effects of a series of non-dimensional parameters on the thermocapillary-buoyancy convection are analyzed.
Research and Development on No-Moving-Part Valves Using Enhancement
Authors: Yang Kai-Shing , Liu Young-Chang , Wang Chi-Chuan , Shyu Jin-Cherng
Published: September 2010

This study characterizes and analyzes the performances of micro diffusers/nozzles with six types of enhancement structures. The pressure drops across the designed micro nozzles/diffusers are found to be increased considerably when the obstacle and fin structure are added. Further, the micro nozzle/diffuser having added circular area reveals the lowest pressure drop, owing to the hydraulic diameter is increased by circular area and lower interface friction. The maximum improvement of the loss coefficient ratio is about 16% for an added 3-fin structure operated at a Reynolds number around 70. Upon this situation, the static rectification efficiency improves 4.43 times than the conventional nozzle/diffuser. Experimental results indicate the performance peaks at a Reynolds number around 70, and an appreciable decline is encountered when the Reynolds number is reduced. It is due to the efficiency ratio of conventional micro nozzle/diffuser significant increases with the Reynolds number.
Fabrication of Dielectrophoretic Microfluidic Device
Authors: Kumar G. Naga Siva , Mitra Sushanta K. , Rao V. Ramgopal
Published: September 2010

Technological needs of the recent times require the improvement in micro-scale devices that manipulate the bioparticles like cells, bacteria, viruses, DNA, proteins, etc. Such devices have diverse and widespread applications in biomedical, drug delivery and diagnostics for separating, trapping, sorting and mixing of particles. Dielectrophoresis (DEP) is one of the techniques used for manipulating the particles in a nonuniform electric field. In the present study, fabrication and characterization of microfluidic device for DEP is analyzed and experimented. An overview of fabrication techniques which can be used for making of DEP device is provided with experimental details. DEP microfluidic device is fabricated by preparing channels and microelectrodes on PDMS and glass materials respectively. Oxygen plasma treatment has been used for bonding the PDMS channel and micro-electrode patterned glass substrate. Further experiments are conducted to demonstrate the DEP principle with polystyrene microbeads. The movement of microbeads towards the high electric field strengths at 12Vpp and 10 MHz frequency is observed. Characterizing equipments like ellipsometer, profilometer, scanning electron microscopy, contact angle measurement systems were used for measuring oxide layer thickness, width and depth of the channels, surface characteristics etc., during fabrication.
Effect of Surface Wetting of Micro/Nano Ratchets on Leidenfrost Liquid Drop Motion
Authors: Ok Jeong Tae , Lopez-On˜a Eugene , Wong Harris , Park Sunggook
Published: September 2010

A micro- and nanoscale asymmetric potential enhances propulsion of liquid drop motion in the Leidenfrost (film boiling) regime. However, the requirement of high operation temperature to levitate a liquid droplet is a critical drawback for practical application of micro- and nanosystems. In this study, we show that the threshold temperature for the droplet motion can be reduced by controlling the wetting properties of ratchet surfaces. Various sizes of metallic ratchets down to sub-micrometer period were fabricated using micro- and nanoscale machining tools. Then, the wettability of the ratchet surfaces was modified by coating hydrophobic fluorinated silane molecules after activating the ratchet surfaces with oxygen plasma. Room temperature contact angle induced by the silane coating increased significantly (∼ 20° for brass ratchets and ∼ 70° for nickel ratchets). We found that the hydrophobic coating of ratcheted surface significantly affected the levitating droplet motion, both decreasing the threshold temperature and increasing the average droplet velocity.