bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface Modifications of Bulk Micromachined Titanium Pillar Arrays: A Wick Material for Thin Flat Heat Pipes
Authors: Ding Changsong , Meinhart Carl D. , MacDonald Noel C.
Published: October 2010

Wicking materials with tunable wettability are of great importance for both fundamental research and practical applications such as heat pipes. In this work, we adopt recently developed titanium bulk micromachining[1] techniques to fabricate pillar arrays. Then we modify the micromachined pillars to form micro- & nano-textured (bitextured) titania structures (BTS). Further, we investigated how to plate gold on the modified surfaces to tune the wettability. A wicking material for heat pipe requires super wetting by common fluids such as water. We show theoretical studies and experimental work to investigate the wetting behavior of two different designs/samples. For heat pipe applications the BTS and plating gold not only increases the capillary pressure which enhances liquid pumping from condenser to evaporator, but also increases the heat transfer performance by extended surface and smaller pore sizes[2]. Testing results show that water can completely wet the micromachined Ti pillars (Design A: 5μm in diameter/5μm gap). The BTS helps increase the wetting speed by over 100% for this design. A second design with much larger diameter and gap (Design B: 100μm in diameter/50μm in gap) is also tested to compare with design A for wetting speed. Results show that Design B gives a wetting speed twice of Design A. Plating method is used to decrease pillar gap (from 50μm to 5μm) by growing gold on surfaces. This will help increase thermal conductivity of wicking material which is preferred for the evaporator and condenser regions of heat pipes. Wetting experiment is done on Sample B after plating with gold. Wetting results after Au plating show that wetting velocity decreases but is still significantly large.
Micro-Scale Transport in the Diffusion Media of Polymer Electrolyte Fuel Cells
Authors: Wang Yun , Feng Xuhui , Thiedmann Ralf , Schmidt Volker , Lehnert Werner
Published: October 2010

This paper reports our recent work on the stochastic-model-based reconstruction of the gas diffusion layer (GDL) of PEFCs and direct numerical simulation and presented the pore-level transport within GDLs of polymer electrolyte fuel cell (PEFC). The carbon-paper-based GDL is modeled as a stack of thin sections with each section described by planar 2D random line tessellations which are further dilated to three dimensions. The reconstruction of the GDL structure is based on given GDL data provided by scanning electron microscopy (SEM) images. Based on the stochastically constructed digital GDL, we further conduct the DNS of the coupled transport processes, including gas flow and species transport, electronic current conduction, and heat transfer. Results indicate remarkable distinction in tortuosities of gas diffusion passage and solid matrix. The numerical tool can be applied to investigate the GDL microstructure and internal pore-level transport in PEFCs.
Advanced Particle-Based Velocimetry Techniques for Microscale Flows
Authors: Lee Sang Joon
Published: October 2010

Recently microscale flows have been receiving large attention in various research areas. However, most conventional imaging techniques are unsatisfactory due to difficulties encountered in the visualization of microscale flows. Recent advances in optics and digital image processing techniques have made it possible to develop several advanced micro-PIV/PTV techniques. They can be used to obtain quantitative velocity field information of various microscale flows from visualized images of tracer particles. As new advanced micro-PIV techniques, the basic principle and typical application of the time-resolved micro-PIV and X-ray micro-PTV methods are explained. As 3D velocity field measurement technique that can be used to measure microscale flows, the holographic micro-PTV method is introduced. These advanced PIV/PTV techniques can be used to reveal the basic physics of various microscale flows and will play an important role in visualizing hidden microscale flow phenomena, for which conventional methods have many difficulties to analyze.
Flow Boiling Heat Transfer Characteristics of a Minichannel Up to Dryout Condition
Authors: Ali Rashid , Palm Bjo¨rn , Maqbool Mohammad H.
Published: October 2010

In this paper the experimental flow boiling heat transfer results of a minichannel are presented. A series of experiments was conducted to measure the heat transfer coefficients in a minichannel made of stainless steel (AISI 316) having an internal diameter of 1.7mm and a uniformly heated length of 220mm. R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 °C and 32 °C. Mass flux was varied from 50 kg/m2 s to 600 kg/m2 s and heat flux ranged from 2kW/m2 to 156kW/m2. The test section was heated directly using a DC power supply. The direct heating of the channel ensured uniform heating and heating was continued until dry out was reached. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux while mass flux and vapour quality have no considerable effect. Increasing the system pressure slightly enhances the heat transfer coefficient. The heat transfer coefficient is reduced as dryout is reached. It is observed that dryout phenomenon is accompanied with fluctuations and a larger standard deviation in outer wall temperatures.
Research on Heat Transfer Performance of Water Film for Outside the Tubes of the Microscale Evaporative Condenser
Authors: Hu Minghui , Zhu Dongsheng , Shen Jialong
Published: October 2010

It is requested to develop a microscale and high performance heat exchanger for small size energy equipments. The heat transfer performance of the water film on the condensing coils of the microscale evaporative condenser was studied for a single-stage compressed refrigeration cycle system. Under various operation conditions, the effects of the spray density and the head-on air velocity on the heat transfer performance of the water film were investigated. The results show that the microscale heat transfer coefficient of the water film αw increases with the increase of spray density and decreases with the increase of head-on air velocity. The results indicate that the key factor affecting the microscale heat transfer of the water film is the spray density. As the results, it is measured that the present device attained high heat transfer quantity despite the weight is light. In addition, via regression analysis of the experimental data, the correlation equation for calculating the microscale heat transfer coefficient of the water film was obtained, its regression correlation coefficient R is 0.98 and the standard deviation is 7.5%. Finally, the correlations from other works were compared. The results presented that the experimental correlation had better consistency with the correlations from other works. In general, the obtained experimental results of the water film heat transfer are helpful to the design and practical operation of the microscale evaporative condensers.
Experimental Measurement of Energy Accommodation Coefficient by Low-Pressure Method
Authors: Yamaguchi Hiroki , Aoki Yuta , Kanazawa Kazuaki , Matsuda Yu , Niimi Tomohide
Published: October 2010

Heat transfer in micro flows has received much attention along with the development in micro- and nano-technology. In micro- and nano-flow fields, the Knudsen number, which is defined as a ratio of the molecular mean free path to the characteristic length of the system, becomes large because of the small characteristic length. In these so-called “high Knudsen number flows”, the number of the collision of gas molecules to the surface is much larger than that of intermolecular collisions. Therefore, it is important for the high Knudsen number flows to understand the gas-surface interaction. Since detailed science of the gas-surface interaction is complicated, the empirical parameter called the accommodation coefficient is widely used for flow analyses of the flows. In this study, the energy accommodation coefficient for metal surface has been measured experimentally by the Low-Pressure method, in which the energy accommodation coefficient is obtained from the pressure dependency of the heat flux in the free-molecular flow regime. It is not easy to realize the free-molecular flow condition, and, thus, the relation between the heat flux and the pressure extended to much higher pressure condition was employed in this study. Experimental geometry was designed as concentric cylinders, and heat flux between two cylinders, whose surface temperatures was different, was measured. Experimental results are reported for argon and oxygen in contact with a platinum surface. The surface temperature dependence of the energy accommodation coefficient was also studied, and verified by the results of previous work.
Air Bubble Bursting Effect of Lotus Leaf
Authors: Wang Jingming , Zheng Yongmei , Nie Fu-Qiang , Zhai Jin , Jiang Lei
Published: October 2010

Superhydrophobic surfaces, especially lotus leaf surface, have been largely explored due to their great importance in fundamental research and abundant potential applications. However, many efforts have been focused on investigating the superhydrophobic surfaces in air instead of in water environment, which are rather crucial to industrial separation progress. A novel air bubble bursting effect on lotus leaf surface was firstly discovered and the underlying mechanism was believed to be related to the micro/nano-hierarchical rough structures. Inspired by air bubble bursting effect on lotus leaf, a superhydrophobic “artificial lotus leaf” with similar micro/nano-hierarchical rough structures was successfully constructed by photolithography and wet etching and also achieved air bubble bursting effect. Smooth and rough silicon surface with the ordered nano-structure or patterned micro-structure were utilized to study the contribution of the micro/nano-hierarchical structures to air bubble bursting, and it was found that air bubble could burst on the superhydrophobic surfaces with micro-structure, but more time was required, while nano-structure could accelerate air bubble bursting. Moreover, the height, width, and spacing of hierarchical structures also affected air bubble bursting, and the effect of the height was more obvious. When the height of hierarchical structures was around the height of lotus papillae, the width and spacing were significant for air bubble bursting. Eventually, an original model was proposed to further evaluate the reason that the micro/nano-hierarchical rough structures had an excellent air bubble bursting effect, and its validity was theoretically demonstrated. It was believed that these findings should spark further theoretical study of some bubble-related interfacial phenomena and find its wide applications in the industrial separation process without any accessional energy and other additives, such as mineral flotation, food processing, textile dyeing, and fermentation.
Thermal Conductivities and Viscosities of Al2O3-Water Nanofluids With Low Volume Concentrations
Authors: Zhao Weilin , Li Jinkai , Liu Zongming , Guan Yanxiang
Published: October 2010

Al2O3-water nanofluids containing low volume concentrations (0.1–0.5 vol. %) of Al2O3 nanoparticles with 40nm and 65nm average particle size were produced using a two-step method with ultrasonication and without any surfactant. The thermal conductivities and viscosities were evaluated by KD2-pro thermal property meter and rotational viscometer respectively at different temperature. Thermal conductivities measurements show that the thermal conductivities of Al2O3-water nanofluids are higher than water. The thermal conductivities with average particle size of 40nm and 65nm are respectively enhanced by 17.9% and 11.2% when approximately 0.5vol.% of Al2O3 nanoparticles are added. Furthermore, the experimental results show the thermal conductivities increased nearly linearly with the nanoparticle volume concentration increasing, and significantly increased with the temperature increasing. Comparison between the experiments and the theoretical models shows that the measured thermal conductivities are much higher than the values calculated from theoretical models, indicating new heat transport mechanisms included in nanofluids. In the contrast to thermal conductivities, the viscosities measurements show that the viscosities of the Al2O3-water nanofluids significantly decrease with increasing temperature, and increased nonlinearly with the nanoparticle volume concentration. As the volume concentration of nanoparticles is increased up to 0.5%, the viscosities of Al2O3-water nanofluids with average particle size of 40nm and 65nm are respectively increased nonlinearly up to 28.3% and 17.5%, which exceed the Einstein model predictions.
Dynamic Condensation Modelling in PEMFC GDL
Authors: Hinebaugh James , Bazylak A.
Published: October 2010

A 2D dynamic pore network model is employed to study the liquid water saturation from condensation originating from a single nucleation site within the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC). A complete derivation of the model along with the physical parameters is provided in this paper. Sensitivity analyses are performed to determine how the overall saturation pattern is affected by the following two parameters: condensation rate and nucleation site location. Our results indicate that when considering an initially dry GDL under typical PEMFC operating conditions, fluctuations in condensation rate have little impact on the overall saturation pattern compared to changes in condensation nucleation site location. We observe significant reductions in both overall saturation and local saturation near the catalyst layer as the condensation nucleation site is placed further away from the catalyst layer.
Preliminary Study on the Dynamic Stability of TiO2-Water Based Nanofluids Flow in Circular Tube
Authors: Yang Junhong , Jiang Qiang , Di Qianqian , Wang Liqiu
Published: October 2010

The preparation of nanofluids with good suspending stability is an important premise for further scientific research and engineering application. However, previous studies mainly focused on the stability of nanofluids in static situation without any flow, and seldom discussed the suspending stability of nanofluids in flow. The objective of this study is to investigate the suspending stability of nanofluids in flow using TiO2-water nanofluids flowing through a circular tube as an example. The study uses the centrifugal sedimentation weight of supernatant suspending solution to characterize the stability of nanofluids. The bigger centrifugal sedimentation weight represents the better stability. This study also characterization of particle size distribution of nanofluids by Malvern laser particle size analyzer. In the present experiment conditions, TiO2-water nanofluids with the mass fraction of 0.5% are driven by a peristaltic pump and flows through a circular tube with 4.8 mm inner diameter. In the present experimental conditions, the volume flow rate of TiO2-water nanofluids is slightly lower than that of pure water. The size distribution of nanoparticle in fluid after circulating flow 2 hours behaves more uniformly than that in a fluid after maintaining static situation 2 hours. with the increasing of Reynolds number of fluid in circulating flow, the suspending stability of nanofluids in circulating flow is significantly higher that that in static situation. When the Reynolds number is up to 2000, the suspending status of nanoparticles in the base fluid is nearly the same as the initial status of sample before circulating. Despite its preliminary, this work demonstrates that the flow pattern is one of influence factors on the suspending stability of nanofluids, which should not be ignored in the study of the suspending stability and heat transfer performance of nanofluids. This study may give some inspiration for further research on relative fields such as the preparation of nanofluids for heat transfer enhancement and so on.
Fractal Characterization of Astragalus Slices Under Various Microwave Assisted Extraction Conditions
Authors: Di Qianqian , Yang Junhong , Sun Mingdi , Wang Liqiu
Published: October 2010

Microwave has been widely used in the extraction of many samples for its unique heating mechanism, short extraction time and high yield of extract. The microstructure characteristics of plant materials are closely related with macro-property and mechanism on mass transfer within matrix. Fractal geometry, with the ability to describe irregular objects provides a new language. Astragalus slice is a special and typical plant porous media. We characterize the microstructure of Astragalus slices irradiated by microwave at 600 and 900 W by using fractal dimension, with the aim to analyze the effect of the microwave power on the extraction yield and discuss further the relationship between the fractal dimensions and microstructure changes of sample during microwave assisted extraction process. It is found that the fractal scaling law of box counting method is not suitable for the apertures on the wall of trachea inside matrix, and Slit Island and mercury injection method may be used. Fractal dimensions of samples at microwave 600 W are smaller than that at 900 W. The smaller fractal dimension is in favor of effective components dissolution inside the slices, which led to extract with the 20 min irradiation of microwave at 600 W is higher than that at 900 W.
Effect of Microwave Irradiation on the Flow of Micro-Particle Suspension (TiO2/H2O) in Micro-Scale Porous Structure
Authors: Di Qianqian , Yang Junhong , Sun Mingdi , Wang Liqiu
Published: October 2010

The mass flow and transport phenomenon in porous media widely exists in nature and engineering applications. Membrane separation is a typical material transfer process in the porous medium. The objective of the present study is to estimate the effect of microwave irradiation on transport of micro-particle suspension through porous structure, using the flowing of TiO2/H2O micro particle suspension through 0.45μm mixed cellulose micro porous membrane as an example. The work mainly compared the filtration behavior of suspension through membranes during microwave assisted filtration and conventional filtration. The particle size distribution of suspension was also characterized by using laser particle size analyzer. As the results, it is measured that the processed volume of suspension during microwave assisted filtration is considerable higher than that during conventional filtration. It is also observed that the average size of particles in suspension is 3.80 μm (10 g/L), and the volume percentage of particles smaller than micro-pore size 0.45μm in micro-filtration membrane is about 11.5%. It is indicated that microwave assisted filtration could considerably enhance the processed volume of suspension with micro-particle, which might be able to give some inspiration to the optimization of some processed technology of suspension chemical industry, food industry and water treatment industry.
Simulation of Gas Flow Over a Micro Cylinder
Authors: Hu Yuan , Sun Quanhua , Fan Jing
Published: October 2010

Gas flow over a micro cylinder is simulated using both a compressible Navier-Stokes solver and a hybrid continuum/particle approach. The micro cylinder flow has low Reynolds number because of the small length scale and the low speed, which also indicates that the rarefied gas effect exists in the flow. A cylinder having a diameter of 20 microns is simulated under several flow conditions where the Reynolds number ranges from 2 to 50 and the Mach number varies from 0.1 to 0.8. It is found that the low Reynolds number flow can be compressible even when the Mach number is less than 0.3, and the drag coefficient of the cylinder increases when the Reynolds number decreases. The compressible effect will increase the pressure drag coefficient although the friction coefficient remains nearly unchanged. The rarefied gas effect will reduce both the friction and pressure drag coefficients, and the vortex in the flow may be shrunk or even disappear.
Two-Phase Heat Transfer Enhancement on Sintered Copper Microparticle Porous Structure Module Surface
Authors: Li Calvin H. , Li Ting , Kanney Brian
Published: October 2010

An experimental study of the pool boiling two-phase heat transfer on a sintered Cu microparticle porous structure module surface is conducted. Enhanced heat transfer capacity of this module surface has been reported, and the boiling characteristics have been investigated. The bubble dynamics and nucleate size distribution have been compared to the theoretical predictions, and the speculated mechanisms have been discussed.
Measuring the Thermal Contact Resistance of a Junction by T Type Probe Method
Authors: Wang Jianli , Gu Ming , Zhang Xing
Published: October 2010

Using a T type probe, the effect of the interstitial material (interposer) on the thermal contact resistance of a junction has been estimated by measuring an individual carbon fiber with different interposers, including the solidified metallic powder, lubricant grease, and dry contact as a comparison. For the metallic powder, the thermal contact conductance was obtained to be 3.0 M W m−2 K−1 by changing the fiber length when the same contact between the fiber and the hot wire was maintained. However, this method can only be applicable to the solidified contact, and the stability of the operating temperature is a must in each length measurement. To estimate the thermal contact resistance of the lubricant Apiezon N grease, even a dry contact, an improved T type probe was employed, by applying an alternative current to the hot wire. This method was verified by measuring the same type of carbon fiber in the frequency range of 0.1 to 1Hz based on a Labview-based virtual lock-in measurement system. The same value of the thermal effusivity of the test fiber was obtained with different interposers, and the thermal contact conductances for the dry contact and high vacuum grease were found to be 0.10 M W m−2 K−1 and 0.26 M W m−2 K−1, respectively.
The Directional Freezing Effects on Biological Cells in a Microfluidic Cell Culture System
Authors: Li Yuhui , Wang Fen , Lu Yanyan , Wang Hao
Published: October 2010

Cryotherapy is a prospective also green method for malignant tumor treatment. At low temperature, the cell viability relates with the cooling rate, temperature threshold, freezing interface as well as ice formation. In this paper a series of directional freezing processes and cell responses in a culture microchip were experimentally investigated. The temperature in the microchip was manipulated by a thermoelectric cooler. The surviving cells, necrotic and apoptotic cells under different cryotreatment (duration of the freezing process, freeze-thaw cycle, post-culture et al) were stained and distinguished by PI and FITC-Annexin V. The locations of the ice front and cell death boundary were observed and recorded through an inverted microscopy. By controlling the cooling process in a microfluidic channel, it is possible to recreate a sketch of biological effect during the process of simulated cryosurgery.
Precursor Film Formation Process Ahead Macroscopic Contact Line of Spreading Droplet on Smooth Substrate
Authors: Ueno Ichiro , Hirose Kanji , Kizaki Yusuke , Kisara Yoshiaki , Fukuhara Yoshizumi
Published: October 2010

The authors pay their special attention to formation process of wafer-thin liquid film, known as ‘precursor film,’ ahead moving macroscopic contact line of a droplet spreading on a solid substrate. The spreading droplet on the solid substrate is accompanied with the movement of a visible boundary line so-called ‘macroscopic contact line.’ Existing studies have indicated there exits a thin liquid film known as ‘precursor film’ ahead the macroscopic contact line of the droplet. The present author’s group has dedicated their special effort to detect the formation process of the precursor film by applying a convectional laser interferometry and a high-speed camera, and to evaluate the spreading rate of the precursor film. In the present study, existing length of the precursor film at a very early stage of the droplet spreading is evaluated by applying a Brewster-angle microscopy as well as the interferometer. The authors extend their attention to the advancing process of the precursor film on inclined substrate.
Proposal of a Micro/Mini Cooling Device Using Fins-Installed Porous Media for High Heat Flux Removal Exceeding 1000W/cm2
Authors: Yuki Kazuhisa , Matsui Akira , Hashizume Hidetoshi , Suzuki Koichi
Published: October 2010

Heat transfer characteristics of micro-sized bronze particle-sintered porous heat sinks and copper minichannel-fins heat sinks are experimentally investigated in order to clarify the feasibility of a newly proposed micro/mini cooling device using fins-installed porous media. Regarding the porous heat sinks, fin effect toward more inside of the porous medium is promoted by sintering the porous heat sink on the heat transfer surface, which results in increasing the heat transfer performance up to 0.8MW/m2K at heat flux of 8.2MW/m2 though there still remains a large pressure loss issue. In addition, the results clarify that the heat exchanging area exists only in the vicinity of the heat transfer surface. As to the minichannel-fins heat sinks, the influence of the channel width and the fin thickness are evaluated in detail. As a result, the minichannel-fins heat sink having the narrower channel width (i.e. scale effect) and lower porosity (i.e. thicker fin thickness with larger heat capacity) achieves higher heat transfer performance up to 0.10MW/m2K at 8.3MW/m2. However, rapid increase of pressure loss, which is occasionally observed in a microchannel due to vapor bubbles choking the narrow channel, still remains as an issue under flow boiling conditions in the minichannel. Finally, heat transfer performance of the fin-installed porous heat sink is numerically predicted by the control volume method. The simulation confirms that the heat transfer coefficient at each wall superheat of 0 and 30 degrees has performance 2.5 times and 2.0 times higher than that of the normal fins, which indicates that this heat sink coupling the micro and mini channels has high potential as efficient cooling method under high heat flux conditions exceeding 10MW/m2.
Subcooled Flow Boiling With Microbubble Emission in a Michrochannel
Authors: Suzuki Koichi , Nomura Tomoyuki , Hong Chungpyo , Yuki Kazuhisa
Published: October 2010

Subcooled flow boiling of water has been investigated for the horizontal multi-microchannel of which hydraulic diameter is 150μm for unit channel. Eleven rectangular microchannels are made on a top of copper heating block of 5.25mm × 5.25mm. The outlet of the channel is opened to the atmospheric surroundings and the maximum pressure in the channel is lower than 25mmHg. The boiling test is performed under the nearly atmospheric condition. The experimental results are discussed compared with subcooled boiling of water in a microchannel of 155μm in hydraulic diameter with Platinum film microheater of 2000μm in length and 200μm in width obtained by Ping Cheng and his co-workers. According to the authors’ previous experiments on subcooled flow boiling in mini and conventional channels, the critical heat flux decreases with decreasing of the hydraulic diameter of the channel. The boiling in the microchannel turns to film boiling after reaching CHF without microbubble emission boiling (MEB) regardless of liquid subcooling and liquid velocity even if the boiling condition is the same as MEB in the minichannels. In the high heat flux region, whole of the microchannels is completely covered with large coalescing bubbles. The results are much different from those of experiments with Platinum film microheater, which have 14.41 MW/m2 of heat flux in MEB. It is difficult to introduce liquid–vapor exchange including MEB for the large capacitance heat sink in microchannel boiling.
Thermal Bubble Microfluidic Gate Based on SOI Wafer
Authors: Tsou Chingfu , Huang Chenghan
Published: October 2010

This paper presents a simple process using an SOI wafer to fabricate a silicon-based vertical microheater to generate thermal bubbles for the applications in microfluidic systems. The fabrication process consists of only two photolithography masks that provide an effective way to manufacture a resistive bulk microheater and high-aspect-ratio microchannel. The electro-thermal property of the proposed microheater has been characterized and verified by finite element analysis and experiment. According to the design concept and experimental results, the largest temperature occurred at the smallest neck section due to the non-uniform property of the resistance along the length of the arch-type microheater, and thus the vapor bubble was generated and attached on the vertical side wall of the microheater. For a typical microheater design, bubble nucleation could be generated under the applied voltage of 5V and the bubble could obstruct the entire 100 μm width of the microchannel when the applied voltage reaches 8V. A switching test has showed the silicon-based microheater has a good thermal-resistance behavior for long-term reliability and the modulation of output flow rate is easy handled by the sizes of thermal bubble. Moreover, the bubble can be formed with a steady growth even when the maximum fluid velocity is larger than 920 μm/s in a microchannel with rectangular cross-section 100 μm wide and 50 μm high. Mixing performance of the thermal bubble to disturb two collateral liquids with lamina flow was also carried out in this work. Experiments show that a high mixing efficiency could be achieved when the vapor bubble was formed in a 100 μm wide, 50 μm deep microchannel with a flow rate of 780 μm/s. These results reveal that the microfluid gate presented here is well designed and bubble sizes are stable and controllable.
Electroosmosis of Dilute Electrolyte Solutions in Microporous Media
Authors: Wang Moran , Kang Qinjun , Viswanathan Hari
Published: October 2010

The multiphysiochemical transport in electroosmosis of dilute electrolyte solutions (<1mM) through microporous media with granular random structures has been modeled in this work by our numerical framework consisting of three steps. First, the three-dimensional microstructures of porous media are reproduced by a random generation-growth method. Then the effects of chemical adsorption and electrical dissociation at the solid-liquid interfaces are considered to determine the electrical boundary conditions, which vary with the ionic concentration, the pH, and the temperature. Finally the nonlinear governing equations for the electrokinetic transport are solved by a highly efficient lattice Poisson-Boltzmann algorithm. The simulation results indicate that the electroosmotic permeability through the granular microporous media increases monotonically with the porosity, the ionic concentration, the pH and the environmental temperature. When the surface electric potential is higher than 50 mV, the permeability increases with the electric potential exponentially. The electroosmotic permeability increases with the pH exponentially, but with the temperature linearly. The present modeling results may improve our understanding of hydrodynamic and electrokinetic transport in geophysical systems, and help guide the design of porous electrodes in micro energy systems.
Effect of Mesoscopic Structure of Interface on Heat and Mass Transfer in a Partially Porous Cavity
Authors: Chen Baoming , Wang Li , Liu Fang , Yun Heming , Geng Wenguang
Published: October 2010

Natural convective heat and mass transfer in a cavity partially filled with a vertical porous layer along the left wall was studied in this paper. Different uniform temperature and concentration were specified at the external vertical walls of the cavity while the horizontal walls are adiabatic and impermeable. Two-domain model together with weak constraint method at the porous/fluid interface was used to simulate the flow, heat and mass transfer in the cavity. The shear stress jump condition at the porous/fluid interface is invoked when the Brinkman-Forhheimer-extended Darcy model is used. The mesoscopic structure is homogeneous (the porosity is constant) at the interior region of porous media while the mesoscopic structure changes acutely at the porous/fluid interfacial location. The effect of the mesoscopic structure changes at the porous/fluid interface region on the macroscopic balance is preserved by prescribing the stress jump condition at the interface. This paper focused on the changes of the stress jump coefficients and their influence on heat and mass transfer at the porous/fluid interface.
Characterization of Surface Roughness Effects on Laminar Flow in Microchannels by Using Fractal Cantor Structures
Authors: Zhang Chengbin , Chen Yongping , Fu Panpan , Shi Mingheng
Published: October 2010

The fractal characterization of the topography of rough surfaces by using Cantor set structures is introduced in this paper. Based on the fractal Cantor surface, a model of laminar flow in rough microchannels is developed and numerically analyzed to study the characterization of surface roughness effects on laminar flow. The effects of Reynolds number, relative roughness, and fractal dimension on laminar flow are all discussed. The results indicate that the presence of roughness leads to the form of the detachment, and eddy generation is observed at the shadow of the roughness elements. The pressure drop in the rough channel along the flow direction is no longer in a linear fashion and larger than that in the smooth channel. The fluctuation characteristic of pressure drop along the stream, which is due to the vortex formation at the wall, is found. Differing from the smooth channel, the Poiseuille number for laminar flow in rough microchannels is no longer only dependent on the cross-sectional shape of the channel, but also strongly influenced by the Reynolds number, relative roughness and fractal dimension of the surface.
Molecular Dynamics Studies of Heat and Stresses in Copper Substrate Scratch With Double-Walled Nano-Cones
Authors: Wang Yun-Che , Chen Jun-Liang , Chen Chuan , Chen Yan-Chi , Hwang Chi-Chuan
Published: October 2010

It is of scientific and industrial importance to obtain detailed understanding of heat transfer and stress distributions in the substrate under nano-scale scratching. In this study, the copper (110) substrate was chosen, and the scratching tip was a double-walled nano-cone. It is found that the double-walled nano-cones are more workable than single-walled nano-cones and carbon nanotubes. Moreover, repeated scratches show high aspect-ratio trenches could be obtained by the manufacturing technique. Time-domain heat transfer and stress analysis was carried out by using a control-volume technique with an atomic spatial resolution, except near the boundaries. It is found the temperature rises locally near the scratch tip, and trailing thermal waves were more prominent than the leading thermal waves. For the case of the scratching temperature at 700 K, the highest temperature during the scratch was found to be about 850 K.
Development of Micro Optical Diffusion Sensor Based on Laser Induced Dielectrophoresis
Authors: Taguchi Yoshihiro , Itani Koichi , Ebisui Akira , Nagasaka Yuji
Published: October 2010

Micro-electro mechanical systems (MEMS) biochips realizing high-speed and high-efficiency of reaction and analysis attract much attention in medical as well as chemical fields. Especially, the miniaturized devices enabling small sample volume, arrayed, and portable measurement may become a powerful tool for material analysis and process control. We have proposed a novel micro optical diffusion sensor (MODS) which enables small sample volume, highspeed and non-contact measurement of diffusion coefficient of liquid sample. MODS consists of a pair of transparent electrodes (Al-doped Zinc oxide: AZO), a photoconductive layer (amorphous silicon: a-Si:H), two MEMS mirrors and excitation and probing fibers for inducing and detecting concentration distribution. The initial concentration distribution of sample is created by an opto-dielectrophoretic (opto-DEP) manipulation along with a sinusoidal pattern of irradiated beam on a photoconductive layer. In the present paper, the measurement principle is proposed, and the preliminary experiment using a bench top apparatus is reported.
The Numerical Investigation of an Interdigital Micromixer With the Circular-Sector Obstacles
Authors: Fan Yan Feng , Hassan Ibrahim
Published: October 2010

In this paper, a passive interdigital micromixer with the circular-sector obstacles is proposed and the mixing performance is estimated by numerical simulation. The tested Reynolds numbers range from 0.01 to 10. Flow recirculation or vortices seems impossible to generate to enhance the mixing at such low Reynolds numbers. Hence, molecular diffusion is the dominant mixing mechanism. Based on the diffusion principle, enlarging the mixing length, reducing the diffusion length and increasing the surface area between species are major methods to obtain mixing enhancement. In order to achieve rapid mixing, shortening the mixing length is necessary. However, the reduced mixing length induces the decreased mixing time which the species take to mix. The circular-section obstacles are placed in the straight microchannels to enlarge the contact surface area between species. The flow path is distorted after passing the obstacles so that the real mixing length increases compared with traditional T-shape micromixers. Furthermore, flow advection takes a part role in mixing since the velocity direction is no longer perpendicular to diffusion direction. Different geometries and layouts of obstacles are analyzed for optimization. The results of optimal design show the worst mixing efficiency, around 50%, occurs at Re = 1. In order to improve the lower limitation of mixing efficiency, the duplicated layouts of obstacles in lateral direction with interdigital inlet are applied to reduce the diffusion path and increase the interface area so that the mixing efficiency could be enhanced. The results show that the mixing efficiency could achieve 85% at Re ≤ 1 with a low pressure drop of 100 Pa. It has the potential to be used in applications with low Reynolds numbers.
Scattering Process of Gas Molecules on Vertically Aligned Single-Walled Carbon Nanotubes
Authors: Kinefuchi Ikuya , Harada Yushi , Kawasaki Jumpei , Ishikawa Kei , Shiomi Junichiro , Takagi Shu , Maruyama Shigeo , Matsumoto Yoichiro
Published: October 2010

Scattering process of helium molecules on vertically aligned single-walled carbon nanotubes (VA-SWNTs) was investigated using the molecular beam technique. The samples were grown by alcohol catalytic CVD method and their thicknesses were approximately 0.1 and 4 μm. We found that the modification of quartz surface with VA-SWNT films significantly enhances the energy transfer between gas molecules and the surface at room temperature and makes the energy accommodation coefficient of helium, which tends to be small even for contaminated surfaces because of the large mass mismatch between helium and surface atom, close to unity. The high porosity of the film enables gas molecules to penetrate into the film and suffer multiple collisions with carbon nanotubes (CNTs), leading to the efficient energy transfer. Although the accommodation coefficient slightly decreases for the thinner film, it is still high enough for practical applications. Our results demonstrate a potential application of VA-SWNTs as nanoscale fin structures to enhance heat transfer between gas phase and solid surfaces in rarefied gas flows. As the surface temperature increases, however, the energy accommodation becomes less efficient. This would be attributed to the small adsorption energy, which could reduce the trapping probability of helium on CNT bundles.
Experimental Study of Fully Developed Convection in a Copper Horizontal Tube With Uniform Heat Flux Using Nanofluids
Authors: Zarezadeh Naser , Saffar-Avval Majid
Published: October 2010

The term of nanofluid refers to a solid-liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. Recent investigations on nanofluids indicate that the suspended nanoparticles markedly change the transport properties and heat transfer characteristics of the suspension. There are less published articles on deriving the forced convective heat transfer of nanofluids than articles on the effective thermal conductivity of nanofluids. Fully developed turbulent flow and heat transfer of two different nanofluids (Al2O3 and TiO2) in water flowing through a circular tube under constant heat flux condition have been experimentally studied. The results showed enhancement of convective heat transfer using the nanofluids. The Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Reynolds numbers. Experimental results emphasize the enhancement of heat transfer due to the nanoparticles presence in the fluid. Nusselt number increases by increasing the concentration of nanoparticles in nanofluid. And values of Nusselt number were calculated and these results have been introduced by experimental correlations for turbulent flow.
Radiative Properties of Nanofluids and Performance of a Direct Solar Absorber Using Nanofluids
Authors: Mu Lijuan , Zhu Qunzhi , Si Leilei
Published: October 2010

Solar energy utilization is very important in the background of global warming and reduction of carbon dioxide emission. Solar air/water collectors are convenient to convert solar energy into heat. Compared with regular solar collectors, direct absorption solar absorbers are not common. However, nanofluids can be used in direct solar absorbers to improve their performances. The objective of this paper is to evaluate the potential of application of nanofluids in direct solar absorbers. Spectral transmittances of nanofluids within the solar spectrum have been measured by a customized spectrophotometer. A direct solar absorber has been built to use nanofluids as the working fluid and its performance has been experimentally investigated. It is found that the radiative properties of nanofluids deviate significantly from that of the base fluid. A remarkable amount of visible light can pass through the SiO2 nanofluid while solar radiation can hardly transmit through the TiO2 nanofluid and ZrC nanofluid. The ZrC nanofluid shows the highest solar absorbance among the studied nanofluids. The temperature increase rate of the nanofluid is faster than that of water when the liquid stagnant in the absorber is illuminated by solar radiation. Furthermore, the temperature differences between the inlet and outlet of the TiO2 nanofluid and ZrC nanofluid are higher than that of the SiO2 nanofluid at the same flow rate. This work is beneficial for the exploration of nanofluids in direct solar absorbers.
Rarefaction Effect on the Characteristics of Micro Gas Journal Bearings
Authors: Zhang Haijun , Yang Qin
Published: October 2010

Journal bearings, which are used to support radial loads in a rotating machine, have somewhat unusual requirements in MEMS deriving from the extremely shallow structures. Thus, the micro gas journal bearings are characterized by a very small length-diameter ratio, defined as the ratio of the bearing length to its diameter and a paradoxically large bearing clearance ratio, defined as the ratio of the average radial clearance to the bearing radius. Given the definition of the reference Knudsen number for micro gas journal bearings, the range of the reference Knudsen number is illustrated according to the viscosity values of air under different temperatures. With the reference Knudsen number being included, the modified Reynolds equation for micro gas journal bearings based on Burgdorfer’s first order slip boundary condition is put forward. The finite difference method (FDM) is employed to solve the modified Reynolds equation to obtain the pressure distribution, load capacities and attitude angles for micro gas journal bearings under different reference Knudsen numbers, bearing numbers and eccentricity ratios. Numerical analysis shows that the pressure profiles and non-dimensional load capacities decrease obviously with gas rarefaction strengthened, and the attitude angle changes conversely. Moreover, when the bearing number is smaller, the effect of gas rarefaction on the non-dimensional load capacity and attitude angle is less.
Impaction of Mineral Nano-Refrigeration-Oil on the Saturated Vapor Pressure of HFC134a
Authors: Li Xinzhong , Wang Ruixiang , Zong Yubo
Published: October 2010

The saturated vapor pressure of HFC134a mixed with mineral nano refrigeration oil was measured by way of the steady-state method. The measurement data were presented as the form of Youbi equation. The nano-partials NiFe2O4, the nano-partials CuO and the mineral lubricant SUNISO 3GS were used to preparation of the mineral nano refrigeration oil. The results show that the saturated vapor pressure of HFC134a mixed with mineral nano refrigeration oil is lower than that of HFC134a mixed with pure mineral lubricant SUNISO 3GS or that of pure HFC134a.
Aerosol Flow Through a Micro-Capillary
Authors: Akhatov Iskander S. , Hoey Justin M. , Thompson Drew , Lutfurakhmanov Artur , Mahmud Zakaria , Swenson Orven F. , Schulz Douglas L. , Osiptsov Alexander N.
Published: October 2010

A combined theoretical/experimental study of micron size aerosol flows through micro-capillaries of diameter about 100 μm and length about 1 cm is presented. It is shown that under proper conditions at a relatively high velocity of about 100 m/s such an aerosol flow reveals a new manifestation of microfluidics: the Saffman force acting on aerosol particles in gas flowing through a micro-capillary becomes significant thereby causing noticeable migration of particles toward the center line of the capillary. This finding opens up new opportunities for aerosol focusing, which is in stark contrast to the classical aerodynamic focusing methodologies where only particle inertia and the Stokes force of gas-particle interaction are typically used to control particle trajectories. A mathematical model for aerosol flow through a micro-capillary accounting for complicated interactions between particles and carrier gas is presented. This model describes the experimental observables obtained via shadowgraphy for aerosol beams exiting micro-capillaries. It is further shown that it is possible to design a micro-capillary system capable of generating a Collimated Aerosol Beam (CAB) in which aerosol particles stay very close to a capillary center line. The performance of such a CAB system for direct-write fabrication on a substrate is demonstrated. The lines deposited by CAB for direct-write fabrication are shown to exhibit widths of less than 5 μm — superior to ink-jet. Materials deposition based upon directed aerosol flow has the potential of finding application in the fields of flexible electronics, sensors, and solar cells. In this paper, the genesis of a new materials deposition method termed Collimated Aerosol Beam Direct-Write (CAB-DW) is discussed.