bluuebunny/crossref_metadata_2025 huggingface dataset:
Improvement of the Performance of a Centrifugal Compressor by Modifying the Volute Inlet
Authors: Kim Semi , Park Junyoung , Ahn Kukyoung , Baek Jehyun
Published: September 2010

In centrifugal compressors, the diffuser and the volute have strong influences on the flow discharged from the impeller and thus also on the performance. In particular, a key parameter is the radial velocity at the volute inlet; it determines the swirl velocity, which is dissipated as a loss, i.e., it results in performance degradation. With the aim of reducing the swirl loss, a new type of volute with a modified inlet height was tested in this study. The volute inlet height was modified to 6 mm and 7 mm from the original height of 5 mm. The reliability of our computations was tested by comparison of the results of a model with this original height with experimental data. Flow analyses were conducted not only at the design mass flow rate but also at lower and higher mass flow rates. A higher total-to-total efficiency was obtained as a result of the linear increases of the volute inlet height from the diffuser outlet to 6 mm and 7 mm. Our detailed investigation of the simulated flow fields shows that the flow characteristics for a volute inlet height of 6 mm are better than those for a volute inlet height of 5 mm. These results clearly show that a greater volute inlet height assists in pressure recovery and reduces swirl loss in the volute. However, the volute inlet height of 7 mm results in larger hub separation and more energy loss, and thus in inferior performance. Therefore, the hub separation and the radial velocity at the volute inlet strongly influence the performance.
An Experimental Investigation of Performance and Exergy Analysis of a Counterflow Vortex Tube Having Various Nozzle Numbers at Different Inlet Pressures of Air, Oxygen, Nitrogen, and Argon
Authors: Kırmacı Volkan , Uluer Onuralp , Dincer Kevser
Published: September 2010

An experimental investigation has been carried out to determine the thermal behavior of cooling fluid as it passes through a vortex tube and the effects of the orifice nozzle number and the inlet pressure on the heating and cooling performance of the counterflow type vortex tube (RHVT). Experiments have been performed using oxygen (O2), nitrogen (N2), and argon (Ar). Five orifices have been fabricated and used during the experimental study with different nozzle numbers of 2, 3, 4, 5, and 6. The orifices used at these experiments are made of the polyamide plastic material. The thermal conductivity of polyamide plastic material is 0.25 W/m K. To determine the energy separation, the inlet pressure values were adjusted from 150 kPa to 700 kPa with 50 kPa increments for each one of the orifices and each one of the studied fluids. The vortex tube that was used during the experiments has L/D ratio of 15 and the cold mass fraction was held constant at 0.5. As a result of the experimental study, it is determined that the temperature gradient between the cold and hot exits is decreased depending on the orifice nozzle number increase. Exergy analyses have been realized for each one of the studied fluids under the same inlet pressures with the experiments (Pi=150–700). The exergy efficiency of the vortex tube is more affected by inlet pressure than nozzle number.
Prediction of Local Heat Transfer in a Vertical Cavity Using Artificial Neutral Networks
Authors: Poulad M. Ebrahim , Naylor D. , Fung A. S.
Published: September 2010

A time-averaging technique was developed to measure the unsteady and turbulent free convection heat transfer in a tall vertical enclosure using a Mach–Zehnder interferometer. The method used a combination of a digital high speed camera and an interferometer to obtain the local time-averaged heat flux in the cavity. The measured values were used to train an artificial neural network (ANN) algorithm to predict the local heat transfer. The time-averaged local Nusselt number is needed to study local phenomena, e.g., condensation in windows. Optical heat transfer measurements were made in a differentially heated vertical cavity with isothermal walls. The cavity widths were W=12.7 mm, 32.3 mm, 40 mm, and 56.2 mm. The corresponding Rayleigh numbers were about 3×103, 5×104, 1×105, and 2.7×105, respectively, and the enclosure aspect ratio (H/W) ranged from A=18 to 76. The test fluid was air and the temperature differential was about 15 K for all measurements. ALYUDA NEUROINTELLIGENCE (version 2.2) was used to generate solutions for the time-averaged local Nusselt number in the cavity based on the experimental data. Feed-forward architecture and training by the Levenberg–Marquardt algorithm were adopted. The ANN was designed to suit the present system, which had 4–13 inputs and one output. The network predictions were found to be in a good agreement with the experimental local Nusselt number values.
Comparison of Lagrangian and Eulerian Simulations of Slurry Flows in a Sudden Expansion
Authors: Frawley P. , O’Mahony A. P. , Geron M.
Published: September 2010

From a review of technical literature, it was not apparent if the Lagrangian or the Eulerian dispersed phase modeling approach was more valid to simulate dilute erosive slurry flow. In this study, both modeling approaches were employed and a comparative analysis of performances and accuracy between the two models was carried out. Due to an impossibility to define, for the Eulerian model already implemented in FLUENT, a set of boundary conditions consistent with the Lagrangian impulsive equations, an Eulerian dispersed phase model was integrated in the FLUENT code using subroutines and user-defined scalar equations. Numerical predictions obtained from the two different approaches for two-phase flow in a sudden expansion were compared with the measured data. Excellent agreement was attained between the predicted and observed fluid and particle velocity in the axial direction and for the kinetic energy. Erosion profiles in a sudden expansion computed using the Lagrangian scheme yielded good qualitative agreement with measured data and predicted a maximum impact angle of 29 deg at the fluid reattachment point. The Eulerian model was adversely affected by the reattachment of the fluid phase to the wall and the simulated erosion profiles were not in agreement with the Lagrangian or measured data. Furthermore, the Eulerian model under-predicted the Lagrangian impact angle at all locations except the reattachment point.
Impact of Fan Gap Flow on the Centrifugal Impeller Aerodynamics
Authors: Lee Yu-Tai
Published: September 2010

The effect of a gap between an inlet duct and a rotating impeller in a centrifugal fan is often neglected in the impeller design calculations or design-related computational fluid dynamics (CFD) analyses. This leads to an arbitrary determination of the gap size for the final fan configuration. Since the gap guides the volute flow back to the impeller flow field near the shroud high-curvature turning area, the low-momentum jet formed by the gap flow could prevent local flow from separation, reducing the local flow turning losses. However, this jet flow has enlarged flow separation in the blade passage, producing shedding vorticity in the downstream passage-flow. The passage-flow separation and the downstream volute flow, which is also affected by the passage-flow separation, have a higher impact on flow losses than the blade leading edge separation. If the gap size is not selected carefully, the combined effect of the passage-flow separation and downstream volute flow losses reduces the fan’s overall performance between 2% points and 5% points as demonstrated in the current study. In this paper, local impeller velocity distributions obtained from both design-CFD and analysis-CFD calculations are compared along the shroud from the gap to the blade trailing edge. The overall impeller flow fields with and without the gap and volute effects are also compared and discussed based on the CFD solutions. Finally, an example of controlling the gap effect is shown.
Analysis of Laminar Flow in the Entrance Region of Parallel Plate Microchannels for Slip Flow
Authors: Sadeghi Arman , Asgarshamsi Abolhassan , Saidi Mohammad Hassan
Published: September 2010

Microscale fluid dynamics has received intensive interest due to the emergence of microelectromechanical systems (MEMS) technology. Fluid flow in microdevices has some characteristics which one of them is rarefaction effect related with gas flow. In this work, the steady state laminar rarefied gas flow in the entrance region of parallel plate microchannels is investigated by the integral method with slip flow conditions at solid surface. The effects of Knudsen number on friction factor and Nusselt number are presented in graphical form as well as analytical form. Also the effect of Knudsen number on hydrodynamic entry length is presented. The results show that as Knudsen number increases the local friction factor and Nusselt number decrease. Also an increment of Knudsen number leads to a larger amount of hydrodynamic entry length.
Evaluation of Imaging Techniques Applied to Water Management Research in PEMFCs
Authors: Daino Michael M. , Kandlikar Satish G.
Published: September 2010

Water management in proton exchange membrane fuel cells (PEMFCs) is critical in efficient operation of fuel cells during normal operation as well as purge and start-up conditions. Insufficient membrane hydration impedes the flow of protons and an overabundance of water obstructs the flow of reactants in the gas diffusion layer (GDL) and in gas distribution channels. These two extremes of water content in PEMFCs significantly reduce performance and efficiency, causing material degradation and potential failure. Visualization and quantitative measurement of water content in PEMFCs lead to greater comprehension of water distribution and transport processes. A wide variety of imaging techniques have been employed in literature to reveal water distribution and transport processes on both macroscale and microscale. The presented techniques utilize visible, infrared, X-rays, fluorescence microscopy, nuclear magnetic resonance (NMR), and neutron radiography to visualize water, measure temperature distributions, and quantify water content. Each imaging technique has intrinsic advantages, disadvantages, and limitations for water detection and will be discussed. A critical evaluation of these techniques and their suitability for visualization of specific components of PEMFC are also discussed.
Effect of Inlet Direction on the Refrigerant Distribution in a Parallel Flow Minichannel Heat Exchanger
Authors: Kim Nae-Hyun , Kim Do-Young , Byun Ho-Won , Choi Yong-Min
Published: September 2010

The refrigerant R-134a flow distributions are experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet orientations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for the mass flux from 70 to 130 kg/m2s and quality from 0.2 to 0.6. In the test section, tubes were flush-mounted with no protrusion into the header. It is shown that normal and vertical inlet yielded approximately similar flow distribution. At high mass fluxes or high qualities, however, slightly better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results.
A Surface-Directed Microfluidic Scheme for Parallel Nanoliter PCR Array Suitable for Point-of-Care Testing
Authors: Ramalingam Naveen , Chen Long-Qing , Yang Xin-Hao , Deng Liqun , Wang Qing-Hui , Huat Eric Yap Peng , Neo Chiew Hoon , Gong Hai-Qing
Published: September 2010

In resource-limited settings, it is impractical to get access to a diagnostic laboratory having sophisticated instruments, and it is desirable to use disposable point-of-care diagnostic chips that do not require liquid handling or pumping instruments for sample distribution among an array of reactors. In addition to the pump-less sample loading method, the challenge to seal an array of reactors without the use of microvalves or mechanical parts still persists. Implementation of microvalve array adds complexity to the chip fabrication and operation processes, and also reduces the space on the microchip. In this paper, we report the development of a high-throughput quantitative PCR chip platform for parallel analyses of multiple gene targets. The PCR mixture distribution among an array of 80 microreactors and subsequent isolation of the reactors were solely realized by a two-step surface tension-based microfluidic scheme, which eliminates the use of pumps, valves and liquid handling instruments. Confinement of the PCR mixture inside the micro reactors was achieved by implementing hybrid flow-restriction passive valves. The microreactors were isolated from each other by the flow of a curable liquid sealant delivered through microchannels by capillary action. We also investigated the effect of detergents that are present in most commercial PCR buffers. Presence of detergents makes the PCR buffer much more wetting on the passive capillary valve surface and this imposes another challenge to the design of the conventional hydrophobic patch valves which has been successfully used for deionized water. We demonstrated a successful capillary valve array with a common air venting channel having a hydrophobic surface for restricting the flow of PCR buffer containing surfactant. The interconnected microreactor array was fabricated on a glass chip substrate with approximate volume of 250 nl microreactor volume for PCR. A different set of PCR primers were preloaded into different microreactor on the PCR array chip for simultaneous amplification of multiple genes. Fluorescent signals from all the microreactors were simultaneously detected at every PCR thermal cycle using EvaGreen fluorescent dye on an in-house real-time PCR instrument. The capability of the scalable PCR array chip was demonstrated by amplifying a fragment of uidA gene for beta-glucuronidase of E. coli genome. Key technical issues related to chip operation such as PCR inhibition on the acid-washed glass substrate, and PCR compatibility of the sealant in both the uncured and cured states have been addressed.
3D Flow Dynamics in a Patterned Round Microchannel
Authors: Qiu Huihe , Zhang Peng
Published: September 2010

The development of MEMS requires deliberate designs for controlling fluids in the low Reynolds number regime. Arranging surface charges in rectangular channels to obtain in-plane or out-of-plane vortices have been studied by previous researchers. However, previous surface modification techniques require different signs of zeta potentials from the other wall surfaces which made it difficult in selecting and coating microchannels. Previously, the opposite polarities are usually adjusted by changing the pH value of the solution with acid chemicals in other researches which made the solution complicated and difficult to simulate a real application. Meanwhile the acid chemicals may also destroy the coating. It is convenient to use same polarity patches if a vortex flow can also be generated. However, it is not clear if the patterned surface charges have the same polarity of zeta potentials as the other walls, what kind flow pattern will be generated and what mechanism behind the flow pattern. Furthermore, the cross-section of previously studied microchannels is usually limited to a rectangular shape. Therefore, the surface charge patterns are usually in 2D since the sidewalls of the rectangular microchannels are difficult to be patterned. However, a channel with round cross-section has better leak-proof performance of the membrane valve. Furthermore, a round channel is also advantageous in mimicking the human vein when a vascular structure is needed in tissue scaffolding, the round microfluidic channel is considered as a good candidate for an artificial capillary vessel. It is anticipated that there will be no stagnation occurs at the corner edges, which occurs at the corners of a rectangular channel, for a round microchannel owing to the perfectly symmetrical velocity profile. This is important when the microfluidic chip is subjected to a separation process such as liquid chromatography. In this paper, effects of patterned surface modification on 3D vortex flows generation in a micro capillary tube under very low Reynolds number have been investigated. Microfabrication technology was successfully employed to pattern surface charges on inner surfaces of round capillary tubes, which form non-uniform zeta-potentials. This technique extends the heterogeneous surfaces from flat surface to curved surface. 3D vortices are visualized and measured at the vicinity of tube walls when an electric field is applied across the surfaces utilizing micro resolution PIV. It demonstrated that 3D vortices can also be generated by the patterned surface charges with a same polarity. Experimental results have been compared with the numerical simulations using CFD-ACE+.
Study on Condensation Heat Transfer of Micro Structured Surfaces
Authors: Ohtake Hiroyasu , Koizumi Yasuo , Miyake Soichiro
Published: September 2010

Condensation heat transfer experiments for steam were performed by using mirror-finished copper surfaces, mirror-finished silicon surfaces and silicon surfaces with micro grooves or micro pins on it. The micro-grooves and the micro-pins were created by the MEMS technology. The film- and also the drop-wise condensation were observed on the copper surface. The film-wise condensation heat flux was in good agreement with the values of the Nusselt equation. It was approximately one-tenth of the drop-wise condensation heat flux. The condensation on the mirror-finished silicon surface was the drop-wise condensation. The heat flux was approximately one-tenth of the drop-wise condensation heat flux on the copper surface. The condensation on the micro-grooved and the micro-pin silicon surfaces was film-wise. The condensation heat fluxes were approximately one-tenth of the copper surface film-wise condensation heat flux. When the contact angle was smaller than 70 degree, the condensation was film-wise and when larger than the value, drop-wise. It seemed that the hollow parts of the micro-grooved or the micro-pin surface were filled with condensate first after the condensation was initiated. It made the surface hydrophilic and the condensation film-wise.
Hydrodynamic Flows in Electrowetting
Authors: Kang Kwan Hyoung
Published: September 2010

In the first part, I will present the hydrodynamic flows generated inside a droplet in electrowetting when an ac voltage is applied. In order to find out the characteristics and origin of the flows, we investigated the flow pattern for a sessile droplet for various conditions. A shape oscillation of a droplet was observed in the low-frequency range by a high-speed camera. The shape oscillation is responsible for the low-frequency flow. The flow at high frequency is caused by some electrohydrodynamic effect. In the second part, an experimental and theoretical work will be presented concerning the shape oscillation of sessile droplet. A set of shape mode equations is derived to describe unsteady motions of a sessile drop actuated by electrowetting. It is revealed that resonance occurs at certain frequencies and the oscillation pattern is significantly dependent on the applied AC frequencies. The domain perturbation method is used to derive the shape mode equations under the assumptions of weak viscous effect and small deformation of drop. The electrical force concentrated on the three-phase contact line is approximated as a delta function, which is decomposed and substituted into each shape mode equation as a forcing term. The theoretical results on the shape and the frequency responses are compared with experimental results, which shows a qualitative agreement. Finally, I will present a novel method to actuate oscillation of a sessile bubble or oil drop in a fluid to produce steady streaming within the fluid. This method is based on time-periodic control of the wettability of the bubble or drop by electrowetting. Jet velocity is proportional to oscillation amplitude and is greatest at natural oscillation frequencies. Analytical and numerical analyses indicate that the jet is produced by steady streaming in the Stokes layer.
Flow Boiling of Carbon Dioxide in Horizontal Mini-Channel and Pattern Dynamics Approach to Study Flow Pattern
Authors: Ozawa Mamoru
Published: September 2010

This paper provides a brief review on experimental and numerical investigations of flow patterns, pressure drop, and heat transfer including critical heat flux (CHF) of flow boiling carbon-dioxide (CO2) at high pressure in mini-channels ranging 0.5mm to 3.0mm in diameter. The flow patterns of CO2 at high pressure with small density difference between vapor and liquid and low surface tension show a slightly different structure from so far observed in mini-channels with air and water. The phase mal-distribution, similar to conventional tubes, in the cross-section becomes rather significant beyond the critical Bond number, which leads to the intermittent dryout at the upper wall of the tube. So far proposed flow pattern transition criteria are ineffective there, and newly developed discrete bubble model demonstrates its high potential in predicting flow patterns. Conventional homogeneous flow model is still available in predicting pressure drop. Based on this fact, flow instability problems, which significantly affect CHF, is discussed focusing on high-pressure CO2 flow.
Natural Convection in a Micro Size Horizontal Cylindrical Annulus With Periodic Volumetric Heat Flux
Authors: Mansour Kamyar
Published: September 2010

We consider the two-dimensional problem of steady natural convection in a narrow (Micro size) Horizontal Cylindrical annulus filled with viscous fluid and periodic volumetric heat flux. The solution is expanded in powers of a single combined similarity parameter, which is the product of the Gap ratio to the power of four, and Rayleigh number and the series extended by means of symbolic calculation up to 16 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is almost zero but Pade approximation lead our result to be good even for much higher value of the similarity parameter.
A Generalized Analysis of Electroosmotically Driven Capillary Flow in Rectangular Microchannels
Authors: Waghmare Prashant R. , Mitra Sushanta K.
Published: September 2010

In this study, the transient response of the fluid with combined electrokinetic and capillary effect in a microchannel is investigated theoretically. A second-order differential governing equation is obtained which represent the electroosmotic and capillary flow in a rectangular microchannel. This governing equation takes care of inertial force, dynamic contact angle, electrokinetic and entrance effect. The electrokinetic effect is modeled through additional electrokinetic force, which varies along the penetration depth of fluid. The non-dimensional analysis is presented by normalizing the gravity, viscous, electrokinetic forces with surface tension force. New non-dimensional group for electrokinetic force is reported which represents the ratio of electrokinetic and surface tension forces. The numerical solution of the governing equation is validated with experimental and analytical results for capillary flows, which shows good agreement. It is observed that for smaller Eo the capillary front advancement follows the similar trend as pure capillary flow but for higher Eo, the electroosmotic force dominates over surface tension force. It is demonstrated that this model provides simple approach to predict the penetration depth in electroosmotically driven capillary flow.
Emission of Liquid Droplets From the Interface of Bidrops Pulled by a Ferrofluid in a Microchannel
Authors: Shibata Yuichi , Takamine Kota , Kawaji Masahiro
Published: September 2010

The field of microfluidics is developing with advances in biotechnology and μ-TAS technologies. In various devices, controlling the flow rate of liquid or gas accurately at micro or nanoliter volume levels is required. By using a ferrofluid, the flow of a liquid or gas in a microchannel can be controlled by the driving power exerted on the ferrofluid. In a previous study, an unsteady flow of a liquid slug caused by the driving force exerted by the ferrofluid was investigated in a 200μm circular microchannel. The velocity of the ferrofluid was found to be affected by the physical properties of the liquids being pulled, such as the dynamic and static contact angles, surface tension and kinematic viscosity of the liquid slugs. At sufficiently high velocities of the ferrofluid, emission of a liquid droplet from the liquid-liquid interface was observed. In the present study, combinations of various liquids with the ferrofluid were examined in two microchannels (130μm and 200μm diameter). The relationship between the emission of liquid droplets and interfacial fluctuation of the bidrops was investigated experimentally and analytically. The emission of liquid droplets from the interface and behavior of the interface were observed using liquids of different viscosities. The interfacial shape changed continuously until a liquid droplet was emitted from the interface of the immiscible liquids. When the ferrofluid velocity was increased, necking of the liquid-liquid interface occurred continuously and some liquid droplets were emitted from the interface. We could study the characteristics of emission of liquid droplets from the interfacial variation.
Kinetics of Colloidal Particle Deposition From Electrokinetic Microfluidic Flows
Authors: Unni Harikrishnan N. , Yang Chun
Published: September 2010

This paper presents a theoretical and experimental investigation on the irreversible deposition of colloidal particles from electrokinetic microfluidic flow in parallel plate channels. The electrokinetic particle transport model presented in this study is based on the stochastic Langevin equation, incorporating the electrical, hydrodynamic, DLVO colloidal interactions and random Brownian motion of colloidal particles. The particle trajectories are computed via the Brownian dynamics simulation technique and the particle deposition is quantified in terms of the surface coverage. Direct videomicroscopic observation using the parallel-plate flow cell technique, is employed to determine the deposition kinetics of polystyrene latex particles in NaCl electrolytes. The theoretical predictions are compared with experimental results, and a reasonable agreement is found.
Thermal Analysis of Micro Capillary Pumped Loop System
Authors: Wang Chin-Tsan , Leu Tzong-Shyng , Yu Jui-Ming , Hu Yuh-Chung
Published: September 2010

A Capillary Pumped Loop is a sort of “two-phase heat transport device”. In this study, the micro capillary pumped loop (MCPL) and temperature sensors embedded in the micro-channels were fabricated using MEMS technology. An open type of MCPL was applied to determine the thermal analysis of MCPL corresponding to different injection volume flow rates under the condition of constant heating power 20W. A series of experiments yielded numerous results and are as follows: first, a larger injection volume flow rate results in a lower system temperature. Second, the thermal bubbles begin to degenerate into smaller bubbles at Q volume = 2 μl/min. In addition, the phenomenon of slug flow is observed with increasing injection volume flow rates, especially for the case of Q volume = 15 μl/min. Although the temperature of MCPL was reduced with the injection volume rate, the MCPL possessed an almost constant temperature difference regardless of injection volume flow rate. These findings will be useful in determining the optimal design of MCPL.
Fuzzy Logic Approach for Controlling Temperature in Electroosmotic Flow Fields
Authors: Movahed Saeid , Eghtesad Mohammad , Kamali Reza
Published: September 2010

By entering technology to the area of micro and nano scales, the design and fabrication of miniaturized instruments such as microelectronic devices, MEMS, NEMS and ..., become very desirable. Many of these devices deal with flow field in micro- and nano-channels. By decreasing the dimensions of channels, the influence of surface effects becomes prominent and cannot be ignored. One of the most charismatic categories of these phenomena is elecrokinetic effect which can result in electroosmotic flow field (EOF) that has many advantages such as being vibration free, being much more compact, having flat-form velocity and etc. These beneficiaries lead to the increasing stimulus of using this type of flow field. Electroosmosis is defined as the motion of ionized liquid relative to the stationary charged surface by an applied electric field. One of the most important disadvantages of EOF is the Joule heating effect, the generation of heat due to the electroosmosis effect. Besides, micro- and nano-channels are usually used as heat sink in miniaturized devices. By considering these facts, it can be concluded that heat characteristics of EOF must be studied carefully in order to manage and control the Joule heating effect for utilizing the cooling characteristics of micro- and nano-channels. Flow field characteristics can be found by solving Navier-Stocks and Energy equations with proper slip boundary conditions. By considering the partial nature of these equations, many conventional model-based control techniques may not be useful. Therefore, one can suggest some non-model based strategies in order to control the properties of flow fields. In the present study, fuzzy logic controllers will be proposed in order to control the temperature and cooling characteristics of micro- and nano-channel heat sinks.
Direct Simulation of Mass Transport in a SOFC Cathode Using Microchannels
Authors: Escobar Jose A. , Pakalapati Suryanarayana R. , Nanduri Jagannath R. , Celik Ismail B.
Published: September 2010

Fuel cells are clean and efficient power generation devices which are being widely investigated under the efforts to reduce the impact of greenhouse gases on the environment. Solid oxide fuel cells (SOFCs), especially, are suitable for stationary power generation using a wide range of alternative fuels. Performance of a SOFC strongly depends on the mass transport inside the porous electrodes which are essentially composed of a network of microchannels. In this study the mass transport inside a SOFC cathode is studied using direct simulation of mass transport in microchannels along with statistical analysis. A virtual cathode is built using microchannels that are representative of continuous flow paths between the cathode/air stream interface and cathode/electrolyte interface of a SOFC. Different representative microchannel flow paths are built with varying tortuosity and channel diameters. The numbers of channels of each kind are chosen according to a normal distribution and they are randomly arranged in an appropriately sized cuboid to construct a unit block of the virtual cathode. The normal distribution is modulated with average and standard deviation values for real world electrodes found in literature. Microchannels are tightly packed to achieve the desired porosity. Mass transport in each of the channels is studied separately using commercial CFD software FLUENT. Three dimensional simulations of momentum and specie transport equations (for oxygen and nitrogen) are performed. The results from individual channel simulations are used to assess the global mass transfer characteristics of the virtual cathode. Results obtained using this approach will be compared with those from a continuum Fick’s law type diffusion model used to simulate mass transport in porous media. The primary objective is to test the assumptions employed within the context of continuum mass transport model.
Experimental Study of Evaporation Heat Transfer Characteristics and Pressure Drops of R410A and R134a in a Multiport Mini-Channel
Authors: Kaew-On Jatuporn , Wongwises Somchai
Published: September 2010

The evaporation heat transfer coefficients and pressure drops of R-410A and R-134a flowing through a horizontal-aluminium rectangular multiport mini-channel having a hydraulic diameter of 3.48 mm are experimentally investigated. The test runs are done at refrigerant mass fluxes ranging between 200 and 400 kg/m2s. The heat fluxes are between 5 and 14.25 kW/m2, and refrigerant saturation temperatures are between 10 and 30 °C. The effects of the refrigerant vapour quality, mass flux, saturation temperature and imposed heat flux on the measured heat transfer coefficient and pressure drop are investigated. The experimental data show that in the same conditions, the heat transfer coefficients of R-410A are about 20–50% higher than those of R-134a, whereas the pressure drops of R-410A are around 50–100% lower than those of R-134a. The new correlations for the evaporation heat transfer coefficient and pressure drop of R-410A and R-134a in a multiport mini-channel are proposed for practical applications.
Heat Transfer and Pressure Drop Characteristics for HFE-7100 Within Microchannel Heat Sinks
Authors: Huang Chun-Min , Jeng Yeau-Ren , Yang Kai-Shing , Wang Chi-Chuan , Wu Yu-Lieh
Published: September 2010

This study examines the heat transfer and pressure drop characteristics of the dielectric fluid HFE-7100 within multiport microchannel heat sink having a square configuration rectangular with a hydraulic diameter of 460 μm. For a lower mass flux of 100 or 200 kg/m2·s, it is found that the heat transfer coefficients are roughly independent of heat flux and vapor quality provided that no flow reversal occurs. However, with the presence of flow reversal at an elevated heat flux, appreciable drop of heat transfer coefficient is encountered. The flow reversal also plays a significant role in the overall pressure drop. Without flow reversal, the pressure drop for higher heat flux always exceeds that of lower heat flux due to acceleration contribution. However, the presence of flow reversal may offset the contribution of acceleration and results in a negligible effect of heat flux. For a higher mass flux like 300 kg/m2·s, the heat transfer coefficients are virtually independent of vapor quality and heat flux.
Flexible Joule-Thomson Micro-Refrigerator
Authors: Kohno M. , Tanabe A. , Kuwamoto Y. , Kubota H. , Takata Y.
Published: September 2010

In this study, a prototype flexible Joule-Thomson micro-refrigerator was fabricated and its cooling power was examined. The micro-refrigerator uses N2, C2H4 or CO2 as a working gas and it consists mainly of a heat exchanger and an evaporator. The outside diameter of the heat exchanger outer tube is 0.9 mm and that of the inner tube is 0.4 mm. The length of the heat exchanger is 450mm. The inner diameter of the evaporator capillary is 0.1 mm. A cooling power of 100 mW at an evaporator temperature of 277 K was attained for inlet and outlet gas (CO2) pressures of 5.0 MPa and 0.1 MPa, respectively. To understand the cooling performance, a numerical analysis of the heat exchanger has been done and the effects of mass flow rate and dimensions of the heat exchanger on temperature profiles and effectiveness were examined.
Microscopic Airway Reopening Through Cascades of Plugs Ruptures
Authors: Baudoin M. , Song Y. , Baroud C. N. , Manneville P.
Published: September 2010

The inner surface of lung airways is covered by a thin layer of mucus whose thickness is usually about 2 or 3% of the total radius of the duct. However certain diseases like asthma, chronic bronchitis or allergies can induce a hypersecretion of mucus, leading to the formation of liquid plugs which occlude the airways. These plugs can considerably alter the distribution of air during the breathing cycle. It is therefore fundamental to understand the propagation of air in the presence of such plugs and in particular airway reopening. Some studies have been performed on real lungs but there was no visualization of the airways, and only information at the entrance was reported. The purpose of this experimental work is to create a synthetic network, reproducing only the main features of the lung airways, to visualize and understand the physics of airway reopening. The human lung is made of about 24 generations with diameters ranging from about 2 cm for the trachea to 100 μm for the smallest ones. As a consequence, the physics is very different for the first and the last generations. The present work focuses on the last micrometric generations for which inertia and gravity can be neglected (small Reynolds and Bond numbers). For this purpose a binary network made of PDMS was designed and fabricated. It is composed of 6 generations with a width of 700 μm for the first generation and a width ratio of 0.8 between the branches of successive generations. A random initial distribution of plugs is inserted inside this network by using syringe pumps and finally some air is introduced inside the airways. The reopening of the network takes place through a series of cascades of plugs ruptures. A single cascade can be explained by a simple model, based on the flow resistance of the plugs and the liquid deposited on the walls. The correlation between successive cascades is extracted from a careful analysis of the data. This study improves considerably our understanding of cascades of plug ruptures, which might be valuable to enhance the treatment of such diseases.
Application of Reactive Molecular Dynamics to Simulate Diffusion and Reaction in a Solid Oxide Fuel Cell Pore
Authors: Carreno-Chavez Rolando , Smirnov Andrei , Nanduri Jagannath , Celik Ismail
Published: September 2010

In a typical solid oxide fuel cell (SOFC), the kinetics of the gas phase reactions in the porous anode and electrochemical reactions at the triple phase boundary are generally unknown. Due to the unavailability of non-destructive experimental methods, factors affecting the performance of SOFC systems, especially the loss in performance due to contaminants, are usually deduced from many days of experiments. In this paper a Reactive Molecular Dynamics (ReMoDy) model based on collision theory is introduced and applied to simulate the behavior of species inside a SOFC pore. Using novel simulation methods, algorithms and visualization techniques ReMoDy has the ability to simulate chemical reactions involving tens of millions of molecules and determining the thermo-physical properties of the fluids from intermolecular energies and forces. In the current work two cases of molecular dynamics simulation inside a micro pore were analyzed. In the first case diffusion of hydrogen molecules was studied inside a 0.03125 μm3 cube. The diffusion coefficients obtained from this simulation are compared to the ones obtained using Chapman-Enskog correlations. In the second case gas phase and surface reactions were modeled for Syngas oxidation in a 1 μm3 cube representing a SOFC electrode pore. For this case detailed gas phase and surface reaction mechanisms involving 13 species and 63 reactions is included. Future studies will include the calculation of diffusion coefficients, rates of formation of different species, and comparison with published data. The results can be used for the verification of continuum models.
Characteristics of Microlayer Thickness Formed During Boiling in Microgaps
Authors: Zhang Yaohua , Utaka Yoshio , Kashiwabara Yuki , Kamiaka Takumi
Published: September 2010

Experiments were performed to measure the thickness of the liquid film formed by growing flattened bubbles in microgaps with laser extinction method for gap sizes of 0.5, 0.3 and 0.15mm. Water, ethanol and toluene were used as working fluids. High-speed camera was also taken to measure the bubble growth process simultaneously. It was confirmed that the gap size and bubble forefront velocity determined the initial microlayer thickness. The variation trend of the microlayer thickness relative to the velocity of interface was divided into two regions: region I where the velocity is small and the thickness increases linearly with increasing velocity; region II where the thickness almost constant or decreased slightly. Furthermore, the non-dimensional correlation for investigating the effects of test materials and gap sizes on micro-layer thickness was presented. With result analysis, the boundaries of the two regions were at Weber number of 80 approximately. And at the region where Weber number was smaller than 80 the thickness of mirolayer was thinner for the liquid whose surface tension coefficient is relatively large. But for the region where Weber number was bigger than 80, the smaller kinematic viscosity of liquid is, the thinner thickness of microlayer became.
Development of a Novel Thermal Sensor to Monitor the Fluid State in a Microchannel
Authors: Choi Sun Rock , Hong Jonggan , Kim Dongsik
Published: September 2010

This work presents development of a novel thermal sensor to measure the fluid phase and composition in a microchannel. As the sensor employs an AC hot-film technique to monitor the fluid state, it is more sensitive and noise-resistant than the conventionally used DC technique. The sensor is composed of a thin-film heater integrated into a PDMS microchannel fabricated on a glass substrate. Monitoring the liquid-gas phase interface in a microchannel is demonstrated by examining water injection and liquid evaporation processes. The results show good agreement with those by optical inspection. Also, real-time monitoring the composition of ethanol/water mixtures flowing in a microchannel is demonstrated. The presented sensor is expected to be used in various potential applications, including multiphase flow sensors, lab-on-a-chip devices, micro heat exchangers and micro fuel cells.
Chatter Stability in Micro Milling Processes Considering Ploughing and Process Damping Effects
Authors: Rahnama Ramin , Sajjadi Mozhdeh , Park Simon S.
Published: September 2010

Micro milling operations utilize miniature tools to remove workpiece material, in order to create the desired 3D miniature components. One of the challenges in a micro milling operation is the unstable phenomenon called regenerative chatter. The occurrence of chatter in the micro domain, as in macro machining, is detrimental to part finishes and significantly reduces the longevity of tools. There are two different cutting mechanisms in micro milling operations, which are determined by the critical chip thickness. When the chip thickness is less than the critical chip thickness, no chip forms and ploughing occurs; whereas, when the chip thickness is greater than the critical chip thickness, a chip forms and shearing cutting happens. During each rotation of the tool, the cutting mechanisms switch from ploughing to shearing and vice versa. This paper introduces a time domain chatter model to investigate the effects of the ploughing and shearing mechanisms on stability. The model also considers the effects of process damping in micro milling, especially at low spindle speeds. Several experimental tests have been performed to validate the model.
The Effect of Tool Surface Treatment on Friction in Hot Forging Process
Authors: Premanond Varunee , Hato Ratchanee , Chumrum Pakorn , Sripraserd Jiraporn
Published: September 2010

This work considers the influences of various types of die surface treatment, lubricants and temperature on friction for hot forging process of brass. Well-known ring compression test were carried out to evaluate friction coefficient for various conditions. Tool material was hot work tool steel H13 and workpiece material was brass C3771. Tool surface condition selected were normal hardening, treated by hard chrome, plasma nitriding and vacuum nitriding. Tests were conducted with and without lubricants at elevated temperature between 400–600°C. The results of experiments without lubricant show that friction coefficient is reduced when using die with treated surface regardless of working temperature. The influences of surface treated on die are not significant when forming under lubricated condition. In other word, lubricant has minor effect to reduce friction when the treated tools are employed. However, graphite in water was found to be almost compatible lubricant to graphite in oil when forming by tool which is treated by vacuum nitriding. The hardness and roughness of tool surfaces are found no relevance to friction coefficient in this work.
Laser Induced High-Strain-Rate Superplastic 3D Micro-Forming of Metallic Thin Film
Authors: Gao Huang , Cheng Gary J.
Published: September 2010

Microforming of metals has always been a challenge because of the limited formability of metals at micro-scales. This paper investigates an innovative micro-forming technique: Laser Dynamic Forming (LDF), which induces 3-D superplastic forming in metal thin films. This forming process proceeds in a sequence of laser irradiation of ablative coating, ionization, shockwave generation and propagation in metal thin films, and conformation of metal thin films to the shape of micro/nanoscale molds. Because the deformation proceeds at ultrahigh strain rate, it is found that materials experience superplastic deformation at microscales. In this paper, experiments are carried out to understand the deformation characteristics of LDF. The shapes of the formed samples are characterized by scanning electron microscopy (SEM) and optical profilometer. The thickness variations are characterized by slicing the cross section using focused ion beam (FIB). The magnitude of deformation depth in LDF is determined primarily by three critical factors: thin film thickness, geometry of molds, and laser intensity. The relationships between laser intensity, film thickness, and mold size are explored in process maps to find out suitable processing conditions of LDF. Nanoindentation testings are conducted to show that the mechanical properties (hardness and yield strength) are increased significantly after LDF.
A New Computationally Efficient Model for Tempering in Multi-Tracks Laser Hardening
Authors: Fortunato Alessandro , Orazi Leonardo , Tani Giovanni
Published: September 2010

The bottleneck in laser hardening principally occurs when large surfaces have to be treated because this process situation leads to multi-tracks laser scanning in order to treat all the component surface. Unfortunately, multi-tracks laser trajectories generate an unwanted tempering effect that depends on the overlapping of two close trajectories. To reduce the softening effects, a simulator capable to optimize the process parameters such as laser power and speed, number and types of trajectories, could sensibly increase the applicability of the process. In this paper an original model for the tempering is presented. By introducing a tempering time factor for the martensitic transformation, the hardness reduction can be predicted according to any laser process parameters, material and geometry. Experimental comparisons will be presented to prove the accuracy of the model.