bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Investigations on Lubricity and Cold Flow Properties of Biodiesel
Authors: Bhale Purnanand V. , Deshpande Nishikant V. , Thombre Shashikant B.
Published: June 2009

The desirability of developing biodiesel from different tree born oil seeds and decreasing the dependency on petroleum based fuels has been discussed by many over the last few decades. However some of the important issues of biodiesel like lubricity, low temperature flow characteristic, material compatibility have not been sufficiently investigated. In this paper, an attempt has been made to explore the lubricity and cold flow properties of biodiesel separately based on the experimental results. These two properties though may not be related with each other but plays significant role towards the long term acceptability of biodiesel in an existing CI engine. For biodiesel, the lubricity parameter is important to evaluate in the context of recent advent of ultra low sulfur diesel fuel (<10 ppm sulfur in Europe) which has raised the concern over the ability of these fuels to sufficiently lubricate diesel engine components in fuel injection system. In the very specific case of biodiesel, the lubricity could be defined as its ability to minimize or completely prevent wear in fuel pumps and injectors. Also, the biodiesel fuels derived from fats or oils with significant amounts of saturated fatty compounds will display higher cloud points and pour points which questions the pumping and injecting of these biodiesel in CI engines under cold climates. The lubricity of Jatropha Oil Methyl Ester, one of the potential specie for India is explored by evaluating the wear characteristics of plunger and barrel of CI engine fuel pump using bench test. Fuel pump plunger test specimen was made of mild steel pin and the rotating disc indicative of barrel in fuel pump was made from EN-31. Several tests were conducted on pin on disc machine using biodiesel for specific wear rate and friction characteristic to establish its lubricity. Results indicated that biodiesel from Jatropha has better lubricity over diesel fuel and has lower specific wear rate over diesel. The cold flow properties of biodiesel were evaluated with and without pour point depressant towards the objective of identifying the pumping and injecting of these biodiesel in CI engines under cold climates. Four different biodiesel were tested to identify the effect of cold flow improver on cloud and pour points. Ethanol has been considered as a cold flow improver in the present investigations. Effect of ethanol on cold flow behavior of this biodiesel was studied. A considerable reduction in pour point has been noticed.
Reduced Friction Losses and Wear by DLC Coating of Piston Pins
Authors: Morgenstern Roman , Kießling Wolfgang , Reichstein Simon
Published: June 2009

A Diamond Like Carbon (DLC) coating is well known to offer superior wear and friction behaviour. This combination of properties makes DLC suitable for many different areas of tribology. This paper concerns itself with usage in the power cylinder environment of automotive diesel engines. To estimate the potential of DLC coatings applied to piston pins in internal combustion engines, linearly reciprocating sliding wear examinations have been performed on uncoated and DLC coated component segments versus different counterpart materials as present in the power cylinder environment, including: piston (aluminium alloy), bushing (brass), piston pin (steel) and connecting rod (steel). Evaluation criteria for the tests include friction and wear performance in dry and lubricated conditions. Test results show how the DLC coatings offer impressive wear reductions for each of the different counterpart materials used. Furthermore, special emphasis is given to the analysis of the friction behaviour. As expected, the coefficient of friction (COF) decreased for aluminium and steel counterparts when the piston pin segments were DLC coated. However, for the combination of DLC with brass the COF increased in the dry condition. This surprising outcome is explained with SEM and EDX investigations of the wear traces. The tests at elevated temperature with lubrication show an inverse relationship with respect to friction criteria when compared with the dry room temperature tests for the DLC with brass combination. Examined engine tests confirm the results of the non-engine wear test rig, showing that DLC coatings applied on piston pins also exhibit properties and good potential to decrease frictional losses and fuel consumption in modern engines.
Electro-Pneumatic Exhaust Valve Modeling and Control for an Internal Combustion Engine
Authors: Ma Jia , Zhu Guoming , Stuecken Tom , Hartsig Andrew , Schock Harold
Published: June 2009

Variable valve actuation of Internal Combustion (IC) engines is capable of significantly improving their performance. It can be divided into two main categories: variable valve timing with cam shaft(s) and camless valve actuation. For camless valve actuation, research has been centered in electro-magnetic, electro-hydraulic, and electro-pneumatic valve actuators. This research studies the control of the electro-pneumatic valve actuator. The modeling and control of intake valves for the Electro-Pneumatic Valve Actuators (EPVA) was shown in early publications and this paper extends the EPVA modeling and control development to exhaust valves for the lift control which is the key to the exhaust valve control since an accurate and repeatable lift control guarantees a satisfactory valve closing timing control. Note that exhaust valve closing timing is a key parameter for controlling engine residual gas recirculation. The exhaust valve lift control challenge is the disturbance from the randomly varying in-cylinder pressure against which the exhaust valve opens. The developed strategy utilizes model based predictive techniques to overcome this disturbance. This exhaust valve lift control algorithm was validated on a 5.4 Liter 3 valve V8 engine head with a pressurized chamber to imitate the in-cylinder pressure. The experimental results demonstrated that the exhaust valve lift tracked the step reference in one cycle with the lift error under 1mm and the steady state lift error was kept below 1mm.
Influence of Physical and Chemical Properties of Two Biodiesel Fuels on Performance, Combustion and Exhaust Emission Characteristics in a DI-CI Engine
Authors: Rao V. Pulagala , Rao Basava V. A.
Published: June 2009

The purpose of this study is to examine the influence of biodiesel (BD) fuel properties on different characteristics of the engine and to compare with the baseline petroleum diesel (PD) fuel. This study consists of two parts, first one is biodiesel characterization and the second one is testing in the engine. Two BD fuels namely, the medium chain (C6–C24) coconut oil methyl ester (COME) and the long chain (C16–C18) sesame seed oil methyl ester (SSME) were selected. It is observed that, the physical and chemical properties such as viscosity, density, bulk modulus, calorific value, C/H ratio, and iodine value of SSME are higher than that of COME, while the cetane number, saturation% and oxygen% of the COME is higher than that of SSME. Experiments were conducted in a naturally aspirated, single cylinder, four-stroke, stationary, water cooled, constant rpm (1500), in-line (pump-high pressure tube-fuel injector) direct injection compression ignition (DI–CI) engine with COME, SSME (with and without preheating), and PD as fuels. The performance was evaluated in terms of fuel consumption (FC), brake specific energy consumption (BSEC), and thermal efficiency (BTE). Except for COME at full load, the BTE of the esters over all load ranges were less than that of PD fuel. Also, a significant improvement in BTE was observed, when the SSME is tested at PD’s viscosity by using preheating technique. At full load, the BSFC of COME and SSME are increased by 16.61% and 18.24% respectively. The minimum BSEC (at full load) of COME is decreased by 1.3% and while that of SSME is increased by 4.5%, as compared to that of PD fuel. The full load peak pressures for COME, SSME and PD fuel are 63.8 bar, 65.8 bar, and 62.9 bar respectively. The high peak pressures of the methyl esters are probably due to dynamic injection advance, caused by their higher bulk modulus. The net heat release rate (HRR) and cumulative heat release (CHR) were calculated from the acquired data. The results show that, at all loads there is a slight increase in peak HRR for COME and large increase in peak HRR for SSME against PD fuel. The higher values of peak HRR indicate better premixed combustion with the methyl esters. However, the peak HRR for preheated SSME (SSME_H) decreases due to late injection and faster evaporation of the fuel. It was observed that, at full load the nitric oxide (NO) emission of SSME is increased by 12.9%, while that of COME is decreased by 13.8% as compared to that of PD fuel. The smoke is increasing linearly with the fuels ‘C/H’ ratio regardless of their molecular structure. The HC emissions of both the esters are very low and are reduced by up to 73%, as compared PD. Also, there is a significant reduction in all exhaust emissions, and in particular the NO emission is observed with preheated SSME, due to change in premixed combustion phase.
Addressing CAM Wear and Follower Jump in Single-Dwell CAM-Follower Systems With an Adjustable Modified Trapezoidal Acceleration CAM Profile
Authors: Flocker Forrest W.
Published: June 2009

Presented is a modified trapezoidal cam profile with adjustable forward and backward acceleration. The profile is suitable for single-dwell cam and follower applications. The main benefit of the profile is that it allows cam designers to choose easily a value for the maximum forward or maximum backward acceleration to achieve design objectives. An additional benefit of the profile is that it has a continuous jerk curve. Follower acceleration is one of the primary factors affecting cam wear and follower jump, two main concerns of cam designers. Large forward acceleration against a load creates cam-follower interface forces that can cause excessive wear. Backward acceleration tends to reduce the cam-follower interface force, and if the backward acceleration is sufficiently large, separation between the cam and follower (“follower jump”) can occur. The cam profile presented in this paper gives cam designers an easy way to adjust the maximum forward or backward acceleration to prevent these problems.
A Theoretical Study on Performance and Combustion Characteristics of HCCI Engine Operation With Diesel Surrogate Fuels: n-Heptane, Dimethyl Ether
Authors: Shahangian Seyed Navid , Keshavarz Mojtaba , Javadirad Ghasem , Bagheri Nader , Jazayeri Seyed Ali
Published: June 2009

HCCI engines have low emission and high efficiency values compared to the conventional internal combustion engines. These engines can operate on most alternative fuels such as dimethyl ether (DME), which has been tested as a possible diesel fuel for its simultaneously reduced NOx and PM emissions. HCCI combustion of both DME and n-heptane fuels display a distinct two-stage ignition reaction with the first stage taking place at fairly low temperatures and the second stage taking place at high temperatures. The second stage is responsible for the main stage of the heat release process. In this study, a single-zone, zero-dimensional, thermo-kinetic combustion model has been developed. MATLAB software is used to predict engine performance characteristics of HCCI engines using two types of diesel fuel: Dimethyl ether and N-heptane. The effects of intake temperature and pressure, fuel loading and addition of EGR gases on auto-ignition characteristics, optimum combustion phasing, and performance of the HCCI engines are considered in this study. Simultaneous effects of these variables for finding the most appropriate regime of HCCI engine operation, considering knock and misfire boundaries, are also investigated.
EGR Oxidation and Catalytic Fuel Reforming for Diesel Engines
Authors: Asad Usman , Zheng Ming
Published: June 2009

Exhaust gas recirculation (EGR) treatment techniques that include combustible substance oxidation, catalytic fuel reforming, and partial bypass-flow control have been experimentally investigated on a single cylinder diesel engine. Application tests are conducted to investigate the effects of the reformed gases on the diesel combustion characteristics and exhaust emissions. This research is aimed at stabilizing and expanding the limits of heavy EGR during steady and transient operations by enhancing the premixed combustion that may significantly alleviate problems with soot formation and cyclic variations. Additionally, the heavy treated EGR is applied to enable in-cylinder low temperature combustion. A preliminary investigation on the effects of water addition to the high temperature catalyst bed is also conducted. The potential of EGR reforming is also examined for possible generation of synthetic EGR (CO2) at low engine loads. The effectiveness of the treated EGR on engine emission and operating characteristics are therefore reported.
Numerical Simulation of Convective Heat Transfer in a Spark Ignition Engine
Authors: Mohammadi Arash , Jazayeri Seyed Ali , Ziabasharhagh Masoud
Published: June 2009

A computational fluid dynamics code is applied to simulate fluid flow and combustion in a four-stroke single cylinder engine with flat combustion chamber geometry. Heat flux and heat transfer coefficient on the cylinder head, cylinder wall, piston, intake and exhaust valves are determined. Result for a certain condition is compared for total heat transfer coefficient of the cylinder engine with available correlation proposed by experimental measurement in the literature and close agreement is observed. It is observed that the value of heat flux and heat transfer coefficient varies considerably in different positions of the combustion chamber, but the trend with crank angle is almost the same.
A Comparison of Ethanol and Butanol as Oxygenates Using a Direct-Injection, Spark-Ignition (DISI) Engine
Authors: Wallner Thomas , Miers Scott A. , McConnell Steve
Published: June 2009

This study was designed to evaluate a ‘what-if’ scenario in terms of using butanol as an oxygenate, in place of ethanol in an engine calibrated for gasoline operation. No changes to the stock engine calibration were performed for this study. Combustion analysis, efficiency and emissions of pure gasoline, 10% ethanol and 10% butanol blends in a modern, direct-injection four-cylinder, spark ignition engine were analyzed. Data was taken at engine speeds of 1000 RPM up to 4000 RPM with load varying from 0 Nm (idle) to 150 Nm. Relatively minor differences existed between the three fuels for the combustion characteristics such as heat release rate, 50% mass fraction burned, and coefficient of variation of indicated mean effective pressure at low and medium engine loads. However at high engine loads the reduced knock resistance of the butanol blend forced the engine control unit to retard the ignition timing substantially, compared to the gasoline baseline and even more pronounced compared to the ethanol blend. Brake specific volumetric fuel consumption which represented a normalized volumetric fuel flow rate, was lowest for the gasoline baseline fuel, due to the higher energy density. The 10% butanol blend had a lower volumetric fuel consumption compared to the ethanol blend, as expected based on energy density differences. Results showed little difference in regulated emissions between 10% ethanol and 10% butanol. The ethanol blend produced the highest peak specific NOx due to the high octane rating of ethanol and effective anti-knock characteristics. Overall, the ability of butanol to perform equally as well as ethanol from an emissions and combustion standpoint, with a decrease in fuel consumption, initially appears promising. Further experiments are planned to explore the full operating range of the engine and the potential benefits of higher blend ratios of butanol.
Thermal Resistance Measurements From a Microchannel Fluorescent Melting Curve Analysis Platform
Authors: Kinahan David J. , Dalton Tara M. , Davies Mark R.
Published: June 2009

Heat Transfer and Thermal Management have become important aspects of the developing field of μTAS systems, particularly in the development of micro PCR thermocyclers. Due to the development of flowing PCR thermocyclers in the field of μTAS, the authors have previously developed a melting curve analysis technique that is compatible with these flowing PCR thermocyclers. The test rig induces a linear temperature gradient along a sample carrying microchannel (0.8mm ID Teflon tubing). Hence, any flow passing through the microchannel is subject to linear heating. The ramp rate seen by the sample is equal to the flow velocity times the thermal gradient across the test rig. This paper presents a characterisation of this test rig, performed by positioning a thermocouple within the teflon tubing. A thermal lag is observed between the temperature of the fluid in the channel and the temperature predicted by 1D conduction heat transfer theory. This lag is as a result of the thermal resistance of the tubing walls and substrate. This lag is measured for three different substrate compositions; each subjected to varying temperature gradients and with sample flow at different velocities. The thermal lag is found to be linearly proportional to flow ramp rate. This finding is supported by a theoretical treatment and numerical simulation of the test rig. Additionally, the contact thermal resistance between the thermal blocks and the fluid within the channels can be made by substituting experimental measurements into the theoretical treatment. This measurement technique is independent of knowledge of substrate dimensions, contact surface quality and substrate composition/material properties.
Programmable Manipulation of a Droplet in a Planar Microchannel
Authors: Jiao Zhenjun , Huang Xiaoyang , Nguyen Nam-Trung
Published: June 2009

Programmable thermocapillary manipulation of liquid droplet in a planar microchannel has been carried out by both theoretical modeling and experimental characterization in this paper. The driving temperature gradients are provided by four micro-heaters at the channel boundaries. In the modeling, the temperature distributions corresponding to both transient and periodic actuation are calculated, and are coupled to the droplet motion through the surface tensions which drives the droplet to move inside the channel. The droplet trajectories and final positions are simulted, and compared with the experimental results, in which a silicon oil droplet was actuated inside a 10 mm×10 mm planar channel with four heater fabricated on the substrate plate. The results show that the droplet can be positioned anywhere in the channel, determined by a heating code related to the heating strengths. Qualitative agreement between the modeling results and experimental data, in terms of temperature distributions, droplet positions and trajectories, has been obtained.
Micro Drop Actuation Using Multiplexer Structures
Authors: Nichols Jackie , Ahmadi Ali , Hoorfar Mina , Najjaran Homayoun , Holzman Jonathan F.
Published: June 2009

Digital microfluidic manipulation is analyzed for applications extending down to micron-scales. A model is introduced to study the relationship between fluid actuation forces and the resulting resistive viscous and capillary forces that dominate the fluid flow on small dimensions. The results are presented through the analysis of a dynamical model of a novel Digital Microfluidic Multiplexer structure. In essence, the multiplexer is an interdigitated structure that is used to apply electrical signals to individual drops confined between two horizontal plates, about 10–50 microns apart. The multiplexer consists of x-electrodes, installed in the bottom plate, perpendicular to the y-electrodes, installed in the top plate. It is shown that such a configuration allows for highly parallel electrode addressability within exceedingly small (micron-scale) integrated structures.
Applications of Nanofluids in Microfluidic Devices
Authors: Chen Ping-Hei , Liou Dar-Sun , Tsai Tsung-Han , Kuo Long-Sheng , Yang Chin-Ting
Published: June 2009

This paper aims to introduce some applications of nanofluids in microfluidic devices. First of all, the fabrication processes of gold nanoparticles in water solution and Fe3O4 nanoparticles in both diesel oil and water solution are described. Fe3O4 nanoparticles in diesel oil and water are called oil ferro-nanofluid and water ferro-nanofluid in this study, respectively. Some special designs of microchannels are presented to perform specific functions, such as the generation of segmented fluid slugs or droplets, and the sieving of nanoparticles in different particle’s size. Measured results show that the mixing performance between a water solution and water ferro-nanofluid in a Y-type microchannel can be enhanced with help of an external magnetic field. In addition, the nanoparticles in different sizes can be easily separated by passing the serpentiform microchannels.
Mixing Enhancement by a Micro-Rotor in Step Microchannel
Authors: Dinh Thien Xuan , Ogami Yoshifumi
Published: June 2009

This paper describes numerically the design and performance of an active mixer which aims to exploit the chaos generated by a micro-rotor in continuous flow. The mixer consists of a step contraction-expansion microchannel and a micro-rotor placed at the step. The micro-rotor can be set into motion by laser power or magnetic field, inducing 3D motion in the surrounding fluid. By tracking streaklines from the inlet, we observed that fluids from the inlet can penetrate into the space between the paddles of the rotor, and then are mixed here. The streaklines also show that two fluids are twisted 90 degrees after passing the rotor region. It implies that mixing in the exit channel occurs in the height instead in the width of the exit channel. It makes the mixer applicable for channels with high aspect ratio of the cross section. The effectiveness mixing of the mixer is measured by the homogeneity distribution of a passive scalar on the outlet of the mixer. The results show that effectiveness of convective mixing induced by the rotor depends on Strouhal number, which is defined as the ratio of tip paddle velocity to mean flow in the channel. Mixing efficiency increases with increasing Strouhal number.
Microstructured Reactors for Gas Phase Reactions
Authors: Schirrmeister Steffen
Published: June 2009

Pilot-scale micro-process technology for heterogeneously catalyzed gas phase reactions is generally highly demanding towards the methods of catalyst coating, flow distribution, reactor manufacturing and assembly, safety issues and other factors. Yet, first cost analysis have shown that economical processes can be developed using micro-technology. For this matter, it is necessary to improve and simplify the laboratory set-up, meaning that the stacked architectures at the meter-scale must be brought down to the micron-scale. This in return calls for specific methods of catalyst coating and a particularly precise assembly of the operation unit.
Two-Phase Flow Considerations in PEMFC Design and Operation
Authors: Owejan Jon P. , Gagliardo Jeffrey J. , Sergi Jacqueline M. , Trabold Thomas A.
Published: June 2009

A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the co-flowing hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in-situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.
Nucleate Pool Boiling on Tubes With Subsurface Mini and Micro Channels
Authors: Stephan Peter , Wondra Frank
Published: June 2009

Tubes with subsurface mini- and micro channels are widely used in boiling devices. Today, the design of these tubes is based on empirical investigations because reliable theoretical or numerical design methods are not available, and the heat transfer performance characteristics vary strongly e.g. for different fluids, system pressures and heat fluxes. The presented study approaches the subject of interest from 3 different paths: (i) macroscopic heat transfer performance measurements; (ii) optical measurements of the thermal and hydrodynamic phenomena inside the subsurface channels with very high spatial and temporal resolution; (iii) development of a computational model. While path (i) is a standard procedure in the scientific community, path (ii) is a completely new approach, and only the combination of (i) and (ii) is a solid basis for the physically based model development in path (iii). Tubes with different subsurface channels and micro pin-fins are used in combination with different fluids at different system pressures. The results are presented and discussed.
Single-Cell Level Measurement of Shape, Shear Stress Distribution and Gene Expression of Endothelial Cells in Microfluidic Chips
Authors: Rossi Massimiliano , Lindken Ralph , Hierck Beerend P. , Westerweel Jerry
Published: June 2009

A measurement technique based on micro-PIV that allows a non-intrusive, non-tactile determination of the topography and shear stress distribution on endothelial cell cultures with a super-cell resolution is presented. The cells are grown in microfluidic perfusion chips where constant steady flow is applied. The expression of shear responsive gene KLF2 of individual cells is measured simultaneously with the shape and shear stress distribution of the same cell. Due to the super-cell resolution of the technique we can study the correlation of the measured quantities in different parts within the cell such as the nucleus or the cytoplasm. Results on human and chicken endothelial cells are shown.
Relationship Between Dimension and Steady Time in Electroosmotic Microchannel Flows
Authors: Yang Dayong , Liu Ying
Published: June 2009

Numerical simulation of transient state based on finite element method is presented aiming at the straight parallel-plate hydrophilic and hydrophobic microchannel. The influence of dimensions of microchannel to velocity in spatial and time is analyzed. The results of numerical simulation show that the transient characteristic of microflow is similar, which develops from the “saddle-like” at the beginning to the “plug-like” at the steady state, while the magnitude of steady time of microflow is proportional to the square of the dimension of microchannel according with viscous flow theory, and the scale is microsecond. The results provide the guidelines for the application of electroosmotic driven in the hydrophilic and hydrophobic microfluidic chips and the precision control of microfluid.
Local Measurement of Forced Convection Heat Transfer in a Micro Glass Tube
Authors: Schilder B. , Yu S. C. M. , Kasagi N. , Hardt S. , Stephan P.
Published: June 2009

The pressure drop and the convective heat transfer characteristics of ethanol and water in a 600 μm diameter tube with and without phase change has been studied experimentally. The test section consists of a glass tube coated with a transparent ITO (indium tin oxide) heater film. For single phase flow it was found that the measured Nusselt numbers and friction factors are in good agreement with the theoretical values expected from Poiseuille flow. Subsequently, the boiling heat transfer of ethanol was studied. It was found that boiling with bubble growth in both upstream and downstream directions leaving behind a thin evaporating liquid film on the tube wall is the dominant phase change process. Local Nusselt numbers are calculated for the two phase flow at different heat fluxes and Reynolds numbers. Compared to single phase flow the heat transfer is enhanced by a factor of 3 to 8.
Contribution of Microlayer Evaporation During Flow Boiling Inside Microchannels
Authors: Mukherjee A.
Published: June 2009

Flow boiling through microchannels is characterized by nucleation and growth of vapor bubbles that fills the entire channel cross-sectional area. As the bubble nucleates and grows inside the microchannel, a thin film of liquid or a microlayer gets trapped between the bubble and the channel walls. The heat transfer mechanism present at the channel walls during flow boiling is studied numerically. These mechanisms are compared to the heat transfer mechanisms present during nucleate boiling and in a moving evaporating meniscus. It is shown that the thermal and the flow fields present inside the microchannels around the bubbles are fundamentally different compared to nucleate boiling or in a moving evaporating meniscus. It is explained that how thin film evaporation is responsible for creating an apparent nucleate boiling heat transfer mechanism inside the microchannels.
Use of Electrokinetic Phenomena in Microfluidic Devices
Authors: Barz Dominik P. J. , Zadeh Hamid Farangis , Meisel Ingo , Ehrhard Peter
Published: June 2009

In microfluidic devices for chemical or biochemical analysis (μTAS) it appears promising to use electrokinetic phenomena since the ratio of surface area to volume is high and therefore interfacial phenomena are pronounced. Electrokinetic phenomena comprise a family of different effects that are all related by the presence of a so-called electrical double layer (EDL). Particularly, two electrokinetic phenomena can be used to fulfil manifold tasks required within microfluidic devices. Electroosmosis, the motion of liquids under the influence of an applied electrical field, can be used to drive liquids. Electrophoresis, the motion of (partially-) charged particles under the influence of an applied electrical field, is another electrokinetic phenomenon applicable for analysing and focusing of species. We present investigations focused on the flow/concentration field in an electrokinetic micromixer and on the performance of a micro-electrophoresis device.
Acoustofluidic Simulations: Ultrasound Handling of Liquids and Particles in Microfluidic Systems
Authors: Skafte-Pedersen Peder , Bruus Henrik
Published: June 2009

Within the field of lab-on-a-chip systems large efforts are devoted to the development of onchip tools for particle handling and mixing in viscosity-dominated liquid flows on the sub-mm scale. One technology involves ultrasound with frequencies in the MHz range, which leads to wavelengths of the order of 0.1–1 mm suitable for mm-sized microchambers. Due to the nonlinearity of the governing acoustofluidic equations, second-order effects will induce steady forces on fluids and suspended particles through the effects known as acoustic streaming and acoustic radiation force. We extend the basic perturbation approach for treating these effects in systems at resonance in various geometries. The first-order eigenmodes are used as source terms for the time-averaged viscous second-order equations. The theory is applied to explain experimental results on aqueous microbead solutions in silicon-glass microchips.
Numerical and Experimental Investigations of Momentum and Heat Transfer in Microchannels
Authors: Favre-Marinet M. , Drobniak S. , Asendrych D. , Gamrat G. , Niklas M.
Published: June 2009

The paper presents the results of a research performed during recent years at LEGI Grenoble with joint participation of CzUT researchers. The special attention is devoted to experimental and numerical studies devoted to three cases involving various aspects of microchannel flow physics (range of microchannel sizes corresponds to the classification of Kandlikar [19]). The first aspect is related to hydraulic properties of a network of parallel triangular microchannels, where experimental investigations revealed the rapid increase of pressure drop for Re exceeding value of 10. The second aspect of the research was the influence of surface roughness, which was investigated both experimentally and numerically for periodically and randomly distributed surface elements. The third research case was devoted to the numerical modelling of heat transfer. As a result, experimental and numerical analyses showed that there was no scale effect for the microchannels considered, i.e. the relevance of the classical continuum flow model was confirmed.
Flow in Channels With Rough Walls: Old and New Concepts
Authors: Herwig H. , Gloss D. , Wenterodt T.
Published: June 2009

In our study we determine the influence of wall roughness on friction for pipe and channel flows by numerically calculating the entropy production in the flow. It turns out, that there is an appreciable influence of wall roughness in laminar flows though this effect often is neglected completely. In addition to the friction factor results we gain an understanding of the physics since we have access to the dissipation distribution in the flow field close to the roughness elements. For a concise description of flows over rough walls there should be a reasonable choice of the wall location as well as the roughness parameter. Various options are discussed and assessed.
Hydrodynamics and Leukocyte Adhesion in Microvessels: State of the Art
Authors: Hanzlik Josa , Cretekos Ellen , Lemkin-Kennard Kathleen A.
Published: June 2009

Leukocyte rolling and adhesion are complex physiological processes that have received a great deal of attention over the past decade. Significant increases in the knowledge base related to how leukocytes adhere in shear flows have occurred as a result of the development of novel experimental and computational techniques. Micro- and nano-fabrication techniques have enabled the development of novel flow devices for studying leukocyte adhesion in simple and complex geometries. Improvements in computer technology have enabled simulations of complex flow processes to be developed. As a result of these advances in knowledge related to leukocyte adhesion, numerous novel devices have been developed that mimic the leukocyte rolling and adhesion process. Examples of these devices include cell separation and enrichment devices and targeted ultrasound contrast agents. Future advances related to leukocyte rolling and adhesion processes hold great promise for advancing our knowledge of disease processes as well as development of novel therapeutic devices.
Thermal and Hydrodynamic Characteristics of Single-Phase Flow and Flow Boiling in a Staggered Micro-Pin-Fin Array
Authors: Qu Weilin
Published: June 2009

This study concerns thermal and hydrodynamic characteristics of water single-phase flow and flow boiling in a micro-pin-fin array. An array of 1950 staggered square micro-pin-fins with a 200×200 μm2 cross-section by a 670 μm height were fabricated into a copper heat sink test section. Two inlet temperatures of 30 °C and 60 °C, and six maximum mass velocities for each inlet temperature, ranging from 183 to 420 kg/m2s, were tested. The corresponding inlet Reynolds number ranged from 45.9 to 179.6. General characteristics of single-phase flow and flow boiling were described. Predictive tools were proposed for single-phase heat transfer coefficient and pressure drop. Unique features of flow boiling heat transfer in the micro-pin-fin array were identified. The classic Lockhart-Martinelli correlation incorporating a single-phase micro-pin-fin friction factor correlation and the laminar liquid–laminar vapor combination assumption was used to predict two-phase pressure drop in the micro-pin-fin array. The predictions agreed well with the experimental data.
Electroosmotic Flow in Tree-Shaped Microchannel Networks
Authors: Barrot Christine , Colin Ste´phane
Published: June 2009

An analytical model of electroosmotic flow in tree-shaped microchannel networks is developed. The aim of the study is to determine the best network architecture to maximize the electroosmotic flowrate for a given electric field and a given total microchannel volume. The network consists of rectangular microchannels with high aspect ratio. The paper shows under what conditions the tree structure offers a higher flowrate than a series of parallel microchannels. The influence of the electric double layer (EDL) thickness is pointed out. As long as the EDL thickness is negligible compared with the microchannel width, the tree-shaped architecture does not present any particular interest. But as soon as the EDL thickness is significant, it is shown that the flowrate can be largely enhanced by increasing the number of bifurcations in the tree-shaped network. Two configurations of bifurcations (V-shaped and U-shaped) are considered and compared. Each bifurcation is composed of a parent microchannel connected to two identical daughter microchannels. The optimal value of the daughter over parent microchannels widths is calculated. The influence of the daughter over parent microchannels lengths and of the number of bifurcations is pointed out. Guidelines for the design of tree-shaped microchannel networks are finally proposed.
Relationship Between Water Transfer Through Membrane and Species Mole Fractions in a Micro PEMFC Channel
Authors: Lee Dongryul , Bae Joongmyeon
Published: June 2009

Two main water transport phenomena occur in the electrolyte of PEMFC. Electro-osmotic drag results from proton movements that promote water transport from anode to cathode. Back diffusion is caused by concentration difference that promotes water diffusion from cathode to anode. Electro-osmotic drag is more dominant than back diffusion when the net water transfer coefficient (α) is over zero, and vice versa. Species mole fractions are determined from net water transfer as a result of water transport phenomena and reaction rates related to current density. The hydrogen mole fraction has an extreme point at α = 0.5, while the extremum of the oxygen mole fraction is located at α = −0.75. The change pattern of species mole fraction varies at these extreme points. Net water transfer through membrane influences membrane conductivity and activation loss which are main factors that influence the performance of PEMFC. The relationship between net water movement and species mole fraction is considered in this paper. It is suggested that criteria predict the species mole fraction.
Modeling of a Gravity Driven Two-Phase Loop
Authors: Agostini Bruno , Yesin Berk
Published: June 2009

This paper proposes an analytical based numerical model of a small channel gravity driven two-phase loop. This model is based on the solving of the mass, momentum and energy conservation equations. Suitable correlations and models are used to calculate the pressure drop, void fraction and heat transfer coefficient in the successive sections of the thermosyphon. Then residuals of these conservation equations are evaluated and minimized with a minimization algorithm. The first simulations show as expected the existence of an optimal filling around 50%, the influence of the heat load and the coolant temperature. This model will enable us to predict the heat transfer performances in a gravity driven two-phase loop with various and changing operating conditions.
Continuum Models With Slip for the Evaporating Meniscus and Comparison With Experimental Results
Authors: Chatterjee Arya , Plawsky Joel L. , Wayner Peter C.
Published: June 2009

The evaporating meniscus is a recurring phenomenon in engineering that has relevance in diverse applications like dip-coating, pool boiling and the micro heat pipe. It is known that the evaporative heat flux at the liquid vapor interface peaks at the interline region. Recent experimental investigations have indicated a rise in the curvature in this region. To match the increased flow predicted by this curvature jump, slip may be required at the solid liquid interface. We use continuum models and experimental data on temperature and thickness to evaluate slip at the liquid solid interface.
Surface Modifications Using Nanofluids for Nucleate Boiling Heat Transfer and CHF Enhancements
Authors: Truong Bao , Hu Lin-Wen , Buongiorno Jacopo
Published: June 2009

Colloidal dispersions of nanoparticles, also known as nanofluids, have shown to result in significant Critical Heat Flux (CHF) enhancement. The CHF enhancement mechanism in nanofluids is due to the buildup of a layer of nanoparticles which occurs upon boiling. Some studies have shown that this layer may also lead to increase in nucleate boiling heat transfer. Therefore, this paper discusses how key surface parameters such as surface wettability and surface roughness can be manipulated and optimized by coating nanoparticles in colloidal dispersions onto the desired surface, to enhance the nucleate boiling heat transfer and CHF.
Liquid Water Transport and Distribution in Fibrous Porous Media and Gas Channels
Authors: Rensink Dirk , Roth Jo¨rg , Fell Stephan
Published: June 2009

In a polymer electrolyte membrane (PEM) fuel cell water is produced by electrochemical reactions in the catalyst layer on the cathode side. The water diffuses through the catalyst layer and a fibrous substrate into gas channels where it is transported away by convection. The fibrous substrate represents the gas diffusion media (GDM). Sometimes the GDM has a thin microporous layer on the side facing the catalyst layer. The same layer structure can be found on the anode side. All layers together are the porous layers of a PEM fuel cell. Under certain operating conditions condensation can occur in the porous layers which might lead to flooding conditions and — if the liquid water forms droplets which grow together in the gas channels — the complete blockage of the channels. Both situations can lead to a local starvation of reactant gases with negative impact on fuel cell performance and durability. The void space of the hydrophobic fibrous substrate in a PEM fuel cell can be interpreted as micro channels in a broader sense, especially if liquid phase transport from the catalyst layer towards the gas channels is in focus. Due to the small dimensions with effective channel diameter in the range of micrometer the flow of liquid water is governed by capillary forces. The same applies for the gas channels at low gas velocities since the Bond and Capillary numbers are well below one. Thus the investigation of liquid water flow and distribution under low gas velocities in the hydrophobic fibrous substrate and the spreading of liquid water along the hydrophilic gas channel walls under capillary action is of special interest for PEM fuel cells and investigated here.
Molecular Dynamics Study on the Effect of Adsorbed Layer on Accommodation Coefficients in Micro Gas Flow
Authors: Sun Jun , Li Zhixin
Published: June 2009

Tangential momentum accommodation coefficient (TMAC) is reported to be less than unity in rarefied gas flow and greatly influenced by many factors such as temperature and adsorbed layers. According to the definition, a proper statistical algorithm in NEMD method is described and verified. Adsorbed layers occur on walls due to strong gas-wall interactions and the effect on TMAC are studied in two dimensional isothermal Poiseuille flow in smooth micro-channels under the conditions of various temperatures, driving forces, and Kn. The simulation results indicate that when gas-wall interactions become stronger, TMAC increases to maximum firstly and decreases a little. Besides, the effects of temperature and Kn on TMAC are not monotonous with the existence of adsorbed layer. In addition, normal momentum accommodation coefficient (NMAC) is almost unity for isothermal flow in smooth microchannels.
Free Convection of Nanofluid in Vertical Annuli
Authors: Falahatpisheh Ahmad , Abouali Omid
Published: June 2009

This paper presents the numerical study of internal free convection in a vertical annulus. To predict the characteristics of nanofluids in heat transfer regime, thermal conductivity, density, and viscosity of clear fluid need to be modified. Numerous models have been disclosed to calculate these properties but recent models for viscosity and thermal conductivity which are in excellent agreement with experiments have been used. These models are thermal conductivity model of Jang and Choi and viscosity correlation of Nguyen et al. for 36nm Al2O3 particles. Inner and outer vertical walls are in constant temperature while horizontal walls are adiabatic. The continuity, Navier-Stokes and energy equation were solved numerically. Effect of nanofluids on buoyancy-driven heat transfer is investigated as a function of geometrical and physical parameters and various particle concentrations for aspect ratio of 0.2<H/L<5, Grashof number of 103<Gr<105 and concentration of 0<φ<0.12. Finally, one correlation is developed to demonstrate the effect of using atom-size particles on Nusselt number.
Experimental Investigation of Factors Affecting Miscible Two-Phase Flow in Fractured and Non-Fractured Micromodels
Authors: Farzaneh Seyed Amir , Kharrat Riyaz , Ghazanfari Mohammad Hossein
Published: June 2009

Micromodel is small-scale artificial model of porous medium which is known as a novel approach for simulating flow and transport in porous media. For better understanding the effect of fracture geometrical properties on oil recovery efficiency, a series of first contact miscible solvent injection process were conducted on horizontal glass micromodels at several fixed flow rate conditions. The micromodels were initially saturated with the heavy crude oil. The produced oil as a function of injected volume of solvents was measured using image analysis of the provided pictures. The concentration calibration curves of solvents in heavy crude oil were used for evaluating the solvents concentration. Several fractured and non-fractured quarter five-spot micromodels were generated by chemically etching process. The result of the experiments show that the produced oil decreased when the flow rate, fracture spacing, fracture discontinuity, fracture overlap, and fracture distribution were increased. In contrast, the produced oil increased, when the solvent viscosity, fracture orientation angles, fracture discontinuity-distribution and the number of fracture were increased. In addition, an optimum solvent composition is proposed.
Experimental Convective Heat Transfer With Nanofluids
Authors: Kabelac S. , Anoop K. B.
Published: June 2009

Nanofluids are colloidal suspensions with nano-sized particles (<100nm) dispersed in a base fluid. From literature it is seen that these fluids exhibit better heat transfer characteristics. In our present work, thermal conductivity and the forced convective heat transfer coefficient of an alumina-water nanofluid is investigated. Thermal conductivity is measured by a steady state method using a Guarded Hot Plate apparatus customized for liquids. Forced convective heat transfer characteristics are evaluated with help of a test loop under constant heat flux condition. Controlled experiments under turbulent flow regime are carried out using two particle concentrations (0.5vol% and 1vol %). Experimental results show that, thermal conductivity of nanofluids increases with concentration, but the heat transfer coefficient in the turbulent regime does not exhibit any remarkable increase above measurement uncertainty.