bluuebunny/crossref_metadata_2025 huggingface dataset:
Convective Heat Transfer Performance of FC-72 in a Pin-Finned Channel
Authors: Chien Liang-Han , Pei S.-Y. , Wu T.-Y.
Published: June 2009

This study investigates the influence of the heat flux and mass velocity on convective heat transfer performance of FC-72 in a rectangular channel of 20mm in width and 2 mm in height. The heated side has either a smooth surface or a pin-finned surface. The inlet fluid temperature is maintained at 30°C. The total length of the test channel is 113 mm, with a heated length of 25mm. The flow rate varies between 80 and 960 ml/min, and the heat flux sets between 18 and 50 W/cm2. The experimental results show that the controlling variable is heat flux instead of flow rate because of the boiling activities in FC-72. At a fixed flow rate, the pin-finned surface yields up to 20% higher heat transfer coefficient and greater critical heat flux than those of a smooth surface.
Numerical Analysis of Electroosmosis in a Micro-Cavity
Authors: Fernandes Dolfred V. , Kang Sangmo , Suh Yong K.
Published: June 2009

The bulk motion of an aqueous solution induced by the application of DC and AC electric fields is studied numerically. The physical model consists of a rectangular micro-cavity filled with dilute, symmetric, binary electrolyte and two completely polarizable cylindrical electrodes. The electric double layer (EDL) model coupled with Navier-Stokes equations governing the electroosmotic flow has been described. The ion-transport in the domain is obtained by solving Poisson-Nernst-Plank equations. We employed IB (immersed boundary) technique for the implementation of boundary conditions and semi-implicit fractional-step method for solving the momentum equations. The Poisson equation for potential distribution is coupled with Nernst-Plank equations for ionic species distribution and solved using CGSTAB iteration solver. Numerical codes are validated using bench-mark problems; driven-cavity-flow and flow over a cylinder. The electric field is almost completely balanced by the accumulation of the counter-ions at the electrodes, at steady state the potential in the most part of domain is zero. The flow field is found predominant in the region near the electrodes.
Characterisation of Flow and Heat Transfer in Sintered Metal Foams
Authors: Sauerhering Jo¨rg , Angel Stefanie , Fend Thomas , Brendelberger Stefan , Smirnova Elena , Pitz-Paal Robert
Published: June 2009

Since sintered metal foam is an innovative material with promising properties such as high porosity, good thermal conductivity, beneficial mechanical properties like strength and weldability, it has been considered to be applied as an open porous wall element in combustion chambers of gas turbines. In this application, the foam serves as a functional material capable to lead cooling air through micro- and minichannels into the inside of the combustion chamber. This cooling technique also known as effusion cooling keeps the combustion chamber walls below critical temperatures and therefore enables the burning process to be more effectively operated at higher temperatures. For a proper design of the wall element, the temperature distribution along the path of the fluid inside the foam must be known. For an exact calculation of the temperature flow and heat transfer processes inside the foam must be known. Therefore in this study the permeability and heat transfer properties of the foam have been characterized experimentally. The methods are described and the results in terms of permeability coefficients, convective heat transfer coefficients and effective thermal conductivity are presented as functions of the foam’s porosity. The method of the calculation is described and finally, the results of the calculation are presented, showing that due to the fine grained structure of the foam, the heat transfer from the solid to the cooling fluid takes place in a thin layer close to the inner surface of the camber wall.
Characteristics of the Water Meniscus at AFM Tip for Various Surface Energy
Authors: Choi Hojin , Kim Jung Yun , Hong Seungdo , Jang Joonkyung , Ha Man Yeong
Published: June 2009

The atomic force microscope (AFM), invented by Binning et al [1], has become a useful tool for its application in various fields of research. Even though AFM shows outstanding performance, one problem is the large adhesion forces due to the formation of a water meniscus in ambient condition. Therefore, it is important to understand the properties of the water meniscus. In previous studies, a theoretical method or Monte Carlo method was used to model the water meniscus problem with the assumption of thermodynamic equilibrium. But the physical phenomenon, occurs in a real AFM environment, is difficult to reach the thermodynamic equilibrium state due to a continuous variation of the water meniscus structure. Through two methods, addressed upon, only shows the equilibrium state, they have many difficulties to make clear the mechanism of a water meniscus formation. Because it is also troublesome to simulate the formation process of a water meniscus, most of researches have only investigated the properties of the water meniscus. Molecular Dynamics Simulation (MD) is the most suitable method for our needs. We can not only obtain the capillary force of the water meniscus, but also visualize the forming mechanism of the water meniscus. For these reasons, the water meniscus is expected to be understood easier. We performed a molecular dynamics simulation of the water meniscus that forms between an atomic force microscope (AFM) tip and a flat solid surface. We obtained the density profile and the capillary force of the nano-scale meniscus. We also examined the structure change in the meniscus by scanning the AFM tip at various distances between the AFM tip and the flat surface. In the case of a hydrophilic tip and a hydrophobic flat surface, the water meniscus changes from convex to concave as the surface energy of the flat surface increases. Using Young-Laplace equation, we obtain the capillary force of the water meniscus. In the case of a hydrophilic tip and a hydrophilic flat surface, the capillary force decreases as the distance between the AFM tip and the flat surface increases. We also examined the radius of the water meniscus. As the distance increases, the radius gets smaller. At a distance over d = 2.5nm, the radius of the water meniscus starts to fluctuate due to the instability of the water meniscus becoming narrow.
Flow Boiling in Transparent Heated Microtube
Authors: Kawanami Osamu , Huang Shih-Che , Kawakami Kazunari , Honda Itsuro , Kawashima Yousuke , Ohta Haruhiko
Published: June 2009

In the present study, a detailed investigation of flow boiling in a transparent heated microtube was performed. The transparent heated tube was made by electroless gold plating method. The enclosed gas-liquid interface could be clearly recognized through the tube wall, and the inner wall temperature measurement and direct heating of the film were simultaneously conducted by using the tube. The experimental conditions were: tube diameter 1 mm, mass velocity 100 kg/m2s, inlet liquid sub-cooling 20 K and heat flux up to 384 kW/m2 in the open system. Flow fluctuation was minimized by employing a twin plunger pump. Among our experimental results, we observed a high-frequency fluctuation of the inner wall temperature and a sharp peak for the heat transfer coefficient with high heat flux conditions, which have not been reported in previous experiments. This abrupt increase in the heat transfer coefficient coincided with a slight rapid axial growth of an elongated bubble found in the observation of the flow behavior. Hence, in low heat flux conditions, the fluctuations of temperature and heat transfer coefficient are strongly suppressed except for the instances when there is no bubble in the tube.
A PIV Study of AC-Electroosmotic Flow in Microchannels
Authors: Wang Yangyang , Kang Sangmo , Suh Yongkweon
Published: June 2009

Microscale mixing is difficult because the small channel-dimensions lead to low Reynolds number and the mixing is due to the diffusion only. This study focus on mixing in a microchannel based on AC electroosmosis by using a PIV technique. The zig-zag electrodes are attached at the bottom of the channel. In the experiment, polymer microspheres red fluorescing particles are used to measure the slip velocity and streamlines of the AC electroosmotic flow. At first, the slip velocity at bottom of the electrodes is measured. We find that when the frequency increase, the slip velocity increase too. The slip velocity of short electrode is faster than the other side. And the maximum slip velocity is about 250 micrometer/s. Then, an velocimetry method is presented now, which can give the vertical dimension from the diffraction pattern variations with the defocusing distances of small particle locations. At first, the lowest grey values of points are recorded. Then, the numerical simulation is done, in which the slip velocity is treated as a subsection function approach to the experiment result. And based on the streamline of the numerical simulation, a easy relationship between the lowest grey level of the particle’s pattern and Z coordinate is built successfully.
High Speed Imaging and Two-Phase Flow Patterns During Flow Boiling in a Single Microchannel
Authors: Barber Jacqueline , Sefiane Khellil , Brutin David , Tadrist Lounes
Published: June 2009

Boiling in microchannels remains elusive due to the lack of full understanding of the mechanisms involved. A powerful tool in achieving better comprehension of the mechanisms is detailed imaging and analysis of the two phase flow at a fundamental level. We induced boiling in a single microchannel geometry (hydraulic diameter 727 μm), using a refrigerant FC-72, to investigate several flow patterns. A transparent, metallic, conductive deposit has been developed on the exterior of rectangular microchannels, allowing simultaneous uniform heating and visualisation to be conducted. The data presented in this paper is for a particular case with a uniform heat flux of 4.26 kW/m2 applied to the microchannel and inlet liquid mass flowrate, held constant at 1.33×10−5 kg/s. In conjunction with obtaining high-speed images and videos, sensitive pressure sensors are used to record the pressure drop profiles across the microchannel over time. Bubble nucleation, growth and coalescence, as well as periodic slug flow, are observed in the test section. Phenomena are noted, such as the aspect ratio and Reynolds number of a vapour bubble, which are in turn correlated to the associated pressure drops over time. From analysis of our results, images and video sequences with the corresponding physical data obtained, it is possible to follow visually the nucleation and subsequent both ‘free’ and ‘confined’ growth of a vapour bubble over time.
Flow Patterns and Heat Transfer in Thin Liquid Films on Walls With Straight, Meandering and Zigzag Mini-Grooves
Authors: Lo¨ffler Karsten , Yu Hongyi , Gambaryan-Roisman Tatiana , Stephan Peter
Published: June 2009

Thin liquid films flowing along solid walls are widely used in technological applications in which high rates of heat and mass transport are required. The transport processes can be further intensified by using structured walls. In the present work hydrodynamics and heat transfer in falling liquid films on heated vertical and inclined walls with mini-grooves are studied experimentally and theoretically/numerically. The experiments are performed with straight, meandering and zigzag mini-grooves. The film dynamics is investigated using a confocal chromatic sensoring (CHR) technique. The flow patterns and the temperature of the liquid-gas interface are visualized using the high-speed infrared thermography. The wall temperature distribution is measured with thermocouples. A numerical model for description of the velocity and temperature fields in the thermal entrance region of the falling films on smooth and structured walls is developed. This model is based on the solution of the Graetz-Nusselt problem for falling films on grooved plates. We show that the mini-grooves significantly affect the flow patterns, film stability and heat transfer in falling liquid films. Using grooved walls leads to the increase of the maximal attainable heat transfer rate.
Numerical Simulation of the Passage of Small Liquid Droplets Through a Thin Liquid Film
Authors: Do-Quang Minh , Stemme Go¨ran , van der Wijngaart Wouter , Amberg Gustav
Published: June 2009

We simulate numerically a novel method for dispensing, mixing and ejecting of picolitre liquid samples in a single step. The system consists of a free liquid film, suspended in a frame and positioned in front of a droplet dispenser. On impact, a picolitre droplet merges with the film, but due to its momentum, passes through and forms a droplet that separates on the other side and continues its flight. Through this process the liquid in the droplet and that in the film is mixed in a controlled way. We model the flow using the Navier-Stokes together with the Cahn-Hilliard equations. This system allows us to simulate the motion of a free surface in the presence of surface tension during merging, mixing and ejection of droplets. The influence of dispensing conditions was studied and it was found that the residual velocity of droplets after passage through the thin liquid film matches the measured velocity from the experiment well.
Design and Particle Image Velocimetry Investigation of a Turbulent Mini-Jet Hemolysis Testing Apparatus
Authors: Fountain Thomas W. R. , Day Steven W.
Published: June 2009

Hemolysis is the break up of red blood cells, and is a condition that is of concern during the design process of blood contacting prostheses. In turbulent flows, hemolysis has been most often correlated to Reynolds shear stress. Mini-scale turbulent jets have been used for hemolysis experiments because they allow for explicit control of shear. Quantitative predictions of hemolysis from shear stress are unreliable, with experimentally determined threshold Reynolds stresses for turbulent shear flow range from 400Pa to 5000Pa, with recent experiments at 800Pa. Reynolds stresses are a statistic of large scale turbulence, and act at spatial scales much larger than that of a red blood cell. It has been suggested in literature that hemolysis may be related to stresses induced by turbulent energy dissipation, which acts as a spatial scale closer to that of a red blood cell. The dissipation of turbulence kinetic energy occurs at the Kolmogorov scales, which is generally similar in scale to that of a red blood cell.
5 Zones Thermocycler for Flow-Through On-Chip PCR
Authors: Felbel Jana , Reichert Anett , Kielpinski Mark , Urban Matthias , Henkel Thomas
Published: June 2009

A microfluidic chip system for flow-through PCR reactions with an optimized thermal profile consisting of five temperature zones was presented. It allows the implementation of one PCR cycle in a half channel loop. In contrast, conventional systems with a three zones arrangement require a complete channel loop per cycle. Therefore, this arrangement increases the level of integration and allows the implementation of a 40 cycles flow-through thermocycler on the footprint of a microscope slide. To obtain a high throughput of samples in a small volume (10–100 nl), the fluidic chip device was designed to operate at segmented-flow conditions for PCR. That way, each droplet may contain a single sample that is independently processed while transported through the microchannel. To achieve stable fluidic conditions, the surface of the micro-channels was modified. In addition to the successful flow-through PCR reaction in the micro reactor, the detection of the tumor suppressor p53 for clinical diagnostics was demonstrated.
Simulation-Based Analysis of a Biologically-Inspired Micropump With a Rotating Spiral Inside a Microchannel
Authors: Koz Mustafa , Yesilyurt Serhat
Published: June 2009

Microorganisms such as bacteria use their rotating helical flagella for propulsion speeds up to tens of tail lengths per second. The mechanism can be utilized for controlled pumping of liquids in microchannels. In this study, we aim to analyze the effects of control parameters such as axial span between helical rounds (wavelength), angular velocity of rotations (frequency), and the radius of the helix (amplitude) on the maximum time-averaged flow rate, maximum head, rate of energy transfer, and efficiency of the micropump. The analysis is based on simulations obtained from the three-dimensional time-dependent numerical model of the flow induced by the rotating spiral inside a rectangular-prism channel. The flow is governed by Navier-Stokes equations subject to continuity in time-varying domain due to moving boundaries of the spiral. Numerical solutions are obtained using a commercial finite-element package which uses arbitrary Lagrangian-Eulerian method for mesh deformations. Results are compared with asymptotic results obtained from the resistive-force-theory available in the literature.
Flow Field Measurements Near a Moving Meniscus of a Capillary Flow With Micro Particle Image Velocimetry (µPIV)
Authors: Nasarek Ralph , Wereley Steve , Stephan Peter
Published: June 2009

Due to the relevance of capillary flows for industrial applications, the dynamics of menisci have been extensively investigated. Contrary to the meniscus velocity, the flow pattern near the meniscus is rather unexplored. The main focus of this study is the visualization of the flow field near the fast moving meniscus with micro particle image velocimetry (μPIV). Therefore an invasive flow of ethanol in a vertical cylindrical glass tube is investigated. The entrance of the tube is located inside a reservoir with a rising liquid surface. Especially near the tube entrance the flow is high dynamical (ca. 120 mm/s) which necessitates high temporal resolution measurement techniques. Flow field measurements at several positions in flow direction have been conducted to compare the meniscus shape and the flow fields near the three phase contact line. The flow pattern near the meniscus is extremely complex. While the typical Hagen-Poiseuille profile with no radial velocity component is to be found far from the meniscus, the flow near the meniscus cannot be a convex velocity profile. As a matter of fact the meniscus movement is characterized by adsorption of the liquid near the wall and liquid transport from the middle of the flow to the three phase contact line. This special flow pattern has been examined. Furthermore it was found that the shape of the meniscus varies over the whole invasion of the tube. The curvature of the meniscus and therefore the contact angle between solid and liquid are getting smaller over the infiltration length which has influence on the capillary force. Measurements near the entrance of the tube show an acceleration of the liquid column until it reaches a certain speed and after that a deceleration. To determine the velocity of the raising meniscus, global measurements of the whole tube have been performed.
A Flexible Multi-Scale Two-Dimensional Model for Calculating the Radial Heat Transfer in Grooved Heat Pipes
Authors: Rossomme Se´verine , Goffaux Ce´cile , Hillewaert Koen , Colinet Pierre
Published: June 2009

This paper describes a multi-scale model for evaluating the radial heat transfer within a grooved heat pipe evaporator. The model is composed of two parts, macroscopic and microscopic, which cannot be decoupled from each other. In the macroscopic part, we solve the heat conduction problem in the solid and in the liquid phases, thanks to a finite-element method allowing high flexibility in the definition of the groove geometry. In order to avoid the classical singularity problem at the contact line (where the liquid-vapor interface meets the groove wall), in addition to taking the solid thermal conductivity into account, we do not impose the saturation temperature but a mixed condition along the interface. We show in particular that the interface temperature equals the saturation temperature (at given vapor pressure), except in the microscopic region where it increases and reaches the solid temperature. In this microscopic zone, a classical lubrication-type theory allows to determine the apparent contact angle, taking into account the influence of small-scale effects, such as the variation of the saturation temperature with the disjoining pressure and with the meniscus curvature. In particular, analytical relationships and correlations are presented for the apparent contact angle, which allow an efficient coupling between macroscopic and microscopic scales. In this paper, attention is devoted to the numerical treatment of both regions, their coupling, and the influence of the macroscopic heat flux and local small-scale effects on the distribution of temperature in the groove.
Analytical Solution of Temperature Field in Micro-Poiseuille Flow With Constant Wall Temperature
Authors: Darbandi Masoud , SafariMohsenabad Salman , Vakilipour Shidvash
Published: June 2009

The analytical study of microchannels has been considered as a preliminary approach to alleviate the difficulties which are normally encountered in numerical and experimental studies. Among the analytical solutions, those with high robustness and low complexities are certainly more attractive. In this work, we present a theoretical approach to predict the temperature field in micro-Poiseuille channel flow with constant wall temperature. The use of power series method simplifies the solution in the current analytical approach. The current analytical derivations are examined for channels with both hot-wall and cold-wall conditions. The current solutions agree well with the numerical solutions for a wide range of Knudsen numbers. Contrary to the past analytical solutions and in spite of using a simple and robust approach, the current formulations predict the temperature field in the channel readily.
Thermodynamic Analysis of Wall Effects on Phase Stability and Homogeneous Nucleation in Nanotubes Containing Superheated Liquid
Authors: Berkeley Pamela , Carey Van P.
Published: June 2009

Rapid heating or sudden depressurization can create highly superheated liquid conditions in stationary or flowing liquid in nanochannels. In these situations, the phase stability of the liquid and the conditions for the onset of bubble nucleation are important factors in the fluid behavior in applications. In the investigation summarized here, a statistical thermodynamics analysis is used to derive a modified version of the Redlich-Kwong fluid property model that accounts for attractive forces between the solid wall surface atoms and liquid molecules in the fluid within a nanochannel. In this model, the wall-fluid attractive forces are quantified in terms of Hamaker constants, which makes it possible to assess the effect of wall-fluid force interactions on the spinodal conditions for a variety of fluid and surface material combinations. This investigation focuses on the application of the aforementioned model to high aspect ratio (parallel plate) channels where a second wall is placed parallel to the first at distances varying in scale from nanometers to micrometers. At large channel widths the fluid properties exhibit wall effects very near each wall with bulk fluid property values being predicted in the center of the channel. As the channel width decreases, the fluid in the center of the channel begins to feel the effects of both walls and the properties deviate from the bulk values throughout the channel; importantly, the spinodal temperature increases significantly above the bulk fluid value. Fluid property profiles are developed for common combinations of walls and fluids, and properties are investigated as a function of non-dimensionalized distance within the channel across various channel widths. The results of this analysis imply that in nano- and micro-passages and near walls with nano-to micro-scale roughness, fluid may experience a stabilizing effect due to its proximity to nearby walls.
Laminar, Transitional and Turbulent Friction Factors for Gas Flows in Smooth and Rough Microtubes
Authors: Lorenzini Marco , Morini Gian Luca , Salvigni Sandro
Published: June 2009

Theoretical and experimental works on microscale transport phenomena have been carried out in the past decade in the attempt to analyse possible new effects and to assess the influence of scaling on the classical correlations which are used in macro-scale heat and fluid flow, following the need to supply engineers with reliable correlations to be used in the design of micro-scale devices. These results were sometimes in mutual contrast, as is the case for the determination of the friction factor, which has been found to be lower, higher or comparable to that for macroscopic channels, depending on the researchers. In this work the compressible flow of nitrogen inside circular microchannels from 26 μm to 508 μm in diameter and with different surface roughness (<1%) is investigated for the whole range of flow conditions: laminar, transitional and turbulence. Over 5000 experimental data have been collected and analysed. The data confirmed that in the laminar regime the agreement with the conventional theory is very good in terms of friction factors both for rough and smooth microtubes. For the smaller microchannels (<100 μm) when Re is greater than 1300 the friction factor tends to deviate from the Poiseuille law because the flow acceleration due to compressibility effect gains in importance. The transitional regime was found to start no earlier than at values of the Reynolds number around 1800–2000. Both smooth and sudden changes in the flow regime have been found, as reported for conventional tubes. Fully developed turbulent flow was attained with both smooth and rough tubes, and the results for smooth tubes seem to confirm Blasius’s relation, while for rough tubes the Colebrook’s correlation is found to be only partially in agreement with the experimental friction factors. In the turbulent regime the dependence of the friction factor on the Reynolds number is less pronounced for microtubes with respect to the prediction of the Colebrook’s correlation and the friction factor tends only to depend on the microtube relative roughness.
Thermal Resistance of the Interface Between Vertically Aligned Multiwalled Carbon Nanotube Arrays and Inconel and SiO2/Si Substrates
Authors: Son Youngsuk , Pal Sunil K. , Vajtai Robert , Ajayan Pulickel M. , Siegel Richard W. , Borca-Tasciuc Theodorian
Published: June 2009

The interface thermal resistance (ITR) of the interface between vertically aligned multiwalled carbon nanotube (MWCNT) arrays and the SiO2/Si and Inconel substrates was investigated. Experimental results were compared with theoretical predictions for the ITR across nanoconstrictions. The model considers classical constriction effects and contributions due to diffuse mismatch thermal resistance. Experimental values of the ITR are much larger than the model predictions. The observed discrepancy may be due to the imperfect mechanical contact between the tubes and substrate or additional contributions to ITR due to the presence of a catalyst layer at the interface.
A Microfluidic Pore Network Approach to Investigate Water Transport in Fuel Cell Porous Transport Layers
Authors: Bazylak A. , Berejnov V. , Markicevic B. , Sinton D. , Djilali N.
Published: June 2009

Pore network modelling has traditionally been used to study displacement processes in idealized porous media related to geological flows, with applications ranging from groundwater hydrology to enhanced oil recovery. Very recently, pore network modelling has been applied to model the gas diffusion layer (GDL) of a polymer electrolyte membrane (PEM) fuel cell. Discrete pore network models have the potential to elucidate transport phenomena in the GDL with high computational efficiency, in contrast to continuum or molecular dynamics modelling that require extensive computational resources. However, the challenge in studying the GDL with pore network modelling lies in defining the network parameters that accurately describe the porous media as well as the conditions of fluid invasion that represent realistic transport processes. In this work, we discuss the first stage of developing and validating a GDL-representative pore network model. We begin with a two-dimensional pore network model with a single mobile phase invading a hydrophobic media, whereby the slow capillary dominated flow process follows invasion percolation. Pore network geometries are designed, and transparent hydrophobic microfluidic networks are fabricated from silicon elastomer PDMS using a soft lithography technique. These microfluidic networks are designed to have channel size distributions and wettability properties of typical GDL materials. Comparisons between the numerical and experimental flow patterns show reasonable agreement. Furthermore, the fractal dimension and saturation are measured during invasion, revealing different operating regimes that can be applied to GDL operation. Future work for model development will also be discussed.
A Microchannel Heat Exchanger for Electronics Cooling Applications
Authors: Dix Joseph , Jokar Amir , Martinsen Robert
Published: June 2009

The power consumption of electronic devices, such as semiconductor diode laser bars, has continually increased in recent years while the heat transfer area for rejecting the associated thermal energy has decreased. As a result, the generated heat fluxes have become more intense making the thermal management of thes systems more complicated. Air cooling methods are not adequate for many applications, while liquid cooled heat rejection methods can be sufficient. Significantly higher convection heat transfer coefficients and heat capacities associated with liquids, compared to gases, are largely accountable for higher heat rejection capabilities through the micro-scale systems. Forced convection in micro-scale systems, where heat transfer surface area to fluid volume ratio is much higher than similar macro-scale systems, is also a major contributor to the enhanced cooling capabilities of microchannels. There is a balance, however, in that more power is required by microchannels due to the large amount of pressure drop that are developed through such small channels. The objective of this study is to improve and enhance heat transfer through a microchannel heat exchanger using the computational fluid dynamics (CFD) method. A commercial software package was used to simulate fluid flow and heat transfer through the existing microchannels, as well as to improve its designs. Three alternate microchannel designs were explored, all with hydraulic diameters on the order of 300 microns. The resulting temperature profiles were analyzed for the three designs, and both the heat transfer and pressure drop performances were compared. The optimal microchannel cooler was found to have a thermal resistance of about 0.07 °C-cm2/W and a pressure drop of less than half of a bar.
Mesochannel Compact Heat Exchangers for Automotive Air Conditioning Applications
Authors: Jokar Amir , Hosni Mohammad H. , Eckels Steven J.
Published: June 2009

Experimental study of the single-phase heat transfer and fluid flow in mesochannels, i.e., between microchannels and minichannels, has received continued interest by researchers in recent years. The studies have resulted in empirical correlations for various geometries ranging from simple circular pipes to complicated enhanced non-circular channels. In spite of these extensive studies, it is still unclear whether the theories and correlations developed for conventional macrochannels are directly applicable for use in microchannels (Dh = 10–200 μm) and minichannels (Dh = 200 μm–3 mm) with heat exchanger applications. A few researchers have agreed that similar results maybe obtained for the laminar flow regime regardless of the channel size; however, no general agreement has been reached for the transitional and turbulent flow regimes yet. In this study, different mesochannel air-liquid compact heat exchangers were evaluated and the experimental results were compared with published empirical correlations. These compact heat exchangers were used in the secondary fluid loops of an automotive air conditioning system that used refrigerant R134a as the working fluid. A modified Wilson plot technique was applied to obtain the heat transfer coefficients, and the Fanning equation was used to calculate the pressure drop friction factors. The uncertainty estimates for the measured and calculated parameters were calculated. The results of this study showed that the well established heat transfer and pressure drop correlations for the macrochannels are not directly applicable for use in the compact heat exchangers with mesochannels.
Evaluation of a Numerical Thrombosis Model for a High Shear Rotating Flow
Authors: Cheek Carlos D. , Day Steven W.
Published: June 2009

Blood clotting, or thrombosis, is an interesting biological application for computational fluid dynamics. Existing numerical thrombosis models have previously been shown to be effective for low shear rates and simple geometries. For these models to be used in biomedical applications such as the design of rotary blood pumps, however, they must first be experimentally validated for high shear rates and complex geometries. In this study, we test the ability of a numerical thrombosis model to predict thrombosis related phenomena in a high shear flow by creating a geometry similar to that of a rotary blood pump. We have applied an existing numerical thrombosis model to an annular gap between rotating concentric cylinders, a geometry that is closely related to rotary blood pumps. Additionally, we created a physical model of the same geometry and exposed blood to a range of shear rates in both the empirical and numerical model. The empirical and numerical results are compared in order to evaluate the ability of the numerical model to predict thrombosis in similar geometries, such as high shear blood handling pumps. Fluent was used to solve the coupled convective-diffusion equations along with user defined equations that include production and consumption of 7 species critical to thrombosis. These equations, along with equations of fluid motion, were solved iteratively within the Fluent solver. All reaction constants were from previously published work. At each of the shear rates and exposure times tested, the numerical model calculated platelet deposition, platelet-platelet aggregation and the two-dimensional distribution of three primary agonists (ADP, thromboxane and thrombin) in addition to the standard fluid variables (velocity, pressure, shear rate, etc.). A physical model was designed and constructed to control the shear rate that to which blood is exposed. An annular gap of 360μm was chosen in order to induce a shear rate of up to 10,000 s-1 while maintaining laminar flow. In a series of experiments, fresh, heparinized, bovine blood was exposed to a constant shear rate ranging from 1,000 to 10,000 s-1 for 120 seconds. Prothrombin time (PT) and activated partial thromboplastin time (APTT) of the blood was then measured for each stress level. While the observed changes in thromboembolitic potential (as measured by PT and APTT) of the whole blood test samples qualitatively correspond to platelet activation and agonist concentration predicted by the numerical model, further work is needed to quantitatively verify the numerical model. Thrombosis models based on coupled convective-diffusion equations show promise, but need further refinement and validation before they can be trusted to authoritatively predict thromboembolitic potential.
Thermo-Hydrodynamic Modeling of a Single Bubble Nozzle-Diffuser Phase Change Micropump
Authors: Saidi Mohammad S. , Saidi Mohammad H. , Pirouzpanah Sahand , Nikparto Ali
Published: June 2009

Nowadays, the potential of phase change process in liquids at micro scale attracts the scientists to fabricate this type of micropumps. Such micropumps have widely found applications in industrial and medical equipments such as recent printers. Not using mechanical parts such as valves, and having small sizes and high and controllable mass flow rates are the advantages of these micropumps. In the nozzle diffuser phase change micropump a heat pulse generates a bubble in a chamber; therefore, the pressure pulse which is generated by the bubble, causes the bubble to expand suddenly with high rate, then the pressure of bubble reduces to the vapor pressure and causes negative rate of expansion to the bubble. After the bubble reaches its maximum size, the bubble collapses and disappears. Due to the existence of difference in pressure drop in the nozzle and diffuser sections, one can see unidirectional flow through diffuser direction. The objective of this article is to analyze theoretically the thermo-hydrodynamic behavior of the Isopropyl Alcohol (IPA) bubble of a phase change micropump. Considering the simultaneous effects of hydrodynamic and thermal characteristics of the bubble in the bubble creation chamber, and temperature-saturation pressure relation of the IPA bubble based on Clausius-Clapeyron equation, the dynamics of the embedded bubble has been modeled. Applying the results of the bubble dynamics, the flow rate of the micropump for one cycle of operation has been attained. The obtained theoretical values for the micropump flow rate show good agreement with the corresponding existing experimental data.
Experimental Study of Particle-Liquid Flow Behaviour in a Microchannel Attached to an Inlet Reservoir
Authors: Kong Clement K. M. , Raghibizadeh Sasan , Kawaji Masahiro
Published: June 2009

An experimental study of a solid particle-liquid mixture in a microchannel was conducted by injecting a particle-liquid mixture into a 3.0 mm diameter cylindrical reservoir connected to a square microchannel. Glass particles with diameters ranging from 8 to 12 μm and density between 0.95 and 1.15g/ml, as well as polystyrene particles with diameters ranging from 8 to 50μm and an average density of 1.03–1.05 g/ml were mixed with de-ionized water to create particle-liquid mixtures. The mixture could be continuously stirred in the reservoir using a small magnetic stir bar. The number of particles entering the microchannel was found to depend on the concentration and density of the particles in the reservoir, stirring and fluid flow rates. The particles with a density larger than the carrier fluid entered the microchannel, settled at the bottom of the channel and became immobile at low fluid flow rates, blocking the subsequent particle flow. In many cases, the particles flowed in small groups or clusters, which were separated by short distances. The average distance between the particles or particle clusters was found to vary both with the stirring rate in the reservoir as well as the fluid flow rate in the microchannel.
Simulation of Droplet Flows Using Lattice Boltzmann Method
Authors: Gupta Amit , Kumar Ranganathan
Published: June 2009

In this work, the mesoscale approach of two-dimensional lattice Boltzmann method (LBM) has been employed to study droplet collision with a dry wall. The impact of drops with solid walls is simulated by using the pseudo-potential method of LBM. Simulations have been conducted for 2<We<162, and it is shown that the maximum spreading of the drop on the solid surface depends on the surrounding density, velocity of impact, surface tension, and the surface wetting characteristics. For a short time interval right after the impact the spreading diameter is shown to follow a parabolic dependence with time. The spread factor is seen to be higher as the Weber number increases. Under certain conditions when the drop has a high impact velocity and/or low surface tension, the kinetic energy of impact dominates over the dissipation and surface energy, leading to breakup of the drop into smaller drops. This breakup is shown to depend upon the wetting/non-wetting nature of the surface used. The spread factor is found to be a maximum at the time of breakup.
Effect of Upstream Viscous Dissipation on Laminar Heat Transfer in Microtubes
Authors: Ghai I. , Biswas G.
Published: June 2009

The effect of viscous dissipation on heat transfer in microtubes is analyzed. In particular, the effect of upstream viscous dissipation on the entrance temperature distribution and thereby on the downstream temperature field and Nusselt number is analyzed. Heat transfer characteristics of a steady laminar flow of hydrodynamically developed and thermally developing Newtonian fluid are presented. The case of adiabatic upstream preparation of the fluid and uniform wall heat flux condition in the downstream region is considered. The temperature distribution and Nusselt number variation in the thermally developing region of the tube is shown. The heat transfer characteristics are also contrasted with the solution available in literature, which assumes a uniform temperature distribution in the entrance cross section. A case study quantifying the viscous dissipation effect on water flowing through a microtube is presented.
Influence of Lateral Boundaries on Evaporative-Driven Convection Patterns
Authors: Iorio C. S. , Kabov O. A.
Published: June 2009

The evolution of convective patterns arising in evaporating liquid layers subject to a flow of inert gas depends on dynamical, thermo-physical and geometrical parameters. To the first group it is possible to associate the average velocity of the inert gas current and the total pressure of the gas phase — inert gas and vapor — insisting on the evaporating layer. The volatile liquid is also of importance in the convective patterns selection especially for what concerns the values of the latent heat of evaporation and of the cinematic viscosity. In this paper, we will focus on the influence that the lateral boundaries of the liquid pool have in the pattern selection process from the numerical point of view. A particular emphasis will be given to the heat transfer characteristics and to their dependence from both the liquid layer depth and the size of the evaporating surface.
On the Resilience of PDMS Microchannels After Violent Optical Breakdown Microbubble Cavitation
Authors: Fernandez Rivas D. , Gardeniers J. G. E.
Published: June 2009

The use of the silicone known as Poly-dimethylsiloxane (PDMS) in microfluidic and other activities has increased enormously. By means of a laser short pulse, optical breakdown of the liquid contained in a microchannel (squared cross-section with 100 μm sides) can occur; as a consequence plasma is generated and a gaseous bubble suddenly expands. The expansion of such a bubble can be so violent that the channel walls deform considerably and in some cases the detachment of the top wall from the PDMS channel is verified. Measurements of channel width reveal a relative increase up to 45% for the tested conditions. Due to the elasticity of PDMS and surface energy values, surprisingly the top wall and channel reattach. Contrary to widely known cavitation where damage to the wall is manifested in form of pitting, we observe no permanent damage for the experimental conditions studied. We envisage the use of PDMS material in other types of applications with high relative volumetric change, such as microexplosions, ultrasonic cavitation and the like, in which there is a cyclic stress load of the PDMS material. We present a compact method to predict the reliability of PDMS for its use in cavitational events occurring inside of microfluidic channels (by force balance and energy conservation analysis).
Implementation of ECP: Benefits, Migration Path and Economics
Authors: Engle Thomas H.
Published: June 2009

I first became aware of the benefits of electrically piloted independent brake cylinder control at the beginning of my railroad career in 1956. I was working for the Rock Island Railroad, and we had Electrically controlled brakes on two of our fastest passenger trains, the Golden State and the Twin Star Rocket. The instantaneous response and the improvement in stopping smoothness and distance that the EP provided was remarkable. For those trains, a big benefit was not needing to “bail off” the locomotive brake. The stop distance from 95 MPH was reduced by about the length of the platform (as we proved on a trip where the EP failed).
Effect of Bolt Strength and Loading Conditions on a 7/8” Bolt Within an End-of-Car Arrangement
Authors: Smyth Andrew D.
Published: June 2009

A cause of failure within end-of-car (EOC) arrangements for cushioned cars with F-shank couplers is that of the yoke bolt failing in shear. This mode of EOC failure is of particular concern due to the concealed nature of the bolt not easily allowing for early detection of the onset of failure. To this end, a finite element analysis (FEA) was performed on a 7/8” bolt and F-bracket assembly to determine the stress state developed within the bolt in an effort to understand the potential cause or causes for the bolt failure. Several parameters, including bolt strength, bolt preload (initial torque), and external loading were varied to determine their effects on bolt performance. The subsequent results indicate that both inherent strength and initial preload have a significant effect on whether a bolt can effectively withstand the various external loading conditions encountered in the field. In addition, it is also apparent that some of the simulation loading scenarios analyzed contain the potential to initiate bolt shearing during operation. From these results, some failure mechanism theories are proposed to describe the type of failure encountered by each bolt grade, either ductile or brittle depending on the inherent material properties.
Design of an Intelligent Remote Monitoring and Control of Bangladesh Railway Transport System Using Information and Communication Technologies (ICTs)
Authors: Chowdhury Md. Abdullah Al Harun Khan
Published: June 2009

In Bangladesh, transport sectors are developing rapidly to meet the increasing demand for transporting passengers and freight inside and outside the country. But there is not such development in railway transport system. The Bangladesh Railway transport system is still using an old technology to monitor and control signalling, scheduling, operations etc. This paper describes various problems in the existing systems and also solutions have been provided considering the existing railway transport systems of Bangladesh. A new system has been developed to control and monitor the total railway transport system from remote locations. While designing the system, various sensors and actuators have been introduced and also Information and Communication Technologies (ICTs) have been applied in the field of railway transport. So a Machatronics aspect of system has been designed to ensure a collision free, safe and efficient operation and management of railway transport system. This system is not only for monitoring and controlling of railway transport but also ensures efficient asset management. As a result face-to-face accidents, cross-road accidents and accidents due to railway line displacements or breakage can be avoided and there will be no loss of assets and valuable human lives.
The TRAXX Platform: A New Way to Build Electric and Diesel Locomotives
Authors: Vitins Janis
Published: June 2009

The opening of the European market for freight and passenger services has initiated the need for cross-border locomotives which fulfill the specific requirements of each country and comply with new European standards. The TRAXX locomotive platform addresses this new market with four locomotive types. Each type is configured with standard building blocks. The TRAXX platform covers both electric and diesel 4-axle locomotives. A high level of standardization allows manufacturing of all types in a single assembly line, thus also reducing manufacturing costs for small production lots. The key to the TRAXX platform is a high level of system integration. Important innovations are in the traction chain, carbody, air supply, in the development of automatic train protection (ATP) systems based on the future European standard ETCS (European Train Control System) and in system integration. Today, the TRAXX platform covers all major mainline traction needs in continental Europe with more than 1′200 locomotives so far sold. Further developments are needed to optimize the ATP systems for new and important cross-border freight corridors and to streamline the homologation procedures throughout Europe.