bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Visualization of Heat Flow and Thermal Mixing in Various Differentially Heated Square Cavities Using Bejan’s Heatlines
Authors: Kaluri Ram Satish , Basak Tanmay
Published: March 2011

A comprehensive analysis of heat distribution and thermal mixing in steady laminar natural convective flow in discretely heated square cavities has been carried out via Bejan’s heatlines. Heatlines are analogous to streamlines and heat energy flow may be visualized by heatlines similar to streamlines which display fluid flow. The trajectories of heatlines indicate direction and magnitude of heat flow and zones of high heat transfer. The heatline approach is implemented to study heat flow in the following three different square cavities which are filled with water (Pr = 7): (1) uniformly heated bottom wall (2) distributed heating with heat sources present on central portions of the walls and (3) multiple heat sources on the walls of the cavity. Top wall is maintained adiabatic in all the cases. Galerkin finite element method with penalty parameter has been used to solve non-linear coupled partial differential equations for flow and temperature fields over a range of Rayleigh numbers (Ra = 103–105). The Galerkin method is further employed to solve the Poisson equation for streamfunctions and heatfunctions. Finite discontinuity exists at the junction of hot and cold walls leading to mathematical singularity. Solution of heatfunction for such type of situation demands implementation of non-homogeneous Dirichlet conditions. Heatlines illustrate that in uniformly heated bottom wall case, the heat from the bottom wall is not adequately distributed to the lower portion of side walls which leads to low temperature in those regions (case 1). In order to improve the heat distribution, the uniform heat sources is divided into three parts and are applied along the central regimes of the walls (case 2). It is observed that, heat distribution and thermal mixing in the cavity is significantly enhanced. However, the lower corner portions are still retained cold. In case 3, multiple heat sources are placed along the walls of the cavity along with heat sources at lower corner regions of the cavity. Heatlines indicate that, the temperature at the core is reduced compared to case 2, but uniform heat distribution results in uniformity of temperature across large area of cavity.
Infrared Imaging Microscope as an Effective Tool for Measuring Thermal Resistance of Emerging Interface Materials
Authors: McNamara Andrew , Sahu Vivek , Joshi Yogendra , Zhang Zhuomin
Published: March 2011

The reduction of interfacial resistance continues to be a significant challenge in thermal management of semiconductor and other microscale devices. Current state-of-the-art thermal interface materials (TIMs) have resistances in the range of 5–10 mm2·K/W. At these values, particularly for the emerging highly nonhomogeneous materials, standard measurement techniques often fail to provide accurate results. This paper describes the use of infrared microscopy for measuring the total thermal resistance across multiple interfaces. The method is capable of measuring samples of wide ranging resistances with thicknesses ranging from 50–250 μm. This steady-state technique has several advantages over other methods, including the elimination of the need for intrusive temperature monitoring devices like thermocouples at the area of interest and the need for a priori knowledge of the specific heat and density of the materials of interest, as in the transient techniques for determining thermal resistances. Results for three different commercially available TIM and uncertainty analysis are presented.
Phase Change Material and the Thermoelectric Effect for Solar Energy Harvesting and Storage
Authors: Zhang Qi , Agbossou Amen , Feng Zhihua , Grillet Anne-Cecile
Published: March 2011

A new method of harvesting solar and ambient energy is presented. The method is based on thermoelectric and latent heat effects and is put into practice in a prototype work unit composed of a thermoelectric generator (TEG) and phase change material (PCM). Numerical and experimental analyses of the work unit are developed. The numerical analysis deals with finite elements modeling on solar radiation, temperature variation and wind-speed effects. The results demonstrate the utility of sensible and latent heat storage in the phase change material and confirm the capacity to generate continuous voltage by day and by night. Subsequently, with an optimized thermodynamic design and with several work units linked together, the proposed energy harvesting system could be utilized as both sensor and actuator in low power applications in buildings. It could replace dry batteries for wireless applications, LED lighting and autonomous systems.
Advanced Power Generation System and Related Thermal Engineering Problems
Authors: Ozawa Mamoru , Matsumoto Ryosuke , Umekawa Hisashi
Published: March 2011

Based on the increased attention to “energy security” and “sustainable development”, it is essential to promote clean use of coal as a fuel. Typical advanced technologies are demonstrated by the pressurized fluidized-bed combined cycle (PFBC) and integrated gasification combined cycle (IGCC). Focusing mainly on these two examples as the advanced energy conversion technology, related problems are reviewed. The PFBC technology is a composite technology of conventional fluidized bed and combined-cycle, in which ash, being a typical component of coal, is not melted but is removed mainly in the fluidized bed. On the other hand, the IGCC is much more complicated and ash removal is conducted by melting in the combustor. Heat released there is utilized for gasification process in the reductor installed just downstream the combustor. Even though both systems have very high potential for clean and efficient use of coal, the commercial plants are limited in a very small number or at the stage of just a demonstration plant. To extend and develop clean-coal technology in the electric power generation market, a strategy of the government on the energy technology as well as the long-term competition in the market are indispensable, otherwise related technologies as well as the engineers involved will be lost.
Acceleration of Recursive Cross-Correlation PIV Using Multiple GPUs
Authors: Tarashima Shuhei , Someya Satoshi , Okamoto Koji
Published: March 2011

A large number of PIV algorithms and systems have been proposed, many of which are highly sophisticated in terms of accuracy and spatial and temporal resolution. However, a general problem with PIV is the time cost to compute vector fields from images, which often imposes specific constraints on the measurement methods. In this paper, focusing on recursive direct cross-correlation PIV with window deformation, which is one of the most popular algorithms for PIV, we propose a technique to accelerate PIV processing using a single Graphics Processing Unit (single-GPU) and multiple GPUs (multi-GPU). In the case of using single-GPU, we show that PIV data can be processed over 100 times faster than using a CPU alone and about 30 PIV image pairs per second can be processed for certain image sizes. The scalability of the algorithm used is also discussed. In the case of using multi-GPU, the image split method and the parallel method of single-GPU-based PIV processing are measured. We show that the effect of multi-GPU can be observed over a certain amount of image data whether either of these two methods is used. Data transfer between CPU and GPUs is shown to be a bottleneck if the number of GPUs used increase.
Effect of Nano-Cluster Compactness on Thermal Physical Properties of Nanofluids
Authors: Feng Zhaozan , Li Wei , Wu Tingting
Published: March 2011

This paper reports an experimental work on the viscosities of 30% (in volume) ethylene glycol water solution based nanofluids, and presents a critical analysis of the experimental reports on thermal conductivity and dynamic viscosity in terms of the mechanism of aggregation and show that, by manipulating the morphology of the aggregates, the first-best trade-off between the thermal conductance and rheological behavior of general nanofluids can be approximately achieved. Attempt to regulate the viscosity of nanosized SiO2 suspensions was made following a semi-empirical method named particle grading.
Experimental Study on Combustion in Miniature Glow Ignition Engine: Detection of Spontaneous Light Emission and Pressure
Authors: Kuno Daisuke , Motozawa Masaaki , Kawaguchi Yasuo
Published: March 2011

Combustion characteristics of a miniature four-stroke glow-ignition engine which has 4.89 cc of swept volume were investigated experimentally with applying an optical technique. As combustion characteristics, crank angle of ignition timing and combustion period were measured directly by the detection of spontaneous light emission. To investigate the effect of engine speed and fuel component on ignition timing and soot formation, elapsed time change of the light intensity was measured under the various engine speed and fuel composition with operating this engine at the largest engine power. Ignition timing can be estimated with comparing light intensity with rate of heat release which was measured at the different time and generation status of soot particles in the engine can be supposed by the light intensity. As a result, when the amount of the nitromethane in the fuel is large, ignition timing advances and soot formation was inhibited.
Influence of Blood Glucose Level on the Scattering Coefficient of the Skin in Near-Infrared Spectroscopy
Authors: Kessoku Sachiko , Maruo Katsuhiko , Okawa Shinpei , Masamoto Kazuto , Yamada Yukio
Published: March 2011

Various non-invasive glucose monitoring methods using near-infrared spectroscopy have been investigated although no method has been successful so far. Our previous study has proposed a new promising method utilizing numerically generated absorbance spectra instead of the experimentally acquired absorbance spectra. The method suggests that the correct estimation of the optical properties is very important for numerically generating the absorbance spectra. The purpose of this study is to measure the change in the optical properties of the skin with the change in the blood glucose level in vivo. By measuring the reflectances of light incident on the skin surface at two distances from the incident point, the optical properties of the skin can be estimated. The estimation is a kind of the inverse problem based on the simulation of light propagation in the skin. Phantom experiments have verified the method and in vivo experiments are to be performed.
Air Cleaning Method With Humidity Swing Operation for Reducing Suspended Particulate Matters
Authors: Onoda Nagisa , Nakabeppu Osamu
Published: March 2011

SPM (suspended particulate matters) removal effects of a humidity swing operation where sample air is continuously humidified with heating and dehumidified with cooling were experimentally studied. The experimental results showed that the humidity swing operation reduced SPM ranging from nanometer to micrometer sizes due to multiple phenomena such as suction flow accompanying condensation and thermophoresis, and the total SPM concentration decreased by 80% under the maximum humidity swing condition. The removal mechanisms were theoretically estimated. It was estimated that contributions of the suction and thermophoresis are significant, gravitational sedimentation works above 5 μm size, and the effect of inertial collision is insignificant. Nuclear condensation that markedly changes the SPM size distribution would not be an effective SPM removal effect. The sum of the theoretical estimations is smaller than the experimental result, and study for clarifying the removal mechanisms is necessary. It was also experimentally confirmed that the SPM reduction effects have no dependence on the concentration. The SPM removal experiment in a circulating system was also conducted. With repeatedly applying the humidity swing operation, the SPM concentration for whole size ranges exponentially decreased and converged at certain value. SPM generation by a mechanical pump might occur. With improving the apparatus, this method is expected to become an effective SPM removal method applicable for the SPM from nanometer to micrometer size.
Importance Sampling in Monte Carlo Ray Tracing Solutions Applied to Radiation in Hypersonic Entry Flows
Authors: Feldick Andrew , Modest Michael F.
Published: March 2011

A method for the application of importance sampling to wavelength selection in ray tracing Monte Carlo solvers is introduced. In this method individual lines are treated independently, and probability distribution and weighting functions are applied to individual lines. The method reduces the number of photons required to reach the same statistical precision as the standard Monte Carlo method.
Absorption and Scattering of the Aggregated Multi-Layered Nanoshells
Authors: Liu Chang H. , Li Ben Q.
Published: March 2011

A generalized mathematical formulation is presented for multi-scattering of electromagnetic fields by an ensemble consisting of arbitrarily-positioned multilayered nanoshells. The model is developed by combining the addition theorem and the efficient recursive procedure for multilayered nanoshells and general procedures for computing the multiple scattered fields and optical properties of the particle ensemble are presented. The enhancement of the electric field and the energy absorbed by the aggregated silica-gold nanoshell and gold-silica-gold nanoshells are analyzed to understand the physics governing the electromagnetic field interaction with aggregated multilayered nanoshells. The mathematical model should be helpful in providing valuable information on optical and radative transfer characteristics needed for the nanoshell-based applications in photothermal therapy, biomedical imaging, biosensing and waveguide for energy transport.
Investigation of Circular Water Jet Impingement Heat Transfer
Authors: Liu Cuicui , Jiang Zeyi , Zhang Xinxin , Ma Qiang , Sun Yusheng
Published: March 2011

Mathematical model combining theoretical analysis approach and differential numerical solving techniques has been set up to predict the free surface water jet impingement heat transfer. Heat transfer properties are obtained and validated by comparison with experiments. The characteristic of Nu-r/d distribution is discussed and the effect of nozzle diameter is analyzed. In addition, nozzle arrangements are studied for water jet equipment designation purpose. The results show that: Reynolds number is the dominate parameter in Nu-r/d distribution and area-averaged Nusselt number increases with increasing nozzle diameter. The best heat transfer effect appears when the aspect ratio of rectangular surface equals to 1. Fewer nozzles and bigger single impinged area could get larger Nusselt number under a given total water flow rate and given total impinged area. At a constant flow rate, larger nozzle diameter and smaller Reynolds number present a larger Nusselt number.
A Generalized Ecological, Internally Irreversible Carnot Cycle Model, as an Optimum Compromise Between Maximum Power and Efficiency for a Given Heat Transfer Inventory
Authors: Ait-Ali Mohand A.
Published: March 2011

A generalization of the so-called ecological cycle introduced by Angulo-Brown for the endoreversible maximum power problem of Curzon-Ahlborn is presented here under the assumption of arithmetic mean temperature differences and Newtonian heat transfer in heat exchangers, but with internal irreversibility and a variable heat transfer entropy penalization factor of maximum power to be produced. This presentation extends that given recently for a case study of an ecological non endoreversible Curzon-Ahlborn cycle, and more recently by this author for general rules of heat conductance allocations in endoreversible and irreversible Carnot like power or refrigeration cycles, and cascades of such cycles. Analytical results are obtained here in terms of heat source and sink temperature ratio, internal irreversibility factor, and ecological entropy penalization factor X defined as the number of times the energy equivalent entropy produced through heat transfer is subtracted from power generated per unit overall heat conductance. The value of X to design for in a given case study will dependent admittedly, on the respective capital cost and expected fuel cost of the particular power cycle to be retained. Finally, a newly defined ratio of maximum power produced to heat rejected by the cycle is presented as an ecology friendly parameter.
Measurements of Heat and Mass Transport Characteristics Through PEMFC GDL
Authors: Araki Takuto , Tachibana Nobuyoshi , Kaneko Haruki , Ota Koichi
Published: March 2011

For further improvement of PEMFC (Proton Exchange Membrane Fuel Cell) performance, an optimal design based not only on empirical knowledge but also systematic physical laws and numerical simulation is desirable. However, even basic parameters such as mass transport and thermal characteristics through layers are not completely clear. A GDL (Gas Diffusion Layer) is one of the key components of PEMFC, and its property strongly affects the temperature distribution, diffusion limitation, and flooding. Thus, we measured the diffusivity and thermal conductivity inside a GDL and at its surface boundaries. Anisotropic diffusivity was observed inside the GDL. The measured diffusivities inside the GDL were 59% lower than the value without a GDL in the thickness direction and 25% lower in plane direction. The measured diffusive resistance at the GDL surface was not considerably different from that achieved through conventional laminar analysis, although some special effects would have been observed at the GDL surface because of its high porosity. Regarding the thermal characteristics, the contact resistance at the GDL surface was measured to be as large as the resistance inside the GDL. However, the contact resistance became extremely small when the GDL contained water.
Channel Flow Along a Wavy Heated Wall
Authors: Barboy S. , Rashkovan A. , Ziskind G.
Published: March 2011

The present study deals with the effects of wall geometry on the fluid flow and heat transfer in a vertical channel with a wavy wall. The waviness is characterized by wave amplitude and period. The wavy wall is heated with a constant heat flux. A detailed parametric investigation of the effect of waviness is performed for different flow conditions. An enhanced version of the turbulence models is required in order to resolve the near-wall region. In particular, a single wall law for the entire wall region can be achieved by blending linear (viscous) and logarithmic (turbulent) laws-of-the-wall. This approach allows the fully turbulent law to be easily modified and extended to take into account other effects such as pressure gradients or variable properties. Second order discretization scheme for momentum equation and turbulence scalar equations was used. SIMPLE pressure-velocity coupling scheme was employed. The results show how the flow and geometry parameters, namely, the Reynolds number and the amplitude and period of waviness, affect such features as the existence of flow separation, its location and size of the recirculation zones. These features determine the temperature distribution on the wavy wall. An attempt is done to assess the effect of flow and geometry parameters quantitatively.
Heat Flow Visualization for Natural Convection in Rhombic Enclosures: Bejan’s Heatline Approach
Authors: Anandalakshmi R. , Basak Tanmay
Published: March 2011

The phenomena of natural convection within a rhombic enclosure filled with air (Pr = 0.71) for (a) isothermally (case 1) and (b) non-isothermally (case 2) heated bottom walls with various aspect-ratios has been studied numerically. In all the cases, top horizontal wall is maintained adiabatic and side walls are maintained cold. Galerkin finite element method with penalty parameter is used to solve non-linear coupled partial differential equations for flow and temperature fields. Poisson equation of streamfunction and heatfunction is also solved using Galerkin method. Simulations are carried out over a range of Rayleigh numbers and numerical results are presented in terms of streamfunction, heatfunction and temperature contours. Streamlines are useful to visualize the fluid flow whereas heatlines are used to study the heat energy distribution within the rhombic cavity. Heatlines are further used to visualize the trajectories of heat flow and zones of high thermal mixing. At lower Ra, heatlines are smooth circular arcs with low magnitude streamfunctions and heatfunctions and thus the heat transfer is conduction dominant. Asymmetric flow is observed for all the cases due to geometrical asymmetry. As Ra increases, buoyant force starts dominating and the magnitudes of streamfunctions and heatfunctions are found to be greater due to enhanced convection effect. Heatlines are distorted greatly showing complex heat distribution inside the cavity. It is observed that primary heat circulation cell is larger for greater tilt angles and thus thermal mixing is high. Heat transfer rates are also studied via local and average Nusselt numbers as functions of Ra and Pr on bottom, left and right walls. Various quantitative and qualitative features of Nusselt numbers have also been explained based on heatlines.
Evaluation of Uncertainties of One-Directional Analytical Model for Thermoreflectance Technique
Authors: Xing Changhu , Hua Zilong , Ban Heng , Hurley David , Kennedy J. Rory
Published: March 2011

One dimensional (1-D) analytical models are generally used for the evaluation of thermal effusivity of film or substrate in thermoreflectance measurement. However, the range of uncertainties associated with the 1-D assumptions needs to be quantified in order to determine the applicability of 1-D models. In the current study, a two-dimensional (2-D) numerical model was created in a commercial software package, COMSOL, to investigate the difference of results obtained by 1-D or 2-D models. The analysis used a frequency lock-in strategy and the result was verified by a comparison of 1-D numerical simulation to 1-D theoretical values. Parametric studies were performed by considering film thickness, heating laser radius, volumetric heating, thermal resistance between layers, and anisotropic thermal conductivities. The results and conclusions provide a general guidance and reference for the parametric design of thermoreflectance measurements.
Intertube Falling-Film Heat Transfer Behavior of Multiwall Carbon Nanotube Suspensions (MWCNT Nanofluids)
Authors: Ruan Binglu , Jacobi Anthony M.
Published: March 2011

The thermal conductivity and viscosity of water-based and ethylene-glycol-based multiwall carbon nanotube (MWCNT) suspensions are measured for MWCNT volume concentrations up to 0.24%. The thermal conductivity is found to increase up to 8.6% and 9.3% for water-based and ethylene-glycol-based nanofluids, respectively. The viscosity of the nanofluids increases compared to that of their base fluids, with larger increases for the ethylene-glycol-based nanofluids. Intertube falling-film heat transfer characteristics of these nanofluids are measured and compared to data for the base fluids. The heat transfer coefficient of the water-based nanofluids decreases at low MWCNT concentrations but increases as the concentration increases. The heat transfer coefficient of the ethylene-glycol-based nanofluids decreases with an increase in MWCNT concentration, with a maximum deviation of 30%.
Thermal Conductivity Augmentation in Polymeric Nanocomposites
Authors: Moreira D. C. , Sphaier L. A. , Nunes L. C. S. , Reis J. M. L.
Published: March 2011

This paper presents an experimental analysis of the thermal conductivity of a nanocomposite system composed of a unsaturated polyester resin as matrix and alumina nanoparticles as filler. The nano particles used are 30–40 nanometers gamma alumina particles. Samples are fabricated using simple molding and no specialized homogenization equipment is used for mixing the particles with the liquid resin. The thermal conductivity is measured using the Fox-50 device, manufactured by LaserComp. Measurements are taken at different temperatures (from 0° to 50°C) for different batches of samples varying the fraction of nanoparticles used in the composite system. Finally, the thermal conductivity measurements are compared with models for predicting the thermal conductivity of composite materials with nanoparticles, showing that the obtained results are over the values estimated by these models.
Measurement of Water, Temperature and Current Distributions in Anode of Polymer Electrolyte Fuel Cell During Low Humidity Operation
Authors: Nishida Kosuke , Yokoi Yoma , Umeda Kazumasa , Tsushima Shohji , Hirai Shuichiro
Published: March 2011

In order to prevent membrane dryout in polymer electrolyte fuel cells (PEFCs) during low humidity operation, it is essential to understand the fundamental phenomena of the water transport and reaction distribution at the anode side in an operating fuel cell. In this study, the water vapor distribution along the anode flow channel of a PEFC under low humidity conditions was quantitatively evaluated by using humidity test paper (HTP), and the effects of flow configuration and inlet humidification on the water distribution in the anode were investigated. HTP is a test paper for detecting water vapor of 20–90% RH, which is coated with a blue surface. This test paper was inserted between the anode electrode and separator in the transparent fuel cell, and the discoloration of HTP was directly visualized using a digital CCD camera. Furthermore, the temperature and current distributions in the anode electrode were measured using IR thermography and segmented cell structure concept. It was found that the water vapor concentration on the anode side increases immediately after the startup because of the back diffusion of the product water from the cathode to anode. In the case of the co-flow configuration with the dry anode and cathode inlets, the water vapor concentration increases monotonically along the anode flow channel. In addition, the anode water distribution affects the temperature and current distributions in the fuel cell largely. The local temperature and current density at the dry anode inlet are lower than those in the downstream region because of the membrane dehydration and low proton conductivity. On the other hand, in the case of the counter-flow pattern, the distributions of water concentration, temperature and current density have the maximum points in the midstream region. The counter-flow configuration is effective in improving the membrane hydration and alleviating the anode dryout.
Experimental Analysis of the Single-Phase Heat Transfer and Friction Factor Inside the Horizontal Internally Micro-Fin Tube
Authors: Tam Hou Kuan , Tam Lap Mou , Ghajar Afshin J.
Published: March 2011

To increase heat transfer, internally micro-fin tubes are widely used in commercial HVAC applications such as flooded evaporators. It is commonly understood that the micro-fin enhances heat transfer but at the same time increases the pressure drop as well. In the previous studies, majority of the works were focused on the development of correlations in a particular flow regime, especially in the turbulent region. There are only a few works that fundamentally studied the continuous change in the characteristic behavior of heat transfer and pressure drop from laminar to transition and eventually the turbulent regions. Therefore, more in-depth study is necessary. In this study, heat transfer and pressure drop were measured simultaneously in a single test section fitted with 2 micro-fin tubes and compared with the data of a plain tube. From the results, the transition from laminar to turbulent was clearly established. The buoyancy effect is present in the laminar region. The transition from laminar to turbulent was found to be inlet dependent. It could be seen that the delay of transition was more obvious for smaller spiral angle while it was not as obvious when large spiral angle tube was used. Furthermore, it was observed that the larger spiral angle had an enhancement of the heat transfer in the upper transition to turbulent regions. Finally, the efficiency index (the ratio of the heat transfer and the friction factor of enhanced tube to those variables for the plain tube) was examined and it was found to have a value larger than one when Reynolds number is larger than 5000 regardless of the type of inlet configuration used. Therefore, the application of the micro-fin tubes used in this study is suitable when Reynolds number is larger than 5000.
Size Control of Hollow Polylactic Acid Microcapsules and Hollow Polyelectrolyte Microcapsules in Bubble Template Method
Authors: Molino Jay J. , Daiguji Hirofumi , Takemura Fumio
Published: March 2011

Biodegradable hollow poly-lactic acid (PLA) microcapsules and hollow polyelectrolyte microcapsules made of poly-allylamine hydrochloride (PAH) were synthesized by directly adsorbing these polymers to N2 (air) and CO2 microbubbles respectively, using the bubble template method. To manufacture PLA microcapsules, droplets of a solution of PLA in methylene chloride (CH2Cl2) were emulsified in water. Then by solvent diffusion, N2 microbubbles nucleated inside the droplets and PLA adsorbed to the bubble surface to form microcapsules. Likewise, for PAH microcapsules, when an aqueous solution of Na2CO3 including PAH is titrated with HCl, within a specific range 7.5 < pH < 9.0, colloidal PAH particles are formed and then adsorbs to the nucleated CO2 microbubbles. This yields to hollow PAH microcapsules. If the solution pH is outside this range, colloidal particles can no longer exist, thus no microcapsules can be synthesized. This document mainly focuses on size control of these two types of microcapsules.
Effect of 1D Cu Nanostructures on Heat Transfer Characteristics of Single Phase Microchannel Heat Sink
Authors: Ali M. Yakut , Yang Fanghao , Fang Ruixian , Li Chen , Khan Jamil
Published: March 2011

This study experimentally assesses single phase heat transfer characteristics of a shallow rectangular microchannel heat sink whose surface is enhanced with copper nanowires (CuNWs). The hydraulic diameter of the channel is 672 μm and the bottom wall is coated with Cu nanowires (CuNWs) of 200 nm in diameter and 50 μm in length. CuNWs are grown on the Cu heat sink by electrochemical synthesis technique which is inexpensive and readily scalable. The heat transfer and pressure drop results of CuNWs enhanced heat sink are compared with that of bare copper heat sink using deionized (DI) water as the working fluid at Reynolds Number (Re) ranging from 106–636. The experimental results indicate an enhancement in Nusselt Number (Nu) at all Re with a maximum enhancement of 24% at Re = 106. The enhanced thermal performance is attributed to two properties of Cu nanowire arrays — improvement in surface wettability characteristics and increased heat transfer surface area.
In-Plane Thermal Conductivity of PEM Fuel Cell Gas Diffusion Layers
Authors: Sadeghi Ehsan , Djilali Ned , Bahrami Majid
Published: March 2011

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a PEM fuel cell. The analysis of this process requires determination of the effective thermal conductivity. This transport property differs significantly in the through-plane and in-plane directions due to the anisotropic micro-structure of the GDL. In the present study, a novel test bed that allows the separation of in-plane effective thermal conductivity and thermal contact resistance in GDLs is described. Measurements are performed using Toray carbon paper TGP-H-120 samples for a range of PTFE content at a mean temperature of 65–70°C. The measurements are complemented by a compact analytical model that achieves good agreement with the experimental data. The in-plane effective thermal conductivity is found to be about 12 times higher than the through-plane conductivity and remains approximately constant, k ≈ 17.5 W/mK, over a wide range of PTFE content.
Finite Element Analysis of Reaction Front Tracking in Lime Calcination
Authors: Sandaka P. Ch. Gourisankar , Nallathambi Ashok Kumar , Specht Eckehard
Published: March 2011

Calcination process is carried out in shaft kilns or rotary kilns for producing the lime. In the present work, the decomposition rates of the limestone in shaft kiln are experimentally measured in laboratory scale equipment. Mathematical model has been developed by using Finite element technique to track the reaction front movement from the surface to the core. The results obtained from the experiments are compared and validated with that of the model. Influences of parameters such as ambient temperature and the particle size are studied. The variation of reaction zone thickness during the process of decomposition has been studied and analyzed. Temperature profiles inside the lime particle are calculated and the influence of the reaction front position on these temperature profiles has been analyzed.
High Speed Transient Temperature Profile Control Using Adjoint-Based Optimal Control Scheme
Authors: Ito Satoru , Suzuki Yuji
Published: March 2011

Optimal control scheme for transient temperature profile inside electronic devices such as pulsed laser diode is developed based on the adjoint equation of one-dimensional heat conduction. Joule heating with a thin-film heater is employed as the control input in order to minimize temperature changes of a thin active layer embedded in a modeled laser diode. In numerical simulations assuming the light-emitting time period of 1 μs, temperature variation of the active layer is successfully suppressed by 80% with the heat input prior to the onset of the laser pulse. It is found that the Fourier number of the layer between the control heater and the active layer is the key parameter to minimize the temperature fluctuations. We also successfully demonstrate suppression of the temperature change in a MEMS-based experimental setup.
Experimental Study and Numerical Analysis of Water Single-Phase Pressure Drop Across an Array of Circular Micro-Pin-Fins
Authors: Mita Jonathan R. , Qu Weilin , Kobayashi Marcelo H. , Pfefferkorn Frank E.
Published: March 2011

This study investigates pressure drop associated with water liquid single-phase flow across an array of staggered micro-pin-fins having circular cross-section. The micro-pin-fins are micro-end milled out of oxygen free copper and have the following dimensions: 180 micron diameter and 683 micron height. The longitudinal pitch and transverse pitch are equal to 400 microns. Seven water inlet temperatures from 22 to 80 °C, and seventeen maximum mass velocities for each inlet temperature, ranging from 159 to 1475 kg/m2s, were tested. The test module was well insulated to maintain adiabatic conditions. The experimental results were compared to those from a micro-pin-fin array having similar size and geometrical arrangement but a square cross-section. The circular micro-pin-fins were seen to yield a significantly lower pressure drop than the square micro-pin-fins. The present experimental results were also compared with the predictions of several friction factor correlations as well as the results from a three-dimensional numerical analysis. Neither was able to accurately predict the experimental data.
Multi-Nozzle Spray Cooling in a Closed Loop
Authors: Lin Lanchao , Leland Quinn
Published: March 2011

A closed two-phase loop system was developed that combined with a multi-nozzle spray cooling unit for the cooling of high heat flux power sources. The fluid circulation was sustained by a magnetic gear pump operating with an ejector pump unit. The motive flow of the ejector shared the pumping liquid flow with the multi-nozzle spray. The use of the ejector stabilized the circulation of the two-phase flow. A multi-nozzle plate with 48 miniature nozzles was designed to generate an array of 4×12 sprays. A closed loop spray cooling experimental setup with a cooling area of 19.3 cm2 was built. The spray nozzle to target distance was 10 mm. Water and FC-72 were used as the working fluids. Spray cooling experiments were performed in three orientations of the spray target surface, namely (a) horizontal facing upward, (b) vertical, and (c) horizontal facing downward. The thermal performance of the horizontal facing downward surface was the best. A comparison with the thermal performance data for a smaller cooling surface area of 2.0 cm2 was made.
Cutoff Radius Effect of Water Configuration Using the Wolf Method
Authors: Takahashi Kazuaki , Narumi Tetsu , Yasuoka Kenji
Published: March 2011

Molecular dynamics simulation has been applied for water to compare the Wolf method to the IPS method and the Ewald sum by evaluating the diffusion coefficient and liquid structure. In our previous study, we applied the IPS method for bulk water and found notable deviation of the radial distribution function g(r). The Wolf method gives a good estimation for the potential energy and the self-diffusion coefficient at a cutoff radius, rc, greater than 2.2 nm while avoiding the notable deviation of g(r) which appeared in the case of IPS. The distance dependent Kirkwood factor Gk(r) was also calculated, and the truncation of a long-range interaction of the cutofflike method (such as cutoff with or without the switch function and the reaction field) show serious shortcomings for dipole-dipole correlations in bulk water systems. This was observed by comparing the shape to that of the Ewald sum. Gk(r) of the cutofflike method greatly deviates from that of the Ewald sum. However, the discrepancy of Gk(r) for the Wolf method was found to be much less than that of other typical cutoff-like methods. We conclude that the Wolf method is an adequately accurate technique for estimating transport coefficients and the liquid structure of water in a homogeneous system at long cutoff distances.
Experimental Investigation of Two-Phase Flow in a Multi-Branch Reduced T-Junction With Co-Current Stratified Gas-Liquid Flow
Authors: Bowden Robert , Hassan Ibrahim
Published: March 2011

Experiments were performed in a horizontal reduced T-junction using a branch diameter of 6.35 mm and an inlet pipe diameter of 50.8 mm. The inlet length was 1.8 m, and three branch orientations were tested at 0, 45, and 90 degrees from horizontal. Air and water, operating at 206 kPa, were used to provide a two-phase environment. Both fluids flowed co-currently within the inlet towards the branch in the smooth-stratified regime. Flow visualization was used to identify the onset of gas entrainment. The critical height at the onset of gas entrainment was quantified as a function of the single phase liquid branch Froude number for the 45, and 90 degree branches, respectively.
Electric Charge of Micro and Nano Bubbles by Interferometric Laser Imaging Technique
Authors: Sekiguchi Tomohiro , Kawaguchi Tatsuya , Satoh Isao , Saito Takushi
Published: March 2011

In this paper we investigated ζ potential of microbubbles by electrophoresis method. The individual bubble diameter measured by means of interferometric laser imaging technique. The experimental results showed that different methods for producing microbubbles which were pressurizing dissolution method, swiveling gas-liquid two-phase flow method and electrolysis method did not make difference in the resultant values of ζ potential. In order to understand the mechanism of charging characteristic of microbubbles, we changed the pH of the deionized water and added alcohols or a surfactant to the deionized water. The results showed that the absolute value of the ζ potential increased when the pH increased. ζ potential was drastically changed by alcohols and surfactant which easily absorbed on the air-water interface. Moreover, simultaneous measurement of ζ potential and diameter of shriking microbubble resulted that bubbles shose diameter is less than 3 μm were observed by using Mie theory.
Thermo-Fluid Analysis of Prismatic Fuel Blocks Using Multi-Dimensional Heat Conduction and One-Dimensional Fluid Model
Authors: Tak Nam-il , Kim Min-Hwan , Lim Hong-Sik , Park Goon-Cherl
Published: March 2011

The recent rapid development of computational fluid dynamics (CFD) technology provides a powerful tool to obtain accurate temperature distribution of prismatic fuel blocks. In general, however, a CFD analysis requires tremendous computational efforts to analyze whole prismatic fuel blocks due to complex geometry and near wall treatment to resolve the boundary layer of the fluid flow. Such efforts might be a huge burden for a designer who wants a large number of calculations with various design options. Recently, a practical method to analyze a thermal behavior of prismatic fuel blocks has been developed by the present authors to overcome the demerits of CFD and system code calculations. The developed method solves three-dimensional heat conduction in prismatic fuel blocks like a CFD code. For the fluid, however, the present method adopts one-dimensional conservation equations like a system code. Such a combination enables significantly reduced computational effort with reasonable computational accuracy. This paper presents an intensive thermo-fluid analysis for prismatic fuel assemblies using the new method. In order to examine the validity and the accuracy of the developed method, at first, single standard and control fuel assemblies have been analyzed and compared with the CFD results. Then the developed method has been applied to a whole core thermal analysis of a prismatic reactor.
Experimental Analysis of Thermal and Electric Transport Characteristics of Nano-Gaps
Authors: Ito Kohei , Hagio Tomoaki , Matsuo Akira , Iwaisako Yasushi , Nakabeppu Osamu
Published: March 2011

We conducted an experiment to demonstrate the thermoelectric nano-gap, which is recently expected to own high performance, in principle, because it does not have conductive heat flow between the high and low temperature region. In this study, the thermoelectric nano-gap is realized with a pair of probe and substrate where they are finely positioned. A temperature difference of ca. 10 K is imposed to the nano-gap under vacuum circumstances. A representative thermoelectric voltage, tunneling-current and gap were 250 μV, 0.3 nA and 50 nm. The obtained voltage and current, with assuming an effective probe-diameter of 10 nm, roughly agreed to a theoretical study (G. Despesse and T. Jager, J. Appl. Phys., Vol.96, p.5026-, 2004). However, the obtained gap was 25 times larger than that from the theoretical study.
Analysis of a Graphene/Ultrathin Graphite Heat Exchanger for Aerospace Thermal Management
Authors: Heltzel Alex , Mishra Columbia , Ruoff Rodney S. , Fleming Andrew
Published: March 2011

Fast evolving techniques for macroscale graphene and ultrathin graphite material production are promising for applications of graphene-based materials in thermal management. A numerical comparison of aluminium and graphene-based plate-fin heat exchangers is conducted. Anisotropic thermal conductivity of graphene-based solution shows an improvement of up to twenty percent in heat rejection over the aluminium design. Thermal and hydraulic performance is characterized for both designs over a range of air flow rates in both laminar and turbulent regimes. Steady and unsteady 3-D conjugate simulations reveal a faster equilibration rate for the graphene-based solution, minimizing thermal lag that must be accounted for in on-demand electronics cooling. The combination of improved heat rejection, rapid response rate, and low material density make a graphene-based solution uniquely suited to aerospace thermal management.
The Effects of Solid Thermal Conductivity and Volume-Fraction in the Natural Convection Inside a Heterogeneous Enclosure
Authors: De Lai Fernando C. , Franco Admilson T. , Junqueira Silvio L. M. , Lage Jose´ L.
Published: March 2011

In this study, the natural convection inside a fluid filled enclosure containing several solid obstructions and heated from the side is simulated numerically as to determine the effects of the solid thermal conductivity and volume-fraction. The solid obstructions are conducting, disconnected square blocks, uniformly distributed inside the enclosure. The mathematical model follows a continuum approach, with balance equations of mass, momentum and energy presented for each one of the constituents (i.e., fluid and solid) inside the enclosure. The equations are then solved numerically via the finite-volume method. The effects of varying the solid-fluid thermal conductivity ratio (K), the fluid volume-fraction or porosity (φ), the number of solid blocks (N) and the heating strength (represented by the Rayleigh number, Ra) on the natural convection process inside the enclosure are investigated parametrically. The Nusselt number based on the surface-averaged heat transfer coefficient along the heated wall is chosen to characterize the convection strength inside the enclosure. The results indicate a competing effect caused by the proximity of the solid blocks to the heated and cooled walls of the enclosures, vis-a`-vis hindering the boundary layer growth, hence reducing the heat transfer effectiveness, and at the same time enhancing the heat transfer when K is large. An analytical estimate of the minimum number of blocks beyond which the convection hindrance becomes predominant is presented and validated by the numerical results.
Specification of Micro-Nanoscale Radiative Patterns Using Inverse Analysis for Increasing Solar Panel Efficiency
Authors: Hajimirza Shima , El Hitti Georges , Heltzel Alex , Howell John
Published: March 2011

This work proposes a comprehensive and efficient optimization approach for designing surface patterning for increasing solar panel absorption efficiency using near field radiation effects. Global and local optimization methods, such as the Broyden–Fletcher–Goldfarb–Shanno Quasi-Newton (BFGS-QN) and Simulated Annealing (SA), are employed for solving the inverse near field radiation problem. In particular, a thin amorphous silicon (a-Si) solar panel with periodic silver nano-wire patterning is considered. The design of the silver patterned solar panel is optimized to yield maximum enhancement in photon absorption. The optimization methods reproduce results found in previous literature but with reduced computational expense. Additional geometric parameters not discussed in previous work are included in the optimization analysis, further allowing for increased absorption enhancement. Both the BFGS-QN and SA methods give efficient results, providing designs with enhanced absorption.
Fractal Aggregation Simulation in the Nanofluid Drying
Authors: Crivoi A. , Duan Fei
Published: March 2011

Nanofluids are kinds of fluids engineered by dispersing nanoparticles in base fluids. After full evaporation drying of nanofluids, the nanoparticles are left on the substrate and can self-organize in different structures on the substrate. The fractal-like branched structures can be formed, these phenomena are simulated in the study. Two dimensional lattice-gas model is applied in the process of the residual thin film evaporation drying. The Monte Carlo simulations show the formation of well-developed fractal aggregates for this configuration. The results of simulations are in good agreement with previously reported experimental results in residual nanofluid thin-film drying.
Entropy Generation Analysis for Mono-Dispersed Droplet Cooling at Critical Heat Flux Regime
Authors: Mahmoudi S. R. , Adamiak K. , Castle G. S. P.
Published: March 2011

In the present work, an analytical expression for entropy generation production for mono-dispersed stream of droplets impacted on the upward heated surface at critical heat flux regime was found. The expression involving the relevant parameters for entropy production contributed to the heat transfer and the entropy flux due to the phase change and the spreading process. The influence of impact velocity and flow rate on entropy generation was explored.