bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat Transfer Augmentation and Suppression in Optimal Rotating Cylinders in Cross-Flow
Authors: Bello-Ochende Tunde , Meyer Josua P. , Ogunronbi Oluseun I.
Published: March 2011

A steady-state two dimensional numerical simulation was carried out to optimize the heat transfer rate density from cylinders under different conditions. The geometric design of the cylinders was varied in two ways. In the first case the cylinders are located on a plane where their leading edges are aligned, and in the second case the cylinders are aligned on a plane which passes through their respective centre-lines. The rotation of the cylinders is within the range of 0 ≤ ω˜ ≤ 1, and the dimensionless pressure drop number, Be, which drives the flow is in the range of 10 ≤ Be ≤ 104. The continuity, momentum and energy equations describing the flow of the coolant, across the cylinders in the computational domain are performed using a computational fluid dynamics code, the results obtained were validated by comparing it with past results in the open literature for stationary cylinders. The effects of the various parameters (dimensionless pressure drop number, rotation) on the maximum heat transfer rate density from the cylinders in terms of augmentation and the suppression were analysed and reported.
Numerical and Experimental Research in Heat Transfer to Screw Compressor Rotors
Authors: Stosic Nikola , Smith Ian K. , Kovacevic Ahmed
Published: March 2011

Due to fast rotation of screw compressor rotors, temperature is uniform in a cross section and temperature field is a function of the axial coordinate only. The rotors in one cross section the rotors are simultaneously heated by hot gas on one side while cooled at another side by cold gas. As a result of identification of the main modes of heat transfer both in the rotors and between the rotors and their surroundings and the relative significance of each, a novel procedure is suggested to cool the rotors by injection of minute quantities of a volatile fluid. By this means the compressed gas should attain higher temperatures without rotor distortion. To confirm these concepts and quantify both the heat transfer rates and the rate of liquid injection required for rotor cooling, both a one dimensional flow study and a more complex 3-D numerical analysis were performed, the latter with the aid of a CFD code. The results indicated that the rotors could be maintained at a far lower temperature than that of the discharged gas by flash evaporative cooling, as a result of injecting a fractional percentage by mass of a volatile fluid. This was confirmed by experiment. This technique may be used to operate dry compressors at substantially higher pressure ratios than are currently possible in such machines. It is also shown that only minor design changes are needed to implement it.
Snow Melting System Around Steel Top of Underground Fire Cistern Using Heat Pipe BACH
Authors: Nagai Niro , Suzuki Kensuke , Maegawa Yoshikazu , Taniguchi Haruki
Published: March 2011

This research aims to develop snow melting system around steel top of underground fire cistern by using Bubble Actuated Circulating Heat pipe (BACH). Water in afire cistern installed underground is heated by underground heat source, 10∼15 °C Heat is transported by BACH from water to snow melting panel near the surface, which contains the steel top, resulting in snow melting around the top. The experimental results show that this system can melt the snow on the snow melting panel in winter season preferably, and has enough anti-freezing ability around the steel top. The numerical simulation program was found to predict temperature variations of the whole system preferably. From a thermal point of view, BACH showed better results compared to those of a conventional thermosiphon heat pipe.
Flashing Flow Mechanisms of HFC-134a and HFC-410a Through Short-Tube Orifices
Authors: Nilpueng Kitti , Wongwises Somchai
Published: March 2011

In this study, the flow mechanisms of HFC-134a and HFC-410A, including flow pattern, pressure distribution, temperature distribution, and mass flow rate inside short-tube orifice are presented and compared under the same working temperature. The test runs are performed at condenser temperature ranging between 35 and 45°C, evaporator temperature ranging between 2 and 12°C, and degree of subcooling ranging between 1 and 12 °C. The results show that the temperature distribution along the short-tube orifice obtained from HFC-410A is slightly higher than that obtained from HFC-134a. On the other hand, the pressure distribution between both refrigerants shows the large difference. It is also found that the tendency of mass flow rate obtained from HFC-134a almost coincides with those obtained HFC-410A as the operating conditions and short-tube orifice size are varied. However, the average mass flow rate of HFC-134a is slightly lower than that of HFC-410A.
Identification of Nucleobases of Single Stranded DNA by Nanopore Force Resolution at Different Film Thickness
Authors: Zolotoukhina Tatiana , Fukui Takeo
Published: March 2011

The model of the molecular translocation of all types of DNA base molecules of cytosine, thymine, adenine and guanine through the nanoporous membrane of a solid thin film has been considered from the point of view of improving the resolution of forces by changing parameters of the membrane itself. The results of simulation of translocation process were compared for all four DNA nucleotides. The molecular dynamics (MD) method with the force field potential has been used for the atomic level modeling of the cytosine (C), thymine (T), adenine (A), and guanine (G) molecules and a configuration of the nanoporous Si membrane. With the planar structure of base molecules and cylindrical symmetry of pore, the two-dimensional projection was used in the simulation. The force field between the base molecule and atoms of nanopore has been estimated. Influence of the Si surface hydrogenization and film thickness on the force resolution for each nucleobase was evaluated vs. possible signal resolution. At 5 layer thickness of the film it was possible to cut thermal fluctuations and distinguish four nucleobase types.
Experimental Study on Polymer Pyrolysis in High-Temperature Air Diluted by H2O and CO2 Using Stagnation-Point Flow
Authors: Ogami Yasuhiro , Mori Makoto , Yoshinaga Kentaro , Kobayashi Hideaki
Published: March 2011

Combustion experiments were performed for polyethylene (PE) in stagnation-point flow to investigate the fundamental characteristics of PE in high-temperature air combustion (HiTAC). Air diluted by nitrogen, water vapor, and carbon dioxide was tested as the oxidizer, and the oxidizer temperature was varied from 300 to 773 K, to investigate the effect of the dilution and temperature on the regression rate, the extinction limit, and the sooting limit of PE. The kinetic parameters of PE pyrolysis under the combustion conditions were estimated from the regression rate experimental data using a new method combining experiments and numerical simulations. It was found that the previously reported kinetic parameters obtained with thermal gravimetric analysis (TGA) were much smaller than those obtained in this study, and that the kinetic parameters of pyrolysis were affected by dilution.
Heat Transfer Enhancement by Means of a Rotation Cylinder
Authors: Hirawa Daiki , Inaoka Kyoji , Senda Mamoru
Published: March 2011

Heat transfer measurement has been done for the low speed channel flow with an insertion of a rotation circular cylinder near the channel wall in order to find the ways to achieve the dynamic control of the wall heat transfer. It was found that the cylinder rotation was quite effective to achieve large heat transfer enhancement including the power loss required for the cylinder rotation. Counter clockwise rotation (the direction to accelerate the gap flow) intensifies the motion of the unsteady vortex shed from the cylinder and this enhances the wall heat transfer over relatively long downstream area of the cylinder. Heat transfer enhancement becomes large as the rotation speed becomes higher and then shows the largest at a certain rotation speed. On the other hand, clockwise rotation (decelerate the gap flow) suppresses the vortex formation and decreases the heat transfer at low speed rotation. However, high speed rotation induces the fast flow approaching near the wall and can achieve large heat transfer there.
Numerical Investigation of the Thermal Behavior in a Hydrogen Tank During Fast Filling Process
Authors: Takagi Youhei , Sugie Naoya , Takeda Kazuhiro , Okano Yasunori , Eguchi Tooru , Hirota Kazuyuki
Published: March 2011

To investigate the thermal behavior during fast hydrogen filling process, the simple one-dimensional analysis considering the heat conduction in tank wall and the three-dimensional numerical simulation dealing with inner gas region were carried out. The numerical analyses were subject to the fast filling test of 35 MPa hydrogen gas into 34 litter tank for 80 seconds. The one-dimensional analysis predicted the temperature rise and the heat loss into surrounding air qualitatively and the averaged temperature of tank wall was underestimated. On the other hand, the three-dimensional simulation overestimated the temperature distribution because of using adiabatic wall condition. However, the effects of buoyant force and convective flow on local thermal profile were fully explained from our numerical results.
BEM Based Solution of Turbulent Flow Over Periodic Hills With Heat Transfer
Authors: Sˇkerget Leopold , Ravnik Jure
Published: March 2011

Detached turbulent flows are difficult to predict numerically and often serve as benchmark cases for developing new numerical schemes and new turbulent models. Turbulent flow over periodic hills is one such examples, since the flow exhibits separation and reattachment on a smoothly and/or sharp curved geometry, strong pressure gradients and fluctuation of the separation point in time. These cases have been chosen by many authors for testing different turbulence simulation approaches. When the bottom wall is heated, the complexity of the problem increased, since convective heat transfer is defined by small scale turbulent structures close to the wall. We developed a Reynolds-Averaged Navier-Stokes and Large Eddy Simulation solver based on the velocity-vorticity formulation of Navier Stokes equations. RANS equations are coupled by a low-Reynolds number turbulent model, while Smagorinsky subgrid model is used for LES. The governing equations are solved with a numerical solution algorithm, which is based on the boundary element method. The pressure field is computed in a post processing step by solving a Poisson equation. The single domain as well as domain decomposition approaches are applied. The developed method was validated using flow over periodic hills test case.
Numerical Simulation for Heat and Fluid Flow Through Porous Media
Authors: Kaneda Masayuki , Matsushima Yusuke , Suga Kazuhiko
Published: March 2011

Heat and Fluid flow through infinite porous media is numerically studied. The pore structure considered is body-centered cubic (BCC) and face-centered cubic (FCC) whose porosity is ranged 0.93–0.98 by changing the pore diameter. For the thermal condition, a constant heat flux from the porous wall is considered. The heat and fluid flow simulations are carried out separately by the lattice Boltzmann method (LBM). It is confirmed that the LBM can simulate them properly and the permeability depends on the pore structure compared at the same porosity. The heat transfer coefficient is found to be affected not only by the permeability but also the pore structure. That is, even at the same permeability, the Nusselt number depends on the structure.
Study on Flow and Heat Transfer Characteristics of Tetra-n-Butyl Ammonium Bromide Hydrate Slurry
Authors: Kumano Hiroyuki , Hirata Tetsuo , Kobayashi Yuki , Kudoh Tomoya
Published: March 2011

Flow and heat transfer characteristics of TBAB hydrate slurry were investigated experimentally. The Reynolds number, diameter of the tubes and solid fraction were varied as experimental parameters. For laminar flow condition, it was found that the ratio of the coefficients of pipe friction and Nusselt numbers increases with solid fraction, and the rate of increase is high in the case of a low Reynolds number. For turbulent flow condition, the ratio of the coefficients of pipe friction and Nusselt numbers was 1 for each condition in the case of a low solid fraction. On the other hand, the ratio of the Nusselt numbers increased with the solid fraction in the high-solid fraction region. Moreover, it was found that the effects of the difference of the size and shape of hydrate particles on the coefficients of pipe friction are large. On the other hand, the effects of the difference of the hydrate particles on the Nusselt number are small.
Thermal Optimization for Internal Embedded Conductive Cooling Schemes Using Topology Optimization Techniques
Authors: Dirker J. , Meyer J. P.
Published: March 2011

Conductive heat transfer has an important role to play in the cooling of, for instance, electronic equipment. Due to its passive nature in relaying heat, internal conductive cooling may have advantages above internal convective heat transfer at small length scales. It does, however, require optimization of the distribution of high heat-conducting material. In this two-dimensional numerical study, the optimum distribution of high conductive material within rectangular heat-generating domains with different aspect ratios is investigated by using a topology optimization algorithm. The volume fraction of the high-conductive material is set at 10% of the total volume. Of interest are the influences of boundary conditions, thermal conductivity and optimization penalization levels on the resulting cooling material distribution. The obtained conducting trees are compared and discussed.
Flow Instability and Critical Heat Flux for Flow Boiling of Water in a Vertical Annulus at Low Pressure
Authors: Haas Christoph , Meyer Leonhard , Schulenberg Thomas
Published: March 2011

We investigated the critical heat flux (CHF) for flow boiling of water in a vertical annulus. The coaxial annulus has a diameter ratio of 1.37 and the inner zircaloy tube is heated directly over a length of 325 mm. CHF can occur prematurely due to flow instabilities. Therefore, we analyzed the flow stability at different heat input conditions using two types of pumps, a rotary and a gear type pump. The unstable CHF occurred at 61% and 90% of the stable value for the rotary and the gear type pump, respectively. Consequently, the following CHF experiments were conducted at stable flow conditions. The outlet pressure was constant at 120 kPa, the mass flux varied from 250 to 1000 kg/(m2s) and the inlet subcooling was at 102, 167, and 250 kJ/kg. The CHF results increase with mass flux from 0.67 to 2.62 MW/m2 and show similar trends compared to literature data. However, the experimental data for flow boiling in annuli at low pressure are limited. Additionally, we measured the dynamic contact angle between the zircaloy tube surface and water using the Wilhelmy method.
Internal Turbine Blade Cooling Simulation: Advanced Models Assessment on Ribbed Configurations
Authors: Guillou Florian , Chedevergne Franc¸ois
Published: March 2011

Many actors of the Gas Turbine (GT) world are today concerned by environmental issues. This is why they are always willing to increase further GT efficiencies. A way to achieve this is rising the turbine entry temperature, providing that the blade turbine is sufficiently cooled to withstand it. This paper investigates a method to use CFD as a design tool, to optimize blades internal coolant channels shapes. It focuses on the use of advanced RANS models combined with a wall law approach to keep CPU costs reasonable. The numerical study relies on experimental data from the MERCI test rig, where a U-shaped coolant duct representative of internal blade turbine channels was tested. The results obtained are encouraging, some need of local mesh refinements have been identified though. The computed heat exchange distributions yield the right trends and well reproduced the effects of rotation, but to go further in the thermal assessment of the models, wall conduction effects will have to be accounted for.
Comparison of Models Used to Predict the Gas Composition of Mixtures Produces by Gasification of Wax Palm Husk Using Air-Steam for Partial Oxidation
Authors: Contreras Mari´a F. , Gordillo Gerardo
Published: March 2011

There are eleven species of wax palm in South America which produce a great amount of biomass per year (∼590 ton/ha). This biomass has not any commercial value and can cause a negative impact on the environment if waste handling systems and storage and treatment structures are not accurately controlled. However, this biomass can serve as feedstock for thermal gasification (partial oxidation and pyrolysis) to produce gaseous and liquid fuels. The current paper presents results on modeling studies to predict the gas composition of mixtures produced by gasification of wax palm husk (WPH) using air–steam for partial oxidation. The parameters investigated by equilibrium model are equivalence ratio and steam-fuel ratio. With the predicted composition of gases, the high heating value was estimated. Atom model was used for comparison purposes. Also, a kinetics model of the pyrolysis of WPH is presented. Thermogravimetric analysis was used to estimate the activation energy and the pre-exponential factor of Arrhenius equation. The results suggest that it is possible to produce gaseous fuels with concentrations of H2 ranging from 5 to 30%, CO from 10 to 19%, and CH4 from 0 to 4%. The activation energy estimated with kinetics model was estimated to be 143.07 kJ mol−1.
Control of Dielectric Fluid Flow Distribution in Micro-Tubes With EHD Conduction Pumping: Numerical Study
Authors: Yazdani Miad , Seyed-Yagoobi Jamal
Published: March 2011

The control of fluid flow distribution in micro-scale tubes is numerically investigated. The flow distribution control is achieved via electric conduction mechanism. In electrohydrodynamic (EHD) conduction pumping, when an electric field is applied to a fluid, dissociation and recombination of electrolytic species produces heterocharge layers in the vicinity of electrodes. Attraction between electrodes and heterocharge layers induces a fluid motion and a net flow is generated if the electrodes are asymmetric. The numerical domain comprises a 2-D manifold attached to two bifurcated tubes with one of the tubes equipped with a bank of uniquely designed EHD-conduction electrodes. In the absence of electric field, the total flow supplied at the manifold’s inlet is equally distributed among the tubes. The EHD-conduction, however, operates as a mechanism to manipulate the flow distribution to allow the flow through one branch surpasses the counterpart of the other branch. Its performance is evaluated under various operating conditions.
Assesment of Vibrational Coupling at Solid-SAM Junctions
Authors: Saltonstall Christopher B. , Duda John C. , Hopkins Patrick E. , Norris Pamela M.
Published: March 2011

Self-assembled monolayers (SAMs) have recently garnered much interest due to their unique electrical and chemical properties. The limited literature detailing SAM thermal properties has suggested that thermal boundary conductance (TBC) at solid-SAM junctions is not only low, but also insensitive to changes in SAM length as the number of methylene groups (-CH2-) along alkanedithiol chains is varied from 8 to 10. The present study investigates the vibrational spectra of alkanedithiol SAMs as a function of the number of methylene groups forming the molecule backbone via Hartree-Fock methods and the subsequent effects on TBC calculated using a diffuse scattering model. In particular, the vibrational overlap between the alkanedithiol and Au is studied. It is found that despite the addition of 9 new vibrational modes per added methylene group, only one of those modes is elastically accessible to Au. It is believed that this “vibrational inaccessibility” is the cause of the insensitivity of thermal conductance to molecule length.
Contact Thermal Resistance Between Individual Multi-Wall Carbon Nanotubes
Authors: Yang Juekuan , Waltermire Scott W. , Yang Yang , Li Deyu , Chen Yunfei
Published: March 2011

Carbon nanotubes (CNTs), because of their superior mechanical, electrical, and thermal properties and possible low-cost, large volume production, have been projected as promising nanostructure additives in polymer composites to achieve tunable and enhanced materials properties. Transport properties of CNT-polymer composites have been widely studied over the past decade and it is well-accepted that when the added CNTs exceed the percolation limit, the electrical conductivity of CNT-polymer composites can usually increase by several orders of magnitude. However, thermal conductivity measurements present mixed results and even for positive results, the enhancement is much lower than that expected from traditional theories. For example, Biercuk et al. [1] demonstrated that 1 wt% of single-wall CNTs (SWCNTs) in industrial epoxy could increase the thermal conductivity by 125% at room temperature, three-times higher than that from 1 wt% loading of carbon nanofibers. However, similar studies [2] showed that thermal conductivity only increased marginally for multi-wall CNT (MWCNT)-epoxy composites and more surprisingly, the thermal conductivity for SWCNT-epoxy composites was even lower than that of pure epoxy.
Simulation on Solar Energy Collection to Power a Thermoacoustic Prime Mover Using Pressurized Hot Steam
Authors: Setiawan Ikhsan , Nohtomi Makoto , Katsuta Masafumi
Published: March 2011

It has been performed a simple simulation and calculation on solar energy collection which is used indirectly to power a thermoacoustic prime mover by producing pressurized hot steam which would supply thermal energy to the prime mover via sealed-off hot heat exchangers. The solar energy collection took place in Yogyakarta City - Indonesia where the average energy of solar global radiation of 4.8 kWh/m2/day (17.3MJ/m2/day) is available around the year. The calculation including the amount of the remaining heat stored, steam pressure, and steam temperature for various areas of the collector unit (Fresnel lens) and volume of water, were done as a function of time for several days. We found that appropriate combinations of lens area and water volume would enable us to operate the thermoacoustic prime mover continuously all day and night.
Measurements of Spectral Transmittance of Porous Cerium Dioxide in the Range 900-1,700 nm
Authors: Ganesan Krithiga , Lipin´ski Wojciech
Published: March 2011

Overall transmittance of porous cerium dioxide is measured in the spectral range of 900–1700 nm using dispersive spectroscopy. Dense and porous samples of cerium dioxide with average porosities of 0.08 and 0.72, respectively, are investigated. The transmittance of both sample types increases with decreasing thickness, and this trend is more pronounced for the dense samples. The on-average spectrally increasing transmittance of the dense samples is attributed to the decreasing absorption by bulk cerium dioxide with radiation wavelength. The transmittance of the porous samples, on the other hand, remains a constant over the spectrum. Porous samples attenuate radiation stronger than the dense samples at any wavelength in the considered range, and it is hypothesized that this effect is due to more intense scattering. Sharp local variations of the transmittance are observed for both sample types.
Thermal Boundary Conductance Between Thin Metal Films and Graphite Substrates
Authors: Smoyer Justin L. , Duda John C. , Norris Pamela M. , Lichtenberger Arthur W.
Published: March 2011

Due to the high intrinsic thermal conductivity of graphitic structures, much interest has developed in incorporating these materials into modern nano-devices for improved thermal abatement. In order to be integrated successfully, thermal energy must be able to transport efficiently through the graphitic materials and into the surrounding structure, most commonly a metal. However, thermal boundary conductance at metal-graphite interfaces is traditionally poor in comparison to non-graphitic substrates, due in large part to the weak van der Waals adhesion force between the metal and underlying carbon structure. To be applicable as thermal abatement materials, an enhanced understanding of the role of the metal-carbon interface is required. This paper reports the changes to phononic thermal transport across the interface between metallic thin films and highly oriented pyrolitic graphite (HOPG) substrates due to changes in interface structure and chemistry. The temperature dependent thermal boundary conductance is measured using transient thermoreflectance from 100 K to 400 K. It is found that the differences in metal-carbon bonding and structure at the interface have a significant impact on the thermal conductance between the metallic thin films and the HOPG substrates.
Char and Tar Production From Dairy Biomass Gasification Using Air-Steam for Partial Oxidation
Authors: Gordillo Gerardo , Annamalai Kalyan
Published: March 2011

The increase in air pollution caused by combustion of fossil fuels demands the exploration of renewable energy sources in order to mitigate the dependence on fossil fuels. Research includes the efforts to partially replace fossil fuels with renewable energy-sources in thermal conversion processes in order to reduce the emission of CO2. The animal wastes can be considered as biomass fuels since their properties are almost similar to ration fed to animals. Concentrated animal feeding operations (CAFOs) such as cattle feedlots and dairies produce a large amount of feedlot manure or feedlot biomass (FB) and dairy manure or dairy biomass (DB), which may lead to land, water, and air pollution if waste handling systems and storage and treatment structures are not properly managed. Both FB and DB are grouped under cattle manure or cattle biomass (CB). The concentrated production of low quality CB at these feeding operations can serve as a good feedstock for locally based gasification for syngas (CO and H2) production and subsequent use in combined heat and power generation. If thermal gasification technology is developed for DB fuels, the environmental impact from both animal feeding operations and fossil-fuels could be mitigated. The current paper presents experimental results obtained from adiabatic fixed-bed gasification of DB using a 10 KW fixed bed counter-flow gasifier and air-steam for partial oxidation. A mass spectrometer (ProLab Thermo ONIX) was used to analyze the gas composition continuously and at real time. The effect of the operating parameters studied, which includes equivalence ratio (1.6 < Φ < 6.4) and steam to fuel (S:F) ratio (0.4 < S:F < 0.8, on the yields of gases, char, and tar are discussed. Also, results from gasification of dairy biomass–ash blend (DB-Ash) and dairy biomass Wyoming coal blend (DB-WYC) is presented for comparison effects. In general, for the set of experiments performed using DB, the gas yield was 1.54 to 5.30 dry tar-free kg of gases per each kg of DAF DB gasified while the char production ranged from 0 to 0.18 kg of char per DAF kg of DB gasified. The average of tar concentration in gases leaving the gasifier was about 80 g/ SATP m3.
First-Principles-Based Interatomic Potential for SI and Its Thermal Conductivity
Authors: Esfarjani Keivan , Chen Gang , Henry Asegun
Published: March 2011

Based on first-principles density-functional calculations, we have developed and tested a force-field for silicon, which can be used for molecular dynamics simulations and the calculation of its thermal properties. This force field uses the exact Taylor expansion of the total energy about the equilibrium positions up to 4th order. In this sense, it becomes systematically exact for small enough displacements, and can reproduce the thermodynamic properties of Si with high fidelity. Having the harmonic force constants, one can easily calculate the phonon spectrum of this system. The cubic force constants, on the other hand, will allow us to compute phonon lifetimes and scattering rates. Results on equilibrium Green-Kubo molecular dynamics simulations of thermal conductivity as well as an alternative calculation of the latter based on the relaxation-time approximation will be reported. The accuracy and ease of computation of the lattice thermal conductivity using these methods will be compared. This approach paves the way for the construction of accurate bulk interatomic potentials database, from which lattice dynamics and thermal properties can be calculated and used in larger scale simulation methods such as Monte Carlo.
Phonon Transport in Asymmetric Sawtooth Nanowires
Authors: Roberts N. A. , Walker D. G.
Published: March 2011

Thermal transport in asymmetric sawtooth nanowires was investigated. The boundaries reflect phonons differently depending on the frequency and momentum of the phonon. These systems show thermally rectifying behavior when the boundary reflections are a function of both the direction the phonon is traveling and the frequency of the phonon. This rectifying effect could be useful for thermal management applications at all size scales, but would have to be built up from the nanoscale because of a strong dependence on the device aspect ratio and the Knudsen number of the system. Monte Carlo simulations show an accumulation of phonons at the boundary which emits phonons in a perceived rough direction where those phonons have some probability of diffuse reflections at the boundary while phonons emitted in the smooth direction only experience specular reflections at the boundary and are eventually thermalized at the opposite boundary. In this study the level of rectification of the system was linearly dependent on the device aspect ratio as long as the length of the device was near or below the phonon mean free path of the phonons.
Thermal Properties of Yttrium Aluminum Garnett From Molecular Dynamics Simulations
Authors: al-Dosari Majid S. , Walker D. G.
Published: March 2011

Yttrium Aluminum Garnet (YAG, Y3Al5O12) and its varieties have applications in thermographic phosphors, lasing mediums, and thermal barriers. In this work, thermal properties of crystalline YAG where aluminum atoms are substituted with gallium atoms (Y3(Al1−xGax)5O12) are explored with molecular dynamics simulations. For YAG at 300K, the simulations gave values close to experimental values for constant-pressure specific heat, thermal expansion, and bulk thermal conductivity. For various values of x, the simulations predicted no change in thermal expansion, an increase in specific heat, and a decrease in thermal conductivity for x = 50%. Furthermore, the simulations predicted a decrease in thermal conductivity with decreasing system size.
Fabrication of a Dye Sensitized Solar Cell and Its Performance Dependence on Temperature and Irradiance
Authors: Peng Edwin , Berberoglu Halil
Published: March 2011

This paper reports the temperature and irradiance dependence of dye-sensitized solar cells (DSSCs) with acetonitrile-based electrolytes. The prototyped DSSCs had nanocrystalline titanium dioxide photoanodes and platinum thin film cathode. The photoanodes were sensitized with N-749 dye. The current-voltage characteristics of the DSSCs were measured at temperatures from 5 to 50 °C and under 500, 1000, and 1500 W m−2 irradiance. The open circuit voltage, VOC, decreased linearly with increasing temperature and had positive, logarithmic relation with irradiance. At temperatures lower than 15 °C, short circuit current density, JSC, was limited by the diffusion of I3− in the electrolyte and increased with increasing temperature. At higher temperatures the recombination of electrons injected into the TiO2 conduction band was dominant over diffusion and JSC decreased with increasing temperature. Moreover, JSC increaed linearly with increasing irradiance. The DSSC photoconversion efficiency did not vary appreciably at temperatures lower than 15 °C but decreased with increasing temperature. Finally, the efficiency increased with increasing irradiance.
Average Nusselt Number for Pressure and Suction Sides of Squealer Turbine Blade Tip
Authors: Ghenai Chaouki
Published: March 2011

Numerical simulations of the flow field and heat transfer of squealer blade tip are performed in this study. The effect of Reynolds number (Re = 10000–40000), the clearance gap to width ratios (C/W = 5%–15%) and the cavity depth to width ratios (D/W = 10%, 20% and 50%) on fluid flow and heat transfer characteristics are obtained. The temperature and velocity distributions inside the cavity, the local heat transfer coefficients, and the average Nusselt numbers for the pressure and suction sides of the turbine blade tip are determined. This paper presents the results of the effects of Reynolds number, clearance gap and width ratios on the Nusslet number for the pressure and suction sides of squealer turbine blade tip. The results show a good agreement with the experimental data obtained by Metzger and Bunker. New correlations for the average Nusselt numbers for turbine blade tip pressure and suction sides are presented.
Slip due to Rarefaction, Surface Roughness, and Intermolecular Interactions for Gases and Liquids
Authors: Ligrani Phil
Published: March 2011

In the present study, slip phenomena are investigated in two different sets of experiments conducted in gases and one Newtonian liquid. Overall, differences in near-surface slip behavior are illustrated for these two different fluid mediums, where the slip is induced surface roughness and rarefaction in the gases, and by surface roughness and intermolecular interactions in the liquid. Within both sets of experiments, flows are induced within micro-fluidic passages by rotation within C-shaped fluid chambers formed between a rotating disk and a stationary surface. When gases are employed, accommodation coefficients are determined in a unique manner from experimental results and analysis based on the Navier-Stokes equations. In all cases, roughness size is large compared to molecular mean free path. When channel height is defined at the tops of the roughness elements, slip is believed to be a result of rarefaction as well as fluid shear. With this arrangement, tangential accommodation coefficients decrease and slip velocity magnitudes increase, at a particular value of Knudsen number, as the level of surface roughness increases. With Newtonian water as the working fluid, hydrophobic roughness is used to induce near-wall slip in the fluid chamber. The magnitudes of slip length and slip velocities increase as the average size of the surface roughness becomes larger. The resulting slip length data show a high degree of organization when normalized using the fluid chamber height, such that experimental data obtained using different chamber heights and different disk roughness magnitudes collapse along a single line, illustrating strong linear dependence of the slip length on the normalized radial-line-averaged shear stress.
Structural Effects of Cathode Catalyst Layer on the Performance of PEFC
Authors: Matsuda Hidetoshi , Fushinobu Kazuyoshi , Ohma Atsushi , Okazaki Ken
Published: March 2011

The structure of cathode catalyst layer (CCL) has strong relationship with the performance of polymer electrolyte fuel cells (PEFCs). We investigated the relationship between the catalyst layer structure and the cell performance experimentally. Multi-layered CCL is used to investigate the effect of the layer design on the cell performance. Membrane side of CCL works, as a reaction area, more actively than the gas diffusion layer (GDL) side at low relative humidity (RH) due to the lower proton conductivity. On the other hand, when the cathode gas has less oxygen partial pressure at high RH, GDL side is more active than membrane side owing to low diffusivity of oxygen. We suggest that the volumetric catalyst concentration of the CCL membrane side should be higher at low RH, however at high RH with lower oxygen partial pressure in cathode gas, the GDL side should have higher concentration. Simple theoretical model is employed to see the behavior of the reaction distribution in the catalyst layer.
Fuel Gas Production by Plasma in a Microwave Oven at Atmospheric Pressure
Authors: Nomura Shinfuku , Eka Putra Andi Erwin , Toyota Hiromichi , Mukasa Shinobu , Yamashita Hiroshi
Published: March 2011

The purpose of this research is to develop a process to use plasma decomposition of hydrocarbon liquids or clathrate hydrates in a microwave oven to produce fuel gas while simultaneously solidifying the carbon and synthesizing it into useful carbonized materials, such as CNTs or activated charcoal. Hydrogen gas with a purity of 60% to 80% can be extracted using a conventional microwave oven. This means that the energy efficiency of hydrogen production using this method is estimated to be approximately 50% of that by electrolysis of alkaline water and approximately 1% of that by the natural gas steam reforming method. However, this process has the added benefit of producing solid carbon at the same time. This method can be applied to a wide variety of waste liquids, or hydrate. Surplus electrical energy could be used to process waste liquids from homes and factories, and the resulting hydrogen energy could be stored and used.
Development of New Heat Exchanger Design
Authors: Couzinet Anthony , Pierrat Daniel , Gros Laurent , Kunc Thierry , Foata Antoine
Published: March 2011

This study deals with a new heat exchanger design developed by DGA Essais propulseurs [inventor: A. Foata - patents 1–4] which should improve heat transfer compared with standard gas to liquid exchangers tube banks, present better compactness without weighing down the drop loss and ensure the behaviour under drastic operating conditions (1600°C and Mach 2). This new design is based on a bundle composed of nine pairs of spiral tubes which are one to one angular drifted. The predictions of the thermal performances are so fussy whatsoever by using experimental test rig or CFD computations. On one hand, the test rig doesn’t allow to fulfil the entire range of operating conditions. And yet the standard operating conditions can shade the influence of temperature gradients on the aero thermal behaviour of the spiral bundles. On the other hand, in order to obtain reliable numerical predictions of heat transfers, the spiral bundle meshing refinement is crucial to simulate accurately the flow structure and the temperature gradients close to the inner walls. Moreover, if the grooves are explicitly taken into account in the numerical model, the computational grids required will be too fine to guarantee realistic computational time. So firstly, experimental and numerical approaches are used jointly in order to show the new heat exchanger design is sustainable. Optimization of the heat exchanger will be performed in the next phase of this study.
Measurement of Current-Density in PEFC With NMR Sensors
Authors: Ogawa Kuniyasu , Yokouchi Yasuo , Haishi Tomoyuki , Ito Kohei
Published: March 2011

In order to improve the power generation performance of polymer electrolyte fuel cells (PEFC), it is necessary to maintain high current density over the whole area of the membrane electrode assembly (MEA) that includes the additional Pt-carbon particles loaded on the Polymer Electrolyte Membrane (PEM) as an electrocatalyst layer. However, the current density generated at the MEA is distributed unevenly due to a lack of hydrogen, flooding, and so on. Therefore, achieving a higher current density in a PEFC requires monitoring the local current density. The local current density in a PEFC can be measured by the frequency shift of the NMR signal received from planar surface coils inserted into the PEFC as sensors. This method is based on the relationship that the spatial gradient of the frequency shifts in NMR signals along the MEA is proportional to the magnetic field strength induced by local current density. In this study, two kinds of MEA were used. One MEA was with the platinum catalyst applied to an area of 50 mm * 50 mm. The other MEA was with platinum catalyst over half this area. The distributions of the frequency shift of NMR signals in PEFCs using these two MEAs were measured. These measured distributions of the spatial gradients were in agreement with those obtained by the theoretical-analysis of magnetic fields in PEFC. The spatial distributions of current density generated in PEFCs were obtained from the spatial gradients and theoretical result.
Interfacial Thermal Resistance Between a Carbon Nanoparticle and Molten Salt Eutectic: Effect of Material Properties, Particle Shapes and Sizes
Authors: Jo Byeongnam , Banerjee Debjyoti
Published: March 2011

The aim of this study is to estimate the interfacial thermal resistance between a carbon nano-particle and alkali molten salt eutectics using molecular dynamics simulations. Additionally the effect of particle shapes and sizes on the interfacial thermal resistance was investigated using three different shapes of the carbon nanoparticles. Transient heat transfer simulation between a carbon particle and molecules of a molten salt was performed with the lumped capacitance method. A carbonate salt eutectic which consists of lithium carbonate (Li2CO3) and potassium carbonate (K2CO3) in 62:38 molar ratio was used as a solvent medium for the nanoparticles. Three carbon particles of a single walled carbon nanotube (SWNT), a fullerene (C60), and a graphite sheet were used to represent different shapes of cylinders, a spheres, and disks, respectively. The interfacial thermal resistance was determined by a correlation with a specific heat of the carbon particle, their surface area, and the time constant of decaying particle temperature. The results show the interfacial thermal resistance values are independent of the particle size for SWNT and graphite particles. For three carbon particles with a similar particle size, similar resistances were obtained in our simulations. The purpose of this study is to design and develop novel high-temperature Thermal Energy Storage (TES) materials in order to improve the operational efficiencies for harnessing solar thermal power at cheaper costs for Concentrated Solar Power (CSP) systems.
Experimental Investigation of Molten Salt Nanofluid for Solar Thermal Energy Application
Authors: Shin Donghyun , Banerjee Debjyoti
Published: March 2011

The overall efficiency of a Concentrated Solar Power (CSP) system is critically dependent on the thermo-physical properties of the Thermal Energy Storage (TES) components and the Heat Transfer Fluid (HTF). Higher operating temperatures in CSP result in enhanced thermal efficiency of the thermodynamic cycles that are used in harnessing solar energy (e.g., using Rankine cycle or Stirling cycle). Particlularly, high specific heat capacity (Cp) and high thermal conductivity (k) of the HTF and TES materials enable reduction in the size and overall cost of solar power systems. However, only a limited number of materials are compatible for the high operating temperature requirements (exceeding 400°C) envisioned for the next generation of CSP systems. Molten salts have a wide range of melting point (200°C∼500°C) and are thermally stable up to 700°C. However, thermal property values of the molten salts are typically quite low (Cp is typically less than ∼2J/g-K and k is typically less than ∼1 W/m-K). To obviate these issues the molten salts can be doped with nanoparticles — resulting in the synthesis / formation of nanomaterials (nanocomposites and nanofluids). Nanofluids are colloidal suspensions formed by doping with minute concentration of nanoparticles. Nanofluids were reported for anomalous enhancement in their thermal conductivity values. In this study, molten salt-based nanofluids were synthesized by liquid solution method. A differential scanning calorimeter (DSC) was used to measure the specific heat capacity values of the proposed nanofluids. The observed enhancement in specific heat is then compared with predictions from conventional thermodynamic models (e.g. thermal equilibrium model or “simple mixing rule”). Transmission Electron Microscopy (TEM) is used to verify that minimal aggregation of nanoparticles occurred before and after the thermocycling experiments. Thermocycling experiments were conducted for repeated measurements of the specific heat capacity by using multiple freeze-thaw cycles of the nanofluids/ nano-composites, respectively. This study demonstrates the feasibility for using novel nanomaterials as high temperature nanofluids for applications in enhancing the operational efficiencies as well as reducing the cost of electricity produced in solar thermal systems utilizing CSP in combination with TES.
Development of the Thermal Flash Method for Characterization of Carbon Nanofibers
Authors: Mahanta Nayandeep K. , Abramson Alexis R.
Published: March 2011

The transient thermal flash technique, originally developed for testing low thermal diffusivity micro/nanofibers, was implemented for measuring the thermal conductivity of vapor-grown carbon nanofibers. The present technique uses a microfabricated strip of gold, which acts both as a heater and a temperature sensor. The modifications were validated against commercially available carbon fibers (Pyrograf® – I from Applied Sciences, Inc. and Mitsubishi K13D2U) and the results obtained were seen to match values previously reported in the literature. The carbon nanofibers reported in this article were also obtained from Applied Sciences, Inc. and are known as PR-25, belonging to the Pyrograf® – III family of nanofibers. The thermal conductivities calculated based on the experimentally determined values of diffusivity along with the specific heat capacity and density of graphite were around 1100 W/m-K and 1700 W/m-K, respectively for the nanofibers heat treated to 1100 °C and 3000 °C.
Case Study for Energy-Efficiency of Liquid-Cooled Data Centers
Authors: Rivera-Lopez Ricardo , Kimber Mark
Published: March 2011

The thermal management of existing data centers is centered on forced convection using air as the transport fluid. A large portion of the energy required for typical data centers is used in maintaining reasonable operating temperatures, and many have looked to liquid cooling as a promising solution to increased energy efficiency. The current work is a case study of making this transition for a single computer board. The energy savings potential is quantified and the removal of heat via liquid cooling is characterized from the chip level to the environment. A thermal solution model is developed and validated through experimentation. The experiment consists of a rack-mounted computer board to simulate a server and cold plates attached at several key locations for cooling. Multiple measurements are made to determine the amount of heat removed and power consumed in the process. The results from this study show that liquid-cooling presents an improved thermal solution to data centers and the energy savings potential is large, which improves the power usage effectiveness since power is mostly used in data processing rather than server cooling.