bluuebunny/crossref_metadata_2025 huggingface dataset:
Scale Analysis of Convective Melting With Internal Heat Generation
Authors: Siahpush Ali , Crepeau John
Published: March 2011

Using a scale analysis approach, we model phase change (melting) for pure materials which generate internal heat for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. We show the time scales in which conduction and convection heat transfer dominate.
Heat-Pump-Based Thermal Storage for Intermittent Electrical and Thermal Sources
Authors: Phelan P. E. , Calhoun R. , Trimble S. , Baker J. E. , Sherbeck J.
Published: March 2011

Thermal storage is advocated as a means for energy storage in some grid-scale electric powerplants, such as for concentrating solar power (CSP). The efficiency of concentrating solar thermal collectors, however, decreases with increasing output temperature, making it difficult to achieve high thermal storage temperatures. A heat pump, as is well known, can operate in either cooling mode or heating mode, and in either case, the coefficient of performance is generally greater than one, enabling a multiplier effect that can serve to either increase or decrease the temperature of thermal storage. A simple steady-state analysis of the “round-trip” system efficiency for storing energy reveals the potential benefits of utilizing a heat pump or refrigerator in such systems. Provided that an inexpensive heat input source is available, the system storage efficiency can reach or even exceed unity, assuming that the energy supplied to the system as heat is neglected. For ice storage at 0 °C, increasing thermal input temperatures above 209 °C increases the system storage efficiency above unity.
Thermal Conductivity of Carbon Nanotubes With Defects
Authors: Chen Haibin , McGaughey Alan J. H.
Published: March 2011

The high thermal conductivities of carbon nanotubes (CNTs) measured experimentally and predicted from theory suggest that they are good candidates for next-generation thermal management materials. The quantities of CNTs needed in applications preclude the use of pristine products. Limited work, however, has been done to study thermal transport in CNTs with defects. In this paper, the thermal conductivities of pristine CNTs and CNTs with various defect types (adatoms, single vacancies, double vacancies, and Stone-Wales) are systematically predicted using molecular dynamics simulation and a direct application of the Fourier law. We investigate the correlation between the thermal conductivity and defect energy.
Experimental Study on Mixed Convection Heat Transfer for Thermally Developing Flow in Horizontal Ducts With Radiation Effects
Authors: Ganesan Rajamohan , Narayanaswamy Ramesh , Gorin Alexander , Perumal Kumar
Published: March 2011

This research focuses on the interaction of surface radiation with laminar mixed convection heat transfer for thermally developing airflow in horizontal ducts. The duct cross section is made of two differentially heated isothermal vertical walls and two adiabatic horizontal walls. In a series of experiments, the Reynolds number is varied from about Re = 800 to Re = 1200, for two aspect ratios of the duct cross section. The hot wall temperature ranges from 27°C to 100 °C, and the emissivity of internal walls are 0.05 and 0.85. The total heat transfer from the hot wall to the cold wall of the duct depends on the mixed convection and also on the surface radiation heat transfer that takes place within the duct. Therefore, it is important that the effects of surface radiation must be properly understood, and accounted for, in the design and analysis of flow and heat transfer through ducts. The results presented here show the effect of surface radiation and mixed convection on the total heat transfer rate within the duct. The flow field within the duct is also made visible by a suitable smoke flow visualization method.
An Experimental Study on Thermal Hydraulic Characteristics of a Steam Condensation Flow in a Circular Microtube
Authors: Iiyama Koji , Kaneko Akiko , Abe Yutaka , Asaba Shingo , Suzuki Yutaka
Published: March 2011

At present, a microchannel heat exchanger is requested to achieve high efficiency in small size energy equipments. In order to clarify the heat transfer mechanism in a microchannel heat exchanger, knowledge on the thermal hydraulic characteristics of condensation flow in channels is essential. However, study on the thermal hydraulic characteristics of steam condensation flow in a microchannel is hardly conducted except visualization of flow patterns. Especially, the number of study on the correlation between the flow structure and heat transfer is small. The objective of the present study is to clarify the thermal hydraulic characteristics of condensation flow in a microtube through the observation of condensation behavior. In order to achieve the purpose, overall heat transfer coefficient in annular flow and slug flow is measured. As the results, the observed flow pattern is transformed from annular flow to bubbly flow through injection flow in a single capillary glassy tube as simulated unit microchannel. And the local overall heat transfer coefficient at slug flow is 10 to 25% lower than that at the annular flow under the particular condition in the present study.
Numerical Simulation of the Cooling Process of Cubic Shells
Authors: Okura Manabu , Ono Kiyoaki
Published: March 2011

In order to keep the environment in an air-conditioned room comfortable, it is important to anticipate the air velocity and temperature fields precisely. The numerical code, solving simultaneously the Navier-Stokes equations governing flow field inside and outside the room and the heat conduction equation applying to walls, are developed. The assumption that the heat transfer coefficient between the fluid and the surface of solids is not used. This code is applied to investigate the cooling process of a cubic shell. The computational results agree with the experimental results. We also investigated the same process of the cubic shells whose walls are internally or externally insulated. The difference of the amount of heat transfer will be discussed.
Entropy Generation Rate in Forced Convection Flow About Inclined Surfaces in a Porous Medium
Authors: Tashtoush Bourhan , Yilbas B. S.
Published: March 2011

Entropy generation rate has been the attraction of research, since it provides information on the thermodynamic irreversibility associated with the thermal systems. The exergy distraction in the thermal system increases entropy generation rate while lowering the second law efficiency of the thermal system. The heat transferring devices, such as heat exchangers, operates better when temperature difference between the transferring device and the heat sink is maintained high. In addition, the use of porous material in these devices enhances the heat transfer rates due to the achievement of high heat transfer coefficients. However, the presence of the porous material also increases the pump power because of the high pressure drop in the flow system. This increases the operational costs. Consequently, entropy generation rate due to pressure drop needs to be minimized to reduce the cost; however, heat transfer rates from the thermal system needs to be enhanced to improve the thermal performance of the heat transferring device. Therefore, a balance between the entropy generation rates due to pressure drop and heat transfer needs to be attained to achieve optimum operating conditions of such devices. To investigate the optimum operating conditions, the forced convection problem about inclined surfaces (or wedges) in saturated porous medium is considered. The flow in the porous medium is described by the Darcy-Brinkman momentum equation. An exact analytical solution of the governing equations using Kummer function is developed for the velocity, temperature, Nusselt Number, and entropy generation rate for the case where the free stream velocity and wall temperature distribution of the inclined surface vary according to the same power function of distance x, along the plate. It is demonstrated that the entropy generation number is weakly dependent on the Brinkman-Darcy number for forced convection flow, which is particularly true near the wall region.
Uncertainties on HTC Measurement of Water Spray Quenching of Aluminum Alloys
Authors: Bikass S. , Andersson B. , Pilipenko A.
Published: March 2011

Water spray cooling of profiles right after extrusion is critical for control over the mechanical properties of high strength alloys. To design the optimum distribution of spray, computer simulation is a powerful tool. For that purpose a quantification of the heat-transfer boundary conditions is challenging, especially as the heat transfer coefficient (HTC) changes with the surface temperature. It is possible to record temperature history during the quenching in laboratory/plant experiments and then HTC values can be calculated by means of inverse modeling. These values are applicable only if they are accurate enough. In this paper, it is assumed the maximum allowed tolerance for calculated HTC to be 5%. This work is based on the computer simulation of the real experiments with thermocouples installed inside the sample to estimate the heat flux at the surface of the sample as well as the sample surface temperature using heat transfer equations. Error sources are typically: inaccurate thermocouple positioning and contact quality, sample geometry, thermocouple accuracy and repeatability, thermal properties, initial temperature and etc. In this study, some of these errors and uncertainty sources are selected and their impact on calculated HTC values is investigated. Finally, maximum allowance for every parameter to achieve calculated HTC within ±5% is calculated. Since HTC is not constant but a curve vs. temperature, the calculated HTC values must be between two parallel curves which represent +5% and −5% of nominal HTC.
Rewetting Processes During Top/Bottom Re-Flooding of Heated Vertical Surfaces
Authors: Ilyas Muhammad , Ahmad Masroor , Hale Colin P. , Walker Simon P. , Hewitt Geoff F.
Published: March 2011

Rewetting of the heated fuel rods is one of the most important phenomena to be considered in analysis of the design basis loss of coolant accident (LOCA) in light water reactors. The rewetting phenomenon is a complex and violent one with the rewetting front moving rather slowly over the heated surface. For water temperature close to saturation, the rate of progression of rewetting front is independent of flow rate of the water approaching the rewetting front; this is an indication of the fact that the rewetting process is governed by events local to the rewetting front [1]. This paper describes an experimental study on the rewetting of heated vertical surfaces during top/bottom reflooding. Through an infrared transparent substrate fixed in the surface, processes occurring locally at the quench front have been studied by using a fast response thermal imaging system (Cedip Titanium 560M). The existence of a cyclic bursting phenomenon at the quench front has been observed. Multiple events of this type gradually remove heat from the metal, allowing the rewetting front to progress slowly over the surface. These intermittent contacts occur over a short axial length. Temperature measurements indicate that the metal surface temperature at the rewetting front is close to the homogeneous nucleation temperature.
Literature Review of Single Phase Internal Flow Heat Transfer Correlations in Straight Circular Conduits
Authors: Abdelmessih Amanie N. , McGuire Erik C.
Published: March 2011

An enormous number of empirical and analytical closed solution, single phase, internal flow heat transfer correlations exist in the open literature. This article is a compilation of single phase internal convective heat transfer correlations in straight, circular conduits. These correlations cover convective internal flow of various Newtonian fluids under a wide range of heating conditions, and orientations for the different flow regimes. In the past some engineers extended the use of some correlations beyond their limits. The purpose of this article is to compile internal flow heat transfer correlations in one source, to alleviate time required by the practicing engineer to research the literature for correlations to meet specific conditions.
Possible Growth Region of Single-Walled Carbon Nanotube in CVD Synthesis on the CHO Components Diagram
Authors: Inoue Shuhei , Tomie Takashi , Matsumura Yukihiko , Kohno Masamichi
Published: March 2011

Considering the massive-scale synthesis of single-walled carbon nanotube (SWCNT), chemical vapor deposition has become a standard process for synthesizing CNTs. In most of these processes, oxygen and hydrogen atoms were included originally or added later; and they are expected to have important roles such that they helped in the removal of amorphous carbon and prevented SWCNTs from containing metal particles. However, whole perspectives for suitable carbon source or ideal balance among carbon, hydrogen, and oxygen have not been reported. We examined a variety of raw materials in our newly developed round-trip-type vacuum furnace in order to determine whether they could be used to synthesize a carbon nanotube. We used Raman spectroscopy for evaluation, and plotted the component ratios of effective and ineffective materials on a C-H-O ternary diagram. Consequently, it is clear that the growth region should satisfy the equation O < C < (H + O) in molar ratio.
A Molecular Dynamics Study for Diffusivity of Proton in Polymer Electrolyte Membrane
Authors: Tokumasu Takashi , Yoshida Taiki
Published: March 2011

This paper describes characteristics of proton transfer in polymer electrolyte membrane by Molecular Dynamics (MD) simulation. Nafion was used as a membrane. Grotthus mechanism as well as Vehicle mechanism was considered in the simulation. To treat Grotthus mechanism, Empirical Valence Bond (EVB) method was used. The parameters or functions of the interaction potential of EVB method were determined so that potential energy barrier of proton hopping obtained by EVB method is consistent with that obtained by Density Functional Theory (DFT) and adjusted so that the diffusion coefficient of hydronium ion in water obtained by MD simulation is consistent with that of experimental results. After annealing the system, radial distribution function or mean square displacements of hydronium ion and water, which correspond to diffusivity of each compound, was obtained. These results show the nanoscale characteristics of proton transfer in polymer electrolyte membrane.
Detecting Burner Instabilities Using Joint-Time Frequency Methods Whilst Co-Firing Coal and Biomass
Authors: Valliappan P. , Thai S. M. , Wilcox S. J. , Ward J. , Tan C. K. , Jagiełło K.
Published: March 2011

Conventional coal-fired burners are designed to operate within specific limits that, in part, result from the need to efficiently burn the fuel. These designs have been developed to ensure stable combustion, lower NOx emissions and increase the combustion efficiency through techniques such as air staging and adding swirl to the combustion air. Recent requirements to reduce CO2 emissions from coal-fired boiler plant has focussed on the co-firing of biomass, primarily wood, either by delivering the pulverised biomass with the coal or through separate burners. To date this approach has typically taken place at substitution levels of around 5% by mass and at these levels the operation of the burners and boiler is not adversely affected. However, as the proportion of biomass increases the fuel characteristics of the blend moves further away from the burner design parameters. This can lead to combustion instabilities and in extreme cases extinction of the flame. In order to co-fire higher concentrations of biomass a system or technique is required that can detect the onset of these instabilities and warn before the combustion conditions become dangerous. In this paper a novel technique based around the Wigner-Ville transform is presented that shows promise at being able to temporarily resolve the conditions that could result in the onset of burner instabilities for three cases; the first will present results from the combustion of 100% bituminous coal, whilst the second and third cases will present the results from experiments where the proportion of biomass was set at 10% and 20% by mass with the same bituminous coal. In each experiment the secondary combustion air was first reduced from a nominal stable condition and then subsequently increased from the same stable condition. It was found that the Wigner transform was able to resolve flicker frequency changes as the airflow rate was reduced. These changes were subsequently used in a neural network to automatically detect drastic changes in the air flow rates to the burner and could provide a means by which utility operators could detect dangerous flame instability conditions in real-time.
Dew and Frost Points Measurements and Estimation for Residual Gases in High-Pressure Hydrogen
Authors: Ishida Kenji , Kosaka Masataka , Monde Masanori
Published: March 2011

Dew and frost points estimation procedure for phase equilibrium systems with solid phases as condensed phases was developed utilizing Peng-Robinson equation of state (PR-EOS), and estimation for H2+H2O systems corresponding to actual hydrogen stations of pressure up to 70 MPa were carried out. It became clear that condensable components in hydrogen stations were only water. Dew and frost point measurement system with high pressure test cell utilizing cooled mirror technique with original visible image analysis were designed and produced. The system was designed available for frost point temperature below −50 °C, and total pressure of system up to 10 MPa. Pressure tight test cell with view port was installed separately from the optics, and had extensible design available for higher pressure experiments up to 40 MPa. Dew and frost points measurements with several compositions of H2+H2O standard gases were carried out. Frost point data obtained by the present measurement system with pressure tight test cell had good repeatability, and obtained binary interaction parameter kij was small value with the negative sign for about 5 MPa, 50 ppm conditions.
Numerical Study of Performance Enhancement of Single-Shot Heat Sinks Using Nano-Enhanced Phase Change Material
Authors: Cotoros Ingrid , Hashemi Ab
Published: March 2011

A numerical model and analysis has been performed on a state-of-the art one shot heat sink including an eicosane/carbon nanotubes (CNT) system of nano-enhanced phase change material (NEPCM). The nanoparticles, dispersed in the PCM liquid phase, can form a network of interconnected ballistic heat transport lines, thus increasing the thermal conductivity of the PCM by almost two orders of magnitude. The results show that the heat sink can be operated with a 40% higher heat load, for an extra 42% time, or with a 29% lighter mass, before the electronics reach the allowable maximum temperature.
Numerical Optimization of a Waste-to-Energy Plant Under Different Operating Conditions
Authors: Samec Niko , Kapitler Miran , Kokalj Filip
Published: March 2011

The combustion process for using municipal solid waste (MSW) as a fuel within a waste-to-energy plant calls for a detailed understanding of the following phenomena. Firstly, this process depends on many input parameters such as MSW proximate and ultimate analysis, the season of the year, primary and secondary air-inlet velocity and, secondly, on output parameters such as the temperatures or mass-flow rates (MFR) of the combustible products. The variability and mutual dependence of these parameters can be difficult to manage in practice. Moreover, another problem is how these parameters can be tuned to achieving optimal combustion with minimal pollutant emissions during the initial plant-design phase. In order to meet these goals, a waste-to-energy plant with bed-combustion was investigated by using a computational fluid dynamics (CFD) approach with ANSYS CFX 12.0 code within a WORKBENCH 2 environment. In this paper, the adequate variable input boundary conditions based on the real measurement and practical calculations of known MSW composition compared with other authors are used and the whole computational work is updated using real plant geometry and the appropriate turbulence, combustion and heat transfer models. Furthermore, the operating parameters were optimized on output parameters through a trade-off study. The different operating conditions were varied and the fluid flow direction, residence time, temperature field, velocity-field, nitric oxide formation and combustion products through the plant’s combustion chamber and preheat intersection in 3D were predicted and visualized. Optimization in real-time has showed the amounts for each input parameter when meeting the optimal operating conditions. Finally, the response charts between the input and output parameters are presented in order to monitor the dependence among these parameters. Further simulations have to be done to include the geometry dimensions as input parameters when applying the CDF simulation and numerical optimization within the project phase.
Synthesis of Single-Walled Carbon Nanotube Containing Platinum Group Element
Authors: Ito Kyohei , Inoue Shuhei , Matsumura Yukihiko
Published: March 2011

To prepare homogeneous nanoparticles is a key issue for catalytic reaction because it directly connects to the control of the reaction. Using the sidewall of SWCNT as a catalyst supporter, the size of nanoparticle can be controlled, because the particle size should be affected by the interaction between SWCNT and metal species and its curvature. In this study, we focused on the direct synthesis of SWCNT with highly dispersed platinum group metal species. As a result, adding an adequate amount of platinum group metals into catalysts never disturbs the synthesis of SWCNT. Referring to TGA measurement, the presence of metal attached and/or metal involved SWCNT is suggested. Furthermore, SEM images show many nanoparticles are on SWCNT. When ruthenium catalyst is used, ruthenium nanoparticles are observed on the surface of nano carbon materials, which looks like SWCNT. These results indicate the possibility of direct synthesis of metal-containing SWCNT in CVD technique.
Comparison of Various Alternative Refrigerants for Vapour Compression Refrigeration Systems
Authors: Dalkilic¸ Ahmet Selim , Wongwises Somchai
Published: March 2011

Single-stage vapour compression refrigeration system was compared with an actual vapour compression cycle, single-stage process with internal heat exchanger, and a two-stage process with economiser using the refrigerants of HCFC-22, CFC-502 and their alternatives such as HFC-134a, HFC-32, HFC-152a, HFC-404A, HFC-407C, HFC-507, HFC-410A. A theoretical performance study on a cascade refrigeration system was performed using two refrigeration cycles connected through the heat exchanger in the middle working as the evaporator for the high pressurized cycle and condenser for the low pressurized cycle. Other performance study was performed using a two-stage cascade refrigeration system having low and high pressure compressors connected through the mixing chamber in the middle. The condensation temperatures were between 30 and 50 °C, evaporation temperatures were between −50 °C and 5 °C and heat exchanger and economiser temperatures were kept as constant for the comparisons. Some of the alternative refrigerants’ coefficients of performance values are found to be higher than their base traditional pure refrigerants. The effects of the main parameters of performance analysis such as refrigerant type, degree of subcooling, and superheating on the performance coefficient, refrigerant charge rate and volumetric refrigeration capacity are investigated for various operating conditions as case studies.
Surface Vortex Simulation at Selected Water Temperatures
Authors: Sˇkerlavaj Aljazˇ , Lipej Andrej , Ravnik Jure , Sˇkerget Leopold
Published: March 2011

Circulating water systems (CW) and safety water systems (SW) in various power plants use vertical pumps to pump water from pump intakes. A properly designed pump intake structure prevents the occurrence of strong surface vortices, which might inhibit the proper functioning of the pump. Although several standards for experimental testing of pump intake structure suitability exist, our goal is to find a way to predict such vortices numerically, from a single-phase simulation. In such a process, we had already eliminated some of the turbulence models. In the current paper we confirm that Scale Adaptive Simulation (SAS) turbulence model with the curvature correction (CC) factor applied is well suited for such flows. By using a methodology for determining the vortex air core length, the SAS-CC turbulence model results were compared to the experimental data for two selected temperatures. The results show better agreement than the laminar simulations in terms of higher mean value accuracy and lower scattering.
Thermal Conductivity of Cage-Like Structures
Authors: Zebarjadi Mona , Esfarjani Keivan , Chen Gang
Published: March 2011

A two dimensional toy model is developed to study thermal transport in cage like structures such a skutterudites and clathrates. The model consists of host atoms on a rectangular lattice with fillers in the center of each rectangle. The thermal conductivity is calculated by using Green-Kubo equilibrium molecular dynamics simulations. It is generally believed that the smaller and the heavier the filler, the lower is the thermal conductivity. We show that the thermal conductivity decreases with atomic displacement parameter while it has local minima versus filler mass. Our study shows that it is very important to include the correct band dispersion to get the right features of the thermal conductivity. We show that by having a double well potential one can further reduce the thermal conductivity.
Interaction of Liquid and Heating Surface During Nucleate Boiling Processes
Authors: Ekmanis Juris , Turlajs Daniels , Zeltin¸sˇ Namejs , Jaundalders Sigurds
Published: March 2011

The article deals with the problems of behavior of heating surfaces during liquids’ boiling processes on “boiling liquid–solid” interfaces. Such processes are taking place in a lot of thermal engineering systems, example, heat tubes, steam generators, evaporators, cryogenic systems etc. Problems of solid heating surfaces and boiling liquids’ interaction are an object for investigators for a long time, but still there are no full explanation both theoretically and experimentally found. The subject of article are the changes of mechanical properties and material structure of solid heating surface, caused by temperature fluctuations in superheated liquid and heating surface near single vapor bubble nucleate centers and beneath them. Experimental studies of heating surface properties in the vicinity of bubble boiling centers were carried out by using of different techniques, including metallography, surface microhardness analysis, optical methods.
Phosphor Thermometry by Using Rare Earth-Doped Yttrium Oxides
Authors: Ishiwada Naohiro , Fujioka Satoko , Ueda Toshihisa , Yokomori Takeshi
Published: March 2011

The development of new temperature-dependent phosphor particles for high temperature thermometry is presented. To develop the phosphor material that can be applied to the intensity ratio method, the photoluminescence of Y2O3:Tb,Tm was investigated with elevating temperature. As a result, Y2O3:Tb,Tm phosphor shows that the peak intensity at 456 nm is still strong at more than 1100 K whereas the other peak intensities decrease due to the thermal quenching. Thus, the intensity ratio I543nm/I456nm has a high gradient and linearity over a wide temperature range, and it is confirmed that Y2O3:Tb,Tm provides a higher resolution in temperature measurement than Y2O3:Tb and Y2O3:Tm. Also, since this phosphor changes its emission color from green to blue with increasing temperature, it is possible to measure temperature not only by analyzing the intensity ratio but also by observing the color change in appearance. It can, therefore, be expected to use as a Visual Thermo-Sensor (VTS).
Effect of a Pair of Horizontal Frame Members on the Convective Heat Transfer With Laminar and Turbulent Flow From a Recessed Window to a Room
Authors: Oosthuizen Patrick H. , Naylor David
Published: March 2011

The horizontal frame members that often protrude from the inner surface of a window can significantly effect the convective heat transfer rate from this inner surface to the room. The purpose of the present numerical study was to determine how the size of a pair of horizontal frame members effect this heat transfer rate. The flow has been assumed to be steady and conditions under which laminar, transitional, and turbulent flows occur are considered. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being dealt with using the Boussinesq approach. The governing equations have been solved using the FLUENT commercial CFD code. The k-epsilon turbulence model with standard wall functions and with buoyancy force effects fully accounted for has been used. The solution has the following parameters: the Rayleigh number, the Prandtl number, the dimensionless window recess depth, and the dimensionless width and depth of the frame members. Results have been obtained for a Prandtl number of 0.74.
Molecular Dynamics Simulation of Homogeneous and Heterogeneous Thermal Bubble Nucleation
Authors: Chen Min , Chen Yunfei , Yang Juekuan , Gao Yandong , Li Deyu
Published: March 2011

Thermal bubble nucleation was studied using molecular dynamics for both homogeneous and heterogeneous systems using isothermal-isobaric (NPT) and isothermal-isostress (NPzzT) ensembles. Simulation results indicate that homogeneous thermal bubble nucleation is induced from cavities occurring spontaneously in the liquid when the temperature exceeds the superheat limit. In contrast to published results using NVE and NVT ensembles, no stable nanoscale bubble exists in NPT ensembles, but instead, the whole system changes into vapor phase. For a heterogeneous system composed of a nanochannel with an initial distance of 3.49 nm between the two solid plates, it is found that if the liquid-solid interaction is equal to or stronger than that between liquid argon atoms, the bubble nucleation temperature of the confined liquid argon can be higher than the corresponding homogeneous nucleation temperature, because of the more ordered arrangement of atoms within two solid walls nanometers apart. This observation is in contradiction to the common understanding that homogeneous bubble nucleation temperature sets an upper limit for thermal phase change under a given pressure. Compared to the system where the liquid-solid interaction is the same as that between liquid argon atoms, the system with reduced liquid-solid interaction possesses a significantly reduced bubble nucleation temperature, while the system with enhanced liquid-solid interaction only has a marginally increased bubble nucleation temperature.
Measuring Ignition Temperature and Heat Generation From Ethanol-Oxygen-Water Mixtures on Platinum
Authors: Gibson Joshua , Steciak Judi , Budwig Ralph , Beyerlein Steve
Published: March 2011

The surface temperature and heat generation from reactions of ethanol-water-oxygen-nitrogen mixtures on a platinum wire catalyst were determined using microcalorimetry. A 127-micron diameter, 99.95% Pt coiled wire was placed crosswise in the quartz tube of a plug flow reactor. A sourcemeter with a four-lead technique allowed the platinum wire to measure its average temperature by serving as a resistance thermometer, and enabled us to determine the amount of heat generated from surface reactions. Ignition temperatures varied from 380 to 570 K and heat generation from 0.8 to 11.8 W/cm2 depending on the absolute amounts of ethanol and oxygen, the ethanol:water ratio, and the fuel-oxygen equivalence ratio. The addition of water to ethanol showed little to no effect on either the ignition temperature or the heat generation on the platinum wire at the maximum 60:40 ethanol:water mole ratio (∼83:17 ethanol:water liquid volume ratio) reported here. These experimental results aid in understanding the heat transfer processes of catalytic igniters used to ignite fuel-lean mixtures.
Utilization of Advanced Working Fluids in Heat Pipes
Authors: Reilly Sean W. , Catton Ivan
Published: March 2011

A novel fluid for use as a working fluid in a heat pipe has been tested at UCLA. The fluid was discovered originally in use with a device consisting of a metal tube charged with the patented inorganic aqueous solution (IAS) which is evaporated when the tube is evacuated before use. According to the patent, this evaporation leaves a thin film which allows the tube to carry high heat flux loads with low temperature drop across the tube in a solid state mode. However, various experiments with these tubes have produced inconsistent results and there is some question as to whether the fluid is completely evaporated. The research on which this work is based, is focused on testing whether the charging fluid will operate as the working fluid in a heat pipe, in order to determine the nature of the IAS fluid. We charged a heat pipe apparatus with a biporous wick in order to investigate if the fluid plays a role in heat transfer. We have extensive data for this experiment using water as the working fluid which will use to compare the two sets of results. Testing has shown positive results in the reduction of the superheat required to drive heat fluxes through a wick compared to water. Some experiments have shown that the operating (temperature) range of the IAS is much larger than a standard heat pipe. It is theorized that the increase in performance of the IAS is due to an increased heat of vaporization. If this fluid is proven to be effective, it would lead to more effective and tunable heat transfer devices.
Improvement of PEFC Cell Performance With a Movable Rib
Authors: Tanigawa Hirofumi , Hakataya Ryoji , Tsuruta Takaharu
Published: March 2011

Water management is very important in the development of the PEFC with high performance and high reliability. We found that the moisturizing effect in the straight-type channel is marked as compared with the serpentine-type channels, and the draining performance in the serpentine-type channel is better than that in the straight-type channel. Then, we have developed the movable rib which can change the position of the rib manually. Experimental study shows that the movable rib can control water behavior and can improve the cell performance.
A Simple Analytical Model for Specific Heat of Nanofluid With Tube Shaped and Disc Shaped Nanoparticles
Authors: Jung Seunghwan , Banerjee Debjyoti
Published: March 2011

In this study we present a simple analytical model for estimating the specific heat capacity of nanofluids containing tube shaped and disc shaped nanoparticles dispersed in a solvent. The model includes the effect of an ordered liquid layer formed at the solid-liquid interface between a tube or disc shaped nanoparticle and the liquid phase. The size and thermo-physical properties of the ordered liquid layer are calculated based on the results of molecular dynamic (MD) simulation. The model is applied to nanofluid dispersed carbon nanotube (CNT) nanoparticles with tube shape in a liquid phase of alkaline metal carbonate salt eutectic mixture (Li2CO3:K2CO3 in 62:38 molar ratio). In addition, the specific heat of nanofluid with graphite nanoparticles with disc shape is calculated using the simple analytical model. The alkaline salt mixture as well as the corresponding nanofluid has potential applications as thermal energy storage (TES) material for solar thermal energy conversion. Hence, the specific heat is an important thermo-physical property for determining the thermal efficiency of the solar thermal energy system. To identify the effect of particle size and mass concentration, the specific heat capacity is plotted as a function of particle size and mass concentration. The experimental data is used to validate the simple analytical model for the effect of particle shape on the specific heat. The results show that the specific heat of nanofluid increases with the mass concentration of nanoparticles. Furthermore, nanoparticles with diameters less than 6 nm can cause anomalous enhancement in the specific heat of nanofluid. The results also show that tube shaped nanoparticles are more effective in enhancing the specific heat capacity of nanofluids than disc shaped nanoparticles due to the higher specific surface area of tube shaped nanoparticles compared to disc shaped nanoparticles of similar mass.
Simulation of Solar Thermo-Chemical Hydrogen Production Techniques
Authors: Almodaris Mohamed , Khorasani Sara , Abraham Jocin James , Ozalp Nesrin
Published: March 2011

This paper presents material and energy process-step models of hydrogen production via concentrated solar energy using Aspen Plus®. The paper provides a thorough comparison of solar cracking and solar reforming of methane processes against conventional steam methane reforming. The material and energy balances show that solar cracking is the most environmentally friendly hydrogen production technique. Some of the primary advantages of solar cracking include (1) elimination of CO2 emission, (2) elimination of costs associated with CO2 sequestration, transportation, and storage, and (3) generation of two commercially viable products, namely carbon black and hydrogen which can be used both as a fuel and a commodity. Considering the hydrogen shortage for different hydrogenation and fuel upgrading processes that the petrochemical industry is facing today, hydrogen production from solar cracking may offer an alternative solution. Therefore, it is important to find less energy intensive and more environmentally friendly hydrogen production techniques to meet the demand of industry. The results show that solar cracking is a more environmentally friendly and commercially competitive process compared to solar reforming and steam reforming considering that it produces virtually no carbon dioxide, but produces the commercially viable carbon black as a by-product.
Steam-Methane Reforming in a Microchannel Under Constant and Variable Axial Surface Temperature Profiles
Authors: Eilers Benn , Narayanan Vinod , Apte Sourabh , Schmitt John
Published: March 2011

An experimental study of steam methane reforming in a microchannel is presented. Palladium nanoparticles, deposited on a porous aluminized FeCrAlY felt, served as catalyst sites for the reforming reactions. Parametric studies of steam-methane ratio, residence time, average reactor temperature, and temperature distribution were performed. Results demonstrated in excess 60 percent conversion of methane at an average reactor temperature of 900°C and the lowest experimented residence time of 26 milliseconds. Methane conversion was found to be strongly dependent on reactor temperature. Ramping temperature distributions demonstrated a 46 percent greater hydrogen output than isothermal reactions performed at the same average temperature.
Visualization of Dispersed Phase Flow in Centrifugal Extractor Using Taylor-Couette Vortex Flow
Authors: Takahashi Hideharu , Kikura Hiroshige , Takeshita Kenji , Aritomi Masanori
Published: March 2011

For studying the designs and running operations of an extractor which uses Taylor-Couette vortex flow, we focused on a metal extraction system as one of the extraction models of heat generating nuclides and observed the flow patterns of dispersed phase by dyeing the phase in the extractor, and we investigated the effects of hydrophobic coating applied to the inner cylinder surface on the flow characteristics. Moreover, for the quantitative measurement and analysis of the flow field, we evaluated the applicability of Ultrasonic Velocity Profiler (UVP) to flow field measurement. Thorough these visualization methods of dispersed phase in a centrifugal extractor using Taylor-Couette vortex flow, we examined the relation between flow field and extraction characteristics of the extractor.
Numerical and Experimental Investigation of Temperature and Flow Field in a Full Vehicle
Authors: Mukutmoni Devadatta , Han Jaehoon , Alajbegovic Ales
Published: March 2011

Accurate CFD simulation of full vehicle enables vehicle development specialists to gain access to detailed flow and temperature field for the entire vehicle and for arbitrary driving conditions. Information of that nature is invaluable to vehicle development and design since it leads to detailed understanding of the problem areas and pointers to how the design and performance can be improved from the thermal management perspective. Presented are simulations of the vehicle Renault Scenic II cruising at 60 kilometers per hour using PowerFLOW. The simulations were performed using a coupling between the flow solver PowerFLOW and the thermal simulation package PowerTHERM that accounts for conduction and radiation effects. The simulation results were compared with the test data for steady state forced convection case. In order to gauge the accuracy of the simulations, extensive validations were made with thermocouple data and flow measurements. Good agreement was observed between the simulation results and the measurements.
The Effect of Fine Particles Added Into Water on the Motion of Bubbles Rising Helically
Authors: Minagawa Hisato , Yasuda Takahiro
Published: March 2011

LDV, PIV and some methods using ultrasonic sound have been often employed to measure multiphase flows. Fine particles are usually added into flows as tracer or scattering particles. The effect of particles added in on the flow characteristics is, however, not examined in detail. Because multiphase flows, especially gas-liquid systems, have gas-liquid interfaces, where impurities are known to aggregate, fine particles may aggregate in gas-liquid interfaces, and may affect the flow situation. Therefore, we measured the movement of helically rising bubbles to investigate the effect of fine particles mixed into the liquid phase. Polyethylene particles of 160μm and 10.6μm medium diameters are used. The reductions of helical sizes and rising and moving velocities are recognized by adding particles. The effect of particle size is also discussed.
Carbon Nanotube Synthesis From Metal Nanoparticles Size-Classified by a Differential Mobility Analyzer
Authors: Tomoda Masahiro , Kawahara Teppei , Tasaki Yohei , Takata Yasuyuki , Hirasawa Makoto , Seto Takafumi , Kohno Masamichi
Published: March 2011

In this study, carbon nanotubes (CNTs) were synthesized from size-classified catalyst metal nanoparticles to observe the effect of the nanoparticles’ diameter on the diameter of the CNTs. The nanoparticles were generated by laser vaporization and classified by a differential mobility analyzer, which sorts them by diameter according to differences in electrical mobility. After classification, the nanoparticles were collected on a Si substrate. CNTs were synthesized from these catalyst metal nanoparticles by using chemical vapor deposition. This experiment synthesized mainly multi-walled carbon nanotubes (MWNTs), with a small amount of single-walled carbon nanotubes (SWNTs). The reason is thought to be that the particles’ diameters were appropriate for MWNT synthesis.
Study on Air Ingress Mitigation Methods in the Very High Temperature Gas Cooled Reactor (VHTR)
Authors: Oh Chang H. , Kim Eung S.
Published: March 2011

An air-ingress accident followed by a pipe break is considered as a critical event for a very high temperature gas-cooled reactor (VHTR) safety. Following helium depressurization, it is anticipated that unless countermeasures are taken, air will enter the core through the break leading to oxidation of the in-core graphite structure. Thus, without mitigation features, this accident might lead to severe exothermic chemical reactions of graphite and oxygen depending on the accident scenario and the design. Under extreme circumstances, a loss of core structural integrity may occur along with excessive release of radiological inventory. Idaho National Laboratory under the auspices of the U.S. Department of Energy is performing research and development (R&D) that focuses on key phenomena important during challenging scenarios that may occur in the VHTR. Phenomena Identification and Ranking Table (PIRT) studies to date have identified the air ingress event, following on the heels of a VHTR depressurization, as very important (Oh et al. 2006, Schultz et al. 2006). Consequently, the development of advanced air ingress-related models and verification and validation (V&V) requirements are part of the experimental validation plan. This paper discusses about various air-ingress mitigation concepts applicable for the VHTRs. The study begins with identifying important factors (or phenomena) associated with the air-ingress accident using a root-cause analysis. By preventing main causes of the important events identified in the root-cause diagram, the basic air-ingress mitigation ideas can be conceptually derived. The main concepts include (1) preventing structural degradation of graphite supporters; (2) preventing local stress concentration in the supporter; (3) preventing graphite oxidation; (4) preventing air ingress; (5) preventing density gradient driven flow; (6) preventing fluid density gradient; (7) preventing fluid temperature gradient; (7) preventing high temperature. Based on the basic concepts listed above, various air-ingress mitigation methods are proposed in this study. Among them, the following one mitigation idea was extensively investigated using computational fluid dynamic codes (CFD) in terms of helium injection in the lower plenum. The main idea of the helium injection method is to replace air in the core and the lower plenum upper part by buoyancy force. This method reduces graphite oxidation damage in the severe locations of the reactor inside. To validate this method, CFD simulations are addressed here. A simple 2-D CFD model was developed based on the GT-MHR 600MWt as a reference design. The simulation results showed that the helium replaces the air flow into the core and significantly reduces the air concentration in the core and bottom reflector potentially protecting oxidation damage. According to the simulation results, even small helium flow was sufficient to remove air in the core, mitigating the air-ingress successfully.