bluuebunny/crossref_metadata_2025 huggingface dataset:
Effectiveness of 3-Row Crossflow Heat Exchangers With Alternating Circuitry
Authors: Chen Tony D.
Published: December 2008

Air-cooled heat exchangers with three tube rows are commonly seen in domestic air-conditioning systems. The analytical solutions of heat exchanger effectiveness for three-row plate fin-and-tube heat exchangers with alternating circuitries have been derived and expressed explicitly in terms of heat capacity ratio and number of transfer units in the recent study. These set of exact solutions serve as a basic tool in designing heat exchanger circuitry to its most accurate possible effectiveness. Comparison of plate-fin-tube heat exchanger effectiveness between airside unmixed and mixed for three-row configurations shows that the effectiveness could be different from 0.3 to 2.4% for the NTUs (Number of Thermal Units) range from 1.0 to 3.0. On the other hand, the result of the comparison of effectiveness between identical and alternating circuiting for 3-row crossflow heat exchangers shows that alternating circuiting could have less effectiveness than identical circuiting from 0.4 to 8.8% in the NTUs range from 1.0 to 3.0. Nevertheless, alternating circuit has its benefit for lower NTU cases, result shows that it could have 1.7 to 0.1% advantages over identical flow arrangement for 2-row heat exchangers with NTUs range from 1.0 to 2.0.
A Simple Heat Transfer Correlation for Three Inlet Configurations Using Artificial Neural Network in the Complex Transition Flow Regime
Authors: Ghajar A. J. , Tam L. M. , Tam S. C.
Published: December 2008

Local forced and mixed heat transfer coefficients were measured by Ghajar and Tam (1994) along a stainless steel horizontal circular tube fitted with reentrant, square-edged, and bell-mouth inlets under uniform wall heat flux condition. For the experiments the Reynolds, Prandtl, and Grashof numbers varied from about 280 to 49000, 4 to 158, and 1000 to 2.5×105, respectively. The heat transfer transition regions were established by observing the change in the heat transfer behavior. The data in the transition region were correlated by using the traditional least squares method. The correlation predicted the transitional data with an average absolute deviation of about 8%. However, 30% of the data were predicted with 10 to 20% deviation. The reason is due to the abrupt change in the heat transfer characteristic and its intermittent behavior. Since the value of heat transfer coefficient has a direct impact on the size of the heat exchanger, a more accurate correlation has been developed using the artificial neural network (ANN). A total of 1290 data points (441 for reentrant, 416 for square-edged, and 433 for bell mouth) were used. The accuracy of the new correlation is excellent with the majority of the data points predicted with less than 10% deviation.
A Microscale Model for Prediction of Breast Cancer Cell Damage During Cryosurgery
Authors: Zhang Aili , Xu Lisa X. , Sandison George A. , Zhang Jiayao
Published: December 2008

The morphology of most ductal carcinoma is characterized by tightly packed groups of small malignant cells. This special structure can make these breast cancer cells have different osmotic responses to freezing and affect the probability of damage from cellular dehydration and intracellular ice formation. A mathematical model has been developed to study the microscale damage of the breast cancer cells during cryosurgery by taking its unique structure into consideration. The model was built based on a spherical unit comprised of an extracellular region that surrounds several layers of cancer cells, as experimentally observed by other researchers [13]. In this model, cell to cell contact and water transportation were both taken into consideration. Temperature transients in the breast cancer undergoing cryosurgery were calculated numerically using the Pennes equation. When subjected to various types of thermal histories, both cell dehydration and intracellular ice formation in the unit structure were examined at the microscale level using the model developed in this study. It was found that the cells in the inner layers hardly dehydrated while those in the outermost layer did greatly. The results were used to explain the experimental phenomena observed in freezing of breast cancer tissues that intracellular ice formation existed even at the slow cooling rate of −3°C/min [13]. In the attempt to better define an optimal freeze-thaw cryosurgery procedure for breast cancer, both serious dehydration and intracellular ice formation (IIF) need to be considered. This study also found that use of constant heat flux is able to induce greater dehydration and higher IIF probability simultaneously. It is recommended that a constant heat flux protocol should be used in cryosurgery to ensure better treatment results.
Comparison of Measured Transient Ceiling Jet Temperature and Velocity Profiles in the Presence of an Upper Layer With Predictions by LAVENT Computer Fire Model
Authors: Motevalli Vahid , Riahi Siamak
Published: December 2008

Development of an upper layer in enclosure fires has a significant effect on the characteristics of the ceiling jet and a direct influence on the placement and performance of fire detection/suppression devices. Detailed transient measurement of ceiling jet velocity and temperature profiles within an upper layer for small-scale fires (2.0 kW and ceiling height of 1 m) are used to analyze the predictions produced by the LAVENT computer fire model. While the model predictions have been compared with large-scale experimental results with fires as large as 33 MW and ceiling height of up to 22m, the large-scale measurements do not have high special resolution and the present work offers a more thorough analysis of the model prediction. Various radiative losses (10–25%) were used to produce predictions that matched the experimental data. Comparison of the small-scale experimental data with the predictions from LAVENT shows that the model, which uses unconfined ceiling jet correlations, does not capture the ceiling jet profile well and over predicts the upper layer temperature during the development of the layer. The peak temperature prediction requires a different radiative loss factor than that which matches the upper layer temperature. In general, the peak temperature measurements are within 10% of the measured value. The velocity is generally over predicted since the retardation of the jet momentum by the upper layer does not seem to be modeled accurately.
Measurement of Windage Losses and Temperature Distribution for a High Speed Composite Rotor in a Stator Assembly at Low Air Pressures
Authors: Hahne Jonathan , Werst Mike , Penney Charles , Liu Hsing-Pang , O’Rarden Jeremy , Bogard David
Published: December 2008

With the advancements in composite technology several innovative applications present themselves that involve high-speed composite rotors spinning in a stator assembly. As rotational speeds and rotor tip speeds increase, these rotors must operate in low air pressure environments to minimize windage losses and thermal effects of being at high speed for long durations. Accurately predicting this windage loss for a specific geometry and operating conditions is very important for a proper design. It is also very important to know the relative heat distribution that is seen by the rotor and stator from this windage loss. Analysis tools to date do not have a coupled link that calculates windage loss and a resultant thermal distribution to the rotor and stator surfaces. This paper presents the design and fabrication of a test setup to measure the total windage loss and temperature distribution from a high-speed composite rotor in a stator structure. Rotor speeds up to 40,000 rpm and rotor tip speeds up to 900 m/s with pressure ranges from 0.1 torr to 10 torr were operating parameters during the testing. The paper will also present experimental data obtained during the testing. Experimental data obtained during the testing will be used to evaluate new analysis methods for predicting the windage loss and thermal distribution in new high-speed rotor applications.
VAT Based Optimization of Heat Transfer in a Flat Channel Filled With a Porous Media
Authors: Catton Ivan , Hu Kunzhong
Published: December 2008

Developments of volume averaging theory (VAT) used to describe transport phenomena in heterogeneous media are applied to optimization of heat dissipation from a heterogeneous media. The media is a porous media representation of a pin fin heat sink (a heterogeneous layer) and the optimization process is accomplished with rigor using the idea of scaled energy transport. The problem is addressed in four steps: 1) determine the parameters needed for optimization from the two temperature VAT equations, 2) use statistical design of experiments (simulating the problem) for the many optimization parameters, 3) perform numerical simulation of the cases that are suggested through the statistical analysis of the optimization parameters, and 4) statistically analyze the numerical results to obtain an optimization response surface. The two applications are enhancement of heat transfer dissipation from a heterogeneous media while minimizing the frictional resistance and minimization of the thermal resistance (a problem of importance to all designers of heat exchangers).
Effects of Gap Geometry and Gravity on Boiling Around a Constrained Bubble in 2-Propanol/Water Mixtures
Authors: Sun Chen-Li , Carey Van P.
Published: December 2008

In this study, boiling experiments were conducted with 2-propanol/water mixtures in confined gap geometry under various levels of gravity. The temperature field created within the parallel plate gap resulted in evaporation over the portion of the vapor-liquid interface of the bubble near the heated surface, and condensation near the cold surface. Full boiling curves were obtained and two boiling regimes — nucleate boiling and pseudo film boiling, the transition condition, and the critical heat flux (CHF), were identified. The observations indicate that the presence of the gap geometry pushed the nucleate boiling regime to a lower superheated temperature range and resulted in correspondingly lower heat flux. With further increases of wall superheat, the vapor generated by the boiling process was trapped in the gap and blanketed the heated surface. This caused premature occurrence of CHF conditions and deterioration of heat transfer in the pseudo film boiling regime. The influence of the confined space was particularly significant when greater Marangoni forces were present at reduced gravity conditions. The value of the CHF for x = 0.025, which corresponded to weaker Marangoni forces, was found to be greater than that of x = 0.015 with a 6.35 mm gap.
Heat Transfer in Gas-Insulated Bus Bars
Authors: Ham Jin-Ki , Kim Young-Ki , Kim Jin-Soo , Song Seok-Hyun
Published: December 2008

Numerical prediction and experimental verification of the temperature rise for a single-phase and a three-phase gas-insulated bus bar with current flow are investigated. Various heat generation rates possibly produced in the gas-insulated bus bar are calculated. To estimate the power loss caused by eddy current, the magnetic field analysis is carried out. The heat balance calculation solving the differential form of an energy balance equation with empirical relations is conducted by using the 5th order Runge-Kutta method. The various cases representing different geometries and current values are investigated by conducting the heat balance calculation. Three-dimensional numerical flow field analysis using finite volume method is performed for the different type of the bus bars. From the flow field analysis based on laminar natural convection, the temperature gradient in the current flowing direction caused by contact heat source is found for both single-phase and three-phase bus bars. In the experiments, temperature rises in each of conductor, contact part, and external tank are measured for a full-scale gas-insulated bus bar. The comparisons of the predicted values of the heat balance calculation and the numerical analyses to results of the experiments are made. From the comparisons, it is concluded that the temperature rise of a bus bar can be predicted quite well by performing laminar natural convection flow analyses.
Effective Thermal Conductivity of Rough Spheres Packed Bed
Authors: Buonanno G. , Carotenuto A. , Giovinco G. , Vanoli L.
Published: December 2008

The effective thermal conductivity, ke, rigorously defined on the basis of the local volume averaging method, is an important parameter in porous media. The experimental and numerical results available in literature demonstrate that the kevalue is influenced by several parameters such as thermal and mechanical properties of the multiphase porous medium, phase volumetric fractions, geometrical shape and spatial distribution of the solid matrix and, in particular, contact area between the solid particles. In the present paper, a numerical method to evaluate the effective thermal conductivity from the packing structure of a packed bed of mono-sized spheres is validated through the comparison with experimental data, obtained by the authors from an apparatus designed and build up for this purpose. The effects of the spheroid surface roughness is examined as the applied contact load and the solid matrix material vary. In particular packed beds of steel and aluminum spheroids saturated by a static gas (air) have been studied. Unfortunately, the lack of published results including an accurate measurement of the particle roughness does not allow the authors to compare their numerical results with other researchers’ experimental data.
Enhancement of Natural Convection and Pool Boiling Heat Transfer via Ultrasonic Vibration
Authors: Kim Ho-Young , Kim Yi Gu , Kang Byung Ha
Published: December 2008

This work experimentally studies the fundamental mechanism by which the ultrasonic vibration enhances natural convection and pool boiling heat transfer. A thin platinum wire is used as both a heat source and a temperature sensor. A high speed video imaging system is employed to observe the behavior of cavitation and thermal bubbles. Experimental results show that the effects of ultrasonic vibration on flow behavior are vastly different depending on the heat transfer regime and the amount of dissolved gas. In the natural convection and subcooled boiling regimes, behavior of cavitation bubbles strongly affects the degree of heat transfer enhancement. In saturated boiling, no cavitation occurs thus the reduced thermal bubble size at departure and acoustic streaming are major factors enhancing heat transfer rate. The highest enhancement ratio is obtained in natural convection regime where no bubbles are present without ultrasonic vibration.
Optimal Design of Compact Heat Exchangers by an Artificial Neural Network Method
Authors: Jia Rongguang , Sunde´n Bengt
Published: December 2008

The artificial neural network (ANN) methods are introduced (mainly for calculation of thermal and hydraulic coefficients) into a computer-aided design code of compact heat exchangers (CCHE). CCHE integrates the optimization, database, and process drawing into a software package. In the code, a strategy is developed for the optimization of compact heat exchangers (CHEs), which is a problem with changeable objective functions and constraints. However, the applicability and/or accuracy of all these methods are limited by the availability of reliable data sets of the heat transfer coefficients (j or Nu) and friction factors (f ) for different finned geometries. In fact, due to expenses and difficulties in experiments, only a limited number of experiments has been carried out for some kinds of heat transfer surfaces. The information, therefore, is usually given by means of correlations. It is well known, however, that the errors in the predicted results by means of correlations are much larger than the measurement errors, being mainly due to the data reduction represented by them. This implies doubts on the optimal solutions. Fortunately, a well-trained network is capable of correlating the data with errors of the same order as the uncertainty of the measurements. This is the main reason for the present introduction of the ANN method to correlate the discrete experimental data sets into continuous formulas. In this study, the ANN method is used to formulate the complex relationship between the thermal and hydraulic coefficients and the other parameters, including the geometry and process data. A specific case on the optimal analysis of a plate-fin heat exchanger (PFH) is presented to show how the trained ANNs can be used for optimal design of heat exchangers. In addition, a case is presented to illustrate how an inverse heat transfer problem is solved by the optimization methodology developed in the present code.
The Two-Flux Model Applied to Interaction of Radiation and Natural Convection in the Laminar Boundary Layer
Authors: Malpica Freddy , Moreno Nathaly , Tremante Andre´s
Published: December 2008

The effects of thermal radiation upon the laminar free convection boundary layer of a vertical flat plate was studied for an absorbing, emitting and scattering gray fluid. The two-flux model was used to represent the radiation term in the energy equation. This method allows to obtain a simpler formulation than required for exact representation of radiation in the boundary layer. The two-flux model reduces the governing equations: continuity, momentum, energy and irradiation, to a set of a coupled partial differential equations. A finite difference scheme is used to transform the resulting equations into an ordinary differential equation system which simplifies the numerical solution. Results for the velocity and temperature profiles and heat fluxes present close agreement with the optical thin and thick limits. Comparison of two-flux results with data obtained using an exact representation of radiation show very small deviations. The method proposed proves to be useful to investigate the effect of the different radiation parameters on natural convection boundary layer, and to obtain numerical results for engineering applications.
Thermal Management of a Heat-Pipe Drill: A FEM Analysis
Authors: Jen Tien-Chien , Jadhav Rajendra
Published: December 2008

Thermal management using heat pipes is gaining significant attention in past decades. This is because of the fact that it can be used as an effective heat sink in very intricate and space constrained applications such as in electronics cooling or turbine blade cooling where high heat fluxes are involved. Extensive research has been done in exploring various possible applications for the use of heat pipes as well as understanding and modeling the behavior of heat pipe under those applications. One of the possible applications of heat pipe technology is in machining operations, which involves a very high heat flux being generated during the chip generation process. Present study focuses on the thermal management of using a heat pipe in a drill for a drilling process. To check the feasibility and effectiveness of the heat pipe drill, structural and thermal analyses are performed using Finite Element Analysis. Finite Element Software ANSYS was used for this purpose. It is important for any conceptual design to be made practical and hence a parametric study was carried out to determine the optimum geometry size for the heat pipe for a specific standard drill.
Modeling of a Solid/Liquid Thermal Storage System
Authors: Mulvey J. S. , Boehm R. F.
Published: December 2008

A computational model utilizing the finite difference method was developed to simulate the behavior of a simple thermal storage system. The system analyzed utilizes the deposition of heat from a fluid to a solid matrix in the initial part of cycle followed by heat removal in the latter part. The storage system was divided into perpendicular slices with respect to the direction of the heat transfer fluid (HTF) flow. To further reduce the area of the slice on which the calculations were performed, the symmetry of the design was then used. Two dimensional conduction and convection calculations were performed within the plane generated by each slice. Interaction between the slices was limited to only the HTF flow rate. It was assumed that the system would experience no losses to the ambient and the HTF contained in each slice would be fully mixed. First and Second Law analysis were incorporated as a means of evaluating different configurations of the storage system design. A technique that allows the designer to choose between design options is discussed.
Numerical and Experimental Studies of Substrate Melting During Thermal Spraying
Authors: Wei G. , Zhang H. , Li L. , Wang X. Y.
Published: December 2008

Melting and re-solidification of the substrate plays an important role in thermal spray coating. A good understanding of this phenomenon will help us to achieve better bonding. A numerical model is developed to investigate the solidification of the droplet, and melting and re-solidification of the substrate. The solidification interface movement is obtained by applying a rapid solidification model on the solid/melt interface. Numerical simulations have been used to study the influence of materials and temperatures of the splat and substrate on substrate melting and re-solidification. In the corresponding experiments, the molybdenum powder is sprayed onto a stainless steel, brass (70%Cu) or aluminum substrate by atmospheric plasma spraying system. The crater depth of the substrates has been measured. Experimental results show that the material properties of the splat and substrate and melting temperature of the substrate play important roles on substrate melting and maximum melting depth. A dimensionless parameter, temperature factor, has been proposed from analysis and can be used as an indicator whether a substrate melting will occur for a certain combination of the droplet and substrate, and this parameter can be correlated with the maximum melting depth of the substrate.
Wall Heat Flux Partitioning During Subcooled Flow Boiling at Low Pressures
Authors: Basu Nilanjana , Warrier Gopinath R. , Dhir Vijay K.
Published: December 2008

In this work a mechanistic model for nucleate boiling heat flux as a function of wall superheat has been developed. The premise of the proposed model is that the entire energy from the wall is first transferred to the superheated liquid layer adjacent to the wall. A fraction of this energy is then utilized for vapor generation. Contribution of each of the heat transfer mechanisms — forced convection, transient conduction, and vapor generation, has been quantified in terms of nucleation site densities, bubble departure and lift off diameters, bubble release frequency, flow parameters like velocity, inlet subcooling, wall superheat, and fluid and surface properties including system pressures. To support the model development, subcooled flow boiling experiments were conducted at pressures of 1.03 to 3.2 bar for a wide range of mass fluxes (124 to 926 kg/m2s), heat fluxes (2.5 to 90 W/cm2) and for contact angles varying from 30° to 90°. Model validation has been carried out with low-pressure data obtained from present work and the wall heat flux predictions are within ± 30% of experimental values. Application of the model to high-pressure data available in literature also showed good agreement, signifying that the model can be extended to all pressures.
Molecular Dynamics Exploration of Properties in the Liquid-Vapor Interfacial Region
Authors: Wemhoff Aaron P. , Carey Van P.
Published: December 2008

Molecular dynamics (MD) simulations of liquid-vapor interfaces were performed to determine mean property variations and property fluctuations in the liquid-vapor interfacial region at various reduced temperatures. The interfacial region typically has a thickness on the order of a few nanometers for systems of practical interest. The system’s initial conditions were specified as a bulk liquid region sandwiched between two bulk vapor regions. Simulations were run using a Lennard-Jones 6-12 potential function between the atoms with appropriate parameters for Argon atoms. As the simulation was performed, interfacial region property data was collected over time. The resulting property data are shown to establish trends similar to those indicated by theoretical and experimental results reported elsewhere. The peak fluctuations of mass density and free energy density were determined to be approximately equal in magnitude when normalized with the difference in their respective bulk values at a given temperature. These fluctuations were found to increase rapidly with temperature. The fluctuations in the interfacial thickness and interfacial position follow a functional dependence on temperature similar to that exhibited by the mean value of interfacial thickness. In addition to exploring fluctuations in the interfacial region, two new methods were developed to determine interfacial tension through methods involving integration of excess free energy density across the interfacial region. These techniques were shown to yield mean results similar to theoretical predictions and those using conventional techniques. In addition, the time required for computation using the new techniques is significantly reduced due to less computational time per step and fewer required steps for convergence to a mean value.
Size Effect on Micro-Droplet Movement Due to Marangoni Effect
Authors: Tseng Yuan-Tai , Tseng Fan-Gang , Chieng Ching-Chang
Published: December 2008

Fundamental physics are studied on the movement of droplets for sizes ranging from 0.1 μl to 1.0 μl on a solid surface subjected to temperature gradients using numerical computations and the comparison with experiments. The receding/advancing contact angles relating to the droplet size and shape are the key parameters of droplet moving and the differences subjected to the temperature gradients induce unbalanced recirculation zones inside the moving droplet, thus induces driving force to drag the droplet. It is found that droplet of smaller size moves faster with smoothly changing speed and the droplet of larger size moves with fluctuating speed and the average moving speed is roughly the same magnitude as that with two-dimensional heating.
Surface Tension Evaluation of Lennard-Jones Fluid System With Untruncated Potentials
Authors: Sinha Shashank , Dhir Vijay K. , Shi Bo , Freund Jonathan B. , Darve Eric
Published: December 2008

In typical atomistic simulations of simple liquids, the Lennard-Jones interatomic pair potential is truncated so that algorithms scale as Natoms rather Natoms2, which would be the case if an interaction were computed explicitly for all atom pairs. However, it is known that interfacial properties are sensitive to the cutoff radius selected. Corrections for the missing ‘tails’ of the potential can reduce the error, but cannot eliminate it because the liquid and vapor densities are also sensitive to the cutoff radius. In light of this, we have developed and implemented a NlogN particle-particle particle-mesh (P3M) algorithm to evaluate the 1/r6 dispersive forces between Lennard-Jones fluid molecules without truncation. Statistical expression for the surface tension also scale as N2 if potentials are not truncated, so we also developed a P3M formulation for computing surface tension. The techniques are demonstrated on a thin liquid film suspended in equilibrium with its own vapor. Simulations at several temperatures between the triple point and the critical point are compared with the available data. The expense of the algorithm is competitive for simple geometries and seems preferable in non-trivial geometries without the possibility of tail corrections.
Air-Water Transient Heat Transfer in a Bubble Column
Authors: Katz Y. , Dubovsky V. , Ziskind G. , Letan R.
Published: December 2008

This work focuses on a direct contact heat transfer between water and air bubbles. A tube 0.2m in outer diameter and 1.4m high was filled with water. Low-humidity air was continuously dispersed into bubbles at the bottom of the tube. The water inside the tube could be heated initially or continuously during the experimental runs, using controllable electrical strip heaters. Temperature measurements were simultaneously performed at several locations inside the column. The results show that the water temperature was almost uniform due to perfect mixing induced by the bubbles movement. The column of water was not insulated, and the cooling by convection and radiation at the walls was accounted for. The heat-transfer rates between the water and the rising air bubbles were estimated for various modes encountered in the experimental study, and mathematically modeled. The model simulations for the batchwise cooled water compare well with the experimental results.
Effect of Compressibility on Gaseous Flows in a Micro-Tube
Authors: Asako Yutaka , Nakayama Kenji
Published: December 2008

The product of friction factor and Reynolds number (f·Re) of gaseous flow in the quasi-fully developed region of a micro-tube was obtained experimentally and numerically. The tube cutting method was adopted to obtain the pressure distribution along the tube. The fused silica tubes whose nominal diameters were 100 and 150 μm, were used. Two-dimensional compressible momentum and energy equations were solved to obtain the flow characteristics in micro-tubes. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The both results agree well and it was found that (f·Re) is a function of Mach number.
Passive Cooling of Heat Generating Cylinders in Narrow Enclosures
Authors: Dubovsky V. , Weiss Y. , Aharon J. , Ziskind G. , Letan R.
Published: December 2008

This paper presents cooling by natural and induced convection of a vertical cylinder and a horizontal array of vertical cylinders in a narrow enclosed space with openings. A laboratory-scale model was studied both experimentally and numerically. The cylinders were 100 mm high and 50mm in diameter. Cartridge heaters were installed in drills along the cylinder axis and connected to an external power source. The cylinders were positioned in a horizontal enclosure which was a box of an inner cross-section of 125mm (height) by 100mm (width). The length of the box was varied with the number of cylinders used, or with the spacing between the cylinders. Temperature was measured at various locations inside the cylinders by fine thermocouples using a multi-channel data acquisition unit. The experiments were performed for one to eight cylinders with various spacing. Numerical simulations were performed for the velocity and temperature fields inside the enclosure, using the Fluent 5.5 software. Both convection and radiation inside the enclosure were accounted for. Effect of various parameters of the system, including the enclosure width, height and the cylinders spacing, on the cylinder temperature has been studied. Comparison of the experimental results with numerical predictions is presented and discussed.
Electronic Thermal Conduction in Thin Gold Films
Authors: Wong Basil T. , Mengu¨c¸ M. Pinar
Published: December 2008

In this work, electronic thermal conduction in thin gold film is modeled via the Boltzmann Transport Equation (BTE). The BTE is solved using a Monte Carlo Method (MCM). Temperature profiles for various film thicknesses are computed. Results show that the electronic thermal transport in gold is still diffusion-like at film thicknesses as small as 100 nm, implying that the Fourier law of conduction can be applied at this scale to predict the steady-state thermal heat transfer without comprising the physics. However, the Fourier law does not predict the temperature profiles accurately if the film thickness is reduced to 10 nm or below.
Analytical/Numerical Hybrid Solution for One-Dimensional Ablation Problem
Authors: Kurokawa Fa´bio Yukio , Diniz Antonio Joa˜o , Campos-Silva Joa˜o Batista
Published: December 2008

Ablation is a thermal protection process with several applications in engineering, mainly in the field of airspace industry. The use of conventional materials must be quite restricted, because they would suffer catastrophic flaws due to thermal degradation of their structures. However, the same materials can be quite suitable once being protected by well-known ablative materials. The process that involves the ablative phenomena is complex, could involve the whole or partial loss of material that is sacrificed for absorption of energy. The analysis of the ablative process in a blunt body with revolution geometry will be made on the stagnation point area that can be simplified as a one-dimensional plane plate problem. In this work the Generalized Integral Transform Technique (GITT) is employed for the solution of the non-linear system of coupled partial differential equations that model the phenomena. The solution of the problem is obtained by transforming the non-linear partial differential equation system to a system of coupled first order ordinary differential equations and then solving it by using well-established numerical routines. The results of interest such as the temperature field, the depth and the rate of removal of the ablative material are presented and compared with those ones available in the open literature.
High Speed Photographic Observation of Flow Patterns During Flow Boiling in Single Rectangular Minichannel
Authors: Balasubramanian Prabhu , Kandlikar Satish G.
Published: December 2008

The present study extends the range of flow patterns observed during boiling of water in a single minichannel of height 197 μm and width 1054 μm. The calculated hydraulic diameter of the channel is 333 μm. High-speed photography is used to obtain video images of the flow field. The top covers of the channels are made of Lexan to permit visual observation, while the minichannel is machined in a copper block heated with cartridge heaters. The flow patterns are studied as a function of mass flux, heat flux, and location along the flow channel length. The results provide important information regarding the boiling behavior in a single channel.
Suppression of Buoyancy in a Prototypical CVD Reactor by Geometry Modification
Authors: Vanka S. P. , Luo Gang , Glumac Nick G.
Published: December 2008

Buoyancy plays a detrimental role in chemical vapor deposition reactors employed for thin film deposition. Buoyancy driven fluid flow causes complex flow patterns which alter the transport of the precursor gases to the substrate, and leads to nonuniform deposition patterns. Consequently, many CVD reactors operate under low pressure to mitigate these flow patterns. However, the growth rates at such pressures are relatively low. Operating a CVD reactor under vacuum conditions is also inconvenient because of the associated hardware that is required. In the present work, we have numerically explored the performance of a new type of stagnation flow CVD reactor at pressures close to atmospheric pressure. The new geometry resembles that of a pancake reactor, but the gases are supplied through a long vertical inlet. The annular wall above the substrate is maintained at a low temperature to avoid deposition on this surface. The substrate is also rotated to improve the hydrodynamic patterns and provide azimuthal symmetry. We report results of a number of high-resolution calculations in this reactor to demonstrate its merits for operation at sub-atmospheric and atmospheric pressures. It is shown that the growth rate is significantly large, in addition to a high degree of film uniformity.
Effect of Inner Wall Rotation on Multicellular Natural Convection Flow in a Vertical Annulus
Authors: Sergent A. , Le Que´re´ P. , Vanka S. P.
Published: December 2008

Numerical simulations have been performed to study the effects of the inner wall rotation on the unsteady multicellular flow of natural convection in the conductive regime (Ra = 8000). We consider a tall air-filled vertical annulus between differentially heated concentric cylinders with the inner cylinder allowed to rotate. The unsteady Boussinesq equations are discretized using a finite volume method with a second order time stepping scheme. The natural convection flow is axisymmetric in this regime, whereas it is known that the mixed convection flow becomes 3D over a range of Reynolds number. We observe the transition in a range of Reynolds number close to the critical Reynolds number of the Taylor-Couette flow. The rotation has a weak influence on the axisymmetric time-periodic natural convection flow before the transition, whereas the flow becomes 3D and chaotic after.
Spectral Thermal Radiation Characteristics of Coal Ashes and Slags: Influence of Chemical Composition and Temperature
Authors: Linka S. , Wirtz S. , Scherer V.
Published: December 2008

During the combustion of pulverized coal, ash particles (formed from inorganic species) can deposit on heat-transfer surfaces, resulting in a decrease in heat transfer rates and system efficiency. In addition to the knowledge of the thermal conductivity of the deposits it is necessary to obtain information on the thermal radiation characteristics of the furnace walls to predict the influence of ash sedimentation on heat transfer. At the Department of Energy Plant Technology investigations on the spectral emissivity of different coal ashes and slags were performed applying a spectral radiometer. The samples were electrically heated. Temperatures were varied between 600 and 1400 °C. Emissivities in the range of wavelengths from 1 to 15 μm have been determined. An essential result is that coal ashes show selective thermal radiation characteristics. The main factor of influence on the emissivity is the chemical composition. Therefore, measurements on the single phases SiO2, Al2O3 and MgO were carried out and compared with the emissivity of typical coal ashes and slags. Furthermore, the emissivity depends on temperature, mainly in the wavelength range from 1 to 6 μm.
Parameter Estimation for Highly Nonlinear Models With Noisy Data
Authors: Emery A. F. , Bardot D.
Published: December 2008

Thermal properties are generally determined through solving inverse problems. Because the temperature is a non-linear function of the properties, the solutions are usually effected by linearizing the equations. The statistics of these linearized estimates are based upon the assumptions that the measurement noise has zero mean and is normally distributed, yielding unbiased and normally distributed parameter estimates. In fact, even for this type of noise, nonlinear functions can lead to bias and nonnormal distributions of estimated properties. We examine these effects and show that for typical thermal systems that while the estimates are unbiased and normal, the confidence limits may be inaccurately defined and the residuals of the fits may not be zero mean and uncorrelated. Characterizing the estimated parameters is critical when nonlinear models are to be used for extrapolation.
Water Flux Coupled to Active Solute Transport Across Amphibian Epidermis
Authors: Guo Peng , Hillyard Stanley D. , Fu Bingmei M.
Published: December 2008

We examine the water flux across amphibian skin driven by both the osmotic gradient and the active transport. Amphibian skin is modeled as a well-stirred compartment bounded by a mucosal barrier and a serosal barrier. The compartment represents the lateral intercellular space between cells in the stratum granulosum. The mucosal barrier is comprised of a cell barrier and a tight junctional barrier. The cell barrier is hypothesized to have the ability to actively transport solute (Na-K-ATPase) and the solute flux is assumed to satisfy the Machaelis-Menton relationship. Our model shows that: 1) there is a substantial water flux coupled to the active solute flux and this coupled water flux is nearly constant when the osmolality of the apical bathing solution is greater than 100 mOsm despite its variation; 2) the predicted rehydration rates from apical bathing solutions are in good agreement with the experiment results in Hillyard and Larsen [2]; 3) the molar ratio of the actively transported solute flux to the coupled water flux is 1:165, which is almost the same as that reported in Nielsen [5].
Heat Transfer Enhancement Using Shaped Polymer Tubes: Fin Analysis
Authors: Li Zhihua , Davidson Jane H. , Mantell Susan C.
Published: December 2008

Uniquely shaped tubes extruded with an inner circular flow passage and an outer streamlined profile are proposed to withstand the inside fluid pressure and simultaneously to reduce the pressure drop across heat exchanger tube bundles comprising hundreds of small-diameter, thin-walled polymer tubes. Heat transfer rates are characterized by treating the tubes as the combination of a base circular tube and longitudinal fin(s) of oval or lenticular profile. Numerical solution of the non-linear differential equation for surface temperature provides the shaped tube efficiency, similar in function to fin efficiency. The effects of Biot number and a dimensionless shape factor on the tube efficiency and the convective heat transfer rate are discussed. A comparison of a circular to an oval shaped tube indicates that convective heat transfer is enhanced for 2000 ≤ Re ≤ 20,000 when Bi < 0.3.
Development of Irregular Interface Morphology During Unidirectional Solidification of Succinonitrile
Authors: Guo Taiming , Li Hongmin , Wang G.-X.
Published: December 2008

This paper reports an experimental investigation on irregular interface morphology patterns developed in thin-film unidirectional solidification of pure succinonitrile. Solidification experiments have been conducted under various temperature gradients and interface velocities. Several irregular patterns have been observed including titled dendrites, degenerate dendrites, and seaweed. It is found that as the temperature gradient increases, steady titled dendrites may evolve into degenerate dendrites and eventually to seaweed. With the same grain orientation, the irregular patterns may transform from one form to another as the growth condition changes. Observations demonstrate that normal dendrites exhibit a higher growth rate than seaweed pattern and would overgrow them. Irregular pattern may also become strongly dynamic and different patterns may evolve into each other during growth within the same experiment. These results should shed a light into the understanding of the interface morphology development during solidification.
Transport in a Microfluidic Catalytic Reactor
Authors: Park Hyung Gyu , Chung Jaewon , Grigoropoulos Costas P. , Greif Ralph , Havstad Mark , Morse Jefffey D.
Published: December 2008

A study of the heat and mass transfer, flow, and thermodynamics of the reacting flow in a catalytic micro-reactor is presented. Methanol reforming is utilized in the fuel processing system driving a micro-scale proton exchange membrane fuel cell. Understanding the flow and thermal transport phenomena as well as the reaction mechanisms is essential for improving the efficiency of the reforming process as well as the quality of the processed fuel. Numerical studies have been carried out to characterize the transport in a silicon microfabricated reactor system. On the basis of these results, optimized conditions for fuel processing are determined.
Molecular Dynamics Simulation of Evaporation of a Thin Liquid Argon Layer in a Closed System
Authors: Wu Y. W. , Pan Chin
Published: December 2008

Evaporation of a thin liquid film is of significant fundamental importance for both science and engineering applications. This work investigates the evaporation of a thin liquid argon layer into vacuum employing molecular dynamics simulation based on the Lennard-Jones potential. The simulation results demonstrate that the net evaporation rate of an ultra-thin liquid film into vacuum in a closed system may be modeled by the balance of evaporation and condensation based on the Schrage model. The evaporation/condensation coefficient and the non-Maxwellian factor may thus be evaluated. However, the coefficient thus obtained is sensitive to the dimension in the direction normal to the surface. It is also found that the mean temperature for the interface region is 2–3 K lower, while the temperature fluctuations are more violent, than that inside the liquid.
Nucleate Boiling Heat Transfer in Silicon-Based Micro-Channels
Authors: Lee P. C. , Li H. Y. , Tseng F. G. , Pan Chin
Published: December 2008

Nucleate boiling heat transfer is of significant interest for the design of a compact evaporator. This work investigates experimentally the nucleate boiling heat transfer in silicon based triangular and trapezoid microchannels with hydraulic diameter of 36.1 μm and 41.3 μm, respectively. Bubble nucleation and interactions are clearly observed in the entrance region, while the two-phase flow pattern in the microchannels is dominated by slug flow with long Taylor bubbles or annular flow. The evaporation of the very thin liquid film is speculated to be the major mode of heat transfer. The two-phase pressure drop is very significant and the corresponding variation in the saturation temperature should be considered in the analysis.
Laser Surface Coating of Metal Matrix Composite on Ti6Al4V Alloy
Authors: Pang W. , Man H. C. , Yue T. M.
Published: December 2008

Laser surface coating of Mo, WC and Mo-WC powders on the surface of Ti6Al4V alloys using a 2kW Nd-YAG laser was performed. The dilution effect, microstructure, microhardness and wear resistance of the fabricated MMC coating were investigated. With a constant thickness of pre-placed powder, the dilution levels of the alloyed layers were found to be increased with the incident laser power. The fabricated MMC layer was metallurgically bonded to the Ti6Al4V substrate. The microhardness of the fabricated surface layer was found to be inversely proportional to the dilution level. The EDAX and XRD spectra results show that new intermetallic compounds and alloy phases were formed in the laser fabricated layer. With increasing weight percentage content of WC particles in the Mo-WC pre-pasted powder, the microhardness and sliding wear resistance of the laser surface coating were increased by 87% and 150 times respectively as compared with the Ti6Al4V alloy.
Thermal Aspects in Resistance Welding of Thermoplastic Composites
Authors: Stavrov Darko , Bersee Harald E. N.
Published: December 2008

This paper presents a comprehensive experimental study on the thermal aspects in resistance welding of thermoplastic composites. A special test set-up was developed to perform the experiments. Glass fiber reinforced polyetherimide was the material used for manufacturing the welding specimens. Stainless steel mesh was used for production of heating elements. The temperature distribution was monitored using type-K thermocouples connected to a data acquisition system. The main objective of the study was investigating a possible solution for the edge effect. Temperature profiles over the weld length and over the weld width were monitored. The focus was on the transient temperature profiles at the edges of the weld. The temperature distribution through the weld thickness was also monitored. The influence of factors like insulation and power level was investigated. Finally, conclusions are drawn and options for improving the temperature distribution and modification of the models are being discussed.
Dependent Scattering and Field Enhancement in Monochromatic Irradiated, Random Porous Media
Authors: Ruan X. L. , Kaviany M.
Published: December 2008

Solving the Maxwell’s equations, an electric filed enhancement is observed in monochromatically irradiated, one- and two-dimensional dispersed dielectric solids with random size distribution. This enhancement is a few orders of magnitude higher than the incident field, for some arrangements. The probability density and the location of enhanced field are determined forgiven solid complex refractive index and particle size distribution. Using the two-flux model, the equation of radiative heat transfer (ERT) is solved for the local intensity. It is shown that ERT can represent a statistical average behavior of the intensity (by introducing corrections to the scattering and absorption coefficients based on dependent scattering). However, ERT is not suitable for predicting field enhancement and photon localization.