bluuebunny/crossref_metadata_2025 huggingface dataset:
Large-Scale Molecular Dynamics Investigation of Nanoscale Laser Material Processing
Authors: Wang Xinwei , Zhu Hua
Published: December 2008

In this work, large-scale molecular dynamics simulation is conducted to explore nanoscale manufacturing with laser-assisted scanning tunneling microscope. Employing a super parallel computer, more than 100 million atoms are modeled to provide substantial details about how the localized thermal and mechanical perturbations result in surface nanostructures. It is found that thermal equilibrium cannot be established due to the small number of atoms. Extremely localized stress accumulation beneath the sample surface results in an explosion of the melted/vaporized material, leaving a nanoscale hole on the sample surface. Normal and shear stress development is observed. Stress propagation in space is strongly influenced by the anisotropic nature of the crystal. The high pressure in the melted/vaporized region pushes the melt adjacent to the solid to move, thereby forming a protrusion at the edge of the hole. More importantly, visible structural destruction is observed in the region close to the bottom of the sample. These destructions are along the direction of 45 degrees with respect to the axial direction, and are attributed to the strong tensile stress. Atomic dislocation is observed in the destructed regions.
A Thermal Model for Concentric-Tube Overfire Air Ports
Authors: Swanson L. W. , Moyeda D. K. , Folsom B. A.
Published: December 2008

A quasi-steady multi-mode heat-transfer model for boiler concentric-tube overfire air ports has been developed that predicts the effect of geometry, furnace heat source and heat sink temperatures, axial injector wall conduction, and coolant flow rate on the tube wall temperature distributions. The model imposes a radiation boundary condition at the outlet tip of the ports, which acts as a heat source. The model was validated using field data and showed that both the airflow distribution in the ports and tube diameter can be used to control the maximum tube wall temperature. This helps avoid tube overheating and thermal degradation. For nominal operating conditions, highly nonlinear axial temperature distributions were observed in both tubes near the hot outlet end of the port.
Experimental Study of Flow Patterns and Regimes of Condensation in Horizontal Three-Dimensional Micro-Fin Tubes
Authors: Chen Qinghua , Amano R. S. , Xin Mingdao
Published: December 2008

During condensation of R134a the flow patterns inside two three-dimensional (3-D) micro-fin tubes with different fin geometries were investigated. The flow patterns and their transitions were visibly observed and recorded. The experimental findings revealed the following results: a comparison of the condensation flow patterns in the 3-D microfin tubes with those in smooth tubes revealed no qualitative differences. The mist flow and the mist-annular flow that appeared in the smooth tube entrance region were not observed in 3-D micro-fin tubes. In the maps of the Mandhane flow regime and the Soliman flow regime, the area of annular flow region of the 3-D micro-fin tube extends towards lower Fr number range in the Soliman map and smaller vapor velocity range in the Mandhane map when compared with that of a smooth tube. The criterion of the flow regime transition between the annular flow and wavy flow decreases from Fr = 7 to Fr = 2 in the Soliman flow regime map. However, no significant effect on the criterion for the plug flow transition was observed. The experimental data points of plug flow in the 3-D micro-fin tubes were also obtained in the same regime of smooth tube in Mandhane flow regime map. The Soliman flow regime map indicates the criterion for plug flow transition to be Fr = 0.4.
Numerical Studies of Injection Process Control With Spatial Gradients of Electrical Conductivity
Authors: Ren Liqing , Li Dongqing
Published: December 2008

The systematic design and precise control of the microfluidic dispenser (crossing microchannels etched into a plastic or glass plate) is key to the performance of many lab-on-a-chip devices. The fundamental understanding of the complicated electrokinetic phenomena in microfluidic dispensers therefore is necessary. In the literature, a few theoretical models studying the transport phenomena in similar crossing microchannels didn’t consider the spatial gradients of conductivity due to its complexity. A new theoretical model was developed in this paper to simulate the transport phenomena in a microfluidic dispenser with the consideration of a large range of spatial gradient of electric conductivity. This developed model was used to simulate the potential field, flow field, and concentration field of the injection processes where the conductivity of the sample-carrying buffer differs significantly from that of the driving buffer. The transport phenomena are found to be very sensitive to the conductivity difference between the sample-carrying buffer and the driving buffer. The developed model can be employed to find the optimal voltages for controlling the dispensed sample size and to provide guidance for designing such a microfluidic dispenser in lab-on-a-chip devices.
Heat and Mass Transport in HDP-CVD Chamber
Authors: Mungekar Hemant , Geoffrion Bruno , Kapoor Bikram , Dubey Naren , Salimian Mak , Cox Michael , Krishnaraj Paddy
Published: December 2008

HDP-CVD reactors are used for Shallow Trench Isolation (STI), Inter Metal Dielectric (IMD) and Inter Layer Dielectric (ILD) applications for logic and memory device fabrication. As device dimension shrinks, the trend has been to use lower pressure and higher plasma density for gap-fill with higher aspect ratio (AR). Higher AR gapfill in addition to higher throughput is achieved by running multiple wafers between a chamber clean, present a unique set of challenges for heat and mass-transfer in an HDP-CVD reactor. This paper describes some of the new state-of-the-art hardware innovations specifically developed to meet these challenges. In particular, heat transfer to plasma facing materials, fluid mechanics, and transport of sub-micron sized particles in the plasma environment of an HDP-CVD reactor are explored.
Effect of Louver Angle in Multi-Louvered Fins at Low Reynolds Numbers
Authors: Guo Dahai , Tafti Danesh K.
Published: December 2008

The paper studies the effect of large louver angles on the performance of large pitch multilouvered fins at low Reynolds numbers. The Reynolds number based on face velocity and louver pitch is varied between 50 and 300. Louver angles are varied from 20° to 60° for fin pitch ratios of 1.5 and 2.0. It is found that increasing louver angle has a favorable effect on flow efficiency up to a certain point, beyond which the flow efficiency decreases. The maximum flow efficiency is realized at smaller louver angles as the Reynolds number increases. The drop in flow efficiency is attributed to the development of recirculation zones which act as blockages. In spite of the decrease in flow efficiency, the heat transfer coefficient increases with louver angle for all the cases studied. It is found that as louver angle increases, impingement heat transfer at the leading surface of louvers becomes a dominant mode of heat transfer. Friction factors also increase with louver angle, primarily due to an increase in form drag.
Review of Thermal Joint Resistance Models for Non-Conforming Rough Surfaces in a Vacuum
Authors: Bahrami M. , Culham J. R. , Yovanovich M. M. , Schneider G. E.
Published: December 2008

The thermal contact resistance (TCR) problem is categorized into three different problems: geometrical, mechanical, and thermal. Each problem includes a macro and micro scale sub-problem; existing theories and models for each part are reviewed. Empirical correlations for microhardness, and the equivalent (sum) rough surface approximation are discussed. Suggested correlations for estimating the mean absolute surface slope are summarized and compared with experimental data. The classical conforming rough contact models, i.e elastic and plastic, as well as elastoplastic models are reviewed. A set of scale (dimensionless) relationships are derived for the contact parameters, i.e. the mean microcontact size, number of micro-contacts, density of microcontacts, and the external load as functions of dimensionless separation, for the above models. These scale relationships are plotted; it is graphically shown that the behavior of these models, in terms of the contact parameters, are similar. The most common assumptions of existing thermal analysis are summarized. As basic elements of thermal analysis, spreading resistance of a circular heat source on a half-space and flux tube are reviewed, also existing flux tube correlations are compared. More than 400 TCR data points collected by different re-searchers during last forty years are grouped into two limiting cases: conforming rough, and elasto-constriction. Existing TCR models are reviewed and compared with the experimental data at these two limits. It is shown that the existing theoretical models do not cover both of the above-mentioned limiting cases.
Numerical and Experimental Investigation of the Effect of Phase Change Materials on Clothing During Periodic Ventilation
Authors: Ghali Kamel , Ghaddar Nesreen , Jones Byron , Harathani Jihad
Published: December 2008

A numerical and experimental investigation is conducted of periodic ventilation process in fabric with microcapsules of phase change materials (PCMs). When microcapsules of phase change materials (PCMs) are added to textiles, they release heat as the liquid changes to solid state and absorb heat as the solid returns to liquid state. In this work, PCMs are incorporated in the numerical three-node fabric ventilation model of Ghali et al. (2002a) to study the transient effect of PCM on body heat loss during exercise when subjected to sudden change in environmental conditions from warm indoor conditions to cold outdoor conditions. The results indicate that the heating effect last approximately 12.5 minutes depending on PCM percentage and cold outdoor conditions. Heat released by PCM decreases the clothed-body heat loss by an average of 2.7 W for a one-layer suit depending on the frequency of oscillation and crystallization temperature of PCM. The experimental results revealed that under steady state conditions of the environment the oscillating PCM fabric has no effect on fabric dry resistance even though the measured sensible heat loss increased with decreasing the air temperature of the chamber. When a sudden change in ambient conditions occurs, the PCM fabric delays and the transient response and decreases body heat loss.
Prediction of Windage Losses of an Enclosed High Speed Composite Rotor in Low Air Pressure Environments
Authors: Liu Hsing-Pang , Werst Mike , Hahne Jonathan J. , Bogard David
Published: December 2008

The frictional windage losses associated with non-ventilated airflows in the air gaps between the rotor and stator of a high speed rotating machine can greatly influence the rotor outer and stator inner surface temperatures. The characteristics of the radial and axial air-gap flows have been of general interest in many engineering applications. A rotating air gap flow is very complex, and in general, can be categorized as a continuum flow, slip flow, and free molecule flow, depending on the ratio of its mean free path to the air gap dimension. For a continuum flow between concentric rotating cylinders, secondary flow of rows of circumferential Taylor vortices in the air gap due to centrifugal flow instability of a curved flow at relatively high rotating speeds will typically be formed. As the air pressure in the air gap drops significantly, rarefied gas flow, departure from continuum flow, occurs when the mean free path becomes relatively large compared to the air gap dimension. This paper has developed and summarized an analytical approach to predict high speed windage losses (rotor tip velocities up to 900 m/s) at low rotor cavity air pressures (0.1 torr to 10 torr). The predicted transient windage losses at various air pressures and high rotor speeds are compared with measured windage losses generated in continuum and slip flow regimes. The agreements between the predicted and measured windage losses are relatively well.
Holographic Interferometric Study of Heat Transfer Associated With a Single Vapor Bubble Sliding Along a Downward-Facing Heater Surface
Authors: Manickam Sathish , Dhir Vijay
Published: December 2008

Heat transfer associated with a single vapor bubble sliding along a downward-facing heater surface was studied experimentally to understand the mechanisms contributing to heat transfer enhancement. Heater surface was made of polished silicon wafer of length 185 mm and width 49.5 mm. Saturated and subcooled performance fluid PF 5060 at atmospheric pressure was used as the test liquid. The heater surface was at 75° inclination to the vertical for the experiments reported here. Single vapor bubbles were generated at an artificially formed cavity at the bottom end of the heater surface. Holographic interferometry was used to obtain the temperature profile around the bubble as it slides along the heater surface near the top end of the heater plate. From the fringe patterns, the temperature gradient around the bubble interface was measured and heat transfer into or out of the bubble was computed. In addition to these experiments, the volumetric growth of the vapor bubble as it slides along the heater surface was obtained using direct high-speed photography for the same experimental conditions. Heat transfer from the wall was estimated utilizing inputs from both interferometry studies as well as the volume growth data. Results are given for a range of liquid subcoolings and wall superheats, and are compared with previously published works.
The Effect of Axial Flow on Heat Transfer in Decaying, Swirling Flow in a Pipe
Authors: McClusky Heather L. , Beasley Donald E.
Published: December 2008

Local Nusselt numbers were experimentally measured in decaying, swirling flow in a pipe. Using a tangential injection mechanism, the two inlet conditions examined in this study were tangential flow and superimposed tangential and axial flow. Local Nusselt numbers at the pipe inlet were greater for tangential flow than for superimposed tangential and axial flow at the same Reynolds number. Local Nusselt numbers increased as the amount of fluid injected tangentially was increased for the superimposed case. For both inlet conditions employed with the present swirl generator, the local Nusselt number approached the fully-developed value in the far field. At the exit of the pipe, L/D = 62.8, local Nusselt numbers were greater than the fully-developed Nusselt number; therefore, heat transfer enhancement was still present at the exit of the pipe. The effect of axial flow on the local Nusselt numbers is explored in this investigation for air and over a Reynolds number range of 12,000 to 29,000.
Assessment of the Thermal Performance of Alternate Firing Schemes in Oxygen-Fired Glass Melting Furnaces
Authors: Jorgensen Kris L. , Ramadhyani Satish , Viskanta Raymond
Published: December 2008

Three firing schemes for an industrial oxygen-fired glass melting furnace were examined to determine the thermal performance and relative merits of each scheme. A comprehensive computer model was used to investigate the effects of each scheme on the combustion and heat transfer in the furnace. The three-dimensional computer model, suitable for predicting and analyzing fluid flow, combustion and heat transfer has been used to simulate the combustion space of the furnace. The turbulent flow field is obtained by solving the Favre averaged Navier-Stokes equations and using the k-ε model to calculate the turbulent shear stresses and close the equation set. The combustion model consists of a single step, irreversible, infinitely fast reaction. A mixture fraction is used to track the mixing of fuel and oxidant and thus reaction progress in this mixing limited model. An assumed shape PDF method is utilized to account for turbulent fluctuations. Radiative heat transfer in the combustion gases and between surfaces is modeled using the discrete ordinates method coupled with the weighted-sum-of-gray-gases model. The model furnace for all three firing schemes was the same size and shape, was charged from the rear end wall and was pulled from the front wall. The three schemes investigated were: 1) non-interlaced side-wall fired, 2) interlaced side-wall fired, and 3) end fired. The results show that all three arrangements provide similar thermal performance and heat transfer characteristics. However, the flow field for the non-interlaced arrangement is very complex in the region where jets from opposing walls meet at the furnace center line. This type of jet interference can lead to unstable flow, particularly at the centerline of the furnace. Unstable flow conditions can affect the heat transfer characteristics of the furnace and make the furnace difficult to operate. Conversely, the interlaced and end-fired schemes do not exhibit the jet interference seen in the non-interlaced arrangement. While the results indicate that the thermal performance of all three arrangements were similar, the possibility of jet interference suggests that an interlaced or end-fired arrangement is preferable.
Parallel Computing of Two Numerical Quadratures for an Integral Formulation of Transient Radiation Transport
Authors: Lu Xiaodong , Hsu Pei-Feng
Published: December 2008

Parallel computing of the transient radiative transfer process in the three-dimensional homogeneous and nonhomogeneous participating media is studied with an integral equation model. The model can be used for analyzing the ultra-short light pulse propagation within the highly scattering media. Two numerical quadratures are used: the discrete rectangular volume (DRV) method and YIX method. The parallel versions of both methods are developed for one-dimensional and three-dimensional geometries, respectively. Both quadratures achieve good speedup in parallel performance. Because the integral equation model uses very small amount of memory, the parallel computing can take advantage of having each compute node or processor store the full spatial domain information without using the typical domain decomposition parallelism, which will be necessary in other solution methods, e.g., discrete ordinates and finite volume methods, for large scale simulations. The parallel computation is conducted by assigning different portion of the quadrature to different compute node. In DRV method, a variation of the spatial domain decomposition is used. In the case of YIX scheme, the angular quadrature is divided up according to the number of compute nodes, instead of the spatial domain being divided. This parallel scheme minimizes the communications overhead. The only communication needed is at the end of each time step when each node shares the partial integrated result of the current time step with all other compute nodes. The angular quadrature decomposition approach leads to very good parallel efficiency. Two new discrete ordinate sets are used in the YIX angular quadrature and their parallel performances are discussed. One of the discrete ordinates sets, called spherical ring set, is also suitable for use in the conventional discrete ordinates method.
Application of a Macroscopic Turbulence Model to Simulation of Flow in a Channel With Equally Spaced Porous Fins
Authors: de Lemos Marcelo J. S. , Tofaneli Luzia A.
Published: December 2008

In this work, numerical solutions are presented for turbulent flow in a channel containing fins made with porous material. The condition of spatially periodic cell is applied longitudinally along the channel. A macroscopic two-equation turbulence model is employed in both the porous region and the clear fluid. The equations of mass continuity, momentum and turbulence transport equations are written for an elementary representative volume yielding a set of equations valid for the entire computational domain. Results are presented for the velocity field as a function of Reynolds, porosity and permeability of the fins. Pressure drop along the channel is compared with the case of solid material.
Experimental Study of the Splayed Electrode Design on Performance of a Microfabricated EHD Thin Film Evaporator: Application in High Heat Flux Cooling
Authors: Moghaddam Saeed , Ohadi Michael , McCaffery Collin
Published: December 2008

This paper presents details of the geometry, fabrication, and pumping performance of an electrohydrodynamically (EHD) driven thin film evaporator with a new electrode design. The operation principle of the device is based on the action of the EHD force on molecules of a dielectric liquid in a highly convergent electric field starting at the end of a pair of electrodes, where the electric field changes from zero to a finite value in between the electrodes. This force drives the liquid into the spacing between the electrodes. Because the electrodes are microfabricated very thinly on a surface, the liquid can be held within microns in thickness over the surface. The new design is a modification of the original device with parallel electrodes. The electrodes in the new design were fabricated in a splayed configuration with the narrow side dipped in the liquid. A relatively large (32 mm × 32 mm) area was covered with the splayed electrodes. The electrodes were fabricated through a lift-off process. It was found that a higher voltage can be sustained between the electrodes prior to breakdown threshold and a pumping head of about three times more than that of the parallel electrode configuration was achieved in the new design.
Thermal and Thermoelectric Measurements of Low Dimensional Nanostructures
Authors: Yu Choongho , Jang Wanyoung , Hanrath Tobias , Kim Dohyung , Yao Zhen , Korgel Brian , Shi Li , Wang Zhong Lin , Li Deyu , Majumdar Arunava
Published: December 2008

Low dimensional materials have unique thermal and thermoelectric properties that can be very different from their bulk counterparts. In a previous work, we and our collaborators have developed a microdevice for measuring thermal and thermoelectric properties of multiwall carbon nanotubes. Here, we used an improved design of the device for measuring single wall carbon nanotubes, Ge nanowires, and SnO2 nanobelts. These nanostructures are trapped between two adjacent symmetric silicon nitride membranes of the micro device using either a wet deposition method or in-situ chemical vapor deposition. The measurements provide the critically needed data of the unique thermophysical properties of these nanomaterials.
A 3-D Numerical Model for Ablation Phenomena and Thermal Stress Evolution During Laser Machining
Authors: Akarapu R. , Li B. Q. , Segall A. E.
Published: December 2008

A full 3-D transient model is developed for the ablation phenomena and thermal stress evolution during laser cutting and/or drilling of ceramic plates. The computational methodology is based on the Galerkin finite element method along with the use of a fixed grid algorithm to treat the thermal ablation resulting from an applied laser source. The present model is able to model any complex ablation operations involving discontinuity in geometries, as encountered in laser cutting and laser drilling operations. This is an advantage over the front tracking method by which the ablation moving interface is precisely tracked in time and which is useful for simple geometries. The laser ablation model is coupled with a thermal stress model to predict the evolution of thermal stresses, which arise due to a rapid change in thermal gradient near the laser beams. Model predictions compare well with the available data in literature for a simple configuration. Results obtained from model for both dual pulsed laser cutting and single laser drilling are discussed.
Numerical Simulation of Transient Boiling Convection in Microchannels
Authors: Fogg D. W. , Koo J. M. , Jiang L. , Goodson K. E.
Published: December 2008

Two-phase microchannel heat exchangers are receiving increasing attention from the microprocessor industry as power density levels in microchips increase. Previous numerical investigations of convective boiling in microchannels assumed steady flow within the channels. However, experimental data shows that two-phase flows in microchannels are highly transient even under steady heat loads. Little work has been done to model the dynamics associated with vapor generation in microchannels. The present work simulates the periodic distribution of vapor within microchannels filled with water by solving one-dimensional homogeneous equations for the mass, momentum and energy transport in conjunction with a transient wall conduction equation. A wall superheat constraint is incorporated to account for the excess superheat temperature required for bubble nucleation. Boiling events reduce the local wall temperature and change the pressure and enthalpy distributions within the flow. The transient pressure fluctuations predicted here are consistent with those observed in experiments. This study provides insight into the significance of bubble nucleation for forced convective boiling in microchannels and will be useful for the optimization of microchannel heat exchangers.
Detailed Phonon Generation Simulations via the Monte Carlo Method
Authors: Pop Eric , Sinha Sanjiv , Goodson Kenneth E.
Published: December 2008

Modeling heat generation at nanometer scales in silicon is of great interest and particularly relevant to the heating and reliability of nanoscale and thin-film transistors. Joule heating is usually simulated as the dot product of the macroscopic electric field and current density [1]. This approach does not account for the microscopic non-locality of the phonon emission near a strongly peaked electric field region. It also does not differentiate between electron energy exchange with the various phonon branches and does not give any information regarding the types of phonons emitted. The present work addresses both of these issues: we use a detailed Monte Carlo (MC) simulation to compute sub-continuum and phonon mode-specific heat generation rates, with applications at nanometer length scales.
Numerical Analysis of Temperature Due to Material Damping in Pultruded Composites
Authors: Mahajerin Enayat , Burgess Gary
Published: December 2008

This paper deals with the temperature rise due to material damping in rotating pultruded composite shafts. Pultrusion is a special fabrication process for manufacturing composite structural components whose cross section does not change along its length. Rotating shafts carrying gears or pulleys are examples. These shafts are subjected to side loads as they rotate. This causes them to bend back and forth in a cyclic manner. As a result, every element of the shaft generates heat due to material damping. The heat generation per cycle of motion is known for many materials and so a mathematical analysis is possible. Mathematical treatment of thermal problems involving pultruded composites can be simplified using two-dimensional Cartesian or Axisymmetric models. An explicit unsteady finite difference scheme with heat generation is used to study rotating shafts. The method is straightforward and capable of handling unsteady heat conduction in anisotropic components having single or multiple fiber types. The scheme can handle nonzero initial conditions and general convection type boundary conditions. Examples are used to illustrate the method and to investigate effects of the fiber-matrix thermal conductivity ratios, shaft rpm, and material damping on the temperature history in the material. The results show that, depending on the stiffness and thermal conductivity ratios in the principal directions, the temperature rise can be significant.
Characterization of Pore Diameters in Highly Porous Media
Authors: Pan H. L. , Pickena¨cker O. , Trimis D.
Published: December 2008

In this paper, a method for the experimental characterization of the equivalent pore diameter of highly porous open structures is presented. The commonly used characterization of such structures through geometrical properties like ppi number (porous per inch) and porosity proves to be not sufficient for the characterization of length scales related to heat and mass transfer. The procedure used here utilizes the quenching limits for flame propagation as characterization criterion. The determined equivalent pore diameter corresponds to the quenching diameter for a tube-geometry filled with the same combustible mixture. The quenching limit was determined by adjusting critical conditions, which are defined by a constant critical Pe´clet number comprising the laminar flame velocity instead of the flow velocity. Variations of oxygen content and air ratio were used in order to change the laminar flame speed and find the quenching limit for a given porous medium. The equivalent pore diameter determined with this method is a characteristic length scale of the porous medium geometry and is related to the heat transfer between the gas phase and the solid porous structure. The validation of the method was performed on sphere packings with well-documented properties. Several practically relevant highly porous media like foams and fabric lamellae structures were characterized and the results are discussed. Based on the effective heat conductivity (EHC) models of Zehner, Bauer and Schlu¨nder [1–3] for packed beds, an adapted model for foam structures was developed. The adapted model utilizes the equivalent pore diameters determined in the paper and predictions are presented.
Arithmetic Mean Temperature Difference and the Concept of Heat Exchanger Efficiency
Authors: Fakheri Ahmad
Published: December 2008

In this paper, it is shown that the Arithmetic Mean Temperature Difference, which is the difference between the average temperatures of hot and cold fluids, can be used instead of the Log Mean Temperature Difference (LMTD) in heat exchanger analysis. For a given value of AMTD, there exists an optimum heat transfer rate, Qopt, given by the product of UA and AMTD such that the rate of heat transfer in the heat exchanger is always less than this optimum value. The optimum heat transfer rate takes place in a balanced counter flow heat exchanger and by using this optimum rate of heat transfer, the concept of heat exchanger efficiency is introduced as the ratio of the actual to optimum heat transfer rate. A general algebraic expression as well as a chart is presented for the determination of the efficiency and therefore the rate of heat transfer for parallel flow, counter flow, single stream, as well as shell and tube heat exchangers with any number of shells and even number of tube passes per shell. In addition to being more intuitive, the use of AMTD and the heat exchanger efficiency allow the direct comparison of the different types of heat exchangers.
Effect of Non-Condensable Gas on the Tube Condenser Operating in Passive Mode
Authors: Oh Seungmin , Revankar Shripad T.
Published: December 2008

An experimental program was developed to study the condensation of steam with noncondensable in a vertical tube for forced flow and complete condensation modes of operation. Test data have been obtained for condensation of air-steam mixture for different mixture flow rate at different system pressure. The design of the test section is based on the passive condenser system in an advanced boiling water reactor. A boundary layer based condensation model was developed for the annular flow regime with inner gas phase and tube wall liquid film. In order to validate the condensation model the model predictions were compared with the pure steam data.
A Numerical Study of Heat Transfer and Fluid Flow in a Hydrothermal Autoclave With a Single- or Multi-Hole Baffle
Authors: Li Hongmin , Evans Edward A. , Wang G.-X.
Published: December 2008

Hydrothermal solution growth is an important technique to grow high quality piezoelectric single crystals. In industry hydrothermal crystal growth, an autoclave is divided into two chambers by a baffle located in the middle height. Industrial practice found that better quality crystals could be grown under certain baffle hole openings or using a multi-hole baffle. This paper presents a numerical study of the effects of the baffle opening, as well as the arrangement of holes on the baffle, on the fluid flow and temperature fields in an industry-size autoclave. A wide range of baffle hole openings from 2% to 25%, together with five hole-arrangements, is investigated. Computational results indicate that changing the baffle hole opening and number of holes on the baffle are effective ways to control the temperature uniformity in the upper growing chamber. With a single-hole baffle, a smaller hole-opening leads to a weaker flow field and more uniform temperature in the growing chamber. With the same opening area, a multi-hole baffle will perform better than a single-hole baffle. The number of holes in a multi-hole baffle shows a strong effect on thermal condition in the upper chamber with 8-hole baffles working better than both baffles with 4 and 16 holes. The hole-arrangement, however, does have significant effect on thermal condition in the growing chamber.
Numerical and Experimental Study of the Slot Film Cooling Jet With Various Angles
Authors: Jia Rongguang , Sunde´n Bengt , Miron Petre , Le´ger Bruno
Published: December 2008

Numerical simulations coupled with LDV experiments were carried out to investigate a slot jet issued into a cross flow, which is relevant in the film cooling of gas turbine combustors. The film cooling fluid injection from slots or holes into a cross-flow produces highly complicated flow fields. In this paper, the time-averaged Navier-Stokes equations were solved on a collocated body-fitted grid system with the V2F turbulence model. The fluid flow and turbulent Reynolds stress fields were compared with the LDV experiments for three jet angles, namely, 30-deg, 60-deg, and 90-deg, and the jet blowing ratio is ranging from 2 to 9. Good agreement was obtained. Therefore, the present solution procedure was also adopted to calculations of 15-deg and 40-deg jets. In addition, the temperature fields, which were difficult to measure by experimental methods, were also computed with a simple eddy diffusivity model to obtain the film cooling effectiveness which was used for evaluation of the various jet-cross-flow arrangements. The results show that a recirculation bubble downstream the jet exists for jet angles larger than 40-deg, but it vanishes when the angle is less than 30-deg, which is in good accordance with the experiments. The blowing ratio has a large effect on the size of the recirculation bubble, and consequently on the film cooling effectiveness. In addition, the influence of boundary conditions for the jet and cross-flow are also addressed in the paper.
Metabolic Mass Transfer Effect in Monotonic and Non-Monotonic Growth of Micro-Organisms
Authors: Vadasz Peter , Vadasz Alisa S.
Published: December 2008

A proposed Neoclassical growth theory of micro-organisms is shown to apply in a much wider variety of cases, from micro-level organisms via animal and human cell growth and up to the macro-level populations encountered in ecology. Including the metabolic mass transfer effects, that are an essential ingredient of the Neoclassical Theory, allows for the recovery of substantial and distinct phenomena observed experimentally. The proposed theory identifies the mechanism controlling the Lag phase, a result that holds impressive future potential in diverse applications. Different theoretical results are presented and compared with experimental data to substantiate the claim that the model based on the Neoclassical Growth Theory is the only available model that produces results, which are consistent with all experimental evidence.
The Impact of Metabolic Mass Transfer and Abiotic Resource Dynamics on Microbial Growth
Authors: Vadasz Johnathan J.
Published: December 2008

Accounting for metabolic mass transfer and abiotic resource dynamics is not common in modeling microbial population growth. In this paper it is demonstrated that the latter is an essential feature that needs to be considered if reliable results are sought. The results of a model that takes the metabolic mass transfer and abiotic resource dynamics into account are shown to capture a variety of features that appear in experiments such as a Lag phase, a Logarithmic Inflection Point, growth followed by decline and oscillations. The results have a wide variety of implications and applications, from food microbiology and wine fermentation, up to human cell growth, where the latter includes tumor growth.
Entropy Generation Minimization of Fully Developed Internal Convective Flows With Constant Heat Flux
Authors: Ratts Eric B. , Raut Atul G.
Published: December 2008

This paper addresses the thermodynamic optimum of single-phase convective heat transfer in fully developed flow for uniform and constant heat flux. The optimal Reynolds number is obtained using the entropy generation minimization (EGM) method. Entropy generation due to viscous dissipation and heat transfer dissipation in the flow passage are summed, and then minimized with respect to Reynolds number based on hydraulic diameter. For fixed mass flow rate and fixed total heat transfer rate, and the assumption of uniform heat flux, an optimal Reynolds number for laminar as well as turbulent flow is obtained. In addition, the method quantifies the flow irreversibilities. It was shown that the ratio of heat transfer dissipation to viscous dissipation at minimum entropy generation was 5:1 for laminar flow and 29:9 for turbulent flow. For laminar flow, the study compared non-circular cross-sections to the circular cross-section. The optimal Reynolds number was determined for the following cross-sections: square, equilateral triangle, and rectangle with aspect ratios of two and eight. It was shown that the rectangle with the higher aspect ratio had the smallest optimal Reynolds number, the smallest entropy generation number, and the smallest flow length.
Natural Convection From Protruding Heat Sources in a Channel
Authors: Icoz Tunc , Wang Qinghua , Jaluria Yogesh
Published: December 2008

Natural convection has important implications in many applications like cooling of electronic equipment due to its low cost and easy maintenance. In the present study, two-dimensional natural convection heat transfer to air from multiple identical protruding heat sources, which simulate electronic components, located in a horizontal channel has been studied numerically. The fluid flow and temperature profiles, above the heating elements placed between an adiabatic lower plate and an isothermal upper plate, are obtained using numerical simulation. The effects of source temperatures, channel dimensions, openings, boundary conditions, and source locations on the heat transfer from and flow above the protruding sources are investigated. Different configurations of channel dimensions and separation distances of heat sources are considered and their effects on natural convection heat transfer characteristics are studied. The results show that the channel dimensions have a significant effect on fluid flow. However, their effects on heat transfer are found to be small. The separation distance is found to be an important parameter affecting the heat transfer rate. The numerical results of temperature profiles are compared with the experimental measurements performed using Filtered Rayleigh Scattering (FRS) technique in an earlier study, indicating good agreement. It is observed that adiabatic upper plate assumption leads to better temperature predictions than isothermal plate assumption.
A Numerical Parametric Study of the Growth of Ice Formation Around a Vertical Tube, During the Full Charging Process of an Indirect, Area-Constrained, Ice-on-Pipe Storage Tank
Authors: Stampa Cleyton S. , Nieckele Angela O. , Braga Sergio L.
Published: December 2008

A parametric numerical investigation regarding the ice layer growth outside a vertical tube is investigated. It encompasses heat transfer and removal of energy, applicable to indirect, area-constrained, ice-on-pipe storage tanks. The study is carried out in a vertical annulus, with the inner vertical wall representing one of the tubes packed into a typical storage tank. Further, the outer vertical wall determines the maximum border for the ice layer growth under the conditions established in the present study, corresponding to a full charging process in such devices. Our task is to provide helpful qualitative results for the investigation of ice storage tank heat transfer performance, considering changes in the following parameters: aspect ratio, radius ratio, Grashof and Stefan numbers. For the mathematical model adopted to simulate transient natural convection of water with phase-change (solidification), it was utilized a model based on the finite volume method to solve the set of coupled conservation equations of mass, momentum and energy.
Transpiration Cooling Using Porous Triple-Laminated Plates
Authors: Wu H. L. , Peng X. F.
Published: December 2008

Transpiration cooling using porous triple-laminated plates was numerically investigated to understand the associated flow mechanism and heat transfer characteristics with/without crossflow. The flow structure and heat transfer behavior are very similar in the two laminate gaps, and crossflow has little influence on them. The cooling performance shows very good uniformity and high efficiency. Violent impingement and turbulent flow inside the plate contribute greatly to local heat transfer intensification. The cooling efficiency might be further improved with enhancement of film cooling effect, by enlarging the discharge holes to decrease the local jet-to-crossflow velocity ratio, or by using inclined discharge holes to increase the film attaching ability.
Experiments on Hydrodynamic and Thermal Behaviors of Thin Liquid Films
Authors: Ozar B. , Cetegen B. M. , Faghri A.
Published: December 2008

An experimental study of heat transfer into a thin film of liquid water on a rotating disk is described. The film was introduced from a flow collar at the center of a heated, horizontal disk at a fixed initial film thickness with a uniform radial velocity. Radial distribution of the disk surface temperatures was measured using a thermocouple / slip ring arrangement. Experiments were performed for a range of liquid flow rates between 3.0 lpm and 15.0 lpm corresponding to Reynolds numbers (based on the liquid inlet gap height and velocity) between 238 and 1188. The angular speed of the disk was varied from 0 rpm to 500 rpm. The local heat transfer coefficient was determined based on the heat flux supplied to the disk and the temperature difference between the measured disk surface temperature and the entrance temperature of the liquid onto the disk. The local heat transfer coefficient was seen to increase with increasing flow rate as well as increasing angular velocity of the disk. Effect of rotation on heat transfer was largest for the lower liquid flow rates with the effect gradually decreasing with increasing liquid flow rates. Semi-empirical correlations are presented in this study for the local and average Nusselt numbers. In addition to the heat transfer characterization, the thickness of the liquid film on the disk surface was measured by an optical method, including the characteristics of the hydraulic jump and the subcritical and supercritical flow regions.
The Effect of Gases Emitted From Batch/Glass Reactions on the Combustion Space Flow Field
Authors: Golchert Brian M. , Chang Shen-Lin , Zhou C. Q.
Published: December 2008

The concept of ‘coupling’ a combustion space CFD code to a code that models the molten glass flow is not a new idea. However, this concept has been limited to an energy coupling; the heat flux calculated in the combustion space model is used to drive the glass melt while the calculated surface temperature is used in the radiative heat transport calculation in the combustion space computation. In reality, there is significant mass (mostly gas) transport from the batch/molten glass into the combustion space. This is an important phenomenon to be modeled since these gases, particulates and volatiles will be removed from the combustion chamber and hence raise environmental considerations. In addition, these released gases have a distinct influence on the flow field in the combustion space. The ANL Glass Furnace Model has been augmented to calculate the chemical reactions that release gases in the batch and in the melt. This work presents preliminary results indicating the effect of these gases on the combustion space.
Bubble Dynamics Around a Vertical Heating Element During Nucleate Pool Boiling
Authors: ElGafy Ahmed Z. , Lafdi Khalid
Published: December 2008

The present work is a description analytical study to express the bubbles dynamic during nucleate pool boiling arises from boiling of water by a vertical cylindrical heating surface under atmospheric pressure conditions. The study includes the description of bubbles formation, detachment, rise and growth around the vertical heating surface. Different formulas are considered to evaluate each of, the number of active nucleation sites on the heating surface, bubbles departure diameter along the heating surface length, bubbles frequency, bubbles rising velocity and growth rate of the diameter of the bubble through its uprising motion around the vertical heating surface. An equation for evaluating the vapor bubble core volume and its thickness around a vertical heating surface is introduced.
An Experimental Study of Heat Transfer With an Impinging Circular Water Jet
Authors: Leng Hao , Guo Liejin , Zhang Ximin , Min Hongbin , Wang G.-X.
Published: December 2008

Impinging jet is widely used in both traditional industrial and new high-tech fields. High efficiency heat transfer in impinging jet cooling makes it an important method for heat transfer enhancement, in particular in cooling of electronic devices with high heat density. This paper presents an experimental study of heat transfer by an impinging circular water jet. A Constantan foil with the size of 5 mm × 5 mm was used to simulate a microelectronic chip with heat generated by passing an electrical current through the foil. A high heat flux over 106 W/m2 was achieved. The surface temperature was measured by a thermocouple glued onto the back surface of the foil. Both a free surface jet and a submerged jet were investigated. Effect of the nozzle-to-surface spacing as well as the jet speed at the exit of the nozzle on cooling was examined. By positioning the jet away from the center of the heating foil surface, the radial variation of the heat transfer coefficients over the foil was also investigated. Quantitative heat transfer data have been obtained and analyzed.
A Suggestion on Heat Sink Simplification on Natural Convection
Authors: Kim Won Nyun , Kim Choong Ki
Published: December 2008

Heat sink is commonly found in electronic systems. For its optimization, numerical computation is introduced. However, narrow gaps between the fins of heat sink have been a troubling factor. That increases the number of grid excessively, and results in increased computation time. The quality of grid can be poor and that halt the accuracy of computed numerical solution. To avoid these problems, many simplification methods are proposed by simplifying complex heat sink. The most popular example is regarding the array of fins as flow resistance from hydraulic point of view and working fluid with different thermal conductivity for thermal equivalence [1]. Its thermal conductivity can be determined according to well-known relationship between Nu, Re, and Pr (see [2, 3]). This simplification presents many advantages but it is not applicable to natural convection. In this paper, a modified model is suggested to extend the simplification to natural convection, which is still popularly applied to electro cooling systems. With the results of [4], thermal conductivity of flow resistance region is iteratively. The modified model is verified by computing flow and thermal fields of PDP. Applying this model to fanless PDP, the number of total grid is reduced by 38.5% percents and corresponding computation time was saved while the accuracy of computed solution is kept undamaged.