bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Flow Orientation on Critical Heat Flux in Subcooled Flow Boiling
Authors: Bower Jason S. , Klausner James F.
Published: March 2009

Recent work has demonstrated that as the bulk convective velocity in subcooled nucleate flow boiling increases, the heat transfer tends to become independent of flow orientation with respect to gravity. There is significant interest in developing heat exchangers for next generation spacecraft that operate in the gravity-independent flow boiling regime. In order to develop such heat exchangers it is important to understand the effect of gravity on the critical heat flux and to determine whether a gravity on the critical heat flux and to determine whether a gravity-independent flow boiling critical heat flux regime exists. This work describes subcooled flow boiling experiments where the critical heat flux is measured over a range of flow orientations with respect to gravity: 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. It has been found that at low bulk flow velocities there is a large variation of critical heat flux with different flow orientations. At large convective velocities, the variation of critical heat flux with different flow orientations is significantly diminished. It appears that with further increases in bulk flow velocity, a gravity-independent critical heat flux regime exists, although the current experimental facility was not capable of operating at those flow conditions.
Cryogenic Two-Phase Flow During Chilldown
Authors: Velat Christopher , Jackson Jelliffe , Klausner James F. , Mei Renwei
Published: March 2009

This work describes an experimental investigation of the transient two-phase flow behavior of liquid nitrogen flowing through a pipeline during the chilldown process. The evaporation process and flow regime transitions were observed during chilldown. Initially pure vapor is observed in the visual test section. Then a moving film boiling front is observed to move through the test section. The length of the film boiling front appears to be very small compared with the length of the facility pipeline. After the film boiling front passes through the test section, a very high velocity stratified two-phase flow appears with a small liquid film thickness. As the pipeline is chilled, the film thickness grows. It appears that the high velocity two-phase flow suppresses nucleate boiling, and the dominant heat transfer mechanism is two-phase bulk turbulent convection. Evaporation occurs at the liquid/vapor interface. A high-speed video camera is used to observe the flow regime transitions during the transient chilldown process. Flow regime comparisons have been made with the Steiner [1], Kattan-Thome-Favrat [2], and Baker [3] flow regime maps.
Theory of Two-Phase Biofluid Flow Dynamics and Selected Applications
Authors: Kleinstreuer C. , Longest P. W. , Zhang Z.
Published: March 2009

Examples of two-phase flows in the human body include particle-hemodynamics in branching arteries and toxic/therapeutic air-particle mixtures in the respiratory system. In this review, the fundamentals of modeling dilute particle suspensions are presented with computer applications to the geometric design of bypass graft-ends and the prediction of local aerosol depositions in the human upper airways. For the latter project, aerosols in the nano- and micro-size ranges, solid and liquid particles as well as evaporating droplets are considered. Specifically, the particle-hemodynamics project deals with the prediction of aggravating two-phase flow events leading to arterial diseases, such as atherosclerosis and hyperplasia, and subsequently the design of bypass grafts mitigating post-operative complications. The lung-aerosol project requires accurate and realistic computations of laminar-to-turbulent airflows and toxic (or therapeutic) particle depositions in the human airways for two applications: dosimetry-and-health-effect assessments of toxic particles and optimal drug aerosol delivery by inhalation. Two-phase flow results from different case studies are presented.
Effects of Flame Development and Structure on Thermo-Acoustic Oscillations of Premixed Turbulent Flames
Authors: Sathiah Pratap , Lipatnikov Andrei N. , Chomiak Jerzy
Published: March 2009

Non-stationary confined premixed turbulent flames stabilized behind a bluff body are studied. A simple kinematic model of such flames was developed by Dowling [9] who reduced the combustion process to the propagation of an infinitely thin flame at a constant speed. The goal of this work is to extend the model by taking into account the structure of premixed turbulent flames and the development of turbulent flame speed and thickness. For these purposes, the so-called Flame Speed Closure model for multi-dimensional simulations of premixed turbulent flames is adapted and combined with the aforementioned Dowling model. Simulations of the heat release rate dynamics for ducted flames due to oncoming flow oscillations have been performed. Typical results show that the oscillations of the integrated heat release rate follow the oncoming flow velocity oscillations with certain time delay. The delays computed using the Dowling and the above approach are different, thus indicating the importance of resolving flame structure when modeling ducted flame oscillations.
Turbulent Heat Transfer to Heavy Liquid Metals in Circular Tubes
Authors: Cheng X. , Batta A. , Chen H. Y. , Tak N. I.
Published: March 2009

The present paper gives a brief literature review on turbulent heat transfer in heavy liquid metals (HLM), especially liquid lead-bismuth eutectic (LBE). Some models available in the open literature on heat transfer and turbulent Prandtl number are assessed. In addition, CFD analysis is carried out for circular tube geometries. The effect of turbulence models, mesh structure and turbulent Prandtl number on the numerical results is studied. Application of ε-type turbulence models with scalable wall function shows less dependence of the numerical results on mesh structure than the ω-type turbulence models with automatic wall treatment. The turbulent Prandtl number affects strongly the heat transfer performance. Comparison between the CFD results, heat transfer correlations and heat transfer test data reveals a decrease in turbulent Prandtl number by increasing Reynolds number. Based on the results achieved, recommendations are made on correlations of heat transfer and turbulent Prandtl number for LBE flows.
Vapor Transport Controlled Process Models for AlN Bulk Sublimation Growth
Authors: Wu Bei , Zhang Hui
Published: March 2009

Sublimation vapor transport method is a widely used technique for the production of optoelectronic materials, such as AlN single crystals. Inductively heated method is most commonly used in high temperature materials processing. In the literature, a one-step reaction with two vapor species, i.e. aluminum (Al) vapor and nitrogen (N2) gas, is usually assumed and a diffusion-controlled growth mechanism is used with thermodynamic equilibrium calculations. In the growth experiments, crystal growth may be in the kinetic controlled region, the interplay between surface kinetics and vapor transport is important. Temperature field with inductively heated method will be simulated in this paper. Afterwards, three growth models are proposed. One model is called the traditional model assuming thermodynamic equilibrium and diffusion as the rate-limiting process, and two other models are developed based on equilibrium partial pressure of either aluminum vapor or reaction nitrogen gas. The predicted growth rates by three models are compared. The advantage and disadvantage of different models are discussed.
Parametric Investigation of Brayton Cycle for High Temperature Gas-Cooled Reactors
Authors: Oh Chang H. , Moore Richard L.
Published: March 2009

The Idaho National Engineering and Environmental Laboratory (INEEL) has investigated a Brayton cycle efficiency improvement on a high temperature gas-cooled reactor (HTGR) as part of Generation-IV nuclear engineering research initiative. In this study, we are investigating helium Brayton cycles for the secondary side of an indirect energy conversion system. Ultimately we will investigate the improvement of the Brayton cycle using other fluids, such as supercritical carbon dioxide. Prior to the cycle improvement study, we established a number of baseline cases for the helium indirect Brayton cycle. The baseline cases are based on a 250 MW thermal pebble bed HTGR. In this study, we used the HYSYS computer code for optimization of the helium Brayton cycle and the balance of plant (BOP). In addition to the HYSYS process optimization, we performed parametric study to see the effect of important parameters on the cycle efficiency. For these parametric calculations, we also used a cycle efficiency model that was developed using the Visual Basic computer language. The results from this study are applicable to other reactor concepts such as a very high temperature gas-cooled reactor (VHTR), fast gas-cooled reactor (FGR), supercritical water reactor (SWR), and others. As part of this study we are currently investigated single-shaft vs. multiple shaft arrangement for cycle efficiency and comparison, which will be published in the next paper. The ultimate goal of this study is to use supercritical carbon dioxide for the HTGR power conversion loop in order to improve the cycle efficiency to values great than that of the helium Brayton cycle. This paper includes preliminary calculations of the steady state overall Brayton cycle efficiency based on the pebble bed reactor reference design (helium used as the working fluid) and compares those results with an initial calculation of a CO2 Brayton cycle.
Three Dimensional Velocity Measurements in an Automotive-Size Evaporator Using Particle Image Velocimetry
Authors: O’Halloran Steven P. , Beck B. Terry , Hosni Mohammad H. , Eckels Steven J.
Published: March 2009

The flow distribution inside of an evaporator is important to fully understand in order to optimize the design of the evaporator. A stereoscopic particle image velocimetry (PIV) system was used to measure single-phase water flow in a Plexiglas model of an automotive-sized evaporator. The evaporator is a “U-shape” type. Flow enters the inlet header and travels through a series of 26 parallel rectangular tubes. The tubes have a width of 15.5-mm, a flow gap (thickness) of 0.9-mm, and a length of 231-mm. The flow then enters the upper header and flows through another series of 26 parallel tubes to the outlet header. PIV measurements were only made within the headers due to the small size of the tubes, however detailed results were observed. In addition to the single-phase experimental results, computational fluid dynamics (CFD) simulations were conducted using the commercially available software Fluent, and the results compare well to the experimental results. Further work was conducted by injecting nitrogen into the flow to obtain two-phase flow under adiabatic conditions. Due to high vapor volume fractions, PIV could not be used for flow measurement, but a volume collection method was used to measure the flow of water through each tube. Significantly different flow distributions were observed at different inlet volume fractions of nitrogen and further investigation is underway.
Nascent Noise Reduction Strategies for Underwater Vehicles
Authors: Bandyopadhyay Promode R.
Published: March 2009

Noise reduction concepts applicable to small underwater vehicles and in the nascent stages of research, are considered. These questions are asked. How to lower the propulsor rotational rate while maintaining forward speed? How to reduce blade tonal? How to supplant the propulsor drive train, which is the main source of hull vibration and radiated noise? The highlights of progress in passive and active approaches are reviewed. The approaches are inspired by unsteady high lift principles employed in flying and swimming animals and by emerging artificial muscle technology.
Flow Friction Characteristics of Aluminum Foam Matrixes
Authors: Wu W. T. , Liu J. F. , Chiu W. C. , Hsieh W. H.
Published: March 2009

A universal correlation of friction characteristic was developed based on the measured pressure drop of air through foam matrixes. The pressure drops of seven types of aluminum foams with different porosities and pore densities were measured. The Reynolds number based on the equivalent spherical diameter of the foam ranged from 32.05 to 1289.32. The range of porosity was 0.87∼0.958. A universal empirical equation was developed to correlate the dimensionless pressure drop with the dimensionless flow velocity for all seven types of aluminum foams. The empirical correlation agreed well with experimental data. According to the empirical correlation and the measured data, the pressure drop introduced by foam matrixes was found to be much lower than that by granular matrixes at the same Reynolds number. The relationship between the friction factor and Re of foam matrixes is found to follow the trend of granular matrixes and can be classified into four regimes: Darcy-flow, inertial-flow, unsteady laminar-flow, and unsteady- and chaotic-flow regimes.
Experimental Investigation of Stratified, Buoyant Wakes
Authors: Kraft Wayne N. , Andrews Malcolm J.
Published: March 2009

The development of a buoyant plane wake has been investigated experimentally. A water channel has been used as a statistically steady experiment to investigate the plane wakes. Parallel streams of hot and cold water are initially separated by a splitter plate. The streams are oriented such that the cold fluid is above the hot fluid, resulting in an unstable stratification. At the end of the splitter plate, the two streams are allowed to mix and a buoyancy driven mixing layer develops. Downstream of the splitter plate, growth of the turbulent buoyancy-driven mix is disrupted by a cylinder. The cylinder is located at the centerline of the mixing layer and associated wake. As a result the dynamic flows of the plane wake and buoyancy driven mixing layer interact. Particle image velocimetry (PIV), and a high-resolution thermocouple system are used to measure the response of the plane wake to buoyancy driven turbulence. Velocity and density measurements are used as a basis from which we describe the transition, and return to equilibrium, of the buoyancy driven mixing layer. We found for wakes where buoyancy is driving the motion, a remarkably fast recovery of a Rayleigh-Taylor mix in the wake region.
Heat Transfer Augmentation by Ion Injection in an Annular Duct
Authors: Grassi Walter , Testi Daniele
Published: March 2009

The thermofluid dynamic effects of ion injection from sharp metallic points added perpendicularly to the inner wire of a short horizontal annulus were experimentally investigated. A dielectric liquid (FC-72 by 3M) was weakly forced to flow in the duct, which was uniformly heated on the outer wall. A d.c. voltage as high as 22 kV was applied to the inner electrode, while the heated wall was grounded. Both the laminar and the turbulent mixed convection regimes were obtained, varying the imposed flow rate. Once an electric field is applied, the flow is dramatically modified by the jets of charged particles, which transfer their momentum to the neutral adjacent ones. Different injection strengths were obtained on the emitters, because the shape of the point tips was not controlled at the micro-scale. Nusselt number distributions were obtained azimuthally and longitudinally, monitoring the wall temperatures. In all cases, heat transfer turned out greatly enhanced in the proximity of the emitters, without a significant increase in pressure drop through the test section and with a negligible Joule heating, making this technique very attractive for application in compact heat exchangers.
The Metal-Dielectric Transition in Short-Pulse Laser Ablation of Metals
Authors: Porneala Cristian , Willis David A.
Published: March 2009

Phase explosion is an explosive liquid-vapor phase change that occurs during short pulse laser ablation. Phase explosion results from homogenous vapor nucleation in a superheated liquid phase as the surface temperature approaches the thermodynamic critical temperature, Tc. For a metastable liquid, the upper limit of superheating is approximately 0.9Tc, above which the rate of homogeneous nucleation rises dramatically. Prior to reaching the superheat limit however, a “dielectric transition” is expected to occur at approximately 0.8Tc. The dielectric transition is the transition of an electrically conductive material to a non-conducting state due to large fluctuations in material properties. One consequence of the dielectric transition is that the material will become semi-transparent. Until now, little work has been performed to understand the role of the dielectric transition in laser ablation, and many questions remain about how the surface will rise above 0.8Tc if the surface is semitransparent and only weakly absorbing. This work investigates the role of the dielectric transition with a one-dimensional numerical model for heat transfer and phase change and includes the effect of the metal to dielectric transition. The model is used to simulate heating of aluminum by a Nd:YAG laser with a 7 nanosecond pulse width (FWHM) at the fundamental wavelength of 1064 nm. Calculations of the transient temperature field, melt depth, and depth of the dielectric layer are obtained. Estimates of the absorption coefficient of a metal surface above the metal-dielectric transition are made from correlations found in the research literature. The value of the absorption coefficient is shown to be a critical parameter for determining the energy density required to reach 0.9Tc.
Ammonia Desorption in Microscale Fractal-Like Branching Flow Networks
Authors: Cullion Rebecca N. , Mouchka Gregory A. , Pence Deborah V. , Liburdy James A. , Kanury A. Murty
Published: March 2009

An experimental study is presented in which ammonia is desorbed from a binary mixture of ammonia and water using a network of microscale fractal-like branching channels. The objectives of the study are (1) to determine feasibility of desorption by boiling the fluid mixture inside a microchannel array, and (2) to quantify the rate of desorption as a function of applied heat flux and strong solution flow rate. The desorber is disk-shaped with an inlet plenum at the center and the flow network branching radially outward toward the edge of the disk. The flow network was chemically etched in a stainless steel disk with a nominal terminal branch hydraulic diameter of 88 μm. Inlet and exit mass flow rates, temperatures, pressures and mass fractions were measured along with the heat flux applied to the surface of the desorber disk. Results indicate that desorption rates and water vapor content in the refrigerant vapor stream increase with increases in heat flux and decrease with strong solution mass flow rate.
Spontaneous Imbibition Into Hele-Shaw Cells Under Temperature Differences
Authors: Sa´nchez M. , Medina A. , Higuera F. J. , Montanes J. L.
Published: March 2009

In this work has been analyzed theoretically and experimentally imbibition under the influence of a longitudinal temperature gradient. Imbibition into vertical and circular cells was analyzed using the lubrication theory. The temperature-dependence of viscosity and surface tension were included in the lubrication equations. In both cases was found that theory describes very well the experimental trends. Moreover, experiments using glycerol show a notable acceleration of the fronts under positive gradients.
Thermal Analysis of Mixed Electroosmotic and Pressure Driven Flows in Two-Dimensional Straight Microchannels
Authors: Horiuchi Keisuke , Dutta Prashanta
Published: March 2009

Analytical solution for the temperature distributions, heat transfer coefficients, and Nusselt numbers of steady electroosmotic flows with an arbitrary pressure gradient are obtained for two-dimensional straight micro-channels. The thermal analysis considers interaction among inertial, diffusive and Joule heating terms in order to obtain the thermally developing behavior of mixed electroosmotic and pressure driven flows. In mixed flow cases, the governing equation for energy is not separable in general. Therefore, we introduced a new method that considers the extended Graetz problem. Heat transfer characteristics are presented for low Reynolds number micro-flows where the viscous and electric field terms are very dominant. Analytical results show that the heat transfer coefficient of mixed-electroosmotic and pressure driven flow is smaller than that of pure electroosmotic flow. For the parameter range studied here (Re<0.7), the fully developed Nusselt number is independent of the thermal Peclet number and pressure gradient. Moreover, in mixed electroosmotic and pressure driven flows, the thermal entrance length increases with the imposed pressure gradient.
Analytical Solution for a Transient Three-Dimensional Temperature Distribution in Laser Assisted Machining Processes
Authors: Gutierrez Gustavo , Araya Juan Guillermo
Published: March 2009

Laser assisted machining is a recent technique for machining brittle ceramic materials by first softening them by heating the material with a laser beam, without reaching the melting point and, in this way, minimizing the damage of the workpiece and tool. The use of a laser source is a common procedure in numerous electronic and optical material processes. This research presents a new analytical solution to determine transient temperature distributions in a finite solid when it is heated by a moving heat source. The analytical solution is obtained by solving the transient three-dimensional heat conduction equation in a finite domain by the method of separation of variables. Previous studies focus on analytical solutions for semi-infinite domains. In this study, for a moving heat source, the temperature field is obtained in a finite domain. The purpose of this study is to obtain an analytical solution to predict transient temperature distribution in a finite solid due to a moving heat source.
Transient Analysis of a Flat Heat Pipe Working as a Passive Air Conditioning System in Residential Buildings
Authors: Gutierrez Gustavo , Catan˜o Juan
Published: March 2009

In this study, a flat heat pipe is proposed to use the enormous heat capacity of the soil as a heat sink to remove heat from the ambient and a the same time provides a heat pathway for the external irradiation so that the thermal insulation of the construction is significantly improved. Heat pipes offer an effective and attractive alternative since they work with a small temperature budget. Heat pipes use the latent heat of vaporization as a heat transfer mechanism and provide a very high effective conductivity. Heat pipes are not thermodynamics cycles then do not suffer from the Carnot limitation. Temperature drop in the vapor core is very small and the vapor provides an almost isothermal condition. The operational temperature of a heat pipe is controlled by the conditions in the condenser (in this case, the soil). So, providing an effective heat transfer pathway in the condenser, the vapor temperature will be closed to the temperature of the soil. Soil temperature does not fluctuate bellow certain distance from the surface and in a tropical climate is always cooler that the ambient and can function as a heat sink. As part of this research, a full transient analysis of the operation of a flat heat pipe is carried out. With the insight of numerical results, a flat heat pipe thermal panel can be constructed and tested to verify the feasibility and actual performance of the flat heat pipe panel.
Effect of Bipolar Plate Cooling on Fuel Cell Heat Generation
Authors: Goodman M. Trent , McNeill S. R. , Van Zee J. W. , Shimpalee S. , Khan J. A.
Published: March 2009

Heat generation inside the fuel cell is dependent on current densities at the fuel cell’s electrolyte membrane. When the moisture levels are too high, there is a flooding of the electrolyte membrane which prevents the hydrogen and oxygen gases from reaching the membrane. Whereas if the moisture levels are too low then an increase in ohmic loses across the membrane results in decreased energy production. A numerical study was performed to understand how the fluid velocity and thermal conductivity of the bipolar plates affects energy production inside the fuel cell. This was accomplished by using two models, one for the fuel cell and the other for the bipolar plate. The fuel cell model included both the conductive and convective heat transfer as well as the electrochemical heat generation. This model included calculations for determining moisture content which was included in the electrochemical reaction. After setting the model’s geometric configuration and fuel flow rates, this model required the temperature of the bipolar plate model as its input. The bipolar plate mode included the conductive and convective heat transfer with a fluid flowing through a set micro channel configuration. After setting its geometric configuration and flow rates, its input was the heat flux from the fuel cell model. An iterative approach was used to reach steady state convergence between the two models. The effect of varying the flow rates through the bipolar plate on power production was studied. These studies show how low flow rates inhibit power production.
Modeling of a Hybrid Renewable Energy System “HyRES”
Authors: Altaii K. , Bradway A. , Al-Jumaily A. M.
Published: March 2009

This paper presents the modeling and simulation of a hybrid renewable-energy system. The sizing, availability, and contribution of solar photovoltaic, wind energy and hydro energy can be simulated to determine the viability, stability, and cost effectiveness of such systems. The model allows the user to enter site specific data (hourly, daily, monthly, and annually) such as solar radiation, wind speed and precipitation. Users can select the type and size of wind turbine, hydroelectric turbine, photovoltaic panel and the electrical load placed on the hybrid renewable system. The simulation will determine the total power that can be produced on an hourly, daily, monthly and annual basis, the optimum combination of renewable energies, and usage/storage of each type of renewable energies, given the specified system and the collected data. With the help of HyRES, the model, one can determine which hybrid renewable energy system would best suit a specific site, and also help to determine which type of wind turbine, hydroelectric turbine, or photovoltaic panels would best complement each other for that site.
Mechanisms of Enhanced Heat Transfer for Oscillating Circular Cylinders
Authors: Pottebaum Tait , Gharib Mory
Published: March 2009

Experiments were conducted to determine the relationship between wake structure and heat transfer for an oscillating circular cylinder in cross-flow. An internally heated cylinder was suspended in a water tunnel and oscillated transverse to the freestream. The cylinder’s heat transfer coefficient was measured over a wide range of oscillation amplitudes and frequencies. By comparing these results to the known wake mode regions in the amplitude-frequency plane, relationships between wake mode and heat transfer were identified. Representative cases were investigated further by using digital particle image thermometry/velocimetry (DPIT/V) to simultaneously measure the temperature and velocity fields in the near-wake. This revealed more detail about the mechanisms of heat transfer enhancement. The dynamics of the vortex formation process, including the trajectories of the vortices during roll-up, are the primary cause of the heat transfer enhancement.
Molecular Dynamics Simulation of Thermal Conductivity of Silicon Films Using Environment-Dependent Interatomic Potential
Authors: Wang Xinwei , Lawrence Cecil
Published: March 2009

In this work, nonequilibrium molecular dynamics is used to predict the thermal conductivity of nanoscale thin silicon films in the thickness direction. Recently developed environment-dependent interatomic potential for silicon, which offers considerable improvement over the more common Stillinger-Weber potential, is used. Silicon films of various thicknesses are modeled to establish the variation of thermal conductivity with the film thickness. The obtained relationship between the thermal conductivity and the film thickness is compared with the results of the Lattice Boltzmann method, and sound agreement is observed.
Heat Transfer With a MicroPCM Suspension in Laminar Tube Flow Using a Realistic Melting Model
Authors: Cassidy Daniel A. , Gould Richard D.
Published: March 2009

A microPCM fluid is a suspension of particles of microencapsulated phase-change-material (PCM) in a carrier heat transfer fluid. Such fluids have potential as pumped loop cooling media for applications in aerospace electronics cooling, terrestrial energy systems, and recently in electric vehicle cooling. The melting process of the phase change material does not occur at a single temperature but rather occurs over a temperature range. In the past, numerical solutions to microPCM fluids have assumed a linear release of latent heat over the phase change region. In this paper four analytic curve fits to differential scanning calorimeter measurements are made to better model the actual melting/solidification behavior. The numerical scheme models hydrodynamically fully developed laminar flow in a circular tube using the enthalpy method. The microPCM fluid contains 23% by weight microencapsulated octacosane particles in a 50/50% by volume ethylene glycol/water carrier fluid. A prescribed uniform heat flux at the tube wall is used. The solutions for these four cases include mixed mean exit temperature, axial tube wall temperature and local heat transfer coefficient.
Melt Flow and Porosity Formation in Pulsed Laser Keyhole Welding
Authors: Zhou J. , Tsai H. L. , Wang P. C. , Menassa R.
Published: March 2009

Instead of CW (continuous wave) mode, pulsed mode laser welding has been popularly used in industry especially for Nd: YAG lasers. In pulsed mode laser keyhole welding, pores have been frequently observed near the root of the solidified weld. Our previous studies have indicated that the formation of porosity is caused by two competing mechanisms during the keyhole collapse process, and they are 1) the speed of solidification process for the melt surrounding the keyhole and 2) the speed of melt backfilling the keyhole. If the solidification process is too fast and completed before the keyhole is filled, pores will be formed. A technique to control the laser power trailing when it is turned off during the keyhole collapse process has been proposed and experimentally validated to postpone the solidification speed and, as a result, to prevent the porosity formation. However, this method fails for a “deep” keyhole. In this study, an electromagnetic force is used to control the melt backfill flow which is proved to be very effective in preventing porosity from occurring. A mathematical model has been developed to calculate the transient heat transfer and fluid flow during the keyhole formation and collapse processes in pulsed laser welding. The continuum model is used to handle the entire domain including solid phase, liquid phase and mush zone. The enthalpy method is employed to handle the absorption and release of latent heat during melting and solidification. The laser induced plasma inside the keyhole due to the Inverse Bremsstrahlung (IB) absorption is considered and the temperature distribution inside the keyhole is calculated. Both the Fresnel absorption and multiple reflections of laser beam energy at the keyhole walls are also considered. Parametric studies to determine the desired strength of the electromagnetic force and its duration under several laser welding conditions have been conducted. Computer animations showing the keyhole formation and collapse, metal flow, and possible formation of pores will be presented.
Phase-Change in Finite Slabs With Time-Dependent Convective Boundary Conditions
Authors: Roday Anand P. , Kazmierczak Michael J.
Published: March 2009

The heat balance integral method is used to solve one-dimensional phase-change problem in a finite slab with time-dependent convective boundary condition, [T∞,1(t)], applied at the left face. The temperature, T∞,1(t), decreases linearly with time; the other face of the slab is subjected to a constant convective boundary condition with T∞,2 held fixed at the ambient temperature. Two initial conditions are investigated: temperature of the solid below the melting point (subcooled), and initially at the fusion temperature (Tf). The temperature, T∞,1(t) at time t = 0 is so chosen such that convective heating takes place and the slab begins to melt (i.e., T∞,1(0)> Tf> T∞,2). Thus the solid-liquid interface proceeds forward to the right. As time continues, and T∞,1(t) decreases with time, the phase-change front slows, stops, and may even reverse direction. Hence this problem features sequential melting and freezing of the slab with partial penetration of the solid-liquid front before reversal of the phase-change process. It should, however, be noted that the study is limited to only one solid-liquid interface at any given time during the phase-change process (either melting or freezing) and that slight subcooling of the melt is allowed. The effect of varying the Biot number at the right face of the slab, for both the initial conditions, is also investigated to determine its impact on the growth/recession of the solid-liquid interface. Temperature profiles in both regions (liquid and solid) are reported in detail. The effect of a slower decay rate of T∞,1(t) on the phase-change process is also analyzed for the initial condition of the slab being at the fusion temperature.
Application of Computational Fluid Dynamics in the Simulation of a Radioactive Waste Vitrification Facility
Authors: Barringer Chris , Berkoe Jonathan , Rayner Chris , Huang Gene
Published: March 2009

The Columbia River in Washington State is at risk of radioactive contamination — a legacy of the cold war. Two hundred thousand cubic meters (fifty-three million US gallons) of radioactive waste is stored in 177 underground tanks at the Hanford site. This waste, which is 60% of the nation’s radioactive waste, is a product of 50 years of plutonium production for national defense. Bechtel National, Inc. has been commissioned by the U.S. Department of Energy to design and build a vast complex of waste treatment facilities to convert this waste into stable glass using a proven vitrification process. In this vitrification process, radioactive waste is mixed with glass-forming materials, then melted at approximately 1200C, and then poured into stainless steel canisters. These canisters are then permanently stored at secure aboveground or belowground facilities. The vitrification process results in a large amount of heat being stored in the hot glass. This heat must be removed within production schedule constraints. In the vitrification facility this glass is cooled in a small room called the Pour Cave. The room contains insulation to protect the concrete, and ventilation and water-cooled cooling panels to facilitate heat removal. The canister heat release rate depends on the thermal properties of the glass (which varies as the glass recipe changes), and the local environment, which includes other hot glass canisters. The cooling process is extremely complex. It is strongly coupled, and is driven by radiation, forced convection, natural convection and conduction heat transfer. Computational fluid dynamics, CFD, was used to predict the heat load to the ventilation system, the cooling panels and to the insulated concrete walls for a variety of operating conditions, providing the data needed for the design of these systems. Of particular interest was the temperature of the concrete, and whether or not design limits would be exceeded. The paper describes the special techniques that were developed to simulate the Pour Cave. This includes description of the modeling of the pouring of the glass, buoyancy modeling, and initialization of the simulation. Results are presented which show the predicted heat transfer characteristics throughout the Pour Cave.
Benchmarking in CFD
Authors: Shiva Prasad B. G.
Published: March 2009

CFD use is spreading fast to all industrial and non-industrial sectors. The progress in the science of Computational fluid Dynamics is not keeping pace with its own technological progress, particularly with reference to applications. Mathematical modeling of fluid flows in most cases is an art which depends on intuition. To gain more credibility in the complex computations of flows in modern machinery, it is not just sufficient to debate about validation, but is becoming increasingly necessary to at least start debating about establishing standards for development, distribution and use of CFD codes. Otherwise, not only the nickname of Colorful Fluid Dynamics might become more permanent, but the rate of growth of the technology of Computational Fluid Dynamics and the development of it’s underlying science might be hampered. This paper discusses the problems in application of CFD for industrial flows and suggests possible solutions and the need for unified action by the CFD community including the concept of ‘Global Benchmarking’.
Correlations Spanning the Entire Biot Number Range to Predict Near Steady State Condition in Transient 1-D Heat Conduction Problems
Authors: Kazmierczak M. , Sharma N.
Published: March 2009

Steady state criteria for vanishing small values of Biot number (lumped case) is well known and is reported in every undergraduate text on heat transfer. The heat conduction time scale for pure thermal diffusion problems (extremely large values of Biot number) is also a very well established fact and common knowledge to all well-schooled thermal engineers. However, to the best of the authors’ knowledge no attempt has been made so far to develop a generalized criterion encompassing the entire Biot number range. Hence, the objective of this paper is to construct a simple, but accurate, correlation to predict the onset of steady state for the three basic configurations (plane layer, cylinder, and sphere) for the complete range of Biot number from the high (Bi → ∞) to the low (Bi →0) Bi value limits, while spanning all values in between. Correlations are developed and reported in this paper such that they predict the transient time duration very close to those obtained from the theoretical solution to the problem. Moreover these proposed correlations are extremely simple in form and, as such, are ideal to be used by practicing thermal engineers in need of a quick estimate for the required time period to achieve steady state for problems that can be modeled from these basic geometries. A more accurate correlation, for the case of slab has also been proposed (containing two additional terms) which can be used if higher accuracy in the intermediate Biot number range were to be desired.
Mass Dispersion Coefficients for Turbulent Flow in an Infinite Porous Medium
Authors: Mesquita Maximilian S. , de Lemos Marcelo J. S.
Published: March 2009

In this work, mass dispersion tensors were calculated within an infinite porous medium formed by a spatially periodic array of longitudinally-displaced cylindrical rods. For the sake of simplicity, just one unit-cell, together with periodic boundary conditions for mass and momentum equations, and Neumann conditions for the mass concentration, was used to represent such medium. The numerical methodology herein employed is based on the control volume approach. Turbulence is assumed to exist within the fluid phase. High and low Reynolds k-e models were used to model such non-linear effects. The flow equations at the pore-scale were numerically solved using the SIMPLE method applied to a non-orthogonal boundary-fitted coordinate system. Integrated mass fraction results were compared with existing data in the literature.
Laminar Free Convection in Inclined Enclosures Filled With a Fluid Saturated Porous Medium
Authors: Braga Edimilson J. , de Lemos Marcelo J. S.
Published: March 2009

Detailed numerical computations for steady-state laminar natural convection within in oblique cavities totally filled with a fluid saturated porous medium is numerically analyzed using the finite volume method in a generalized coordinate system. The inclined walls are maintained at constant but different temperatures, while the horizontal walls are kept insulated. Governing equations are written in terms of primitive variables and are recast into a general form. Flow and heat transfer characteristics, (streamlines, isotherms and average Nusselt number), are investigated for Rayleigh number ranging from 103 to 104 and inclined angles ranging from 0° to 45°. In general, present results show good agreement with previous works. Analyses of important environmental and engineering flows can benefit from the derivations herein and, ultimately, it is expected that additional research on this new subject be stimulated by the work here presented.
Structure of the Rotor Tip Flow in a Highly Loaded Single-Stage Axial-Flow Pump Approaching Stall: Part II — Stall Inception — Understanding the Mechanism and Overcoming Its Negative Impacts
Authors: Goltz I. , Kosyna G. , Wulff D. , Schrapp H. , Stark U. , Saathoff H. , Bross S.
Published: March 2009

When reaching the stall point of an axial-flow pump, the pump head characteristic becomes unstable and the pump head suddenly drops. Before this happens however, at even higher flow rates the NPSH3 and the pump body and shaft vibrations increase dramatically. For effectively increasing the available operating range, it is essential to find a solution for all three problems without reducing the pump efficiency at design. The paper describes an experimental investigation on the outlined subject that gives insight into the flow phenomena leading to stall. Based on this knowledge a very simple type of casing treatment was chosen and investigated. It was found to satisfy all mentioned requirements. Subject to the investigations is a highly loaded axial-flow pump having a nq of 150 (SI units). The overall pump performance was investigated measuring pump head, efficiency, NPSH3, and casing as well as shaft vibrations. Further-more, oil flow pictures taken at the pump casing and at the rotor blades, and video captures of the cavitating core of the tip clearance vortex were analyzed for understanding the flow phenomena leading to stall (see also related paper Part I, Schrapp et al. (2004)). From the video captures it was realized that the behavior of the tip clearance vortex which was found to perform so-called spiral-type vortex breakdown is triggering stall inception in this machine.
Transport Through Single-Hole Baffles in Industry Hydrothermal Autoclaves With Three-Dimensional Flows
Authors: Li Hongmin , Braun Minel J. , Evans Edward A. , Wang G.-X.
Published: March 2009

Hydrothermal growth is the industrial preference to obtain high quality piezoelectric crystals. The industry growth process is carried out in autoclaves, cylindrical containers filled with an aqueous solution. The solution flows in industry autoclaves during growth are usually three-dimensional. A baffle is normally used to partition an autoclave into two chambers and reduce flow strength. In this paper transport through single-hole baffles of various are a openings in the three-dimensional flow is investigated systematically. It was found that a single-hole baffle is effective in controlling the fluid exchange and heat transfer between the two chambers. A smaller baffle opening leads to a more uniform thermal environment for growth. Flow structure and heat transfer data show that there is a pair of steady flow streams between the two chambers. However the heat exchange carried by this pair of streams, as well as heat exchange through molecular diffusion, is negligibly small. Transport through baffle opening is dominated by turbulence diffusion. Heat transfer analysis shows that heat flow rate depends on both the baffle opening area and the area of the chamber walls.
Steady Laminar Flows in Lower Half Heated Upper Half Cooled Rectangular and Cylindrical Enclosures: Comparison of the Transport Mechanism
Authors: Li Hongmin , Braun Minel J. , Evans Edward A. , Wang G.-X.
Published: March 2009

Flows in enclosures have been studied for various applications. Transport in lower half heated upper half cooled enclosures becomes a focus of research recently due to its application on hydrothermal crystal growth. Natural convection flows in hydrothermal autoclaves are driven by the temperature differential on the enclosure walls; lower half hot and upper half cooled. Due to the difficulties associated with visualization of flow in cylindrical enclosures, flows were experimentally visualized in a rectangular model autoclave. In this study, flows and transport mechanisms in rectangular and cylindrical enclosures are studied numerically in the steady laminar flow regime. Flow structures and transport mechanisms are analyzed and compared. Parametric studies on aspect ratios are carried out for both enclosures. Results show that flows in rectangular and cylindrical enclosures have the same transports mechanism. Fluid in the wall layers in one half form streams that feed into the center of the other half. However, for two kinds of enclosures quantitative differences exist on the stream formation.
A Finite Element Method for the Solution of Free Boundary Problem
Authors: Rai K. N. , Rai D. C.
Published: March 2009

A finite element method is presented for the solution of a free boundary problem which arises during planar melting of a semi-infinite medium initially at a temperature which is slightly below the melting temperature of the solid. The surface temperature is assumed to vary with time. Two different situations are considered (I) when thermal diffusivity is independent of temperature and (II) when thermal diffusivity varies linearly with temperature. The differential equation governing the process is converted to initial value problem of vector matrix form. The time function is approximated by Chebyshev series and the operational matrix of integration is applied, a linear differential equation can be represented by a set of linear algebraic equations and a nonlinear differential equation can be represented by a set of nonlinear algebraic equations. The solution of the problem is then found in terms of Chebyshev polynomial of second kind. The solution of this initial value problem is utilized iteratively in the interface heat flux equation to determine interface location as well as the temperature in two regions. The method appears to be accurate in cases for which closed form solutions are available, it agrees well with them. The effect of several parameters on the melting are analysed and discussed.