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IN THIS VOLUME


Heat Transfer

2004;():7-12. doi:10.1115/IMECE2004-59076.

Thermoacoustic engines and refrigerators use the interaction between heat and sound to produce acoustic energy or to transport thermal energy. Heat leaks in thermal buffer tubes and pulse tubes, components in thermoacoustic devices that separate heat exchangers at different temperatures, reduce the efficiency of these systems. At high acoustic amplitudes, Rayleigh mass streaming can become the dominat means for undesirable heat leak. Gravity affects the streaming flow patterns and influences streaming-induced heat convection. A simplified analytical model is constructed that shows gravity can reduce the streaming heat leak dramatically.

Commentary by Dr. Valentin Fuster
2004;():13-20. doi:10.1115/IMECE2004-59149.

The nonequilibrium molecular dynamics (NEMD) method has been used to calculate the lattice thermal conductivities of Ar and Kr/Ar nanostructures in order to study the effects of interface scattering, boundary scattering, and elastic strain on lattice thermal conductivity. Results show that interface scattering poses significant resistance to phonon transport in superlattices and superlattice nanowires. The thermal conductivity of the Kr/Ar superlattice nanowire is only about 1/3 of that for pure Ar nanowires with the same cross sectional area and total length due to the additional interfacial thermal resistance. It is found that nanowire boundary scattering provides significant resistance to phonon transport. As the cross sectional area increases, the nanowire boundary scattering decreases, which leads to increased nanowire thermal conductivity. The ratio of the interfacial thermal resistance to the total effective thermal resistance increases from 30% for the superlattice nanowire to 42% for the superlattice film. Period length is another important factor affecting the effective thermal conductivity of the nanostructures. Increasing the period length will lead to increased acoustic mismatch between the adjacent layers, and hence increased interfacial thermal resistance. However, if the total length of the superlattice nanowire is fixed, reducing the period length will lead to decreased effective thermal conductivity due to the increased number of interfaces. Finally, it is found that the interfacial thermal resistance decreases as the reference temperature increases, which might be due to the inelastic interface scattering.

Commentary by Dr. Valentin Fuster
2004;():21-25. doi:10.1115/IMECE2004-59297.

Pool boiling heat transfer phenomenon of artificial micro-cavity enhanced surfaces by wet etching MEMS fabrication immersed in a saturated dielectric fluid has been experimentally studied. The present research is to investigate pool boiling behavior including heat transfer performance and flow pattern of “artificial micro cavities” heating surfaces simulating microelectronic devices at atmospheric pressure with FC-72 as the working fluid. The test surfaces are the solid silicon based blocks with 200 μm diameter circular cavities with flat plane, 16 × 16, 25 × 25, 33 × 33 array and 50 μm depth. Effects of this double enhancement technique on critical heat flux (CHF) and nucleate boiling heat transfer in the horizontal orientation (microcavities are vertical) were also investigated. Results indicated that, in general, increasing the number of micro cavities also increase the enhanced surface area and it could increase the critical heat flux. The pronounced increase of boiling heat transfer coefficients with the application of the artificial micro-cavity to the heat surface were also investigated in this paper.

Commentary by Dr. Valentin Fuster
2004;():27-35. doi:10.1115/IMECE2004-59429.

Thin liquid films on solid surfaces are seen in a variety of systems including bubble growth during nucleate boiling and microgroove heat pipe evaporators and condensers. The small thickness of such films leads to difficult experimental observation of phenomena within various regions of the film: the wall-affected region, the bulk liquid, and the liquid-vapor interfacial region. A novel hybrid simulation methodology is used that combines a deterministic molecular dynamics simulation of the liquid regions with a stochastic treatment of the far-field vapor region boundary. In this simulation scheme, the imposed far-field pressure is iterated as the simulation is advanced in time until the mass in the system stabilizes at the specified temperature. This establishes the equilibrium saturation vapor pressure for the specified temperature as dictated by the intermolecular force interaction models for the fluid and molecules near the solid surface. Simulation results are presented for an argon liquid film on a metallic surface. The simulated surface tension values compare favorably with those from ASHRAE tables, although the simulated saturation density and pressure values behave as though the system is at a slightly higher temperature. The method presented here is a viable tool for simulating thin films on solid surfaces for systems operating far from the critical point.

Commentary by Dr. Valentin Fuster
2004;():37-46. doi:10.1115/IMECE2004-59463.

This paper presents numerical results for two-dimensional steady-state natural convection in a square cavity. The upper and lower walls are kept at different constant temperatures, whereas the lateral walls have certain thickness and thermal conductivity and are externally insulated. Under these conditions we deal with a conjugate natural convection problem in which the heat conduction in the lateral walls is coupled with the internal convection. The continuity, momentum and energy equations were solved by using the finite volume method. The results here presented include: (i) the temperature distribution in the lateral walls and in the fluid, (ii) the velocity field, and (iii) the average Nusselt number at the upper and lower walls. It was found that the steady state fluid flow is strongly dependent on the initial temperature condition, when the fluid is initially at rest. The PIV technique allowed us to get some experimental data by measuring the velocity field in a two-dimensional square cavity. A good agreement between numerical and experimental results was found.

Commentary by Dr. Valentin Fuster
2004;():47-51. doi:10.1115/IMECE2004-59503.

Experiments are performed to study the heat transfer characteristics during the power-on transient period from an array of 4 × 1 discrete heat sources in a vertical rectangular channel using air as the working fluid. The heat flux ranges from 1000 W/m2 to 5000 W/m2 . For 2 mm protrusion of the heater, the effect of heat fluxes and chip numbers are investigated and observed that the transient Nul strongly depends on the number of chips. Correlations are presented for individual chips as well as for overall data in the transient regime.

Commentary by Dr. Valentin Fuster
2004;():53-60. doi:10.1115/IMECE2004-59730.

Even though the theory of thermionic emission of electrons from bulk metals is well understood, discrete electron energy states exist when material length scales approach one nanometer, and the traditional treatment must be revised. This paper presents a theoretical development of thermionic emission from nanoscale materials. A general expression for the emitted current as a function of field, temperature and work function is established for a quantum wire. The results differ from those of 3-D bulk materials. Simulation of thermionic emission from a quantum wire is achieved with the non-equilibrium Green’s function (NEGF) method, which includes relevant mesocopic physics and has been widely applied to transport problems in nanostructures. The NEGF approach provides a powerful solution to modeling problems when interfacial transport effects between bulk and confined conductors are important. Both the theoretical and simulated results indicate a higher current density and thus higher energy conversion capacity than that of a bulk material with the same work function. Thus the quantum confined materials may provide a method for improving the capacity of direct energy conversion devices and systems.

Commentary by Dr. Valentin Fuster
2004;():61-65. doi:10.1115/IMECE2004-59840.

The investigation into possible applications of the thermal wave conduction theory to explain the spectacular enhancement of heat flux by a factor of between 1.4 to 2.5 in nanofluid suspensions is presented. While other possible explanations have been proposed to settle this discrepancy they were not investigated into sufficient detail for providing a definite answer and they all apply at the nano-scale level rather than bridging between the nano-scale effects and the macro-system investigated. The possible mechanisms proposed so far are Brownian motion, liquid layering at the liquid/particle interface, ballistic phonon effects, nanoparticle clustering as well as convection and wave effects. Furthermore, most available methods for measuring thermal conductivity assume and make use explicitly of the Fourier mechanism of heat transfer. If somehow the nano-level heat transfer effects impact profoundly on the resulting heat flux at the macro-level, possibly via wave phenomena, the whole concept behind the measurement device might be flawed. The present paper presents a possible way by which the transitions from nano-scale via the micro-scales towards the macro-scale occur, hence bridging the gap from nano devices to macro systems performance.

Commentary by Dr. Valentin Fuster
2004;():67-73. doi:10.1115/IMECE2004-59900.

Thermographic phosphors have emerged as a new technique for measuring heat fluxes, which relies on the temperature dependent intensity decay of thermographic phosphors. However, instead of reducing the intensity data to temperatures, heating rate is estimated. It has been shown that the heating rate can provide significantly better heat flux estimates than temperature measurements. Because the technique is new, little is known about the quality of heating rate estimates. Further, the heating rate estimation depends on the introduction of additional free parameters, which increases the uncertainty of the estimates. The analysis presented here indicates that sample rates must be one to two orders of magnitude greater than the frequency at which the heat flux must be known. Also, the sensitivity of the intensity to higher-order derivatives is small suggesting that derivatives beyond the heating rate are not accessible with single-shot data.

Topics: Heat , Phosphors
Commentary by Dr. Valentin Fuster
2004;():75-83. doi:10.1115/IMECE2004-59928.

The Loop Heat Pipe (LHP) under development is a next generation micro heat transfer device that utilizes the latent heat of a working fluid and has excellent transfer capacity compared with that of standard metallic cooling devices. A typical LHP consists of an evaporator, a reservoir (also called the compensation chamber), vapor and liquid lines, a subcooler, and a condenser. As heat is applied to the evaporator, all of the input energy goes into the evaporation of the liquid in the pores of the primary CPS wick or leak to the bottom. The nucleate boiling, which occurs beneath the primary wick in the evaporator, is a very significant phenomena. It affects critical operating issues, such as dry out of the primary wick. Using a clear evaporator machined from Pyrex glass, the nucleation, which occurred in the evaporator, was studied. De-ionized water was utilized as the working fluid.

Commentary by Dr. Valentin Fuster
2004;():85-92. doi:10.1115/IMECE2004-60080.

Critical heat flux enhancement by the electrohydrodynamic (EHD) effect has been analyzed quantitatively based on the increased frequency of liquid-vapor interface oscillations around the edge of the bubble. The majority of heat transfer occurs when the liquid film thickness becomes less than 50 μ m, which only occurs once per period. The main mechanism of heat flux enhancement induced by the EHD effect would be a result of an increase in surface tension due to the effect of electric lines of force. By representing the terms of the forces for a change in curvature and the surface tension resulting from the electric lines of force, the equation of the liquid-vapor instability was obtained and analyzed. Experimentally it has been shown that as the applied voltage increased, the periodic time interval of the thickness change was shortened. This effect reduces the potential for dryout of the liquid film by making the minimum thickness time period shorter. By measuring the pressure oscillation on the boiling surface, the change of the thin liquid film thickness and the dynamic shape of bubbles, the relationship among the pressure, the liquid film thickness and the bubble shape was clarified. Consequently, this model successfully explains the relationship between the applied voltage and the enhancement of the critical heat flux.

Commentary by Dr. Valentin Fuster
2004;():93-101. doi:10.1115/IMECE2004-60116.

An experimental study has been performed in order to determine the thermal characteristics of a specific concrete formula to be used for a large-scale tank-grouting project. The experimental results were incorporated into finite-element numerical simulations aimed at predicting local concrete temperatures over the duration of multiple concrete pours. The pours will occur in a stepwise fashion whereby each additional concrete layer will be added while the previous layers are still undergoing the curing process. The experimental portion of the project included a series of laboratory-scale tests aimed at determining the time-dependent adiabatic temperature rise of several concrete samples and the corresponding time-dependent concrete internal heating rates. Results of the experiments were incorporated directly into the finite-element thermal model. The finite-element simulations indicated that the pour schedule did not have a strong influence on the maximum temperature in the concrete.

Commentary by Dr. Valentin Fuster
2004;():103-110. doi:10.1115/IMECE2004-60243.

The objective of this fundamental study is to numerically predict the temperature along a fin cooled by natural convection and radiation and to compare with measurements. The physical situation considered is a horizontal fin with a cylindrical cross-section. One end of the fin is maintained at a constant elevated temperature, and the fin is sufficiently long so that heat loss from the tip is negligible. Heat is transferred by conduction along the fin and dissipated from the surface via natural convection and radiation. The effect of natural convection is described with a published correlation for a horizontal cylinder, and a simple model is used for the radiative heat transfer. A finite difference formulation that allows for variable fluid property effects is used to determine the temperature distribution along the fin. A comparison is made to experimental results, and the agreement between the model and experiment is very good. Results show that the heat loss due to radiation is typically 15%–20% of the total.

Commentary by Dr. Valentin Fuster
2004;():111-118. doi:10.1115/IMECE2004-60294.

A two-dimensional, mathematical model is adopted to investigate the development of buoyancy driven circulation patterns and temperature contours inside a rectangular enclosure filled with a compressible fluid (Pr=1.0). One of the vertical walls of the enclosure is kept at a higher temperature then the opposing vertical wall. The top and the bottom of the enclosure are assumed insulated. The physics based mathematical model for this problem consists of conservation of mass, momentum (two-dimensional Navier-Stokes equations) and energy equations for the enclosed fluid subjected to appropriate boundary conditions. The working fluid is assumed to be compressible through a simple ideal gas relation. The governing equations are discretized using second order accurate central differencing for spatial derivatives and first order forward finite differencing for time derivatives where the computation domain is represented by a uniform orthogonal mesh. The resulting nonlinear equations are then linearized using Newton’s linearization method. The set of algebraic equations that result from this process are then put into a matrix form and solved using a Coupled Modified Strongly Implicit Procedure (CMSIP) for the unknowns (primitive variables) of the problem. A numerical experiment is carried out for a benchmark case (driven cavity flow) to verify the accuracy of the proposed solution procedure. Numerical experiments are then carried out using the proposed compressible flow model to simulate the development of the buoyancy driven circulation patterns for Rayleigh numbers between 103 and 105 . Finally, an attempt is made to determine the effect of compressibility of the working fluid by comparing the results of the proposed model to that of models that use incompressible flow assumptions together with Boussinesq approximation.

Commentary by Dr. Valentin Fuster
2004;():119-128. doi:10.1115/IMECE2004-60296.

Earlier studies have shown that for cavities present on any heater surface to become active nucleation sites during boiling, they should entrap gas. The liquid penetrates the cavity due to the capillary and surface forces, but the exact physical mechanisms have not been fully quantified. The physical mechanisms of the gas entrapment process in closed-end microchannels, representing nucleation sites, are investigated in this study. Aside from the fluid properties, the width, length and depth of the cavities, as well as the static contact angle of the test liquid with the solid are considered as main parameters that influence the gas entrapment process. Test pieces consisted of micromachined silicon dices with glass bonded on top. Widths of 50, 30, 15 and 5μm were chosen based on size distribution probability. The mouth angle was 90° in all cases. Test pieces were held horizontally under a microscope equipped with a CCD camera. A drop of liquid was placed at the entrance of the microchannel and capillary and surface forces drive the liquid into the microchannel. Experiments show two main filling behaviors: (1) A uniform meniscus forms at the entrance and moves inwards, (2) Two menisci: one at the entrance and the other at the closed end of the microchannel. In some cases droplet formation at the walls was observed. A single meniscus typically forms for higher contact angles, while two menisci form for lower contact angles. In all cases, after a sufficient time interval (hours to days) the microchannel was completely flooded. In general, for a given depth, wider microchannels take longer to fill. Surface cleanliness and fabrication process also play a role in modifying the contact angle and hence the time taken to fill the microchannel. A comparison of the interface advancement in the microchannel with a simple mass diffusion model shows reasonable agreement.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2004;():129-135. doi:10.1115/IMECE2004-60578.

The generation and propagation of thermoacoustic waves in mildly supercritical carbon dioxide are investigated by solving the fully compressible form of the Navier-Stokes equations. Mildly supercritical fluids have high thermal conductivity; however the diffusion of heat in such fluids is very slow. Due to the high compressibility of the mildly supercritical fluids, the boundary layer along any heated surface expands and compresses adiabatically the whole fluid. We investigate these interesting phenomena via a high order numerical scheme. A square enclosure filled with supercritical carbon dioxide is considered as the computational domain. Thermally induced pressure waves are generated by heating the left wall. The thermodynamic properties of the slightly supercritical carbon dioxide are calculated via the NIST Standard Reference Database 12 [1].

Topics: Waves , Carbon dioxide
Commentary by Dr. Valentin Fuster
2004;():137-146. doi:10.1115/IMECE2004-60691.

The analytical solutions of unsteady heat conduction with variable thermal properties (thermal conductivity, density and specific heat are functions of temperature or coordinates) are meaningful in theory. In addition, they are very useful to the computational heat conduction to check the numerical solutions and to develop numerical schemes, grid generation methods and so forth. Such solutions in rectangular coordinates have been derived by the authors; some other solutions for unsteady point symmetrical heat conduction in spherical coordinates are given in this paper to promote the heat conduction theory and to develop the relative computational heat conduction.

Commentary by Dr. Valentin Fuster
2004;():147-153. doi:10.1115/IMECE2004-61054.

Investigators have been surprised with new thermal phenomena behind the recently discovered nanofluids, fluid with unprecedented stability of suspended nanoparticles although huge differences in the density of nanoparticles and fluid. For example, nanofluids have anomalously high thermal conductivities at very low volume fraction, strongly temperature-dependent and size-dependent conductivity, and three-fold higher critical heat flux than that of base fluids. In this paper, the thermal characteristics of free convection in a rectangular cavity with nanofluids such as water-based nanofluids containing 6nm copper and 2nm diamond are theoretically investigated with a new model of the thermal conductivity for nanofluids presented by Jang and Choi. In addition, based on theoretical results, the effects of various parameters such as the volume fraction, the temperature, and the size of nanoparticles on free convective instability and heat transfer characteristics in a rectangular cavity with nanofluids are suggested.

Commentary by Dr. Valentin Fuster
2004;():155-159. doi:10.1115/IMECE2004-61161.

Infrared (IR) thermography and, more recently, thermosonics have proven to be viable means of qualitative nondestructive evaluation (NDE). However, structural defects such as cracks observed through thermosonics can only be identified as “hot spots” indicating a general location of a defect without an accurate depiction of the dimensions or shape of the defect. This paper introduces a new technique dubbed Laser Scanning Thermal Probe, LSTP, which combines thermography with the use of heat application in strategic locations to observe the spatial heat flow patterns. LSTP provides the ability to record heat propagation across a defect area with temperature discontinuities forming due to differences in defect thermal diffusivity, thus providing information which can be used to characterize the defect such as crack length.

Commentary by Dr. Valentin Fuster
2004;():161-172. doi:10.1115/IMECE2004-61323.

Transient heat conduction in solid prismatic bars of constant cross-sectional area having uniform heat generation and unsteady momentum transport in infinitely long ducts of arbitrary but constant cross-sectional area are examined. In both cases the solutions are mathematically modeled using a transient Poisson equation. By means of scaling analysis a general asymptotic model is developed for an arbitrary non-circular cross-section. Further, by means of a novel characteristic length scale, the solutions for a number of fundamental shapes are shown to be weak functions of geometry. The proposed models can be used to predict the dimensionless mean flux at the wall and the area averaged temperature or velocity for the tube, annulus, channel and rectangle for which exact series solutions exist. Due to the asymptotic nature of the proposed models, it is shown that they are also applicable to other shapes at short and long times for which no solutions or data exist. The root mean square (RMS) error based on comparisons with exact results is between 2.2–7.6 percent for all data considered.

Topics: Heat conduction
Commentary by Dr. Valentin Fuster
2004;():173-180. doi:10.1115/IMECE2004-61381.

Flow in a rectangular enclosure with a square vertical cross-section normal to the longitudinal coordinate direction and having a strip on the lower horizontal surface which is heated to a uniform high temperature has been numerically studied. Two wall thermal boundary conditions have been considered. In one, the longitudinal vertical side walls are cooled to a uniform low temperature and the horizontal top surface is adiabatic while in the other the longitudinal vertical side walls and the horizontal top surface are cooled to a uniform low temperature. In both cases, the square vertical end walls of the enclosure are adiabatic. It has been assumed that the flow is laminar and that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces. The unsteady, three-dimensional governing equations, expressed in dimensionless form, have been solved using a finite-difference procedure. The solution was started with no flow in the enclosure. The solution, in general, has the following parameters: the Rayleigh Number, Ra, the Prandtl number, Pr, the dimensionless longitudinal length of the enclosure relative to the size of the square cross-section, Ay , the dimensionless width of the heated strip on the lower surface relative to the size of the square cross-section, wH , and the thermal boundary condition on the upper surface. Results have only been obtained for a Prandtl number of 0.7 and only results for wH = 1/3 will be presented. Results have been obtained for values of Ay between 0.5 and 2 for Rayleigh numbers up to 5×105 . In all cases, three-dimensional unsteady flow has been found to exist at the higher Rayleigh numbers. The conditions under which this unsteady flow develops and the effect of Ay on the variation of the mean Nusselt number with Rayleigh number and the effect of the wall surface boundary condition on these results has been investigated.

Commentary by Dr. Valentin Fuster
2004;():181-190. doi:10.1115/IMECE2004-61382.

Free convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat surfaces (termed “slats”) near the heated surface, these surfaces being, in general, inclined to the heated surface. The slats are pivoted about their center-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The temperature of the vertical isothermal surfaces has been assumed to be greater than the ambient temperature. Various cases have been considered to examine the effect of the geometry of the adiabatic surfaces above and below the heated plate, the effect of heat conduction in the slats and the effect of heat generation in the slats. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air being considered. The heat generation that can occur in the slats is then the result of solar energy passing through the window and being absorbed by the slats. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated surface has been examined.

Topics: Convection
Commentary by Dr. Valentin Fuster
2004;():191-197. doi:10.1115/IMECE2004-61701.

Despite its importance as a canonical two-dimensional flow, the laminar wall jet has not been extensively studied using modern computational fluid dynamic methods. As in the laminar boundary layer, existence of analytical self-similar solutions make the problem particularly attractive for validating CFD code, yet we have found little archival work in which it has been used for this purpose. In the present study, we present a numerical investigation of the steady, laminar, and two-dimensional plane wall jet with constant properties. A finite-volume approach is used to solve the governing equations using self-similar inlet boundary conditions for the velocity and temperature profiles. The thermal solution is investigated for isothermal boundary condition at the wall. Velocity and temperature profiles are reported at various locations downstream and show an excellent agreement with the similarity solution obtained by Glauert [1] and Schwarz, et al. [2] respectively. In addition, the skin friction coefficient and the Nusselt number are investigated and compared with the analytical solutions presented by Glauert [1] and Mitachi, et al. [3] respectively, and very good agreement is observed. Despite its simplicity, it is shown that proper convergence of the numerical solutions of the wall jet to the expected analytical solutions requires care in specification of the jet inlet conditions, and the boundary conditions on the computational domain boundaries.

Commentary by Dr. Valentin Fuster
2004;():199-210. doi:10.1115/IMECE2004-62113.

Giant Magnetoresistance (GMR) head technology is one of the latest advancement in hard disk drive (HDD) storage industry. The GMR head superlattice structure consists of alternating layers of extremely thin metallic ferromagnet and paramagnet films. A large decrease in the resistivity from antiparallel to parallel alignment of the film magnetizations can be observed, known as giant magnetoresistance (GMR) effect. The present work characterizes the in-plane electrical and thermal conductivities of Cu/CoFe GMR multilayer structure in the temperature range of 50 K to 340 K using Joule-heating and electrical resistance thermometry in suspended bridges. The thermal conductivity of the GMR layer monotonously increased from 25 Wm−1 K−1 (at 55 K) to nearly 50 Wm−1 K−1 (at room temperature). We also report the GMR ratio of 17% and a large negative magnetothermal resistance effect (GMTR) of 33% in Cu/CoFe superlattice structure. The Boltzmann transport equation (BTE) is used to estimate the GMR ratio, and to investigate the effect of repeats, as well as the spin-dependent interface and boundary scatting on the transport properties of the GMR structure. Aside from the interesting underlying physics, these data can be used in the predictions of the Electrostatic Discharge (ESD) failure and self-heating in GMR heads.

Commentary by Dr. Valentin Fuster
2004;():211-218. doi:10.1115/IMECE2004-62124.

In this article we examine the augmentation of classic Rayleigh-Bénard convection by the addition of periodically-spaced tranverse fins attached to the heated, lower plate. The respective impacts of the fin size, the fin spacing and the thermal conductivity of the fin material are examined through numerical simulations for different laminar Rayleigh numbers and reported it terms of the Nusselt number. With the exception of very closely spaced fins, the heat transport is observed to exceed that of the idealized Rayleigh-Bénard case. It is found that local heat transport maxima and minima do exist for specific fin spacings and that the maxima become more pronounced at higher Rayleigh numbers. For ‘small fins’ the fin spacing corresponding to maximum heat transport is such that the fin spacing is approximately equal to the enclosure height.

Topics: Cavities , Fins
Commentary by Dr. Valentin Fuster
2004;():219-224. doi:10.1115/IMECE2004-62147.

Much attention has been paid in recent years to the use of nanoparticle suspensions for enhanced heat transfer. The majority of this work has focused on the thermal conductivity of these nanofluids, which can be as much as 2.5 times higher than that of the plain base fluid. The present work moves beyond measurements of non-flowing liquids, to explore the role that nanofluids can play in enhancing convective heat transfer within microscale channels. A unique pseudo-turbulent flow regime is postulated, which simulates turbulent behavior at very low Reynolds numbers, in what are nominally laminar flows. The resulting fluid mixing has the potential to raise the average convective heat transfer coefficient within the channel. Numerical modeling, using the lattice Boltzmann method, confirms the existence of the pseudo-turbulent flow regime. Finally, experimental results are presented which demonstrate a significant heat transfer enhancement when using nanofluids in forced convection. The current results are especially relevant to microchannel heatsinks, where the low Reynolds numbers impose limitations on the maximum Nusselt number achievable.

Commentary by Dr. Valentin Fuster
2004;():225-229. doi:10.1115/IMECE2004-62150.

The amorphous/crystalline phase formation during writing or erasure of the written marks, in the rewritable phase change (PC) optical recording media, is controlled by the temperature distribution in the media and its variation with time. Temperature distribution, on the other hand, strongly depends on the thermal properties of its constituent layers in particular the ZnS-SiO2 dielectric layer that separates the phase change media from the substrate and aluminum heat sink. The reported values for the thermal conductivity of thin dielectric layers are however limited in the literature. In this manuscript, we report thermal conductivity data for dielectric layers of thickness near 50, 100 and 225 nm using the steady sate Joule-heating and electrical resistance thermometry technique. The boundary resistance at the interface is estimated to be near 7.0×10−8 m2 K W−1 , which would limit the thermal time constant for cooling of PC layer and potentially impact data rate and jitter in optical recording technology.

Commentary by Dr. Valentin Fuster
2004;():231-235. doi:10.1115/IMECE2004-62213.

This paper presents the fabrication and results of an experimental study carried out to determine the thermal fluid performance of a 3-D, active micro convective heat sink having high surface-to-volume ratio geometry. The heat sink consists of an array of elemental units arranged in parallel. Each unit is constructed as a network of nearly fractal geometry. The design of each unit uses the constructal method to minimize the point-to-point temperature difference within the heat sink and Murray’s Law to minimize pressure drop across the device. One elemental unit of the heat sink was manufactured using the tape casting fabrication method with thick silver film techniques. An experiment was conducted using water as the coolant under laminar flow conditions to obtain the pressure drop and heat transfer characteristics of the 3-D micro convective heat sink. The results were then compared with theoretical calculations.

Topics: Heat sinks , Plumbing
Commentary by Dr. Valentin Fuster
2004;():237-240. doi:10.1115/IMECE2004-62216.

The lattice Boltzmann method (LBM) as a relatively new numerical scheme has recently achieved considerable success in simulating fluid flows and associated transport phenomena. However, application of this method to heat transfer problems has been at a stage of infancy. In this work, a thermal lattice Boltzmann model is employed to simulate a two-dimensional, steady flow in a symmetric bifurcation under constant temperature and constant heat flux boundary conditions. The bifurcation effects on the heat transfer and fluid flow are investigated and comparisons are made with the straight tube. Also, different bifurcation angles are simulated and the results are compared with the work of the other researchers.

Commentary by Dr. Valentin Fuster
2004;():241-248. doi:10.1115/IMECE2004-62406.

Computations for turbulent natural convection within an inclined cavity totally filled with a fluid saturated porous medium are presented. The finite volume method in a generalized coordinate system is applied. 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 a wide range of values of Rayleigh number and inclined angle. The turbulent model used is the standard k-ε model with a wall function. In this work, the turbulence model is first switched off and the laminar branch of the solution is found. Subsequently, the turbulence model is included so that the solution merges to the laminar branch for a reducing Ram . This convergence of results as Ram decreases can be seen as an estimate of the so-called relaminarization phenomenon. Present solutions are compared with published results and the influence of the inclination angle on Racr is analyzed. For Ram greater than around 104 , both laminar and turbulent flow solutions deviate, indicating that such critical value for Ram was reached.

Commentary by Dr. Valentin Fuster
2004;():249-258. doi:10.1115/IMECE2004-59369.

The sensitivity coefficients for analyzing the interstitial properties during phase change in porous media are presented. Computation of the sensitivity coefficients is the main objective of this study. Experimentally measured temperature data provide an estimate of the phase front locations used as the state variable for this study. The derivations are based on the assumption that the phase front, X, at a given time, t, is a function of interstitial properties τt and τq with all other parameters remaining constant. The properties τt and τq are the lag-time in temperature and heat flux, respectively. The analysis includes two types of boundary conditions: prescribed temperature of phase change materials and prescribed temperature for solid matrix. Results for the first case show that for any given ratio τtq , the sensitivity coefficient decreases asymptotically to zero at large times. Furthermore, the sum of the sensitivity coefficients St + Sq = 0 when τtq ≈ 1. This is significant information because the variables St and Sq have the same magnitude with opposite signs. This implies that these two sensitivity coefficients become linearly dependent and it will be difficult to predict the values of τt and τq in the neighborhood of τtq ≈ 1. A similar trend but with different sensitivity values are reported for the second case.

Commentary by Dr. Valentin Fuster
2004;():259-270. doi:10.1115/IMECE2004-59453.

The importance of equation of state models is fundamental to new technologies such as supercritical water oxidation for the destruction of organic pollutants. In order to be able to perform hazard and risk assessments, the parameters ofthermodynamic models are considered as information characteristics of chemicals that store the knowledge on their thermodynamic, phase and environmental behavior. Considering the extremely large number of existing chemicals, it is obvious that there is need for developing theoretically sound methods for the prompt estimation of their phase behavior in aquatic media at supercritical conditions. Recent developments of the global phase equilibria studies of binary mixtures provide some basic ideas of how the required methods can be developed based on global phase diagrams for visualization of the phase behavior of mixtures. The mapping of the global equilibrium surface in the parameter space of the equation of state (EoS) model provides the most comprehensive system of criteria for predicting binary mixture phase behavior. The main types of phase behavior for environmentally significant organic chemicals in aqueous environments are considered using structure-property correlations for the critical parameters of substances. Analytic expressions for azeotropy prediction for cubic EoS are derived. A local mapping concept is introduced to describe thermodynamically consistently the saturation curve of water. The classes of environmentally significant chemicals (polycyclic aromatic hydrocarbons - PAH, polychlorinated biphenyls - PCB, polychlorinated dibenzo-p-dioxins and furans, and selected pesticides) are considered and main sources of the property data are examined. Vapor pressure, heat of vaporization, and critical parameter estimations for pure components were chosen for seeking a correlation between the octanol–water partition coefficients KOW and the EoS binary interaction parameters - k12 . The assessment of thermodynamic and phase behavior of representatives for different pollutants is given.

Topics: Water
Commentary by Dr. Valentin Fuster
2004;():271-280. doi:10.1115/IMECE2004-59511.

In this paper we present the solution of the inverse problem of simultaneously estimating the heat and mass transfer coefficients at the surface of a drying one-dimensional body. The physical problem is formulated in terms of the linear Luikov equations and the unknown functions are time dependent. The inverse problem is solved by using the conjugate gradient method of function estimation with adjoint problem. Results are presented for the estimation of discontinuous functions by using simulated measurements of local temperature, local moisture content and/or total moisture weight. The main objective of the paper is to examine the effects of different types of measurements on the inverse problem solution.

Commentary by Dr. Valentin Fuster
2004;():281-289. doi:10.1115/IMECE2004-59638.

Baking has historically been a trial and error method of cooking. Little research has been conducted to determine the heat transfer characteristics that promote good baking results, and previous research studies have focused on commercial baking applications and the quantities of radiation, convection and conduction that are delivered to the food after a favorable baking process has been defined. The objective of the present work is to experimentally explore the feasibility of modifying a residential oven to mimic commercial baking products. The first step in the solution process was to define the thermo-physical conditions that promote favorable baking results. Next, by defining the current residential oven’s baking characteristics through experimentation, the optimal geometric and material properties were determined. Experimentation included single thermocouple testing, multiple thermocouple testing, and ‘bake’ testing. It was found that a stacked wall structure created by layering various materials in a sandwich like configuration, placed between the lower resistive heating element and the oven cavity, improved the heat transfer characteristics of the oven.

Topics: Heat transfer , Ovens
Commentary by Dr. Valentin Fuster
2004;():291-296. doi:10.1115/IMECE2004-59711.

The direct general identification method of the radiative properties of high porosity media, developed and validated for virtual statistically isotropic media in [1], has been applied to a real statistically anisotropic medium. This medium has a transparent fluid phase and an opaque gray diffuse solid phase. It is modelled by a semi-transparent equivalent medium characterized by extinction and absorption coefficients β and κ. These quantities are directly determined from the morphology data obtained by X-ray tomography and from the absorptivity of the solid phase. The application of this approach to a mullite sample has established that β and κ are homogeneous but depend on direction. This last feature has to be accounted for by a radiative transfer method for this type of medium.

Commentary by Dr. Valentin Fuster
2004;():297-306. doi:10.1115/IMECE2004-59722.

The objective of this present work is to provide a new approach for the radiative characteristics computation of semitransparent media containing spherical bubbles. The bubble size is considred much larger than the wavelength. First, previous models from the literature based on the independent theory are reviewed and established in the Geometric optic limit. Second, a predictive model using the Monte Carlo method is developed. The results obtained from the independent theory models and the Monte Carlo approach are compared. In addition, by varying the bubbles volume fraction, we investigate the limit of validity of the independent theory in such medium.

Commentary by Dr. Valentin Fuster
2004;():307-320. doi:10.1115/IMECE2004-60129.

New compact analytical models for predicting the effective thermal conductivity of regularly packed beds of rough spheres immersed in a stagnant gas are developed. Existing models do not consider either the influence of the spheres roughness or the rarefaction of the interstitial gas on the conductivity of the beds. Contact mechanics and thermal analyses are performed for uniform size spheres packed in SC and FCC arrangements and the results are presented in the form of compact relationships. The present model accounts for the thermophysical properties of spheres and the gas, contact load, spheres diameter, spheres roughness and asperities slope, and temperature and pressure of the gas. The present model is compared with experimental data for SC and FCC packed beds and good agreement is observed. The experimental data cover a wide range of the contact load, surface roughness, interstitial gas type, and gas temperature and pressure.

Commentary by Dr. Valentin Fuster
2004;():321-328. doi:10.1115/IMECE2004-61163.

Electron field emission is the process by which electrons tunnel from a cathode to an anode, usually through vacuum, by the application of a large voltage bias. Field-emission devices find applications in flat panel displays, electrical circuit breakers, and power diodes. A significant amount of heat can be transferred to the anode from high-energy electrons as they impact the anode surface. This study investigates the heating of a thin, disc-shaped steel anode by field-emitted electrons from a single carbon nanotube. Experiments have demonstrated a temperature rise of more than 11.0 C at the anode center at an energy deposition rate of 16 mW. A finite-difference model is employed to predict the steady-state anode temperature profile resulting from electron field emission, and this profile is compared to that obtained from measurements taken with an infrared camera. The comparison yields information regarding the diameter of the electron beam as it strikes the anode. The paper also discusses significant experimental challenges, which include attaching individual nanotubes on a tungsten needle to withstand the applied electric field and obtaining consistent results during repeated testing.

Commentary by Dr. Valentin Fuster
2004;():329-336. doi:10.1115/IMECE2004-61337.

This paper presents experimental measurements of some effective thermophysical properties of slurries consisting of microencapsulated phase change materials (MCPCMs) suspended in distilled water. The related apparatus and procedures are also presented and discussed. The MCPCMs considered here consist of a core of phase-change material (PCM), in this case a substance akin to octadecane, surrounded by a solid shell. The effective density of the slurries was measured using hydrometers. The effective thermal conductivity of the slurries was measured using an in-house designed apparatus. The effective kinematic viscosity of the slurries was measured using a series of glass capillary viscometers. A differential scanning calorimeter (DSC) was used to obtain the effective specific heat, melting and freezing temperatures of the core PCMs, and the latent heat of the slurries. Slurry concentrations between 0% (pure distilled water) and 20% by mass of the MCPCMs were considered in this investigation, at temperatures ranging from 5°C to 65°C. Where possible, the results have been compared to predictions obtained using available analytical expressions with properties of the constitutive materials as inputs.

Commentary by Dr. Valentin Fuster
2004;():337-342. doi:10.1115/IMECE2004-62027.

Thermal interface materials (TIMs) are widely used in electronics packaging. Increasing heat generation rates require lower values of the TIM thermal resistance, which depends on the material thermal conductivity and the TIM thickness, or the bond line thickness (BLT). The variation of the TIM thickness is not well understood. The major difficulty comes from the complexity of TIMs as condensed particle systems, especially when the TIM thickness is squeezed to several multiples of the filler particle diameter. This confined heterogeneous structure makes the behavior of TIMs different from that of homogeneous fluids. In this study, we propose a two-medium model for the BLT. The variation of BLT with attachment pressure is modeled using two parameters: the viscidity of the fluids and the interactions of particles. The predictions are compared with the measurements for TIMs made of aluminum oxide particles (sizes: 0.6–6 microns, volume fractions: 30%–50%) and silicon oil (kinematic viscosity: 100 cst and 1000 cst). Reasonable agreement is obtained for different applied pressures. Results indicate that the impact of the particle interactions is an important factor governing the variation of the TIM BLT, especially when the BLT is small.

Commentary by Dr. Valentin Fuster
2004;():343-345. doi:10.1115/IMECE2004-62332.

The fundamental study of phase transformations continues to be a key for successful implementation of metals and alloys in micro- and nano-scale structures in integrated circuitry and magnetic recording devices and systems. The thermodynamic and thermokinetic properties of extremely thin layers can be altered due to the relative effect of boundaries and interfaces on the volume of the material. Calorimetry at the nano-scale requires measurement sensitivity on the order of 1 nJ or better, which requires improved thermal design, development of thermal modeling, and development of experimental measurement techniques. In this report, the specific heat of 144 nm thick CoFe layer is measured, using frequency-domain Joule heating and thermometry (3ω-technique), on Cu/SiO2 and Cu/SiO2 /CoFe suspended bridges. Analyses of the heat transfer in suspended structures are performed to establish guidelines for design and fabrication of small-scale differential scanning calorimeters.

Commentary by Dr. Valentin Fuster
2004;():347-352. doi:10.1115/IMECE2004-59134.

Temperature distributions and thermal stress distributions in a semi-transparent GSO crystal during Czochralski (CZ) single crystal growth were numerically investigated by thermal radiation heat transfer analysis and anisotropy stress analysis. As GSO has special optical properties, such as semi-transparency at a wavelength shorter than 4.5 μm, thermal radiation heat transfer was calculated by the Monte Carlo method. These calculations showed that thermal stress is caused by the radial temperature distribution on the outside of the upper part of the crystal. To reduce this temperature distribution, the following three manufacturing conditions were found to be effective: use a sharp taper angle of the crystal, install a lid to the top of the insulator, and install a ring around the tapered part of the crystal.

Commentary by Dr. Valentin Fuster
2004;():353-361. doi:10.1115/IMECE2004-59169.

In this paper, an integrated model considering induction heating, transient heat transfer and crystal growth has been developed to study dynamic response of temperature and powder sublimation in an aluminum nitride (AlN) growth system. The electromagnetic field and induction heat generation are calculated by the Maxwell equations. Transient temperature distribution in the growth chamber is simulated by energy equation accounting for conduction/radiation within and between various components. In order to provide proper temperature control during sublimation growth, dynamic responses of temperature and temperature difference of the bottom and top external surfaces of the growth crucible to power variation and coil position movement are simulated. Finally the crystal and powder shapes as a function of time are predicted and compared with dynamic experimental observation.

Commentary by Dr. Valentin Fuster
2004;():363-369. doi:10.1115/IMECE2004-59204.

Wetting behavior of liquid and dynamics of the meniscus are important in many meniscus-controlled material processes such as Edge-defined Film-fed Growth (EFG), fiber pulling growth, micro/nano tube growth, etc. Understanding dynamic responses of meniscus to perturbations is essential to the improvement of the quality of products from these processes. In this paper, symmetric meniscus structure as well as asymmetric structure and the break-up of the steady-state meniscus are studied experimentally using the techniques of pulling from shaper (TPS). The break-up conditions for the steady-state meniscus are obtained. A theoretical model is also proposed to study the dynamic response of the meniscus shape with a given ribbon thickness to the change in pulling rate and meniscus height. This model is analyzed for symmetric meniscus structure and its transition to an asymmetric system.

Commentary by Dr. Valentin Fuster
2004;():371-377. doi:10.1115/IMECE2004-59241.

Fusion arc welding processes often generate substantial residual stresses, which may alter the performance of welded structures. Residual stresses are the results of incompatible elastic and plastic deformations in a body. Destructive techniques are generally used to experimentally determine residual stresses. Employment of these methods would not often be possible or practical in industry. In this study, three-dimensional (3D) and two-dimensional (2D) finite element simulations and experimental work have been performed to analyze the thermomechanical problem of GMAW and to obtain a full-field view of the residual stress field. One of the purposes of this study is to examine the formation of residual stresses upon cooling of a weldment. Comparisons of the results of 2D and 3D finite element models reveal many three-dimensional features in the thermomechanical problem of GMAW. The magnitude of longitudinal residual stresses obtained from the 2D model, however, compares well with the results obtained from the 3D model.

Commentary by Dr. Valentin Fuster
2004;():379-387. doi:10.1115/IMECE2004-59286.

Zinc-coated steels are used extensively in the auto industry because they are inexpensive, durable and have high corrosion resistance. Lasers are being used to weld zinc-coated steels due to high welding speed, small seam and narrow heat affected zone. However, it is difficult to laser weld lap-joint zinc-coated steel sheets under a very small gap condition between the metal interfaces since there is a considerable amount of zinc vapor generated. This vapor must be vented out; otherwise it will be trapped in the weld pool leading to different welding defects, such as large voids at the tip of the weld and porosities in the form of small bubbles in the weld. These defects can significantly decrease the strength of the weld. In this paper, a mathematical model and the associated numerical techniques have been developed to study the transport phenomena in laser welding of zinc-coated steels. The volume-of-fluid (VOF) method is employed to track free surfaces. The continuum model is used to handle the liquid phase, solid phase and mushy zone of the metal. The enthalpy method is employed to account for the latent heat during melting and solidification. The transient heat transfer and melt flow in the weld pool during the keyhole formation and collapse processes are calculated. The escape of zinc vapor through the keyhole and the interaction between zinc vapor and weld pool are studied. The aforementioned weld defects are found to be caused by the combined effects of zinc vapor-melt interactions, keyhole collapse and solidification process. By controlling the laser pulse profile, it is found that the keyhole collapse and solidification process can be delayed, allowing the zinc vapor to escape, which results in the reduction or elimination of weld defects.

Commentary by Dr. Valentin Fuster
2004;():389-398. doi:10.1115/IMECE2004-59288.

This study develops a quantum mechanical model to investigate energy absorption in ultrafast laser of dielectrics. The model investigates the optical property variations, electron temperature, and density changes at femtosecond scales. The ionizations and electron heating are two major factors considered for pulse absorption occurring within the pulse duration. The flux-doubling model is employed to calculate the free electron generation mainly through impact ionization and photoionization. The quantum mechanical treatments are used to account for the specific heat and the relaxation time for free electrons. The time and space dependent optical properties of the dense plasma generated by the ultrafast laser pulse are calculated. The predictions of ablation threshold and ablation depth of fused silica and barium aluminum borosilicate (BBS) are in good agreements with published experimental data. The model greatly improves the accuracy in predicting the ablation depth and can predict the crater shape.

Commentary by Dr. Valentin Fuster
2004;():399-409. doi:10.1115/IMECE2004-59338.

Melt casting of energetic materials is investigated, and a numerical model formulated for the analysis of the coupled fluid flow, heat transfer, and stress fields involved in this phase-change process. The numerical model is based on a conservative multi-block control volume method. The SIMPLE algorithm is employed along with an enthalpy method approach to model the solidification process. Results from the model are verified against experimental data as well as published numerical results for simplified cases. In the melt casting of RDX-binder mixtures, the very high viscosity of the melt leads to the influence of melt convection being very limited. The impact of different cooling conditions on the velocity, temperature and stress distributions, as well as on the solidification time, are discussed. The model can be used to improve the quality of cast explosives, by optimizing and controlling the processing conditions.

Topics: Casting , Explosives
Commentary by Dr. Valentin Fuster
2004;():411-418. doi:10.1115/IMECE2004-59348.

A thermometric technique has been developed to study the thermal characteristics of the foam-metal interaction in the lost foam casting process. A cylindrical foam pattern and heated steel block have been used to estimate the endothermic losses associated with the thermal degradation of the expanded polystyrene at the metal front. Thermocouple readings have been analyzed to determine the temperature of the kinetic zone between the advancing metal front and the receding foam pattern. The heat transfer coefficient between the metal front and the foam pattern has been calculated from the thermal data at the simulated metal front. The results confirmed that the endothermic degradation of the polystyrene pattern at the metal front introduced a steep thermal gradient in the metal and a consistently increasing heat flux. It is found that the heat transfer coefficient, initially 150 W/m2 ·K increases to 220 ~ 300 W/m2 ·K during the process. Foam density has marginal effect on the heat flux and heat transfer coefficient, whereas the increase of simulated metal front velocity enhances the heat transfer at the metal front. The kinetic zone temperature is measured to be in the range of 150 to 290°C with an average of 200°C and a gaseous gap size of 1 to 4 cm.

Commentary by Dr. Valentin Fuster
2004;():419-425. doi:10.1115/IMECE2004-59565.

A discontinuous Galerkin finite element computational methodology is presented for the solution of the coupled phase-field and heat conduction equations for modeling microstructure evolution during solidification. The details of the discontinuous formulation and the solution procedures are given. A major difference between the current method and those used in the literatures is the application of higher-order localized formulation and unstructured mesh, which holds a great promise in both parallel computing and adaptive meshing. The accuracy of the discontinuous model is checked with the analytic solution for a simple 1-D solidification problem. Numerical simulations and selected results are given for more complex 2-D dendrite structures formed during solidification. The calculated results are consistent with those reported in literature.

Commentary by Dr. Valentin Fuster
2004;():427-435. doi:10.1115/IMECE2004-59895.

A two-dimensional numerical model has been developed to investigate the induction electromagnetic (EM) field and the thermo-fluid field in a radio frequency inductively coupled plasma (RF-ICP). Various physical and chemical phenomena have been considered such as the induction heating, plasma generation, and the in-flight particle interaction with the plasma jet. This model has been applied to the induction plasma spray process operated in a vacuum chamber. The partially stabilized zirconia powder (PSZ) has been used as an example for the feedstock. The effects of power input, standoff distance and powder injection position on the plasma and particle behaviors have also been investigated and discussed.

Commentary by Dr. Valentin Fuster
2004;():437-445. doi:10.1115/IMECE2004-59907.

Optical crystals, such as YAG, GGG, sapphire, CaF2 , are of high melting temperature. As a result, the radiative heat transfer during the crystal growth process is an important factor to influence the temperature distribution and hence the quality of as-grown crystals. However, the radiation effect is complicated in the optical crystal growth system. It depends on the system geometry and properties, such as the spectral transmittance of the oxide crystals, and the emissivity of the ampoule and the furnace inner wall. In this paper, the analysis of heat transfer coupled with radiation effect has been carried out using one-dimensional thermal resistance network method for optical crystals growth. The results show that radiation plays a significant role in the heat transfer and temperature distribution during growth, and the subsequent quality of the grown crystal. Two-dimensional numerical simulations have also been conducted and the simulation results have been compared with the one-dimensional results. Effects of system configuration and physical properties on crystal quality are also analyzed and discussed.

Commentary by Dr. Valentin Fuster
2004;():447-455. doi:10.1115/IMECE2004-59969.

Advanced ceramics are not easily fabricated and consolidated by the plasma spray technique because of their extremely high melting temperature. Zirconium diboride (ZrB2 ) has been successfully plasma sprayed, but the coatings are quite porous. The high levels of porosity are usually a result of unmelted ZrB2 particles that have been incorporated into the coating during deposition. Applying a laser surface treatment to reduce both the porosity and the coating surface roughness, and to improve the coating quality, is of great interest. A laser based surface treatment technique provides a well-controlled heat input, with minimal or no distortion. In this study, a two dimensional mathematical model is developed to investigate the effects of laser power, beam diameter and level of porosity on the coating quality, incorporating melting, solidification, and evaporation phenomena. A continuum model is used to solve Navier-Stokes equations for both solid and liquid phases. Volume-of-Fluid (VOF) is incorporated to track the free surface. The surface force is incorporated as a body force instead of a boundary condition. The porosity level and surface roughness before and after the laser surface treatment are simulated and compared with experimental results.

Commentary by Dr. Valentin Fuster
2004;():457-467. doi:10.1115/IMECE2004-60196.

Despite immense advances in Laser Aided Direct Metal/Material Deposition (LADMD) process many issues concerning the effects of process parameters on the stability of variety of properties and the integrity of microstructure have been reported. Modeling of heat flow seems to be a standard practice to couple heat flow calculations to related macroscopic phenomena such as fluid flow in the melt and solid-liquid mushy region, macrosegregation and thermal stresses. A key component in these models is the coupling between thermal and solute fields. Like macrostructural phenomena even microstructural features such as phase appearance, morphology, grain size or spacing are certainly no less important. The focus of this paper is the solute transport, in particular the manner in which process scale transport is coupled to transport at the local scale of the solid-liquid interface which requires a modeling of the redistribution of solutes at the scale of the secondary arm spaces in the dendritic mushy region. Basic microsegregation models which assume either no mass diffusion in the solid (Gulliver-Scheil) or complete diffusion in the solid (equilibrium lever rule) in a fixed arm space are inappropriate in high energy beam processes involving significantly high cooling rates. This paper aims at incorporating a model that accounts for finite mass diffusion and coarsening of the arm space. Due to the complexity and nonlinearity of LADMD process, analytical solutions can rarely address the practical manufacturing process. Consequently, this is an attempt towards a methodology of finite element analysis to predict solidification microstructure and thermal stresses. The simulation has been carried out for H13 tool steel deposited on a mild steel substrate. However, the program can easily be extended to a wide variety of steels.

Commentary by Dr. Valentin Fuster
2004;():469-474. doi:10.1115/IMECE2004-60290.

A new experimental configuration for the casting of metal matrix composites (MMCs) using Al-4.5 wt pct Cu have been used to obtain finer microstructures around the fiber reinforcement. The new configuration allows the fibers to be extended out the mold and cooled by a heat sink. By doing so, the solidification can be made more rapid, and more primary alpha-aluminum phase can be formed on the surface of the fibers. It is believed that this can lead to improvement in the properties of the composite. CFD simulation of the solidification of Al-4.5 wt pct Cu in the casting process has been carried out by using commercial CFD code. Parametric studies on the effects of different processing parameters on solidification time have been simulated using the CFD code. These parameters include, but are not limited to, the pouring temperature of the liquid melt, sink temperature, fiber length extended out of the mold, the mold initial temperature, fiber conductivity, applied pressure, and fiber bundle diameter. Selected simulation results are compared with the available experimental data obtained from the UWM Center for Composites.

Commentary by Dr. Valentin Fuster
2004;():475-482. doi:10.1115/IMECE2004-60338.

Glass tempering has relied on radiant heaters to transfer heat to glass sheets. This has a direct correlation to the length of time that the glass is subject to heating in the furnace and its related line speeds. This paper presents the design and development of a supplementary convection heating system with the objective of reducing glass residence time and increasing production efficiency. It will document the methodology of retrofitting a current furnace in a cost effective manner while increasing heat transfer to the glass and increasing throughput. This study will address factors affecting the systems’ performance such as current draw, surface compression test, break pattern, warp configuration, and nozzle design. The retrofitted system has achieved design goals and promises to increase heat transfer rates to the glass as well as increase furnace productivity.

Commentary by Dr. Valentin Fuster
2004;():483-489. doi:10.1115/IMECE2004-60421.

Laser ablation presents a promising technique for material processing. The quality of products is strongly influenced by the properties of the laser induced plume. In compressible flow, the ambient conditions can be transmitted upstream. Therefore, the laser ablation process is strongly affected by the ambient conditions. In this paper, the effects of laser intensity, back pressure and temperature on the laser induced plume were studied using a numerical model, which calculates the density, pressure and temperature of the laser induced plume at different laser intensity and ambient conditions. The results are in agreement with experimental results available in the literature and can be used for the optimization of the pulsed laser deposition process.

Commentary by Dr. Valentin Fuster
2004;():491-497. doi:10.1115/IMECE2004-60640.

In order to understand the mechanism of the surface patterns on silicon melt in Czochralski furnaces, we conducted a series of unsteady three-dimensional numerical simulations of silicon melt flow in a rotating shallow annular pool in the counter-clockwise direction under micro-gravity. The pool is heated from the outer cylindrical wall and cooled at the bottom of an inner cylinder. The temperature differences between the vertical outer wall and the inner wall are 16 K, 21 K, 26 K and 32 K. Bottom and top surfaces of the melt pool are adiabatic. When the rotation rate is very slow, the hydrothermal waves are dominant in the pool and propagate in a direction opposite to the pool rotation. When the rotation rate exceeds the first critical value, the phase velocity of the hydrothermal waves increases rapidly and its propagating direction becomes same as that of the pool rotation. With much larger rotation rate, the flow becomes an axisymmetric steady flow. Details of the flow and temperature disturbances are discussed and the critical rotation rates are determined.

Commentary by Dr. Valentin Fuster
2004;():499-505. doi:10.1115/IMECE2004-60723.

The particle velocity in cold gas dynamic spraying (CGDS) is one of the most important factors that can determine the properties of the bonding to the substrate. In this paper, the acceleration process of micro-scale and sub micro-scale copper (Cu) and platinum (Pt) particles inside and outside De-Laval-Type nozzle is investigated. A numerical simulation is performed for the gas-particle two phase flow with particle diameter ranging from 100nm to 50μm, which are accelerated by carrier gas Nitrogen in a supersonic De-Laval-type nozzle. The carrier gas velocity and pressure distributions in the nozzle and outside the nozzle are illustrated. The center-line velocity for two types of particles, Pt and Cu, are demonstrated. It is observed that the existence of the bow shocks near the substrate prevents the smaller size particles (less than 0.5 μm) from penetrating, thus leads to poor coating in the actual practices.

Commentary by Dr. Valentin Fuster
2004;():507-511. doi:10.1115/IMECE2004-60770.

The physical and mathematical models of the thermocapillary convection in liquid bridge with liquid encapsulation are established in the present paper. A numerical simulation of the thermocapillary convection in liquid bridge with liquid encapsulation is performed by employed vorticity-stream function method and the Alternative Direction Implicit scheme in finite difference. The distribution of temperature and flow in liquid columns is then obtained. It is verified that liquid encapsulation can reduce the thermocapillary convection in liquid bridge and can improve the quality of crystal growth in float zone. The influence law of the thickness of liquid encapsulation on the thermocapillary convection in liquid bridge is obtained, the more thickness of liquid encapsulation decreases, the more the thermocapillary convection in the inner liquid and the outer liquid diminishes. It is found that the flow profile of two liquid columns is much more complex than that of single liquid column.

Topics: Convection
Commentary by Dr. Valentin Fuster
2004;():513-518. doi:10.1115/IMECE2004-60774.

This paper focuses on the convection of molten gallium contained in an enclosure of aspect ratios 5:1:1.3 subject to horizontal temperature gradient and an external constant magnetic field. Flows without magnetic field are also solved for comparisons by the spectral method. Steady flows and oscillatory flows show different flow structures. The symmetries are strictly maintained for steady flow and the symmetries are broken for oscillatory flow. Near the onset, stable periodical oscillations appear and the symmetries are satisfied between the present state and the state half a period later, which agrees well with the analytical results. The phase characters agree well with the experimental results. Energy analysis shows that the production of fluctuating kinetic energy by shear of mean flow is the main production to the change of the total fluctuating kinetic energy, while viscous effects give the main dissipation term. The dissipation of the fluctuating kinetic energy by magnetic field keeps negative.

Commentary by Dr. Valentin Fuster
2004;():519-525. doi:10.1115/IMECE2004-60875.

Phase-field models of solidification with convection often assume the existence of a single (mixture) velocity at any location inside the diffuse interface, and the phase-field, φ, is advected by this mixture velocity. In this paper, the advection of the phase-field is examined for a one-dimensional normal flow to a solidification front induced by a density difference between the solid and liquid. It is found that the results from a phase-field model that assumes a single velocity inside the diffuse interface are generally not in agreement with the sharp interface condition for the kinetic undercooling of the front in the presence of unequal densities, regardless of the interface width. By introducing a two-phase approach, where the solid and liquid are assumed to coexist inside the diffuse interface with different velocities, good agreement with the sharp interface condition is obtained irrespective of the density ratio between the two phases.

Commentary by Dr. Valentin Fuster
2004;():527-533. doi:10.1115/IMECE2004-61088.

The objectives of this article are to suggest a way to evaluate the quality of polycrystalline silicon film from the thin film optics analysis and also to investigate the heat transfer characteristics in a rapid thermal annealing system for LCD manufacturing. The characteristic transmission matrix method is used to calculate the transmittance, and the predictions are compared with the experimental data for two different samples. The transient and one-dimensional conductive and radiative heat transfer equations are additionally solved to predict the surface temperatures of thin films. The two-flux method is also used for radiation and the ray-tracing method is utilized to consider the wave interference. As the film thickness increases, the peak transmittance increases and the wavelength for the peak becomes longer due to wave interferences. These characteristics can be used for in-situ and practical estimation of the silicon film quality during the crystallization process. From thermal analysis, it is shown that the selective heating in the multilayer film structure acts as an important mechanism during the annealing of silicon film deposited on the glass.

Commentary by Dr. Valentin Fuster
2004;():535-540. doi:10.1115/IMECE2004-61253.

Radiation absorption of an infinitely long hollow cylinder with Fresnel boundary is studied using the ray tracing method. Since the radiative heat transfer is the dominant heat transfer mode in optical fiber drawing, the current radiative transfer model can provide insights into physical processes for microstructured optical fiber fabrication. Effects of refractive index, optical thickness and geometry on radiative heat transfer are studied. The results of this study can also serve as benchmark solutions for radiative heat transfer for other methods, such as the finite volume method and the discrete ordinates method for participating media with Fresnel boundary.

Commentary by Dr. Valentin Fuster
2004;():541-546. doi:10.1115/IMECE2004-61263.

High intensity femtosecond laser ablation processes are computed by molecular dynamics (MD) simulations. The procedure of MD modeling and related numerical techniques are introduced. The volumetric phase separation is illustrated. The emphasis is to understand the thermodynamic state of a material which is heated with an extremely high heating rate, and the formation of particles with nanometer size in the laser-ablated plume. It is revealed that nanoparticles are originated from expansion of a supercritical fluid in a reduced temperature and pressure environment. The parameters of the nanoparticles (temperature, size and velocity) are shown. The results will help to understand femtosecond laser materials processing and applications where nanoparticles are of interest.

Commentary by Dr. Valentin Fuster
2004;():547-555. doi:10.1115/IMECE2004-61331.

The production of stable thin-film photovoltaic devices requires tight control of temperature uniformity of the glass substrates during the vacuum deposition process. As a first step towards developing an optimized control system for maintaining thermal uniformity as the substrates traverse multiple stations during the deposition process, a finite element thermal model of a single deposition station has been developed. The model couples cavity radiation processes with conduction within the graphite sources and the substrate itself. The effect of adjustable parameters such as radiation shielding, addition of a radiation spreader and varying power distribution among the radiation lamps has been studied. It is concluded that while radiation shielding substantially improves the uniformity, it cannot bring the temperature variation down to the very low levels necessary for producing stable devices. Addition of a radiation spreader improves the uniformity. Seeking and applying the optimum power distribution among the radiation lamps results in more incremental gain in uniformity but a change in lamp configuration is required for attaining the desired uniformity levels.

Commentary by Dr. Valentin Fuster
2004;():557-563. doi:10.1115/IMECE2004-61460.

Phase change problems are encountered in several manufacturing and material processing applications. Such problems are computationally challenging because it is necessary to solve a non-linear heat conduction equation and take into considerations the conditions needed to produce material ablation, varying continuously the heat source position, thermo physical properties and physical shape of the domain. This research presents a numerical simulation of the temperature field and the removed material resulting from the impingement of a moving laser beam on a ceramic surface. A finite volume approach has been developed to predict the temperature field including phase changes generated during the process. The model considers heat losses by convection and radiation due to the high temperatures involved and uses a coordinate system affixed to the workpiece; therefore no quasi-steady conditions are assumed, as in the majority of previous works. Numerical predictions were compared with former three-dimensional numerical models considering a semi-infinite solid and from experimental data found in the literature. This study gives insight into the interactions between the laser beam and a silicon nitride workpiece during the cutting.

Commentary by Dr. Valentin Fuster
2004;():565-573. doi:10.1115/IMECE2004-61707.

This paper presents results from ongoing research on thermal-model based feedforward specification of laser power in a laser powder deposition process. The goal of this algorithm is to compute, before deposition of a layer, the laser power sequence and distribution that would produce a desired temperature distribution over that layer. This in turn will enable uniform cooling of the layer and avoid build up of residual stresses. In this paper, results based on a simplified thermal model and second-order spatial discretization are presented. Two types of discretization in the time domain are examined. The matrix-exponential-based discretization is expected to be more accurate at lower laser speeds. The desired laser power sequence and the resulting temperature histories for a prescribed laser speed are discussed within the context of a thin-walled part.

Commentary by Dr. Valentin Fuster
2004;():575-580. doi:10.1115/IMECE2004-61743.

The solidification of binary mixture (NH4 Cl–H2 O) inside a trapezoidal cavity is investigated experimentally in this study. The experiments are carried out in a trapezoidal cavity measuring 65 mm × 130 mm × 150 mm with inclined angle of 69°. Solidification of ammonium chloride occurs on the left inclined copper wall held under constant heat rate condition while the other walls are maintained at adiabatic conditions. Particle image velocimetry was used in this study for visualization of the dynamic field during the solidification process. The temperatures of the solution inside the cavity and the boundary walls were measured by 32 thermocouples during the solidification process. Convective flow field, temperature distribution and frozen layer thickness were obtained for different initial concentrations of ammonium chloride varying from 0 to 19.8 % (sub-eutectic and near-eutectic growth) and various boundary conditions (Tcold = −30 °C to 0 °C). The results obtained in the course of study reveal that (1) the solidification rate is higher during initial stages of the solidification process, (2) the process of solidification is slower with increase in the initial concentration levels of the ammonium chloride and (3) the initial concentration play a significant role in the evolution of convection flow patterns.

Commentary by Dr. Valentin Fuster
2004;():581-589. doi:10.1115/IMECE2004-62365.

Phase change in a simulated low gravity environment is the central topic of this research work. The application of a transverse magnetic field gives rise to Lorentz forces that can dampen the convective flows especially the buoyancy driven flows. The flow suppression depends on a dimensionless parameter namely the Hartmann number. This paper presents the experimental results for sidewall solidification and melting and therefore, addresses the fixed solid phase conditions. Gallium is used as phase change material (PCM) and both melting and solidification processes are investigated. The effects of an applied magnetic field on phase change rate and on the shape of the solid/melt interface are studied. The solid thickness is measured via ultrasonic techniques and the solid/melt interface is mapped using florescent light shadowgraphy through a transparent window. The presented data consist of temperature history, ultrasonic detection of the interface, florescent light shadowgraphy and solid phase volume fraction. The presence of the magnetic field had a marked effect on melting and natural convection whereas; phase change convection was noticeable in the solidification cases.

Commentary by Dr. Valentin Fuster

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