Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2000"

Forced Convection in High Porosity Metal Foams

Abstract: We report an experimental and numerical study of forced convection in high porosity (e∼0.89-0.97) metal foams. Experiments have been conducted with aluminum metal foams in a variety of porosities and pore densities using air as the fluid medium. Nusselt number data has been obtained as a function of the pore Reynolds number. In the numerical study, a semi-empirical volume-averaged form of the governing equations is used. The velocity profile is obtained by adapting an exact solution to the momentum equation. The energy transport is modeled without invoking the assumption of local thermal equilibrium. Models for the thermal dispersion conductivity, k d , and the interstitial heat transfer coefficient, h sf , are postulated based on physical arguments. The empirical constants in these models are determined by matching the numerical results with the experimental data obtained in this study as well as those in the open literature. Excellent agreement is achieved in the entire range of the parameters studied, indicating that the proposed treatment is sufficient to model forced convection in metal foams for most practical applications

Flow and heat transfer correlations for porous fin in a plate-fin heat exchanger

Abstract: The present experimental study investigates the impact of porous fins on the pressure drop and heat transfer characteristics in plate-fin heat exchangers. Systematic experiments have been carried out in a simplified model of a plate-porous fin heat exchanger at a controlled test environment. The porous fins are made of 6101 aluminum-alloy foam materials with different permeabilities and porosities. Comparison of performance between the porous fins and the conventional louvered fins has been made. The experimental results indicate that friction and heat transfer rate are significantly affected by permeability as well as porosity of the porous fin. The porous fins used in the present study show similar thermal performance to the conventional louvered fin. However, the louvered fin shows a little better performance in terms of pressure drop. For compactness of the heat exchanger, the porous fins with high pore density and low porosity are preferable. Useful correlations for the friction factor and the modified j-factor are also given for the design of a plate-porous fin heat exchanger.

Analysis of Variants Within the Porous Media Transport Models

Abstract: An investigation of variants within the porous media transport models is presented in this work. Four major categories in modeling the transport processes through porous media, namely constant porosity, variable porosity, thermal dispersion, and local thermal non-equilibrium, are analyzed in detail. The main objective of the current study is to compare these variances in models for each of the four categories and establish conditions leading to convergence or divergence among different models. To analyze the effects of variants within these transport models, a systematic reduction and sensitivity investigation for each of these four aspects is presented. The effects of the Darcy number, inertia parameter, Reynolds number, porosity, particle diameter, and the fluid-to-solid conductivity ratio on the variances within each of the four areas are analyzed. For some cases the variances within different models have a negligible effect on the results while for some cases the variations can become significant. In general, the variances have a more pronounced effect on the velocity field and a substantially smaller effect on the temperature field and Nusselt number distribution

Pool Boiling Heat Transfer From Plain and Microporous, Square Pin-Finned Surfaces in Saturated FC-72

Abstract: The present research is an experimental study of double enhancement behavior in pool boiling from heater surfaces simulating microelectronic devices immersed in saturated FC-72 at atmospheric pressure. The term double enhancement refers to the combination of two different enhancement techniques: a large-scale area enhancement (square pin fin array) and a small-scale surface enhancement (microporous coating). Fin lengths were varied from 0 (flat surface) to 8 mm. Effects of this double enhancement technique on critical heat flux (CHF) and nucleate boiling heat transfer in the horizontal orientation (fins are vertical) are investigated. Results showed significant increases in nucleate boiling heat transfer coefficients with the application of the microporous coating to the heater surfaces. CHF was found to be relatively insensitive to surface microstructure for the finned surfaces except in the case of the surface with 8-mm-long fins. The nucleate boiling and CHF behavior has been found to he the result of multiple, counteracting mechanisms: surface area enhancement, fin efficiency, surface microstructure (active nucleation site density), vapor bubble departure resistance, and re-wetting liquid flow resistance

Molecular Dynamics Study of Solid Thin-Film Thermal Conductivity

Abstract: This study uses the molecular dynamics computational technique to investigate the thermal conductivity of solid thin films in the direction perpendicular to the film plane. In order to establish a benchmark reference, the computations are based on the widely used Lennard-Jones argon model due to its agreement with experimental liquid-phase data, its physically meaningful parameters, and its simple two-body form. Thermal conductivity increases with film thickness, as expected from thin-film experimental data and theoretical predictions. The calculated values are roughly 30 percent higher than anticipated. Varying the boundary conditions, heat flux, and lateral dimensions of the films causes no observable change in the thermal conductivity values. The present study also delineates the conditions necessary for meaningful thermal conductivity calculations and offers recommendations for efficient simulations. This work shows that molecular dynamics, applied under the correct conditions, is a viable tool for calculating the thermal conductivity of solid thin films. More generally, it demonstrates the potential of molecular dynamics for ascertaining microscale thermophysical properties in complex structures.

The onset of flow instability in uniformly heated horizontal microchannels

Abstract: Onset of nucleate boiling and onset of flow instability in uniformly heated microchannels with subcooled water flow were experimentally investigated using 22-cm long tubular test sections, 1.17 mm and 1.45 mm in diameter, with a 16-cm long heated length. Important experimental parameter ranges were: 3.44 to 10.34 bar channel exit pressure; 800 to 4,500 kg/m{sup 2}s mass flux (1 to 5 m/s inlet velocity); 0 to 4.0 MW/m{sup 2} channel wall heat flux; and 7,440--33,000 Peclet number at the onset of flow instability. Demand curves (pressure drop versus mass flow rate curves for fixed wall heat flux and channel exit pressure) were generated for the test sections, and were utilized for the specification of the onset of nucleate boiling and the onset of flow instability points. The obtained onset of nucleate boiling and onset of flow instability data are presented and compared with relevant widely used correlations.

Laminar natural convection in isosceles triangular enclosures heated from below and symmetrically cooled from above

Abstract: A numerical study of natural convection in an isosceles triangular enclosure with a heated horizontal base and cooled upper walls is presented. Nearly every previous study conducted on this subject to date has assumed that the geometric plane of symmetry is also a plane of symmetry for the flow. This problem is re-examined over aspect ratios ranging from 0.2 to 1.0 and Grashof numbers from 10 3 to 10 5 . A pitchfork bifurcation occurs at a critical Grashof number for each of the aspect ratios considered, above which the symmetric solutions are unstable to finite perturbations and asymmetric solutions are instead obtained. Results are presented detailing the occurrence of the pitchfork bifurcation in each of the aspect ratios considered, and the resulting flow patterns are described. A flow visualization study is used to validate the numerical observations. Computed local and mean heat transfer coefficients are also presented and compared with results obtained when flow symmetry is assumed. Differences in local values of the Nusselt number between asymmetric and symmetric solutions are found to be more than 500 percent due to the shifting of the buoyancy-driven cells

Optical Properties in the Visible of Overfire Soot in Large Buoyant Turbulent Diffusion Flames

Abstract: Nonintrusive measurements of the optical properties of soot at visible wavelengths (351.2-800.0 nm) were completed for soot in the overfire region of large (2-7 kW) buoyant turbulent diffusion flames burning in still air at standard temperature and pressure, where soot properties are independent of position and characteristic flame residence time for a particular fuel. Soot from flames fueled with gaseous (acetylene, ethylene, propylene, and butadiene) and liquid (benzene, cyclohexane, toluene, and n-heptane) hydrocarbon fuels were studied. Scattering and extinction measurements were interpreted to find soot optical properties using the Rayleigh-Debye-Gans/polydisperse-fractal-aggregate theory after establishing that this theory provided good predictions of scatter-ing patterns over the present test range. Effects of fuel type on soot optical properties were comparable to experimental uncertainties. Dimensionless extinction coefficients were relatively independent of wavelength for wavelengths of 400-800 nm and yielded a mean value of 8.4 in good agreement with earlier measurements. Present measurements of the refractive index function for absorption. E(m), were in good agreement with earlier independent measurements of Dalzell and Sarofim and Stagg and Charalampopoulos. Present values of the refractive index function for scattering, F(m), however, only agreed with these earlier measurements for wavelengths of 400-550 nm but otherwise increased with increasing wavelength more rapidly than the rest. The comparison between present and earlier measurements of the real and imaginary parts of the complex refractive index was similar to E(m) and F(m).

Heat transfer and friction characteristics of internal helical-rib roughness

Abstract: This paper provides heat transfer and friction data for single-phase flow in seven 15.54-mm inside diameter tubes having internal helical-rib roughness. The range of geometric parameters were number of rib starts (18 to 45), helix angle (25 to 45 deg), and rib height (0.33 to 0.55 mm). These geometries provide data on a new class of internal enhancement that is typical of commercially rough tubes presently used. The tested geometries provide enhancement by flow separation at the ribs, and by a significant surface area increase. The data were taken with water having 5.08≤Pr≤6.29. Two different correlations were employed to predict the Stanton number and friction factor as a function of geometric variables and Reynolds number. The average deviation of the multiple regression heat transfer and correlations were 2.9 percent and 3.8 percent, respectively. Heat transfer and friction correlations based on the heat-momentum transfer analogy for rough surfaces yielded standard deviations of 1.4 percent and 5.4 percent, respectively. The correlations were shown to reasonably predict the heat transfer and friction for commercially used helical-rib roughened tubes.

Film Cooling From Shaped Holes

Abstract: Local and spatially averaged magnitudes of the adiabatic film cooling effectiveness, the iso-energetic Stanton number ratio, and film cooling performance parameter are measured downstream of (i) cylindrical round, simple angle (CYSA) holes, (ii) laterally diffused, simple angle (LDSA) holes, (iii) laterally diffused, compound angle (LDCA) holes, (iv) forward diffused, simple angle (FDSA) holes, and (v) forward diffused, compound angle (FDCA) holes. Data are presented for length-to-inlet metering diameter ratio of 3, blowing ratios from 0.4 to 1.8, momentum flux ratios from 0.17 to 3.5, and density ratios from 0.9 to 1.4. The LDCA and FDCA arrangements produce higher effectiveness magnitudes over much wider ranges of blowing ratio and momentum flux ratio compared to the three simple angle configurations tested

An Experimental Study of Molten Microdroplet Surface Deposition and Solidification: Transient Behavior and Wetting Angle Dynamics

Abstract: The basic problem of the impact and solidification of molten droplets on a substrate is of central importance to a host of processes. An important and novel such process in the area of micromanufacturing is solder jetting where microscopic solder droplets are dispensed for the attachment of microelectronic components. Despite the recent appearance of a few numerical studies focusing on the complex transient aspects of this process, no analogous experimental results have been reported to date to the best of our knowledge. Such a study is reported in this paper. Eutectic solder (63Sn37Pb) was melted to a preset superheat and used in a specially designed droplet generator to produce droplets with diameters in the range 50-100 μm. In a first series of experiments, the size, temperature, and impacting speed of the molten droplets were maintained constant. The primary variable was the temperature of the substrate that was controlled in the range from 48°C to 135°C. The dynamics of molten solder microdroplet impact and solidification on the substrate was investigated using a flash microscopy technique. The time for the completion of solidification from the moment of a solder droplet impact on the substrate varies between 150 μs and 350 μs. The dynamic interaction between the oscillation in the liquid region and the rapid advance of the solidification front was visualized, quantified, and presented in this paper. In a second series of experiments, the evolution of the wetting angle between the spreading drop and the substrate was recorded and analyzed. No quantitative agreement with Hoffman's correlation for wetting was found. It was established that the wetting angle dynamics is strongly coupled with the evolution of the droplet free surface. Two successive regimes were distinguished during the spreading. The influence of the initial impact velocity and substrate temperature on the dynamics of the measured wetting angle was described in both regimes. To the best of our knowledge, this study presents the first published experimental results on the transient fluid dynamics and solidification of molten microdroplets impacting on a substrate at the above-mentioned time and length scales that are directly relevant to the novel solder jetting technology.

The Use of an Organic Self-Assembled Monolayer Coating to Promote Dropwise Condensation of Steam on Horizontal Tubes

Abstract: Hydrophobic coatings have been created through self-assembled monolayers (SAMs) on gold, copper, and copper-nickel alloy surfaces that enhance steam condensation through dropwise condensation. The monolayer is formed by chemisorption of alkylthiols on these metal surfaces. Due to their negligible thickness (10-15 A), SAMs have negligible heat transfer resistance, and involve a minuscule amount of the organic material to pose any contamination problem to the system from erosion of the coating. The coating was applied directly to copper and 90/10 copper-nickel tubes, and to previously gold-sputtered aluminum tubes. The quality of the drops on SAMs, based on visual observation, was found to be similar for the three surfaces, with the gold surface showing a slight superiority. When compared to complete filmwise condensation, the SAM coating increased the condensation heat transfer coefficient by factors of 4 for gold-coated aluminum, and by about 5 for copper and copper-nickel tubes, under vacuum operation (10 kPa). The respective enhancements under atmospheric conditions were about 9 and 14. Comparatively, the heat transfer coefficient obtained with a bare gold surface (with no organic coating) was 2.5 times that of the filmwise condensation heat transfer coefficient under vacuum, and 3.4 at atmospheric conditions.

Pool Boiling Heat Transfer in Aqueous Solutions of an Anionic Surfactant

Abstract: Saturated nucleate pool boiling of aqueous surfactant solutions on a horizontal cylindrical heater has been experimentally investigated. Sodium dodecyl or lauryl sulfate (SDS or SLS), an anionic surfactant, is employed. Boiling performance, relative to that for pure water, is found to be enhanced significantly by the presence of SDS, with an early onset of nucleate boiling. An optimum level of enhancement is observed in solutions at or near critical micelle concentration of the surfactant; the enhancement, however, decreases considerably in higher concentration solutions. The dynamic surface tension measurements indicate a substantial influence of temperature on the overall adsorption isotherm. The diffusion kinetics of surfactant molecules and micelles is, therefore, expected to be quite different at boiling temperature than at room temperature. This greatly modifies the boiling mechanism that is generally characterized by the formation of smaller-size bubbles with increased departure frequencies, and a decreased tendency to coalesce which causes considerable foaming.

Heat Transfer and Fluid Flow in a Square Duct With 12 Different Shaped Vortex Generators

Abstract: Detailed local Nusselt number distributions in the first pass of a sharp turning two-pass square channel with various configurations of longitudinal vortex generator arranged on one wall were measured using transient liquid crystal thermography. Flow patterns and friction factors were measured by the use of laser-Doppler velocimeter and pressure transducer, respectively. The Reynolds number, based on channel hydraulic diameter and bulk mean velocity, was fixed at 1.2 × 10 4 . The vortex generator height-to-hydraulic diameter ratio and pitch-to-height ratio were 0.12 and 10, respectively. Comparisons in terms of heat transfer augmentation and uniformity and friction loss are first performed on 12 configurations of single longitudinal vortex generator. The fluid dynamic mechanisms and wall confinement relevant to heat transfer enhancement are then documented for three-selected vortex generator models. In addition, the differences in fluid flow and heat transfer characteristics between a single vortex generator and a vortex generator array are addressed for the delta wing I and 45 deg V (with tips facing upstream) models which provide better thermal performance among the 12 configurations examined

Fractional-Diffusion Solutions for Transient Local Temperature and Heat Flux

Abstract: Applying properties of the Laplace transform, the transient heat diffusion equation can be transformed into a fractional (extraordinary) differential equation. This equation can then be modified, using the Fourier Law, into a unique expression relating the local value of the time-varying temperature (or heat flux) and the corresponding transient heat flux (or temperature). We demonstrate that the transformation into a fractional equation requires the assumption of unidirectional heat transport through a semiinfinite domain. Even considering this limitation, the transformed equation leads to a very simple relation between local timevarying temperature and heat flux. When applied along the boundary of the domain, the analytical expression determines the local time-variation of surface temperature (or heat flux) without having to solve the diffusion equation within the entire domain. The simplicity of the solution procedure, together with some introductory concepts of fractional derivatives, is highlighted considering some transient heat transfer problems with known analytical solutions. S0022-14810001002-1

From Heat Transfer Principles to Shape and Structure in Nature: Constructal Theory

Abstract: This lecture reviews a relatively recent body of heat transfer work that bases on a deterministic (constructal) principle the occurrence of geometric form in systems with internal flows. The same principle of global optimization subject to constraints allow us to anticipate the natural (animate and inanimate) flow architectures that surround us. The lecture starts with the example of the optimal spatial distribution of material (e.g., heat exchanger equipment) in power plants. Similarly, void space can be allocated optimally to construct flow channels in the volume occupied by a heat generating system. The lecture continues with the optimization of the path for heat flow between a volume and one point. When the heat flow can choose between at least two paths, low conductivity versus high conductivity, the optimal flow structure for minimal global resistance in steady flow is a tree. Nearly the same tree is deduced by minimizing the time of discharge in the flow from a volume to one point. Analogous tree-shaped flows are constructed in pure fluid flows, and in flow through a heterogeneous porous medium. The optimization of trees that combine heat transfer and fluid flow is illustrated by means of two-dimensional trees of plate fins

Laminar Flow Heat Transfer and Pressure Drop Characteristics of Power-Law Fluids Inside Tubes With Varying Width Twisted Tape Inserts

Abstract: Results of an experimental investigation of heat transfer and flow friction of a generalized power-law fluid in tape generated swirl flow inside a 25.0 mm i.d. circular tube, are presented. In order to reduce excessive pressure drops associated with full width twisted tapes, with less corresponding reduction in heat transfer coefficients, reduced width twisted tapes of widths ranging from 11.0 to 23.8 mm, which are lower than the tube inside diameter are used. Reduced width twisted tape inserts give 18%--56% lower isothermal friction factors than the full width tapes. Uniform wall temperature Nusselt numbers decrease only slightly by 5%--25%, for tape widths of 19.7 and 11.0 mm, respectively. Based on the constant pumping power criterion, the tapes of width 19.7 mm perform more or less like full width tapes. Correlations are presented for isothermal and heating friction factors and Nusselt numbers (under uniform wall temperature condition) for a fully developed laminar swirl flow, which are applicable to full width as well as reduced width twisted tapes, using a modified twist ratio as pitch to width ratio of the tape. The reduced width tapes offer 20%--50% savings in the tape material as compared to the full width tapes.

Three-Dimensional Sintering of Two-Component Metal Powders With Stationary and Moving Laser Beams

Abstract: Melting and resolidification of a mixture of two metal powders with significantly different melting points under irradiation of a stationary or a moving Gaussian laser beam were investigated numerically and experimentally. The liquid motion driven by capillary and gravity forces as well as the shrinkage of the powder bed caused by the overall density change were taken into account in the physical model The liquid flow was formulated by using Darcy's law, and the energy equation was given using a temperature transforming model. Prediction were compared with experimental results obtained with nickel braze and AIS1 1018 steel powder. The effects of laser properties and the scanning velocity on the laser sintering process were also investigated. An empirical correlation that can be used to predict the cross-sectional area of the heat affected zone is proposed.

Transport Phenomena and Droplet Formation During Pulsed Laser Interaction With Thin Films

Abstract: This work investigates transport phenomena and mechanisms of droplet formation during a pulsed laser interaction with thin films. The surface of the target material is altered through material flow in the molten phase induced by a tightly focused laser energy flux. Such a process is useful for developing a laser-based micromachining technique. Experimental and numerical investigations of the laser-induced fluid flow and topography variations are carried out for a better understanding of the physical phenomena involved in the process. As with many machining techniques, debris is often generated during laser-material interaction. Experimental parametric studies are carried out to correlate the laser parameters with the topography and droplet formations. It is found that a narrow range of operation parameters and target conditions exists for clean structures to be fabricated. The stop action photography technique is employed to capture the surface topography variation and the melting development with a nanosecond time resolution and a micrometer spatial resolution. Numerical simulations of the laser-induced surface deformation are also performed to obtain the transient field variables and to track the deforming surface. The comparison between the numerical and experimental work shows that, within the energy intensity range investigated in this work, the surface deformation and droplet formation are attributed to the surface-tension-driven flow, and the recoil pressure effect plays an insignificant role in the surface topography development.

Mist/steam cooling in a 180{degree} tube bend

Abstract: An experimental study on mist/steam cooling in a highly heated, horizontal 180{degree} tube bend has been performed. The mist/steam mixture is obtained by blending fine water droplets (3{approximately}15 microns) with the saturated steam at 1.5 bar. The test section consists of a thin wall ({approximately}0.9 mm), welded, circular, stainless steel 180-degree tube (20 mm ID) with a straight section downstream of the curved section, and is heated directly by a DC power supply. The experiment was conducted with steam Reynolds numbers ranging from 10,000 to 35,000, wall superheat up to 300 C, and droplet to steam mass ratio at about 2%. The results show that the heat transfer performance of steam can be significantly improved by adding mist into the main flow. Due to the effect of centrifugal force, the outer wall of the test section always exhibits a higher heat transfer enhancement than the inner wall. The highest enhancement occurs at a location on the outer wall about 45{degree} downstream of the inlet of the test section. Generally, only a small number of droplets can survive the 180{degree} turn and be present in the downstream straight section, as observed by a Phase Doppler Particle Analyzer (PDPA) system. The overallmore » cooling enhancement of the mist/steam flow ranges from 40% to 300%. It increases as the main steam flow increases, but decreases as the wall heat flux increases.« less

Inverse design model for radiative heat transfer

Abstract: Inverse solution techniques are applied to the design of heat transfer systems where radiation is important. Various solutions using inverse methods are demonstrated, and it is argued that inverse design techniques provide an alternative to conventional iterative design methods that is more accurate and faster, and can provide a greatly improved first estimate of a thermal design. This estimate can then he used as a trial design in more complete thermal analysis programs for predicting system behavior, eliminating many faulty first design trials

Fourier Versus Non-Fourier Heat Conduction in Materials With a Nonhomogeneous Inner Structure

Abstract: Distinct non-Fourier behavior in terms of finite propagation velocity and a hyperbolic wave like character of heat conduction has been reported for certain materials in several studies published recently. However, there is some doubt concerning these findings. The objective of this note is to present experimental evidence for a perfectly Fourier-like behavior of heat conduction in those materials with nonhomogeneous inner structure that have been under investigation in the other studies. This controversy needs to be settled in order to understand the physics of heat conduction in these materials

Melting and surface deformation in pulsed laser surface micromodification of Ni-P disks

Abstract: The nanosecond pulsed laser-induced transient melting and miniature surface deformation of Ni-P hard disk substrates has been investigated experimentally. A photothermal displacement method has been developed to detect the transient melting and surface deformation process with nanosecond time resolution. The deflection signals show the variation of the feature shape in response to different pulse energies of the near-infrared pulsed nanosecond heating laser beam. A laser flash photography system is also developed to visualize the growth dynamics of the entire feature with nanosecond time resolution and submicron spatial resolution. The feature formation is explained as a result of surface tension driven flow. The surface tension depends not only on the surface temperature, but also on the surfactant concentration. Competition between the thermocapillarity and a surfactant concentration effect is revealed in the course of the bump formation process. Smaller features with diameters of 5 μm are obtained by using visible pulsed laser radiation. On-line monitoring of the transient growth process of such small features is achieved by a new laser flash deflection microscope

Effect of Double Dispersion on Mixed Convection Heat and Mass Transfer in Non-Darcy Porous Medium

Abstract: Similarity solution for the problem of hydrodynamic dispersion in mixed convection heat and mass transfer from vertical surface embedded in porous media has been presented. The flow induced by the density variations is comparable with the freestream flow. The heat and mass transfer in the boundary layer region for aiding and opposing buoyancies in both aiding and opposing flows has been analyzed. The structure of the flow, temperature, and concentration fields in the Darcy and non-Darcy porous media are governed by complex interactions among the diffusion rate (Le) and buoyancy ratio (N) in addition to the flow driving parameter (Ra/Pe). The flow, temperature, and concentration fields are analyzed and the variation of heat and mass transfer coefficients with the governing parameters are presented

Diffusion Layer Modeling for Condensation With Multicomponent Noncondensable Gases

Abstract: Many condensation problems involving noncondensable gases have multiple noncondensable species, for example, air (with nitrogen, oxygen, arid other gases); and other problems where light gases like hydrogen may mix with heavier gases like nitrogen. Particularly when the binary mass diffusion coefficients of the noncondensable species are substantially different, the noncondensable species tend to segregate in the condensation boundary layer. This paper presents a fundamental analysis of the mass transport with multiple noncondensable species, identifying a simple method to calculate an effective mass diffusion coefficient that can be used with the simple diffusion layer model. The results are illustrated with quantitative examples to demonstrate the potential importance of multicomponent noncondensable gas effects.

Simulations of Three- Dimensional Flow and Augmented Heat Transfer in a Symmetrically Grooved Channel

Abstract: Navier-Stokes simulations of three-dimensional flow and augmented convection in a channel with symmetric, transverse grooves on two opposite walls were performed for 180 ≤Re≤ 1600 using the spectral element technique. A series of flow transitions was observed as the Reynolds number was increased, from steady two-dimensional flow, to traveling two and three-dimensional wave structures, and finally to three-dimensional mixing

An experimental investigation of the transient characteristics on a flat-plate heat pipe during startup and shutdown operations

Abstract: This work presents an experimental investigation of the thermal performance of a flatplate heat pipe during startup and shutdown operations. Using the analytical model developed in a previously study, analytical and experimental results on the effect of input power and cooling heat transfer coefficient on the thermal performance of the heat pipe are presented and discussed. The results indicate that the wick in the evaporator section provides the largest resistance to the heat transfer process followed by the wick in the condenser section. It is found that the heat transfer coefficient has an insignificant effect on the maximum temperature difference across the heat pipe where this difference refers to the maximum difference on the outside surfaces of the flat-plate heat pipe. However, as expected, the input heat flux has a substantial effect on the temperature rise where the temperature rise refers to the temperature increase on the outside surface of the heat pipe. It is found that the temperature difference across the heat pipe depends mainly on the input power. The heat transfer coefficient strongly affects the time it takes to reach steady state while input power has a substantially smaller effect. Empirical correlations for the maximum temperature rise, the maximum temperature difference and the time constants are obtained. The experimental results are compared with the analytical results and are found to be in very good agreement.@S0022-1481~00!01803-X#

The numerical and experimental study of non-premixed combustion flames in regenerative furnaces

Abstract: A numerical procedure is presented to predict the turbulent non-premixed combustion flames in regenerative furnaces. A moment closure method with the assumed β probability density function (PDF) for the mixture fraction is used in the present work. The procedure is applied to an experimental regenerative slab reheat furnace developed in NKK to demonstrate its predictive capability. The predictions are compared with the experimental data. The comparison is favorable.

Nonequilibrium Natural Convection in a Differentially Heated Cavity Filled With a Saturated Porous Matrix

Abstract: Steady-state natural convection in an enclosure filled with a saturated porous medium is investigated numerically. Brinkman-Forchheimer's extension of Darcy flow with a nonequilibrium model is used in the analysis. The paper intends to address the validity of the equilibrium model for natural convection. The predicted results indicated that the equilibrium model is difficult to justify for non-Darcy regime and when the solid thermal conductivity is higher than the fluid thermal conductivity. The maximum differences in the temperatures between the two phases take place at the bottom-left corner and due to skew-symmetry of the problem at the upper-right corner

Neck Down and Thermally Induced Defects in High-Speed Optical Fiber Drawing

Abstract: The drawing speeds employed in the manufacturing of optical fibers have been rising in recent years due to growing worldwide demand. However, increasing speeds have placed stringent demands on the manufacturing process, mainly because of large temperature gradients that can generate thermally induced defects and undesirable variations in fiber characteristics. Heat transfer and glass flow that arise in drawing fibers of diameters 100-125 microns from cylindrical silica preforms of diameters 5-10 cm play a critical role in the success of the process and in the maintenance of fiber quality. This paper presents an analytical and numerical study of the optical fiber drawing process for relatively large diameter preforms and draw speeds as high as 20 m/s. The free surface, which defines the neck-down profile, is not assumed but is determined by using a balance of forces. An iterative numerical scheme is employed to obtain the profile under steady conditions. The transport in the glass is calculated to obtain the temperature, velocity and defect distributions. A zone radiation model, developed earlier, is used for calculating radiative transport within the glass. Because of the large reduction in the diameter of the preform/fiber, the velocity level increases dramatically and the geometry becomes complicated. A coordinate transformation is used to convert the computational domains to cylindrical ones. The numerical results are compared with experimental and numerical results in the literature for smaller draw speeds for validation. The effects of high draw speeds and of other physical variables on defects generated in the fiber, on the neck-down profile, and on the feasible domain for the process are determined.