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RESEARCH PAPERS: 1995 Max Jakob Lecture

J. Heat Transfer. 1996;118(3):518-527. doi:10.1115/1.2822662.
Abstract
Topics: Two-phase flow
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Analytical and Experimental Techniques

J. Heat Transfer. 1996;118(3):528-531. doi:10.1115/1.2822663.
Abstract
Topics: Heat transfer
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):532-538. doi:10.1115/1.2822664.

The process of parameter estimation and the estimated parameters are affected not only by measurement noise, which is present during any experiment, but also by uncertainties in the parameters of the model used to describe the system. This paper describes a method to optimize the design of an experiment to deduce the maximum information during the inverse problem of parameter estimation in the presence of uncertainties in the model parameters. It is shown that accounting for these uncertainties affects the optimal locations of the sensors.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Heat Conduction

J. Heat Transfer. 1996;118(3):539-545. doi:10.1115/1.2822665.

Heat transfer around nanometer-scale particles plays an important role in a number of contemporary technologies such as nanofabrication and diagnosis. The prevailing method for modeling thermal phenomena involving nanoparticles is based on the Fourier heat conduction theory. This work questions the applicability of the Fourier heat conduction theory to these cases and answers the question by solving the Boltzmann transport equation. The solution approaches the prediction of the Fourier law when the particle radius is much larger than the heat-carrier mean free path of the host medium. In the opposite limit, however, the heat transfer rate from the particle is significantly smaller, and thus the particle temperature rise is much larger than the prediction of the Fourier conduction theory. The differences are attributed to the nonlocal and nonequilibrium nature of the heat transfer processes around nanoparticles. This work also establishes a criterion to determine the applicability of the Fourier heat conduction theory and constructs a simple approximate expression for calculating the effective thermal conductivity of the host medium around a nanoparticle. Possible experimental evidence is discussed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):546-554. doi:10.1115/1.2822666.

A Boundary Element Method (BEM) implementation for the solution of inverse or ill-posed two-dimensional Poisson problems of steady heat conduction with heat sources and sinks is proposed. The procedure is noniterative and cost effective, involving only a simple modification to any existing BEM algorithm. Thermal boundary conditions can be prescribed on only part of the boundary of the solid object while the heat sources can be partially or entirely unknown. Overspecified boundary conditions or internal temperature measurements are required in order to compensate for the unknown conditions. The weighted residual statement, inherent in the BEM formulation, replaces the more common iterative least-squares (L2) approach, which is typically used in this type of ill-posed problem. An ill-conditioned matrix results from the BEM formulation, which must be properly inverted to obtain the solution to the ill-posed steady heat conduction problem. A singular value decomposition (SVD) matrix solver was found to be more effective than Tikhonov regularization for inverting the matrix. Accurate results have been obtained for several steady two-dimensional heat conduction problems with arbitrary distributions of heat sources where the analytic solutions were available.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Forced Convection

J. Heat Transfer. 1996;118(3):555-561. doi:10.1115/1.2822667.

This paper presents results of an experimental study on the heat transfer enhancement in laminar flow of non-Newtonian fluids, aqueous Carbopol-934 solutions through a small-scale square duct. The square duct is a top-wall heated configuration with a hydraulic diameter of 0.4 cm. The aqueous Carbopol solutions examined are those neutralized, and have a polymer concentration range of 1000–2000 wppm. It is shown that the enhanced heat transfer behavior of the Carbopol solutions within low Reynolds number range is different from that within relatively high Reynolds number range. There exists a limiting polymer concentration, Cmax , at which the non-Newtonian fluid possesses the maximum ability to enhance heat transfer. If the polymer concentration becomes too high, the minimum Reynolds number required to enhance heat transfer increases with the increasing polymer concentration.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):562-569. doi:10.1115/1.2822668.

The local heat transfer coefficient distribution on a square heat source due to a normally impinging, axisymmetric, confined, and submerged liquid jet was computationally investigated. Numerical predictions were made for nozzle diameters of 3.18 and 6.35 mm at several nozzle-to-heat source spacings, with turbulent jet Reynolds numbers ranging from 8500 to 13,000. The commercial finite-volume code FLUENT was used to solve the thermal and flow fields using the standard high-Reynolds number k–ε turbulence model. The converged solution obtained from the code was refined using a post-processing program that incorporated several near-wall models. The role of four alternative turbulent Prandtl number functions on the predicted heat transfer coefficients was investigated. The predicted heat transfer coefficients were compared with previously obtained experimental measurements. The predicted stagnation and average heat transfer coefficients agree with experiments to within a maximum deviation of 16 and 20 percent, respectively. Reasons for the differences between the predicted and measured heat transfer coefficients are discussed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):570-577. doi:10.1115/1.2822670.

This is an experimental numerical and theoretical study of the heat transfer on a pin-finned plate exposed to an impinging air stream. The pin fins are aligned with the air approach velocity. The base plate and the fin cross section are square. It is demonstrated experimentally that the thermal conductance between the plate and the air stream can be maximized by selecting the fin-to-fin spacing S. Next, a simplified numerical model is used to generate a large number of optimal spacing and maximum heat transfer data for various configurations, which differ with respect to fin length (H), fin thickness (D), base plate size (L), fluid type (Pr), and air velocity (ReL ). Finally, the behavior of the optimal spacing data is explained and correlated theoretically based on the intersection of asymptotes method. The recommended correlations for optimal spacing, Sopt /L ≅ 0.81 Pr−0.25 ReL −0.32 , and maximum thermal conductance, (q/ΔT)max /ka H ≅ 1.57 Pr0.45 ReL 0.69 (L/D)0.31 , cover the range D/L = 0.06 − 0.14, H/L = 0.28−0.56, Pr = 0.72−7, ReD = 10−700, and ReL = 90−6000.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):578-584. doi:10.1115/1.2822671.

This paper presents experimental heat transfer results in a two-pass square channel with smooth and ribbed surfaces. The ribs are placed in a staggered half-V fashion with the rotation orthogonal to the channel axis. The channel orientation varies with respect to the rotation plane. A change in the channel orientation about the rotating frame causes a change in the secondary flow structure and associated flow and turbulence distribution. Consequently, the heat transfer coefficient from the individual surfaces of the two-pass square channel changes. The effects of rotation number on local Nusselt number ratio distributions are presented. Heat transfer coefficients with ribbed surfaces show different characteristics in rotation number dependency from those with smooth surfaces. Results show that staggered half-V ribs mostly have higher heat transfer coefficients than those with 90 and 60 deg continuous ribs.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):585-591. doi:10.1115/1.2822672.

In this paper, the enhancement of heat transfer due to unsteady flow in channels with in-line and staggered baffles is investigated through the numerical solution of the governing unsteady fluid flow and energy equations with periodicity in the stream wise direction. For the inline configuration, the flow becomes naturally unsteady at a critical Reynolds number (Q/v ) around 110. For the staggered case, this value is around 200. Significant increases in heat transfer rate are observed once the flow becomes unsteady. Results for several Reynolds numbers up to 500 are presented. The present results can be valuable to the design and operation of compact heat exchangers used in process industry.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):592-597. doi:10.1115/1.2822673.

An experimental and numerical study has been carried out for laminar forced convection in a long pipe heated by uniform heat flux and subjected to a reciprocating flow of air. Transient fluid temperature variations in the two mixing chambers connected to both ends of the heated section were measured. These measurements were used as the thermal boundary conditions for the numerical simulation of the hydrodynamically and thermally developing reciprocating flow in the heated pipe. The coupled governing equations for time-dependent convective heat transfer in the fluid flow and conduction in the wall of the heated tube were solved numerically. The numerical results for time-resolved centerline fuid temperature, cycle-averaged wall temperature, and the space-cycle averaged Nusselt number are shown to be in good agreement with the experimental data. Based on the experimental data, a correlation equation is obtained for the cycle-space averaged Nusselt number in terms of appropriate dimensionless parameters for a laminar reciprocating flow of air in a long pipe with constant heat flux.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Natural and Mixed Convection

J. Heat Transfer. 1996;118(3):598-605. doi:10.1115/1.2822674.

Natural convection heat transfer in enclosed horizontal N × N arrays (N = 3, 5, and 7) of electrically heated rods with a pitch-to-diameter ratio (P/d) of 1.35 has been experimentally investigated. Each array was positioned in an isothermal square enclosure with a width-to-diameter ratio (W/d) of 20.6. Pressurized air or helium was used as the working fluid. It was observed that the bottom-tow rods were relatively insensitive to increases in the array size, as they exhibit only slight temperature variations, but the top-row rods demonstrated substantial temperature increases. Natural convection correlations in the form of Nusselt number (Nud ) as a function of modified Rayleigh number (Rad *) were obtained for each rod in each array. The correlations cover three flow regimes of conduction, transition, and convection in the range of 6.45 < Rad * < 3.08 × 105 . A generalized enclosure Nusselt number was correlated as a function of enclosure modified Rayleigh number and the array size (N). Comparison of the data with previous numerical prediction showed that this correlation may be readily used to obtain a conservative estimate of the maximum temperature in the arrays with N = 3, 5, 7, and 9.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):606-615. doi:10.1115/1.2822675.

This paper concerns an experimental study of the mixed convection flow and heat transfer inside a divergent channel formed by two plane walls. One of the side walls is oriented vertically and is heated uniformly, and the opposite wall is tilted at an angle of 3 deg with respect to the vertical position and is insulated. The ratio of the height to wall spacing at the flow inlet, which is at the smaller opening of the channel is 15. The Reynolds number of the main forced flow ranges from 100 to 4000 and the buoyancy parameter, Gr/Re2 , varies from 0.3 to 907. Flow reversal is found to occur for both assisted and opposed convection. The effect of channel divergence on the occurrence and structure of the reversed flow and the heat transfer is presented and discussed. It is found that the divergence of the channel decelerates the mainstream such that flow reversal is initiated at a much lower buoyancy parameter. The adverse pressure gradient tends to push the reversed flow upstream and leads to a deeper penetration of the reversed flow into the channel The destabilization effect of the divergent channel can lead to breakdown of vortices and to transition to turbulent flow. This can significantly enhance the heat transfer. Temperature fluctuation measurements at different locations are used to indicate oscillations and fluctuations of the reversed flow. The effect of the buoyancy parameter on the Nusselt number and the reversed flow structure is discussed. The average Nusselt number is determined and correlated in terms of relevant nondimensional parameters for pure forced and mixed convection, respectively.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Falling Film Heat Transfer

J. Heat Transfer. 1996;118(3):616-625. doi:10.1115/1.2822676.

When a liquid film falls from one horizontal tube to another below it, the flow may take the form of discrete droplets, jets, or a continuous sheet; the mode plays an important role in the wetting and heat transfer characteristics of the film. Experiments are reported that explore viscous, surface tension, inertial, and gravitational effects on the falling-film mode transitions. New flow classifications, a novel flow regime map, and unambiguous transition criteria for each of the mode transitions are provided. This research is part of an overall study of horizontal-tube, falling-film flow and heat transfer, and the results may have important implications on the design and operation of falling-film heat exchangers.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):626-633. doi:10.1115/1.2822678.

When a liquid film falls from one horizontal tube to another below it, the flow may take the form of discrete droplets, jets, or a continuous sheet; the mode plays an important role in the heat transfer. Experiments are reported that explore the local heat transfer behavior for each of these flow patterns, and the results are related to the important features of the flow. Spatially averaged Nusselt numbers are presented and discussed, and new mode-specific design correlations are provided. This research is part of an overall study of horizontal-tube, falling-film flow and heat transfer.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):634-641. doi:10.1115/1.2822679.

The coupled heat and mass transfer between a falling triethylene glycol (TEG) desiccant film and air in crossflow have previously been presented and solved numerically for the cases of regeneration and dehumidification. Here, correlations for the effects of independent variables on the rate of regeneration in the regenerator and on the rate of dehumidification and sensible cooling in the absorber are developed by statistical analysis of the numerical results. The functional correlations developed should be useful in the design of regenerators and absorbers having falling liquid desiccant films and air in crossflow.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Radiative Transfer

J. Heat Transfer. 1996;118(3):642-649. doi:10.1115/1.2822680.

This study analyzes and optimizes a new design for a four-fin radiating fin array system in a multi-objective fuzzy optimization environment. The conflicting objectives of minimizing both weight and horizontal size are considered simultaneously in the fuzzy optimization logic. A fuzzy feasible domain is constructed by considering all of the fuzzy sets individually defined by the fuzzy objectives and constraints. The system is optimized by maximizing the fuzzy decision function. A systematic procedure of the fuzzy optimization is discussed in detail.

Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Boiling and Condensation

J. Heat Transfer. 1996;118(3):654-661. doi:10.1115/1.2822682.

Pool boiling heat flux versus wall superheat boiling curves were obtained for horizontal 3.18-mm-dia thin-walled brass tubes heated by an internal high-speed flow of ethylene glycol. The boiling liquids were saturated n-pentane, R-113, acetone, methanol ethanol, benzene, and isopropanol. Boiling results include nucleate and transition boiling in all the test liquids, but film boiling was achieved only with methanol. The measured peak heat fluxes are well correlated by available predictions. The methanol experiments clearly display two transition boiling curves, one obtained on increasing the cylinder temperature from nucleate boiling, the other on decreasing the cylinder temperature from film boiling. For the cases in which the highest cylinder temperature reached only into the transition regime, a single transition boiling curve resulted.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):662-667. doi:10.1115/1.2822683.

The effects of surface treatments and “gassy-subcooling” on pool boiling heat transfer are quantified by testing both smooth and treated surfaces at gassy-subcooling levels from O°C to 40°C (1 atm) and 40°C to 85°C (3 atm). Incipient and nucleate boiling wall superheats decrease over this range of gassy-subcooling. At gassy-subcooling levels greater than 20°C, the boiling curves for the smooth surface indicate two distinct regions governed by different heat transfer mechanisms, one in which the boiling process is influenced by the presence of dissolved gas, the other by boiling of the pure liquid. The critical heat flux (CHF) for each surface continually increases with increased levels of gassy-subcooling and the CHF sensitivity to gassy-subcooling is higher for the treated surface. The CHF increase due to combined surface treatment and gassy-subcooling (85°C) is ~400 percent (78 W/cm2 ).

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):668-671. doi:10.1115/1.2822684.

An experimental study to determine the effect of liquid and secondary gas flow in droplet impingement cooling is presented. The nucleate boiling regime in particular is analyzed. A correlation to predict the Nusselt number based on the liquid film thickness is derived and compared with the experimental data.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):672-679. doi:10.1115/1.2822685.

Spray cooling of a hot surface was investigated to ascertain the effect of nozzle-to-surface distance on critical heat flux (CHF). Full cone sprays of Fluorinert FC-72 and FC-87 were used to cool a 12.7 × 12.7 mm2 surface. A theoretical model was constructed that accurately predicts the spray’s volumetric flux (liquid volume per unit area per unit time) distribution across the heater surface. Several experimental spray sampling techniques were devised to validate this model. The impact of volumetric flux distribution on CHF was investigated experimentally. By measuring CHF for the same nozzle flow rate at different nozzle-to-surface distances, it was determined CHF can be maximized when the spray is configured such that the spray impact area just inscribes the square surface of the heater. Using this optimum configuration, CHF data were measured over broad ranges of flow rate and subcooling, resulting in a new correlation for spray cooling of small surfaces.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):680-688. doi:10.1115/1.2822686.

In this study, a quantitative analysis of critical heat flux (CHF) in rectangular heated channels was carried out based on new flow models and the analytical results were compared with existing experimental results at pressures of about 0.1 to 14 MPa with a water mass flux of 3.9 to 28,000 kg/m2 s and inlet water subcooling ranging from 0 to 328 K. The flow models proposed for CHF were a completely separated two-phase flow model with a macroliquid sublayer under conditions of comparatively low velocity and zero water subcooling at outlet of the channel and a subcooled boiling flow model with a macroliquid sublayer under the conditions of high water subcooling and high velocity, respectively. It could be shown that the analytical CHF results gave good predictions for over 800 existing experimental results, identifying the effects of predominant parameters as regards CHF.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):689-693. doi:10.1115/1.2822687.

This work investigates the effects of vapor shear during pure vapor external condensation on horizontal integral-fin tubes. More than 220 experimental data points in a wide range of operative conditions and enhanced surface geometries are reported together with the visual observation of the condensate flow patterns. The effects of vapor shear are relevant only for vapor Reynolds numbers greater than 70,000–100,000, while heat transfer enhancement is linked to the geometry of the extended surface. A simple semi-empirical equation was developed to account for the shear stress contribution in forced-convection condensation: this equation, applied in conjunction with the model by Briggs and Rose (1994) for stationary vapor condensation, displays a good ability in reproducing all the available data with relevant vapor velocities.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Phase-Change and Multiphase Heat Transfer

J. Heat Transfer. 1996;118(3):694-701. doi:10.1115/1.2822688.

The present paper deals with a new defrosting method for application to a cold heat exchanger by means of a frost sublimation phenomenon. The frost sublimation phenomenon occurs under the condition of temperature below the triple point of water (0.01°C, 0.6105 kPa). The present experimental study has focused on the heat and mass transfer processes at a horizontal frost layer in a moist airflow. It is found that the mass flux from the frost layer increases with the increase of water vapor mass concentration difference between the frost and main moist air flow, main air flow velocity, and the infrared radiant heat intensity. Nondimensional correlations for mass transfer, heat transfer, and defrost completion time have been derived.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):702-708. doi:10.1115/1.2822689.

The thermodynamics of the rapid vaporization of a liquid on a solid surface heated by an excimer laser pulse is studied experimentally. The transient temperature field is measured by monitoring the photothermal reflectance of an embedded thin film in nanosecond time resolution. The transient reflectivity is calibrated by considering a temperature gradient across the sample based on the static measurements of the thin film optical properties at elevated temperatures. The dynamics of bubble nucleation, growth, and collapse is detected by probing the optical specular reflectance. The metastability behavior of the liquid and the criterion for the onset of liquid–vapor phase transition in nanosecond time scale are obtained quantitatively for the first time.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):709-714. doi:10.1115/1.2822690.

A simple analytical model to predict the onset of Ledinegg instability in vertical channels under downflow conditions has been developed and evaluated. The model divides the flow field into three regions based upon the fluid temperature. The pressure drop is then found by solving an appropriate set of equations for each region. The theoretical results are compared to an extensive set of experimental data covering a range of channel diameters and operating conditions. Agreement is excellent, and the prediction of the velocity at which the minimum point in the demand curve occurs is within 12 percent over the range of experimental results. A parameter, the ratio between the surface heat flux and the heat flux required to achieve saturation at the channel exit for a given flow rate, is found to be a very accurate indicator of the minimum point velocity.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):715-724. doi:10.1115/1.2822691.

In this study, an investigation was carried out to clarify the mechanism of countercurrent flow limitation (CCFL) or flooding, that is, limitations in the falling water mass flux in countercurrent two-phase flow in vertical channels, and to identify the effects of predominant parameters regarding CCFL, adopting the criterion that the CCFL condition be given by an envelope of momentum equation applied for the entire length of the channel with respect to any void fraction. As a result, it was found that the analytical model proposed could adequately predict all existing experimental results investigated in this study. In the channel configuration, circular, rectangular, and annular or planar channels, channel dimensions of diameter, gap size, width or circumference, and length, interfacial and wall friction, water injection mode, and inlet water subcooling were dominant parameters. Therefore, both the mechanism and the quantitative effects of CCFL have been identified.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):725-730. doi:10.1115/1.2822692.

A mathematical model of the evaporating liquid–vapor meniscus in a capillary slot has been developed. The model includes two-dimensional steady-state momentum conservation and energy equations for both the vapor and liquid phases, and incorporates the existing simplified one-dimensional model of the evaporating microfilm. The numerical results, obtained for water, demonstrate the importance of accounting for the fluid flow in calculating the effective evaporative heat transfer coefficient and the superheat of the vapor over the liquid–vapor meniscus due to the heat transfer from the heated wall. With higher heat fluxes, a recirculation zone appears in the vapor near the heated wall due to extensive evaporation in the thin-film region of the liquid–vapor meniscus.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Heat Pipes

J. Heat Transfer. 1996;118(3):731-739. doi:10.1115/1.2822693.

A mathematical model for predicting the minimum meniscus radius and the maximum heat transport in triangular grooves is presented. In this model, a method for determining the theoretical minimum meniscus radius was developed and used to calculate the capillary heat transport limit based on the physical characteristics and geometry of the capillary grooves. A control volume technique was employed to determine the flow characteristics of the micro heat pipe, in an effort to incorporate the size and shape of the grooves and the effects of the frictional liquid–vapor interaction. In order to compare the heat transport and flow characteristics, a hydraulic diameter, which incorporated these effects, was defined and the resulting model was solved numerically. The results indicate that the heat transport capacity of micro heat pipes is strongly dependent on the apex channel angle of the liquid arteries, the contact angle of the liquid flow, the length of the heat pipe, the vapor flow velocity and characteristics, and the tilt angle. The analysis presented here provides a mechanism whereby the groove geometry can be optimized with respect to these parameters in order to obtain the maximum heat transport capacity for micro heat pipes utilizing axial grooves as the capillary structure.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):740-746. doi:10.1115/1.2822694.

An experimental investigation was conducted and a test facility constructed to measure the capillary heat transport limit in small triangular grooves, similar to those used in micro heat pipes. Using methanol as the working fluid, the maximum heat transport and unit effective area heat transport were experimentally determined for ten grooved plates with varying groove widths, but identical apex angles. The experimental results indicate that there exists an optimum groove configuration, which maximizes the capillary pumping capacity while minimizing the combined effects of the capillary pumping pressure and the liquid viscous pressure losses. When compared with a previously developed analytical model, the experimental results indicate that the model can be used accurately to predict the heat transport capacity and maximum unit area heat transport when given the physical characteristics of the working fluid and the groove geometry, provided the proper heat flux distribution is known. The results of this investigation will assist in the development of micro heat pipes capable of operating at increased power levels with greater reliability.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):747-755. doi:10.1115/1.2822695.

An analytical investigation of the heat transfer characteristics for evaporating thin liquid films in V-shaped microgrooves with nonuniform input heat flux was conducted. This investigation assumed that the capillary pressure difference caused by the receding of the meniscus is responsible for the axial liquid flow along the groove, and that the disjoining pressure difference along the groove side wall provided the driving force for the flow up the groove wall. The combined heat transfer mechanisms of both liquid conduction and interfacial vaporization were used to describe the local interracial mass flux in the interline region. Based on this approach, a local heat transfer coefficient was defined. The local and average heat transfer coefficients were both found to be sensitive to the characteristic thermal resistance ratio. In addition, when the film superheat was constant, the primary factor affecting the length of the evaporating interline region was found to be the heat flux supplied to the bottom plate, and for high heat flux conditions, the highest heat transfer coefficient did not necessarily exist at the axial dryout point. The expression developed for the evaporating film profile was shown to assume an exponential form if the heat flux distributed on the active interline region was assumed to be uniform.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Porous Media

J. Heat Transfer. 1996;118(3):756-761. doi:10.1115/1.2822696.

Thermal dispersion in convective flow in porous media has been numerically investigated using a two-dimensional periodic model of porous structure. A macroscopically uniform flow is assumed to pass through a collection of square rods placed regularly in an infinite space, where a macroscopically linear temperature gradient is imposed perpendicularly to the flow direction. Due to the periodicity of the model, only one structural unit is taken for a calculation domain to resolve an entire domain of porous medium. Continuity, Navier–Stokes and energy equations are solved numerically to describe the microscopic velocity and temperature fields at a pore scale. The numerical results thus obtained are integrated over a unit structure to evaluate the thermal dispersion and the molecular diffusion due to tortuosity. The resulting correlation for a high-Peclet-number range agrees well with available experimental data.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Heat Transfer in Manufacturing

J. Heat Transfer. 1996;118(3):762-773. doi:10.1115/1.2822697.

Unsteady, axisymmetric transport of mass, momentum, energy, species, and magnetic field intensity with a mushy-zone phase change in workpieces and temperature, and magnetic fields in electrodes during resistance spot welding, are systematically investigated. Electromagnetic force, joule heat, heat generation at the electrode–workpiece interface and faying surface between workpieces, different properties between phases, and geometries of electrodes are taken into account. The computed results show consistencies with observed nugget growth, electrical current, and temperature fields. The effects of the face radius and cone angle of the electrode, parameters governing welding current, electrical contact resistance, magnetic Prandtl number, electrical conductivity ratio, and workpiece thickness on transport phenomena are clearly provided.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):774-780. doi:10.1115/1.2822698.

Approximate, quasi-one-dimensional conduction models have been developed to predict the changing shape of holes, single grooves, or overlapping grooves carved by ablation into a thick solid that is irradiated by a moving laser source. For CW or pulsed laser operation a simple integral method is presented, which predicts shapes and removal rates with an accuracy of a few percent, while requiring one order of magnitude less CPU time than a three-dimensional, numerical solution. For pulsed operation a “full-pulse” model is presented, computing the erosion from an entire pulse in a single step, and reducing computer time by another order of magnitude.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):781-786. doi:10.1115/1.2822699.

This work applies fractal percolation theory to examine the impact of anomalous diffusion in short time-scale applications of random media. It is shown that there exist three regimes of heat transport corresponding to transport over the basic percolation unit (particle), the fractal cluster, and the homogeneous medium. Scaling is performed to determine the characteristic time scales of anomalous diffusion. The dependence of these time scales on both material properties and structure is examined to assess the impact of the anomalous diffusion regime on short time-scale energy transport. Additional criteria that determine the importance of anomalous diffusion relative to other transport phenomena and properties, such as radiation and thermal boundary resistance, are established.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 1996;118(3):805-809. doi:10.1115/1.2822706.
Abstract
Topics: Heat exchangers
Commentary by Dr. Valentin Fuster

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