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Research Papers: Photogallery

J. Heat Transfer. 2013;135(8):080901-080901-1. doi:10.1115/1.4024183.

The Sixteenth Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for both the 2012 Summer Heat Transfer Conference held in Puerto Rico, USA, on July 8–12, 2012, and the 2012 International Mechanical Engineering Congress and Exhibition (IMECE) held in Houston, Texas in USA, on Nov. 9–15, 2012. Both Photogallery sessions presented a total of 19 entries and the peer-reviewed evaluation conducted by the participants has identified the 11 entries for publication in this ASME Journal of Heat Transfer August issue of 2013.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster

Research Papers: Conduction

J. Heat Transfer. 2013;135(8):081301-081301-7. doi:10.1115/1.4024276.

Control of transport processes in composite microstructures is critical to the development of high-performance functional materials for a variety of energy storage applications. The fundamental process of conduction and its control through the manipulation of granular composite attributes (e.g., grain shape) are the subject of this work. We show that athermally jammed packings of tetrahedra with ultrashort range order exhibit fundamentally different pathways for conduction than those in dense sphere packings. Highly resistive granular constrictions and few face–face contacts between grains result in short-range distortions from the mean temperature field. As a consequence, ‘granular’ or differential effective medium theory predicts the conductivity of this media within 10% at the jamming point; in contrast, strong enhancement of transport near interparticle contacts in packed-sphere composites results in conductivity divergence at the jamming onset. The results are expected to be particularly relevant to the development of nanomaterials, where nanoparticle building blocks can exhibit a variety of faceted shapes.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):082201-082201-11. doi:10.1115/1.4024262.

Synthetic jets are generated by an equivalent inflow and outflow of fluid into a system. Even though such a jet creates no net mass flux, net positive momentum can be produced because the outflow momentum during the first half of the cycle is contained primarily in a vigorous vortex pair created at the orifice edges; whereas in the backstroke, the backflow momentum is weaker, despite the fact that mass is conserved. As a consequence of this, the approach can be potentially utilized for the impingement of a cooling fluid onto a heated surface. In previous studies, little attention has been given to the influence of the jet's origins; hence it has been difficult to find reproducible results that are independent of the jet apparatus or actuators utilized to create the jet. Furthermore, because of restrictions of the resonators used in typical actuators, previous investigations have not been able to independently isolate effects of jet frequency, amplitude, and Reynolds number. In the present study, a canonical geometry is presented, in order to study the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner in which it is produced. The unsteady Navier–Stokes equations and the convection–diffusion equation were solved using a fully unsteady, two-dimensional finite volume approach in order to capture the complex time dependent flow field. A detailed analysis was performed on the correlation between the complex velocity field and the observed wall heat transfer. Scaling analysis of the governing equations was utilized to identify nondimensional groups and propose a correlation for the space-averaged and time-averaged Nusselt number. A fundamental frequency, in addition to the jet forcing frequency, was found, and was attributed to the coalescence of consecutive vortex pairs. In terms of time-averaged data, the merging of vortices led to lower heat transfer. Point to point correlations showed that the instantaneous local Nusselt number strongly correlates with the vertical velocity v although the spatial-temporal dependencies are not yet fully understood.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2013;135(8):081501-081501-8. doi:10.1115/1.4023879.

This paper presents the visualization results obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel used was a fused silica tube having an internal diameter of 781 μm, a heated length of 191 mm, and was coated with a thin, transparent, and electrically conductive layer of indium-tin-oxide (ITO) on the outer surface. The operating parameters during the experiments were: mass flux 100–400 kg/m2 s, heat flux 5–45 kW/m2, saturation temperatures 25 and 30 °C, corresponding to saturation pressures of 6.65 bar and 7.70 bar and reduced pressures of 0.163 and 0.189, respectively. A high speed camera with a close up lens was used to capture the flow patterns that evolved along the channel. Flow pattern maps are presented in terms of the superficial gas and liquid velocity and in terms of the Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in the literature. Rigorous boiling and increased coalescence rates were observed with an increase in the heat flux.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):081502-081502-13. doi:10.1115/1.4023678.

For a steam generator (SG) in a commercialized sodium-cooled fast breeder reactor (FBR), flow instability in the water side is one of the most important items needing research. As the first step of this research, thermal-hydraulic experiments using water as the test fluid were performed under high pressure conditions at the Japan Atomic Energy Agency (JAEA) by using a circular tube. Void fraction, pressure drop, and heat transfer coefficient data were obtained under 15, 17, and 18 MPa. This paper discusses the steam-water pressure drop and void fraction. Using the obtained data, we evaluated existing correlations for void fraction and two-phase flow multipliers under high pressure. As a result, the drift flux model implemented in the TRAC-BF1 code was confirmed to suitably predict the void fraction well under the present high pressure conditions. For the two-phase flow multiplier, the Chisholm correlation and the homogeneous model were confirmed to be the best under the present high-pressure conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2013;135(8):081701-081701-12. doi:10.1115/1.4024066.

Thermoacoustic waves in near-critical supercritical carbon dioxide are investigated experimentally on acoustic time scales using a fast electrical heating system along with high speed pressure measurements. Supercritical carbon dioxide (near the critical or the pseudocritical states) in an enclosure is subjected to fast boundary heating with a thin nickel foil and an R-C circuit. The combination of very high thermal compressibilities and vanishingly small thermal diffusivities of the near-critical fluid affect the thermal energy propagation, leading to the formation of acoustic waves as carriers of thermal energy (the so called piston effect). The experimental results show that under the same temperature perturbation at the boundary, the strength of the acoustic field is enhanced as the initial state of the supercritical fluid approaches criticality. The heating rate, at which the boundary temperature is raised, is a key factor in the generation of these acoustic waves. The effect of different rates of boundary heating on the acoustic wave formation mechanism near the critical point is studied. The thermoacoustic wave generation and propagation in near-critical supercritical fluid is also investigated numerically and compared with the experimental measurements. The numerical predictions show a good agreement with the experimental data.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):081702-081702-8. doi:10.1115/1.4023937.

The fundamental problem of heat and mass transfer from a slightly deformed sphere at low but finite Peclet numbers in Stokes flow is solved by a combined regular and singular perturbation method. The deformed sphere is assumed to be axisymmetric and its shape is described by a power series in a small parameter; the correction to the Nusselt number due to the deformation of the sphere is obtained through a regular perturbation with respect to this parameter. On the contrary, the correction to the Nusselt number due to the small Peclet number is derived by applying a singular perturbation method. The analytical solution is derived for the averaged Nusselt number in terms of the Peclet number and the deformation parameter.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2013;135(8):081901-081901-9. doi:10.1115/1.4024278.

In the present paper, rectangular channels with six types of elliptic scale-roughened walls for heat transfer enhancement are numerically studied. Heat transfer and fluid flow characteristics for sixteen different scale-roughened models (with the scale height varying in the range from 1 mm to 2.5 mm) are numerically predicted using commercial computational fluid dynamics (CFD) code, Ansys cfx. The turbulent model employed is the k–ω based shear–stress transport (SST) model with automatic wall function treatment. In the performance evaluation, we use a “universal” porous media length scale based on volume averaging theory (VAT) to define the Reynolds number, Nusselt number, and friction factor. It is found that heat transfer performance is most favorable when the elliptic scales are oriented with their long axis perpendicular to the flow direction, while the scales elongated in the flow direction have lower Nusselt numbers and pressure drops compared with the circular scale-roughened channels. Results indicate that the scale-shaped roughness strongly spins the flow in the spanwise direction, which disrupts the near-wall boundary layers continuously and enhances the bulk flow mixing. With the flow marching in a more intense spiral pattern, a 40% improvement of heat transfer enhancement over the circular scale-roughened channels is observed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):081902-081902-8. doi:10.1115/1.4023883.

The results of a computational study into the thermal performance of thermally radiating fractal-like fins are presented. Previous experimental studies have shown that fractal patterns increase the heat transfer surface area while simultaneously reducing mass. Two fractal patterns were used for comparison, the modified Koch snowflake and the Sierpinski carpet. For an isothermal base fin radiating to free space, the fin effectiveness and fin efficiency are presented for the zeroth and first four fractal iterations in order to quantify the performance. Emissivity, width/thickness ratio, base temperature, and fin material were varied to better understand their impact on the performance of fractal-like fins. Based upon the observed results, fractal-like fins greatly improve the fin effectiveness per unit mass. In certain cases, fin effectiveness per unit mass was found to increase by up to 46%. As the cost of access to space is significant, this reduction in mass could lead to savings for spacecraft thermal management applications.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):081903-081903-11. doi:10.1115/1.4024017.

In this study, efficiencies for partially wetted fins for the uniform cross section spine, conical spine, concave parabolic spine, and convex parabolic spine are presented using an analytical method. Depending on the set of boundary conditions, there are two methods for deriving the efficiencies of partially wet fins for each spine. The eight equations for fin efficiencies were investigated. Fin efficiency is a function of the length of the dry portion. Thus, the equations for calculating the length of the dry portion are also presented. The findings indicate that a larger cross-sectional fin results in a higher conduction heat transfer rate. Contrarily, the fin efficiency is lower. This is different from the longitudinal fin, for which the trend lines of heat transfer rate and fin efficiency are the same. This converse relationship is due to the effect of the ratio of the cross-sectional area to the surface area. Moreover, partially wet fin efficiencies decrease with increased relative humidity. For convenience, the approximate equation for efficiencies for partially wet fins, which is derived from the equations for fully wet and fully dry fin efficiencies, is also presented.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2013;135(8):082001-082001-9. doi:10.1115/1.4024352.

Two-dimensional non-Fickian diffusion equation is solved analytically under arbitrary initial condition and two kinds of periodic boundary conditions. The concentration field distributions are analytically obtained with a form of double Fourier series, and the damped diffusion wave transport is discussed. At the same time, the numerical simulation is carried out for the problem with homogeneous boundary condition and arbitrary initial condition, which shows that the concentration field gradually changes from the initial distribution to the steady distribution and it changes faster for the smaller Vernotte number. The numerical results agree well with the experimental results.

Commentary by Dr. Valentin Fuster

Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2013;135(8):082202-082202-10. doi:10.1115/1.4024280.

Laminar to weakly turbulent mixed convection in a square duct heated from the bottom side is highly strengthened by ionic jets generated by an array of high voltage points, opposite to the heated strip. Negative ion injection is activated within the dielectric liquid HFE-7100. Local temperatures on the heated wall are measured by liquid crystal thermography. Distributions of the Nusselt number are obtained at different forced flow rates, applied heat flows, and transiting electrical currents. In correspondence of the point emitters, higher Nusselt numbers in the impingement areas are measured and an analogy with the thermo-fluid dynamic behavior of an array of submerged impinging jets in a crossflow is drawn. The diameter of the ionic jets is evaluated and an electrohydrodynamic Reynolds number is employed for correlation and similarity purposes. Potential applications of the technique are high-efficiency compact heat exchangers and heat sinks.

Commentary by Dr. Valentin Fuster

Research Papers: Porous Media

J. Heat Transfer. 2013;135(8):082601-082601-8. doi:10.1115/1.4024281.

A well-known set of Berkovsky–Polevikov (BP) correlations have been extremely useful in predicting the wall-averaged Nusselt number of “wide” enclosures heated from the side and filled with a fluid undergoing natural convection. A generic form of these correlations, dependent on only two coefficients, is now proposed for predicting the Nusselt number of a heterogeneous (fluid–solid), porous enclosure, i.e., an enclosure filled not only with a fluid but also with uniformly distributed, disconnected and conducting, homogeneous solid particles. The final correlations, and their overall accuracies, are determined by curve fitting the numerical simulation results of the natural convection process inside the heterogeneous enclosure. Results for several Ra and Pr, and for 1, 4, 9, 16, and 36 solid particles, with the fluid volume-fraction (porosity) maintained constant, indicate the accuracy of these correlations to be detrimentally affected by the interference phenomenon caused by the solid particles onto the vertical boundary layers that develop along the hot and cold walls of the enclosure; the resulting correlations, in this case, present standard deviation varying between 6.5% and 19.7%. An analytical tool is then developed for predicting the interference phenomenon, using geometric parameters and scale analysis results. When used to identify and isolate the interference phenomenon, this tool is shown to yield correlations with much improved accuracies between 2.8% and 9.2%.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):082602-082602-10. doi:10.1115/1.4024091.

This paper proposes and implements a new methodology for optimizing finned-tube heat exchangers (FTHEs) using a volume-averaging theory (VAT) hierarchical physical model and a genetic algorithm (GA) numerical optimizer. This method allows for multiple-parameter constrained optimization of FTHEs by design of their basic morphological structures. A consistent model is used to describe transport phenomena in a FTHE based on VAT, which allows for the volume-averaged conservation of mass, momentum, and energy equations to be solved point by point, with the morphology of the structure directly incorporated into the field equations and full conjugate effects included. The equations differ from those often presented in porous media modeling and are developed using a rigorous averaging technique, hierarchical modeling methodology, and fully turbulent models with Reynolds stresses and fluxes in every pore space. These averaged equations have additional integral and differential terms that must be dealt with in order for the equation set to be closed, and recent work has provided this closure for FTHEs. The resulting governing equation set is relatively simple and is discretized and quickly solved numerically. Such a computational solution algorithm is fast running, but still able to present a detailed picture of the temperature fields in both of the fluid flows as well as in the solid structure of the heat exchanger. A GA is integrated with the VAT-based solver to carry out the FTHE numerical optimization, which is a ten parameter problem, and the FTHE is optimized subject to imposed constraints. This method of using the VAT-based solver fully integrated with a GA optimizer results in a new all-in-one tool for performing multiple-parameter constrained optimization on FTHEs.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2013;135(8):082701-082701-9. doi:10.1115/1.4024279.

The performance of tree-like fins with varying bifurcation angle, surface emissivity, material, width-to-thickness ratio, and base heat rate was examined. Overall system performance was examined computationally. The computational results have been validated, verified, and cast in terms of commonly defined dimensionless parameters. Tree-like fins were found to be more effective and more efficient than the rectangular fins. Fin efficiency and effectiveness were found to increase with increasing bifurcation angles while base temperatures were found to decrease with increasing bifurcation angles. As expected, base temperatures were highest for the largest width-to-thickness ratios and smallest for materials with relatively higher thermal conductivities.

Commentary by Dr. Valentin Fuster

Photogallery

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):080903-080903-1. doi:10.1115/1.4024185.
Abstract
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):080904-080904-1. doi:10.1115/1.4024186.
Abstract
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):080908-080908-1. doi:10.1115/1.4024190.
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):080909-080909-1. doi:10.1115/1.4024191.
Abstract
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):080910-080910-1. doi:10.1115/1.4024192.
Abstract
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):080911-080911-1. doi:10.1115/1.4024193.
Abstract
Topics: Nanofluids
Commentary by Dr. Valentin Fuster

Announcement

J. Heat Transfer. 2013;135(8):080913-080913-1. doi:10.1115/1.4024584.
Abstract
Commentary by Dr. Valentin Fuster

Technical Briefs

J. Heat Transfer. 2013;135(8):084501-084501-4. doi:10.1115/1.4024385.

Recently, it has become possible to conduct line-by-line (LBL) accurate radiative heat transfer calculations in spectrally highly nongray combustion systems using the Monte Carlo method. LBL accuracy, in principle, adds little to the computational load as compared to gray calculations. However, when employing the Monte Carlo method, the original scheme for choosing appropriate emission wavenumbers for statistical photon bundles is numerically expensive. An improved wavelength selection scheme has been applied to hypersonic plasmas for Monte Carlo solvers. However, directly applying this improved scheme to combustion gases may cause significant errors. In this paper, a hybrid scheme for wavenumber selection is proposed, significantly decreasing CPU requirements compared to previous work. The accuracy of the new method is established and its time requirements are compared against the previous method.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):084502-084502-7. doi:10.1115/1.4024277.

A voltage applied across a uniform plate results in a uniform ohmic heat dissipation, useful for conducting heat transfer experiments or preventing unacceptably low temperatures on spacecraft components. Most experiments to date involve application of a known uniform heat flux to the surface of a model. Measurement of the resulting temperature distribution facilitates calculation of the heat transfer coefficient, h. The dependence of h on the boundary condition, however, may necessitate a specified nonuniform heat flux. In this paper, a novel methodology is developed for designing a nonuniform thickness heat flux plate to provide a specified spatially variable heat flux. The equations are derived to solve the two dimensional heat flux with a variable cross sectional area. After showing that this inverse heat transfer problem cannot be readily linearized, a methodology utilizing a smooth surface polynomial was applied. Then, for a prescribed, desired heat flux distribution, a 7th order polynomial (including 36 terms) yielded a normalized root mean squared error of 1% over the surface. This distributed heat flux could result in significant power and thus cost savings for a variety of applications.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):084503-084503-4. doi:10.1115/1.4024282.

Boundary-layer forced convective heat transfer at a moving flat surface parallel to a moving stream is presented for the case where the plate is subjected to a variable heat flux. In particular, we assume that the surface heat flux varies with spatial variable x according to a power-law rule. The similarity solutions for the problem are obtained by solving the reduced ordinary differential equations numerically, while exact solutions are provided for certain parametric values. It is noted that even in the case of prescribed surface heat flux, dual solutions exist when the surface and the fluid move in opposite directions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):084504-084504-4. doi:10.1115/1.4024016.

The concept of relative inverse admittance applied to composite fins optimization in the case of longitudinal rectangular fins under 2D heat conduction is presented in this work. Here, different values for convective conditions at the fin and composite layer surfaces are used and the influence of the kc/kf ratio and composite thickness in optimum geometry is determined. The optimization process is carried out through universal graphs in which the range of parameters covers most of the practical cases a designer will find. Relative inverse admittance is applied in a general form and emerges as an easily used tool for optimizing composite fins.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(8):084505-084505-3. doi:10.1115/1.4024283.

A mathematical method is described for the analytical solution of the convective heat transfer rates from a rotating isothermal and porous disk in a uniform flow field. By applying the appropriate velocity component of the fluid in the energy equation, a similarity solution was derived showing an increase in the rates of heat transfer with increasing rotational Reynolds number and with decreasing flow Reynolds number. Effects of natural convection and viscous dissipation were assumed negligible.

Commentary by Dr. Valentin Fuster

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