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### Photogallery

J. Heat Transfer. 2008;130(8):080801-080801-1. doi:10.1115/1.2937158.
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The Twelfth Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for the 2007 International Mechanical Engineering Congress and Exhibition (IMECE) held in Seattle, Washington on November 11–16, 2007. The peer-reviewed evaluation process for the presented entries identified the seven entries from 2007 IMECE and two additional entries from 2007 ASME Summer Heat Transfer Conference, held in Vancouver, Canada, for publication in the ASME Journal of Heat Transfer August issue of 2008.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080901-080901-1. doi:10.1115/1.2937164.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080902-080902-1. doi:10.1115/1.2937176.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080903-080903-1. doi:10.1115/1.2937179.
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Topics: Cooling , Water
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080904-080904-1. doi:10.1115/1.2937184.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080905-080905-1. doi:10.1115/1.2937187.
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Topics: Waves , Bubbles , Water
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080906-080906-1. doi:10.1115/1.2937181.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080907-080907-1. doi:10.1115/1.2937182.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080908-080908-1. doi:10.1115/1.2937185.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):080909-080909-1. doi:10.1115/1.2937174.
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Commentary by Dr. Valentin Fuster

### Research Papers: Conduction

J. Heat Transfer. 2008;130(8):081301-081301-8. doi:10.1115/1.2927403.

A new multiscale model of thermal contact resistance (TCR) between real rough surfaces is presented, which builds on Archard’s multiscale description of surface roughness. The objective of this work is to construct the new model and use it to evaluate the effects of scale dependent surface features and properties on TCR. The model includes many details affecting TCR and is also fairly easy to implement. Multiscale fractal based models often oversimplify the contact mechanics by assuming that the surfaces are self-affine, the contact area is simply a geometrical truncation of the surfaces, and the pressure is a constant value independent of geometry and material properties. Concern has grown over the effectiveness of frequently used statistical rough surface contact models due to the inadequacies in capturing the true multiscale nature of surfaces (i.e., surfaces have multiple scales of surface features). The model developed in this paper incorporates several variables, including scale dependent yield strength and scale dependent spreading resistance to develop a new model that can be used to evaluate TCR. The results suggest that scale dependent mechanical properties are more influential than scale dependent thermal properties. When compared to an existing TCR model, this very inclusive model shows the same qualitative trend. Results also show the significance of capturing multiscale roughness when addressing the thermal contact resistance problem.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):081302-081302-8. doi:10.1115/1.2928013.

A method using either a one-dimensional analytical or a two-dimensional numerical inverse technique is developed for measurement of local heat fluxes at the surface of a hot rotating cylinder submitted to the impingement of a subcooled water jet. The direct model calculates the temperature field inside the cylinder that is submitted to a given nonuniform and time dependent heat flux on its outer surface and to a uniform surface heat source on an inner radius. In order to validate the algorithms, simulated temperature measurements inside the cylinder are processed and used by the two inverse techniques to estimate the wall heat flux. As the problem is improperly posed, regularization methods have been introduced into the analytical and numerical inverse algorithms. The numerical results obtained using the analytical technique compare well with the results obtained using the numerical algorithm, showing a good stable estimation of the available test solutions. Furthermore, real experimental data are used for the estimation, and local boiling curves are plotted and discussed.

Commentary by Dr. Valentin Fuster

### Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2008;130(8):081501-081501-7. doi:10.1115/1.2909081.

A mathematical model predicting the oscillating motion in an oscillating heat pipe is developed. The model considers the vapor bubble as the gas spring for the oscillating motions including effects of operating temperature, nonlinear vapor bulk modulus, and temperature difference between the evaporator and the condenser. Combining the oscillating motion predicted by the model, a mathematical model predicting the temperature difference between the evaporator and the condenser is developed including the effects of the forced convection heat transfer due to the oscillating motion, the confined evaporating heat transfer in the evaporating section, and the thin film condensation in the condensing section. In order to verify the mathematical model, an experimental investigation was conducted on a copper oscillating heat pipe with eight turns. Experimental results indicate that there exists an onset power input for the excitation of oscillating motions in an oscillating heat pipe, i.e., when the input power or the temperature difference from the evaporating section to the condensing section was higher than this onset value the oscillating motion started, resulting in an enhancement of the heat transfer in the oscillating heat pipe. Results of the combined theoretical and experimental investigation will assist in optimizing the heat transfer performance and provide a better understanding of heat transfer mechanisms occurring in the oscillating heat pipe.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2008;130(8):081701-081701-9. doi:10.1115/1.2909615.

This paper experimentally investigated the rotational effects on heat transfer in a two-pass rectangular channel $(AR=1:4)$, which is applicable to the channel near the leading edge of the gas turbine blade. The test channel has radially outward flow in the first passage through a redirected sharp-bend entrance and radially inward flow in the second passage after a $180deg$ sharp turn. In the first passage, rotation effects on heat transfer are reduced by the redirected sharp-bend entrance. In the second passage, under rotating conditions, both leading and trailing surfaces experienced heat transfer enhancements above the stationary case. Rotation greatly increased heat transfer enhancement in the tip region up to a maximum Nu ratio $(Nu∕Nus)$ of 2.4. The objective of the current study is to perform an extended parametric study of the low rotation number (0–0.3) and low buoyancy parameter (0–0.2) achieved previously. By varying the Reynolds numbers (10,000–40,000), the rotational speeds $(0–400rpm)$, and the density ratios (inlet density $ratio=0.10–0.16$), the increased range of the rotation number and buoyancy parameter reached in this study are 0–0.67 and 0–2.0, respectively. The higher rotation number and buoyancy parameter have been correlated very well to predict the rotational heat transfer in the two-pass, 1:4 aspect ratio flow channel.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):081702-081702-10. doi:10.1115/1.2909611.

The thermohydraulic performance of turbulent flow of air through rectangular and square ribbed ducts with twisted-tape inserts has been experimentally studied. The performance is influenced by the twisted-tape-generated swirl flow and the boundary layer separation, reattachment, and flow recirculation due to the ribs. Correlations developed for friction factor and Nusselt number satisfactorily predict the experimental data. The performance of the ribbed ducts with full-length twisted-tape inserts is found to be better than only ribbed ducts and ducts with only twisted-tape inserts. The regularly spaced twisted-tape elements in specific cases significantly perform better than their full-length counterparts. However, the short-length twisted-tape performance is worse than the full-length twisted tapes.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Exchangers

J. Heat Transfer. 2008;130(8):081801-081801-5. doi:10.1115/1.2927396.

This paper analyses the heat exchange behavior in systems characterized by Y-shaped fins through a numerical approach based on a CFD software. Starting from individual Y profiles, as optimized in a previous work in relation to the dimensionless conductance and to the performance parameter of efficiency, it has been here investigated the advantage of a modular use of profiles. The analysis has been performed by superimposing some dimensional constraints to make immediately comparable the results obtained in the different configurations faced. Each module considered has a number of fins depending on the angle $α$ between the two arms of the Y profile. This number depends therefore also on the horizontal width occupied by the whole system and it is upperly limited by the value allocated to the best performing individual fin. The results showed a significant increase of the dimensionless conductance and therefore of the exchanged thermal power for those multifin configurations with low values of $α$. This result validates the new optimization criterion proposed.

Commentary by Dr. Valentin Fuster

### Research Papers: Melting and Solidification

J. Heat Transfer. 2008;130(8):082301-082301-11. doi:10.1115/1.2928010.

A detailed experimental and analytical study has been performed to evaluate how copper porous foam (CPF) enhances the heat transfer performance in a cylindrical solid/liquid phase change thermal energy storage system. The CPF used in this study had a 95% porosity and the phase change material (PCM) was 99% pure eicosane. The PCM and CPF were contained in a vertical cylinder where the temperature at its radial boundary was held constant, allowing both inward freezing and melting of the PCM. Detailed quantitative time-dependent volumetric temperature distributions and melt/freeze front motion and shape data were obtained. As the material changed phase, a thermal resistance layer built up, resulting in a reduced heat transfer rate between the surface of the container and the phase change front. In the freezing analysis, we analytically determined the effective thermal conductivity of the combined PCM/CPF system and the results compared well to the experimental values. The CPF increased the effective thermal conductivity from $0.423W∕mKto3.06W∕mK$. For the melting studies, we employed a heat transfer scaling analysis to model the system and develop heat transfer correlations. The scaling analysis predictions closely matched the experimental data of the solid/liquid interface position and Nusselt number.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2008;130(8):082401-082401-8. doi:10.1115/1.2927400.

Anisothermal flow prevails in a heated microchannel. It is desirable to understand the influence of temperature-dependent physical properties on the flow and heat transfer characteristics for natural convective gas microflow. In this study, formulas for the shear viscosity, thermal conductivity, constant-pressure specific heat, density, and molecular mean free path are proposed in power-law form and validated through experimental data. Natural convective gas flow with variable physical properties in a long open-ended vertical parallel-plate microchannel with asymmetric wall temperature distributions is further investigated. The full Navier–Stokes equations and energy equation combined with the first-order slip∕jump boundary conditions are employed. Analysis process shows that the compressibility and viscous dissipation terms in balance equations are negligible. Numerical solutions are presented for air at the standard reference state with complete accommodation. It is found that the effect of variable properties should be considered for hotter-wall temperatures greater than $306.88K$. The effect is to advance the velocity slip and temperature jump as well as the velocity symmetry and temperature nonlinearity. Moreover, it tends to reduce the mass flow rate and the local heat transfer rate excluding on the cooler-wall surface where the temperature-jump effect prevails over the temperature-nonlinearity effect. Increasing the cooler-wall temperature magnifies the effect on flow behavior but minifies that on thermal behavior.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):082402-082402-9. doi:10.1115/1.2909608.

Presented here is a new molecular dynamics simulation approach for visualizing multidimensional acoustic wave-packet propagation in anisotropic materials. This approach allows examination of longitudinal wave propagation in a selected frequency range and may also be extended to track transverse motions. The obtained results agree with analytical predictions and experimental measurements of quasilongitudinal wave front propagation in the literature. Additionally, spectral analysis reveals minor levels of frequency redistribution as the wave packet propagates, which is indicative of phonon-phonon scattering. The present approach provides new capabilities for phonon-focusing studies and offers an alternative to existing experimental and Monte Carlo techniques used for these studies.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):082403-082403-8. doi:10.1115/1.2928014.

A technique to extract in-plane thermal conductivity of thin metallic films whose thickness is comparable to electron mean free path is described. Microscale constrictions were fabricated into gold films of thicknesses $43nm$ and $131nm$. A sinusoidal voltage excitation across the constriction results in a local temperature rise. An existing technique known as scanning joule expansion microscopy, measures the corresponding periodic thermomechanical expansion with a $10nm$ resolution and determines the local temperature gradient near the constriction. A three-dimensional finite-element simulation of the frequency-domain heat transfer fits the in-plane thermal conductivity to the measured data, finding thermal conductivities of $82±7.7W∕mK$ for the $43nm$ film and $162±16.7W∕mK$ for the $131nm$ film, at a heating frequencies of $100kHz$ and $90kHz$, respectively. These values are significantly smaller than the bulk thermal conductivity value of $318W∕mK$ for gold, showing the electron size effect due to the metal-dielectric interface and grain boundary scattering. The obtained values are close to the thermal conductivity values, which are derived from electrical conductivity measurements after using the Wiedemann–Franz law. Because the technique does not require suspended metal bridges, it captures true metal-dielectric interface scattering characteristics. The technique can be extended to other films that can carry current and result in Joule heating, such as doped single crystal or polycrystalline semiconductors.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):082404-082404-8. doi:10.1115/1.2909614.

Submicron metallic slit arrays with different geometry were designed and fabricated on silicon substrates. Their infrared radiative properties (transmittance, reflectance, and absorptance) were investigated both experimentally and theoretically. The normal transmittance of three fabricated Au slit arrays was measured at wavelengths between $2μm$ and $15μm$ using a Fourier-transform infrared spectrometer. The experimental results were compared to the values calculated from the rigorous coupled-wave analysis. The applicability of the effective medium theory for modeling radiative properties was also examined. The agreement between the measurement and modeling results demonstrates the feasibility of quantitative tuning of the radiative properties by employing periodic micro/nanostructures.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):082405-082405-13. doi:10.1115/1.2909617.

The friction numbers for laminar flows of water in microtubes, determined from the temperature rise due to the viscous dissipation heating assuming a velocity slip, show a strong dependence on the diameter and aspect ratio. The calculated values compare well with those determined from experimental data for water flows in glass and diffused silica microtubes ($16–101μm$ in diameter $D$ and aspect ratios $L∕D=499–1479$). With a slip, the friction number almost exponentially decreases as $D$ decreases and, to a lesser extent, as $L∕D$ increases. For $D>400μm$, the friction number approaches the theoretical Hagen–Poiseuille for macrotubes (64) when $L∕D>∼1500$, but higher values at smaller $L∕D$. The developed semiempirical analytical expression for calculating the friction number is in good agreement with the numerical and experimental results. The results suggest the presence of a velocity slip in the experiments and the plausible presence of a thin nanolayer at the walls of the microtubes. For $D>200μm$, this layer, if exists, is estimated to be $∼18.9nm$, but increases to $∼21.5nm$ for $D<200μm$, when $R¯e=800$ and $L∕D=1479$.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):082406-082406-7. doi:10.1115/1.2928050.

The influence of particle anisotropy on the effective thermal conductivity of a suspension is experimentally investigated. Suspensions of micron-sized, silicon-carbide particles with varying aspect-ratio distributions were prepared and measured. It is shown that the conductivity of the silicon-carbide suspensions can be quantitatively predicted by the effective medium theory of Nan (1997, “Effective Thermal Conductivity of Particulate Composites With Interfacial Thermal Resistance  ,” J. Appl. Phys.81(10), pp. 6692–6699), provided the volume-weighted aspect ratio of the particles is used. Recent experimental data on multiwalled-nanotube-in-oil suspensions by Yang (2006, “Thermal and Rheological Properties of Carbon Nanotube-in-Oil Dispersions  ,” J. Appl. Phys., 99(11), 114307) are also analyzed and shown to be in at least qualitative agreement with the effective-medium-theory prediction that the thermal conductivity of suspensions is enhanced by large aspect-ratio particles.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2008;130(8):082701-082701-8. doi:10.1115/1.2909612.

The full-spectrum $k$-distribution (FSK) approach is a promising model for radiative transfer calculations in participating media. FSK achieves line-by-line (LBL) accuracy for homogeneous media at a tiny fraction of LBL’s high computational cost. However, inhomogeneities in gas temperature, total pressure, and component-gas mole fractions change the spectral distribution of the absorption coefficient and can cause inaccuracies in the FSK approach. In this paper, a new hybrid FSK method is proposed that combines the advantages of the multigroup FSK (MGFSK) method for temperature inhomogeneities in a single gas species and the multiscale FSK (MSFSCK) method for concentration inhomogeneities in gas mixtures. In this new hybrid method, the absorption coefficients of each gas species in the mixture are divided into $M$ spectral groups depending on their temperature dependence. Accurate MGFSK databases are constructed for combustion gases, such as $CO2$ and $H2O$. This paper includes a detailed mathematical development of the new method, method of database construction, and sample heat transfer calculations for 1D inhomogeneous gas mixtures with step changes in temperature and species mole fractions. Performance and accuracy are compared to LBL and plain FSK calculations. The new method achieves high accuracy in radiative heat transfer calculations in participating media containing extreme inhomogeneities in both temperature and mole fractions using as few as $M=2$ spectral groups for each gas species, accompanied by several orders of magnitude lower computational expense as compared to LBL solutions.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Heat Transfer. 2008;130(8):084501-084501-4. doi:10.1115/1.2927399.

Pool nucleate boiling heat transfer experiments were performed for water using heat transfer surfaces having unified cavities. Cylindrical holes of $10μm$ in diameter and $40μm$ in depth were formed on a mirror-finished silicon wafer of $0.525mm$ in thickness using Microelectromechanical systems technology. The test heat transfer surface was heated by a semiconductor laser beam. Experiments were conducted in the range of up to $4.54×104W∕m2$. The temperature of the back side of the heat transfer surface was measured by a radiation thermometer. When the spacing between cavities was $S∕Lc<0.8$, the horizontal and declining coalescence of bubbles on the neighboring cavities were dominant. Strong thermal and bubble coalescence interactions between nucleation sites were observed in this situation. This vigorous bubble coalescence created strong convection. The heat carried by this convection accounted for a large part of the heat transfer. As the cavity interval became wide, $S∕Lc≥1.2$, the horizontal and the declining coalescence of the bubbles ceased. The coalescence was limited to the vertical or no coalescence. The thermal and bubble coalescence interactions between the nucleation sites became quite low, to the extent of being negligible. The bubbles themselves were key in carrying heat away from the heat transfer surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):084502-084502-4. doi:10.1115/1.2927404.

The concept of heat exchanger efficiency is extended to the heat exchanger networks. General expressions that can be used for determining the overall efficiency and effectiveness of heat exchangers connected in series regardless of the heat exchanger type have been presented. A simple, accurate, approximate algebraic expression is provided for determining the efficiency of the heat exchanger. The approach presented is far more general compared to the traditional approaches, providing the designer the flexibility to select the efficiency of the individual heat exchangers, the overall system efficiency, the number of heat exchangers, as well as the type of heat exchanger to select, by utilizing a single equation. An expression is derived for determining the minimum number of shells, as a convenient alternative for the traditional methods currently in use.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):084503-084503-5. doi:10.1115/1.2928006.

This paper presents the fundamental approaches of modeling thermal-hydraulic component briefly. A set of lumped parameter mathematical models is developed, which are based on conservation of mass and energy. Subsequently, the connection rule for basic thermal-hydraulic components and the method to automatically generate the complete thermal-hydraulic system model are put forward. The integration methods for solving the cross-coupling thermal-hydraulic equations are also discussed for a position-controlled thermal-hydraulic system. Simulation results show the interaction between pressure and temperature. The simplified representations of thermal-hydraulic differential equations are also proposed in this paper, which can reduce simulation time. The validity of the simplified representations is judged by simulation.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):084504-084504-5. doi:10.1115/1.2909610.

The exergy (second-law) efficiency is formulated for a condensation process in a shell and one-path tube exchanger for a fixed control volume. The exergy efficiency $ηex$ is expressed as a function of the inlet and outlet temperatures and mass flow rates of the streams. This analysis is utilized to assess the trend of local exergy efficiency along the condensation path and evaluate its value for the entire condenser, i.e., overall exergy efficiency. The numerical results for an industrial condenser, with a steam-air mixture and cooling water as working fluids, indicate that $ηex$ is significantly affected by the inlet cooling water and environment temperatures. Further investigation shows that other performance parameters, such as the upstream mixture temperature, air mass flow rate, and ratio of cooling water mass flow rate to upstream steam mass flow rate, do not have considerable effects on $ηex$. The investigations involve a dimensionless ratio of the temperature difference of the cooling water and environment to the temperature difference of condensation and the environment. Numerical results for various operational conditions enable us to accurately correlate both the local and overall exergy efficiency as linear functions of dimensionless temperature.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):084505-084505-4. doi:10.1115/1.2909616.

Heat transfer enhancement in combined convection around a rotating horizontal cylinder using nanofluids is presented. The transport equations are solved numerically using a second-order finite volume scheme. Water-based nanofluid containing various volume fractions of different types of nanoparticles is used. The nanoparticles used are Cu, Ag, $Al2O3$, and $TiO2$. In the region outside the plume, the Nusselt number increases by increasing the volume fraction of nanoparticles. However, in the plume region, the effect of the volume fraction of nanoparticles on the Nusselt number is less pronounced.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(8):084506-084506-4. doi:10.1115/1.2928015.

In the present investigation, the steady state performance of a rectangular single phase natural circulation loop (NCL) with end heat exchangers is studied. One-dimensional governing equations are considered in developing the mathematical model. Analytical expressions are derived for the circulation rate and temperature profile. However, the individual performance parameters are to be computed iteratively as the equations are strongly coupled. A suitable iterative procedure is given to evaluate the important loop parameters such as steady state flow rate, and riser and downcomer temperatures. Few special cases are discussed where analytical expressions for circulation rate and temperature distribution can be obtained directly without any iterative procedure. It is also shown that both the hot and cold end heat exchangers should have equal conductance $(UA)$ for maximization of circulation rate. This feature of NCL is identical with heat power cycle and can be explained in light of equipartition principle.

Commentary by Dr. Valentin Fuster

### Errata

J. Heat Transfer. 2008;130(8):087001-087001-2. doi:10.1115/1.2928056.
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In this paper there were a number of errors that we have corrected below.

Commentary by Dr. Valentin Fuster