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### Guest Editorial

J. Heat Transfer. 2015;137(2):020301-020301-1. doi:10.1115/1.4029009.

The eighteenth Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for the 11th AIAA/ASME Thermophysics and Heat Transfer Conference held in Atlanta, GA, on June 16–20, 2014. The Photogallery session presented eight entries in total and the peer-reviewed evaluation conducted by both the participants and selected HTD K-22 Technical Committee members has identified the five final entries for publication in this issue of ASME Journal of Heat Transfer.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster

### Photogallery

J. Heat Transfer. 2015;137(2):020901-020901-1. doi:10.1115/1.4029011.

Frequency domain thermoreflectance (FDTR) imaging is used to create quantitative maps of both in-plane thermal conductance and cross-plane thermal boundary conductance (TBC) for graphene multilayers encased between titanium and silicon dioxide. A graphene flake is encased between a metal layer and a thermally oxidized p-type silicon wafer and a piezo stage is used to raster scan the sample for imaging. For each image pixel, a periodically modulated continuous-wave laser (the red pump beam) is focused to a Gaussian spot, less than 2 um in diameter, that locally heats the sample, while a second beam (the green probe beam) monitors the surface temperature through a proportional change in reflectivity. The pump beam is modulated simultaneously at six frequencies and the thermal properties of the graphene flake are extracted by minimizing the error between the measured probe phase lag at each frequency and an analytical solution to the heat diffusion equation in a multilayer stack of materials. Phase images at six frequencies for the sample are shown in b. Different layers of the graphene flake are clearly shown in 9.9 MHz and 11.3 MHz images. The six phase data points at every pixel are then fitted to our thermal model to generate two thermal property maps of the graphene flake: in-plane thermal conductance and TBC, shown in c. The in-plane thermal conductance map shows an increased conduction of heat in graphene with the number of layers, while the TBC map indicates a constant cross-plane conduction along the flake. Our imaging technique can be used to study thermal transport in graphene and has implications for thermal management in graphene based electronic devices.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(2):020902-020902-1. doi:10.1115/1.4029012.

Frequency domain thermoreflectance (FDTR) imaging is used to create quantitative thermal conductivity maps of porous Aluminum Nitride (AlN) particles embedded in epoxy. The AlN-epoxy composite is polished and coated with a metal layer. A piezo stage is used to move the sample for imaging with our FDTR system. For each pixel, a periodically modulated continuous-wave laser (the red pump beam) is focused to a Gaussian spot, less than 2 um in diameter, to locally heat the sample, while a second beam (the green probe beam) monitors the surface temperature through a proportional change in the metals' reflectivity. The pump beam is modulated simultaneously at six frequencies and the thermal properties of the AlN composite are extracted by minimizing the error between the measured probe phase lag at each frequency and an analytical solution to the heat diffusion equation in a multilayer stack of materials. A schematic of the AlN sample in our measurement system and an optical image of the polished surface of the AlN-epoxy composite before coating with metal is shown in a. Two scanning electron microscope images of the porous AlN particles prior to embedding in epoxy are shown in b. One of the six simultaneously collected phase images of the probe laser is shown in c. The dark blue regions in the phase image are pits on the sample surface. We fit the six phase images to our thermal model and obtain thermal conductivity maps. The conductivity maps of four particles are shown in d. A log color bar is used to highlight the contrast of thermal conductivity in a single particle. The thermal conductivity of the AlN particles ranges from 80W/mK in the dense regions to 5W/mK in the porous regions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(2):020903-020903-1. doi:10.1115/1.4029013.

Wetting characteristics of the vertically aligned carbon nanotube (VACNT) array surface can be modified by atmospheric plasma treatment with masks. The plasma treatment of the VACNT array surface was performed by moving an atmospheric pressure plasma jet system (Plasmatreat GmbH, Steinhagen) at a fixed speed of 0.167 m/s through a mask with holes. The hole diameter is fixed equal to 250 µm. The distance between holes varies between 0.5 mm to 3.0 mm.The Easy Drop Shape Analyzer (KRUSS GmbH) was used for contact angle measurement. Experimental results show that the wetting characteristics of the VACNT array surface can be modified from a contact angle of 140° to 0° depending on the distance between holes.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(2):020904-020904-1. doi:10.1115/1.4029014.

A thermally representative tank-to-tank transfer line was designed and tested at SMiRF at NASA Glenn to simulate liquid hydrogen transfer from a Low Earth Orbit depot storage tank to a customer receiver tank. The line was equipped with three external skin silicon diode measurements, one internal stream temperature measurement, and a high speed imaging system to view flow profiles as the system chilled down from 250K to LH2 temperatures. Image: Two phase flow video stills correlated with temperature measurements shows the time evolution of chill down. Skin measurements indicate chill down is nearly complete within 20s due to annular flow and liquid layer along wall, but internal stream temperature doesn't bottom out until >140s due to bubbly flow. Majority of chill down is spent in annular and bubbly flow regimes. SD1-3 are successive skin diodes mounted along the transfer line.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(2):020905-020905-1. doi:10.1115/1.4029015.

Nucleate boiling heat transfer depends on various aspects of the bubble ebullition, such as the bubble nucleation, growth and departure. In this work, a synchronized high-speed optical imaging and infrared (IR) thermography approach was employed to study the ebullition process of a single bubble on a hydrophilic surface. The boiling experiments were conducted at saturated temperature and atmospheric pressure conditions. De-ionized (DI) water was used as the working fluid. The boiling device was made of a 385-um thick silicon wafer. A thin film heater was deposited on one side, and the other side was used as the boiling surface. The onset of nucleate boiling (ONB) occurs at a wall superheat of ΔTsup= 12 °C and an applied heat flux of q" = 35.9 kW/m2. The evolution of the wall heat flux distribution was obtained from the IR temperature measurements, which clearly depicts the existence of the microlayer near the three-phase contact line of the nucleate bubble. The results suggest that, during the bubble growth stage, the evaporation in the microlayer region contributes dominantly to the nucleate boiling heat transfer; however, once the bubble starts to depart from the boiling surface, the microlayer quickly vanishes, and the transient conduction and the microconvection become the prevailing heat transfer mechanisms.

Commentary by Dr. Valentin Fuster

### Research Papers: Conduction

J. Heat Transfer. 2015;137(2):021301-021301-9. doi:10.1115/1.4028920.

When heat flux flows in a given medium, its path will solely be determined. This implies that material parameters determined by the predesigned path of heat flux will guide heat to flow along the designed path. Based on this idea, we develop a new method for the design of the cylindrical thermal cloak which can make heat flux detour the cloaked object. For the inhomogeneous anisotropic medium, we derive the relation between the path trajectory of heat flux and material parameters and obtain two differential equations and one boundary condition which are used to determine material parameters in the cylindrical cloak. The transient behavior on the flow of heat flux is simulated by Comsol Multiphysics and the transient thermal protection of the cylindrical cloak for the cloaked object is examined. The effect of the product of density and specific heat on the dynamic diffusion process of heat flux is analyzed. Since one can flexibly design the path of heat flux in the cloak, it has the large degree of freedom to construct thermal cloaks with the specific distributions of material parameters. The present method provides a new blue print for the transient thermal protection of a specific target.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2015;137(2):021501-021501-9. doi:10.1115/1.4028808.

This paper numerically investigates the heat transfer characteristics of a mesoscale liquid film slurry flow containing micro-encapsulated phase change material (MEPCM) in the presence of evaporation. The two-phase evaporating liquid film flow is modeled using one-fluid volume-of-fluid (VOF) formulation. During the evaporation process of the base fluid, the concentration of MEPCM in the slurry film increases as it flows along a heated plate, resulting in a continuous variation of its effective thermal properties. The effect of MEPCM on the evolution of the liquid film thickness under different operating conditions is presented. It is shown that the MEPCM suppresses the rate of decline in the liquid film thickness, which results in a higher heat transfer coefficient compared to that of pure liquid film under similar operating conditions. This study also provides an understanding towards delaying of the dry-out condition in slurry liquid film flow evaporation compared to that of the pure fluid.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(2):021502-021502-12. doi:10.1115/1.4029019.

Dry-out is an essential phenomenon that has been observed experimentally in both slug and annular flow regimes for flow boiling in mini and microchannels. The dry-out leads to a drastic drop in heat transfer coefficient, reversible flow and may cause a serious damage to the microchannel. Consequently, the study and prediction of this phenomenon is an essential objective for flow boiling in microchannels. The aim of this work is to develop an analytical model to predict the critical heat flux (CHF) based on the prediction of liquid film variation in annular flow regime for flow boiling in a horizontal uniformly heated circular microtube. The model is developed by applying one-dimensional (1D) separated flow model for a control volume in annular flow regime for steady, and sable saturated flow boiling. The influence of interfacial shear and inertia force on the liquid film thickness is taken into account. The effects of operating conditions, channel sizes, and working fluids on the CHF have been investigated. The model was compared with 110 CHF data points for flow boiling of various working fluids, (water, LN2, FC-72, and R134a) in single and multiple micro/minichannels with diameter ranges of $(0.38≤Dh≤3.04 mm)$ and heated-length to diameter ratios in the range of (117.7 $(117.7≤Lh/D≤470)$470). Additionally, three CHF correlations developed for saturated flow boiling in a single microtube have been employed for the model validation. The model showed a good agreement with the experimental CHF data with mean absolute error (MAE) = 19.81%.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(2):021503-021503-9. doi:10.1115/1.4028974.

In the present work, the characteristic atmospheric saturated heat flux controlled pool boiling curves for zirconia–water and silver–water nanofluids have been reproduced by the coupled map lattice (CML) method using a two-dimensional (2D) boiling field model. The heater is a long horizontal flat plate of thickness 0.44 mm. The pool height is 0.7 mm. The stirring action of the bubbles is modeled by increasing the fluid thermal diffusivity by an enhancement factor. The thermal conduction in the plate is also incorporated into the model. The basic advantage of CML is that individual bubbles are not tracked, and yet the effects of bubbles are reflected qualitatively in the final solution. In the simulation of atmospheric saturated pool boiling of water minimum cavity diameter taken is 0.8 μm based on which a random distribution of cavity sizes has been specified. In the boiling of ZrO2–water nanofluid there is a deposition of nanoparticles in the cavities on the heated surface resulting in reduction of surface roughness. This feature is taken care of by proportionate decrease in minimum cavity diameter. The CML model predicts decrease in heat transfer coefficient and increase in critical heat flux (CHF) with increase in zirconia nanoparticle concentration. In the case of Ag–water nanofluid no such deposition of nanoparticles has been reported; rather surface oxidation occurs which increases the surface roughness. This is simulated by proportionately increasing the minimum cavity diameter with weight fractions of nanoparticles. The present CML model predicts increase in the heat transfer coefficient and decrease in CHF with increase in silver nanoparticle concentration. Thus, the CML results for the boiling of the aforesaid two nanofluids match qualitatively with the published experimental works.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;137(2):021504-021504-7. doi:10.1115/1.4028975.

A new interpretation of the characteristic area and frequency appearing in the wall nucleation source from the point of view of the stochastic nature of this phenomenon is presented in this paper. This analysis shows important drawbacks in the standard interpretation of these terms, such as a strong bias in the characteristic area and high sensitivity to experimental conditions for the frequency. Finally, methods to improve the measurement of the corresponding mean values as well as estimators for their uncertainties based on the definition of a generalized probability density function (PDF) are provided.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2015;137(2):021901-021901-11. doi:10.1115/1.4028903.

For spray cooling using nanofluid as the working fluid, a nano-adsorption layer is formed on the heated surface and affects the heat transfer performance of the cooling system. This study performs an experimental investigation into the formation of this nano-adsorption layer and its subsequent effects on the spray heat transfer performance of a cooling system using Al2O3–water nanofluid as the working fluid. The experiments consider four different nanoparticle volume fractions (i.e., 0 vol. %, 0.001 vol. %, 0.025 vol. %, and 0.05 vol. %) and two different surface roughnesses (i.e., 0.1 μm and 1.0 μm). The experimental results show that the 0.001 vol. % nanofluid yields the optimal heat transfer performance since most of the nanoparticles rebound from the heated surface directly on impact or are washed away by subsequently arriving droplets. The surface compositions of the spray-cooled specimens are examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results reveal that for all of the nanofluids, a nano-adsorption layer is formed on the surface of the spray-cooled test pieces. Moreover, the layer thickness increases with an increasing nanoparticle concentration. A greater nano-adsorption layer thickness not only results in a higher thermal resistance but also reduces the effect of the surface roughness in enhancing the heat transfer performance. In addition, the nano-adsorption layer absorbs the nanofluid droplets under the effects of capillary forces, and therefore reduces the contact angle, which induces a hydrophilic surface property.

Commentary by Dr. Valentin Fuster

### Research Papers: Porous Media

J. Heat Transfer. 2015;137(2):022601-022601-11. doi:10.1115/1.4028864.

We introduce a new class of ventilated brake disk which incorporates an open cellular core: wire-woven bulk diamond (WBD). Transient and steady-state thermofluidic characteristics are presented. As reference, a commercially available pin-finned brake disk is also considered. At a braking power of 1.9 kW, representative of a medium sized truck descending a 2% gradient at a vehicle speed of 40 km/h (i.e., 200 rpm), the WBD cored brake disk reduces the overall brake disk temperature by up to 24% compared to the pin-finned brake disk. Results also reveal that in typical operating ranges (up to 1000 rpm), the WBD core provides up to 36% higher steady-state overall cooling capacity over that obtainable by the pin-finned core. In addition, the three-dimensional morphology of the WBD core gives rise to a tangentially and radially more uniform temperature distribution. Although the WBD core causes a higher pressure drop, this is balanced by the benefit of a stronger suction of cooling flow. Flow mixing in an enlarged heat transfer area by the WBD core is responsible for the substantial heat transfer enhancement. The WBD core is mechanically strong yet light while providing a substantial reduction in a brake's operating temperature.

Commentary by Dr. Valentin Fuster

### Research Papers: Two-Phase Flow and Heat Transfer

J. Heat Transfer. 2015;137(2):022901-022901-9. doi:10.1115/1.4025430.

Heat pipes are well known as simple and effective heat transport devices, utilizing two-phase flow and the capillary phenomenon to remove heat. However, the generation of capillary pressure requires a wicking structure and the overall heat transport capacity of the heat pipe is generally limited by the amount of capillary pressure generation that the wicking structure can achieve. Therefore, to increase the heat transport capacity, the capillary phenomenon must be either augmented or replaced by some other pumping technique. Electrohydrodynamic (EHD) conduction pumping can be readily used to pump a thin film of a dielectric liquid along a surface, using electrodes that are embedded into the surface. In this study, two two-phase heat transport devices are created. The first device transports the heat in a linear direction. The second device transports the heat in a radial direction from a central heat source. The radial pumping configuration provides several advantages. Most notably, the heat source is wetted with fresh liquid from all directions, thereby reducing the amount of distance that must be travelled by the working fluid. The power required to operate the EHD conduction pumps is a trivial amount relative to the heat that is transported.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2015;137(2):024501-024501-3. doi:10.1115/1.4028807.

This note introduces a rescaling approach that greatly simplifies the evaluation of flow and physical parameters such as skin friction and heat transfer rate in recent single phase nanofluids research for which the nanofluids begin to create a non-Newtonian fluid after 5–6% volumetric concentration of nanoparticles. Much of the task is hence reduced to a particular case for a chosen fluid. By the help of specified transformations, the nanofluid results can be obtained from known properties of regular fluid. Such rescaling is shown to work well for the rotating disk boundary layer flow in nanofluid, and thus sheds light upon the future studies of this kind when further physical mechanism are taken into account.

Commentary by Dr. Valentin Fuster

### Errata

J. Heat Transfer. 2015;137(2):027001-027001-1. doi:10.1115/1.4030039.
FREE TO VIEW

The above referenced paper is being retracted from the Journal of Heat Transfer.

Commentary by Dr. Valentin Fuster

### Announcements

J. Heat Transfer. 2015;137(2):028002-028002-1. doi:10.1115/1.4029010.
Abstract
Commentary by Dr. Valentin Fuster