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### In Memoriam

J. Heat Transfer. 2016;138(8):080101-080101-3. doi:10.1115/1.4033348.

It is with great sadness that we share with you the loss of a dear colleague and friend, Dr. Richard H. Pletcher. He was one of the pioneers and an internationally renowned expert in computational fluid mechanics and heat transfer, and was not only inspirational in directing the careers and lives of so many of us, but a mentor, who was extremely supportive and kind to all. Richard, or Dick, as we all personally and collegially addressed him, passed away on Saturday, Sept. 12, 2015, having been diagnosed with terminal cancer. His 80 years of life are marked with multifaceted contributions to engineering, as his work helped advance computational techniques to solve challenging problems in aerospace and mechanical engineering. Dick is remembered for his large body of work in heat transfer, fluid mechanics, buoyant jets and plumes, turbulence modeling, separated flows, viscous–inviscid interactions, computational fluid mechanics and heat transfer, and large eddy simulation (LES) of complex turbulent flows.

Commentary by Dr. Valentin Fuster

### Guest Editorial

J. Heat Transfer. 2016;138(8):080301-080301-1. doi:10.1115/1.4033795.
FREE TO VIEW

The 21st Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for the 2015 International Mechanical Engineering Congress and Exhibition (IMECE) held in Houston, TX on Nov. 13–19, 2015. The Photogallery sessions at the 2015 IMECE presented a total of 19 entries, and the peer-reviewed evaluation conducted by the participants has identified the 11 final entries for publication in this ASME Journal of Heat Transfer August issue of 2016.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster

### Photogallery

J. Heat Transfer. 2016;138(8):080901-080901-6. doi:10.1115/1.4033822.
OPEN ACCESS

The control of propellant boil-off is essential in long-term space missions. However, a clear understanding of propellant cryogenic condensation/evaporation in microgravity is lacking. One of the key factors in designing such systems is the location of liquid surfaces and the relation to wettability. The BT-2 Neutron Imaging Facility located at the National Institute of Standards and Technology (NIST), Gaithersburg, MD, is used to image evaporation and condensation of hydrogenated propellants inside of an aluminum 6061 container. Liquid hydrogen has larger neutron cross-section area than the aluminum, allowing the visualization of the liquid-vapor interface. The test cell has a conical section that enables determination of a contact angle with enhanced accuracy. If the contact angle is equal to the angle of the cone, a flat liquid-vapor interface is expected. The test cell has the cone angle of 10o and a flat interface was not observed. Using the Laplace-Young equation to fit the interface, the contact angle for hydrogen and aluminum was between 0° and 4°. The theoretical Laplace curves with contact angles of 2o and 10o are plotted on the liquid-vapor interface. The of 2o curve is a closer fit as compared to the 10o curve. The uncertainty arises from resolution limits of the neutron imaging setup and edge detection. More details on the neutron imaging mechanism and relevant physics can be found from the authors' other publication of Cryogenics, 74, pp131-137, 2016: doi:10.1016/j.cryogenics.2015.10.016.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080902-080902-6. doi:10.1115/1.4033824.

Formation of dry-out area could lead to a sharp increase in surface temperature during droplet impingement cooling. The objective of this study is to figure out an effective way of suppressing dry-out formation in droplet impingement cooling. In this work, HFE-7100 droplet train was produced using a piezo-electric droplet generator at a frequency of 6000 Hz with a droplet Weber number of 280. A translucent substrate was coated with a thin film ITO, which was used as a heater in the experiments. A copper screen laminates with a single punched hole (diameter = 3 mm) was placed over the heater surface at a distance of 0.3 mm to enhance surface heat transfer. Optical images showed that screen laminates effectively suppressed the formation of the dry-out area. It was also found that heat transfer was greatly improved when screen laminates were used. The heat transfer improvement could be attributed to the enhanced surface tension effects, which keep the whole surface wet at high surface temperatures. In summary, the results show that screen laminates effectively suppress the formation of dry-out area and greatly improve surface heat transfer.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080903-080903-6. doi:10.1115/1.4033819.

Drop condensation and coalescence is visualized using high-speed Surface Plasmon Resonance (SPR) reflectance microscopy. SPR microscopy is a label-free technique that can characterize thin films (less than 1µm) by detecting the changes in the refractive index of the test medium. The sensing surface is a 50 nm thick gold film on a 2.5 nm thick Ti layer is deposited on a borosilicate substrate. P-polarized monochromatic light (632 nm) is incident on the gold film in a total internal reflection mode. Free electrons in the gold film are excited by the incident light when a resonance condition is met. The result is a decrease in the reflected intensity. Resonance depends upon wavelength, incident angle, and refractive index of prism and test medium. To induce condensation, a water bridge is created between the SPR gold film and an ITO coated glass slide. When the ITO coated slide is heated water evaporates from the bridge and condenses on the gold film. The sequence of images on the process of droplet deposition and drop coalescence are captured at 1500 frames per second. Experiments were conducted at an SPR angle of 44o, which is slightly above the minimum intensity angle for air at 43.8o. Therefore, the brightest and darkest regions correspond to the areas on the gold film covered with bulk water and a very thin film of water, respectively. The thickness of the film is proportional to the intensity of reflected light.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080904-080904-6. doi:10.1115/1.4033826.

The effect of jet-to-jet distance of two adjacent jets on boiling heat transfer was investigated by a simultaneous boiling visualization and heat transfer measurement. The hydrodynamics of two adjacent jets were visualized by the high-speed imaging and the 4K camera on unheated surface. The surface temperature of the hot steel plate was estimated by formulating 2-D inverse heat conduction, which was measured through the flat-plate heat flux gauge. The jet Reynolds number was fixed at Re = 17,000 and the jet-to-jet distance of two adjacent jets was set to the three different jet-to-jet distances (p/d = 10, 20 and 30). The wall jet flow interference between two adjacent impinging jets forms a vertical fountain at its boundary. The more scattered water droplets are observed from the fountain due to the higher intensity at the p/d = 10. The flow interaction becomes to reduce its momentum as increasing the p/d. The boiling visualization shows which boiling mode starts to occur and turns to be disappeared from film boiling to nucleate boiling on the hot surface. The measured surface temperature has a good agreement with the corresponding boiling visualization. The closer jet distance (lower p/d) shows the rapid temperature gradient along the interaction, which provides much higher cooling rate in a multiple jet nozzle when designing an intensive quenching device.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080905-080905-6. doi:10.1115/1.4033818.

We demonstrated suppressed dry-out on structured surfaces during flow boiling in microchannels. We designed and fabricated microchannels with well-defined silicon micropillar arrays (heights of ~25 µm, diameters of 10 µm and pitches of 40 µm) coated with silicon dioxide on the bottom heated channel wall. We visualized the flow fields inside a smooth and structured surface microchannel during the annular flow boiling regime with a high speed camera at a frame rate of 2000 fps. Time-lapse images revealed two distinct dry-out dynamics for the two types of surfaces. For the smooth surface, the thin liquid film broke-up into smaller liquid drops/islands and the surface stayed in a dry state after the drops evaporated. The microstructured surface, on the other hand, preserved the thin liquid film initially due to capillary wicking. Dry patches eventually formed at the center of the microchannel which indicated wicking in the transverse direction (from the sidewalls inward) in addition to wicking in the flow direction. Overall, the structured surface showed less instances of dry-out both spatially and temporally. These visualizations aid in the understanding of the stability of the thin liquid film in the annular flow boiling regime and provide insight into heat transfer enhancement mechanisms by leveraging surface structure design in microchannels.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080906-080906-6. doi:10.1115/1.4033827.

With the ever increasing cooling demands of advanced electronics, thin film evaporation has emerged as one of the most promising thermal management solutions. High heat transfer rates can be achieved in thin films of liquids due to a small conduction resistance through the film to the evaporating interface. In thin film evaporation, maintaining a stable liquid film to attain high evaporation rates is challenging. We investigated nanoporous anodic aluminum oxide (AAO) membranes to supply liquid to the evaporating surface via capillarity. In this work, we achieved enhanced experimental control via the creation of a hydrophobic section within the nanopore. By creating a non-wetting section, the liquid is confined within the membrane to a region of well-controlled geometry. This non-wetting section also prevents flooding, where the formation of a thick liquid film degrades device performance. When heat flux is applied to the membrane surface, the liquid wicks into the membrane from the bottom and becomes pinned at the onset of the hydrophobic layer. As a result, the wetting in the membrane is controlled, flooding is prevented, and a stable evaporating surface in achieved. With this approach, thin film evaporation from nanoporous media can now be studied for varying parameters such as pore size, porosity, and location of the meniscus within the pore.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080907-080907-6. doi:10.1115/1.4033823.

The present study experimentally investigates the evaporative characteristics for a nanofluid droplet on heated surface. For experiments, the alumina (Al2O3) nanoparticles having a 50 nm average diameter were distributed in deionized (DI) water. The equilibrium contact angles (ECA) of DI-water on bare (without texturing) and hole-patterned textured (by µ- CNC machine) copper surfaces were 60o and 82o. Also, advancing and receding contact angles were 73.3o and 25.8o for bare surface, and 101.3o and 55.2o for textured surface. Surface temperature was fixed as 100±0.2oC, measured by resistance temperature detector (RTD) sensors with data logger. During the experiments, the ambient temperature was 22oC with the relative humidity of 32%. At the initial stage, the dynamic contact angle (DCA) of 0.01 vol.% nanofluid droplet on the textured surface drastically increased over its own ECA due to the generation of large bubbles inside the droplet. However, the contact angle of 0.1vol.% nanofluid droplet at t = 5 s was smaller than that of 0.01vol.% case because the increase in nanofluid concentration caused the reduction of surface tension. After that, DCA gradually decreased until dried out, and total evaporation time was significantly delayed in the case of textured surface. Moreover, the heat transfer characteristics during evaporation phenomenon was affected by the nanofluid concentration and the contact area with the heated surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080908-080908-6. doi:10.1115/1.4033825.

Visualization of the flow structure generated by passive vortex generators continues to be a matter of research in the fluid mechanics and heat transfer communities. In this study, self-sustaining counter-rotating vortex pairs (CVP) generated from a series of vortex generators (VG) have been characterized numerically and experimentally to understand the effects of the VG parameters on vortical flow structure formation. Four different types of VGs were considered by varying the taper angle from 0° to 19.3° at a fixed inclination angle of 24.5° and a Reynolds number of 1965. Flow fields were experimentally visualized using a smoke technique. Each VG induced a coherent CVP flow structure in the wake region despite the fact that the upstream flow was laminar. CVPs initially dominate flow dynamics over a certain streamwise length; however, Kelvin-Helmholtz (KH) instability appears to affect the spatial evolution of CVP longitudinally. The CVP within the stability region were reconstructed digitally in 3D by interpolating several 2D smoke images taken at various spanwise planes. The smoke results indicate that as taper angle decreases, the onset location of KH instability decreases. Furthermore, the CVP trajectory within the stability region was observed to be predominantly controlled by a two-dimensional inviscid process, while the effects by the free stream were not significant. Based on the experimental observations and the numerically reconstructed 3D CVP flow structures, VG with smaller taper angle results in CVPs with higher circulation, which is a positive aspect for mass and heat transfer applications. Preliminary numerical simulations based on RANS have shown that heat transfer enhancement is about 50% in the region near the rectangular vortex generator.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080909-080909-6. doi:10.1115/1.4033816.

Depicted are sequences of water drop impacts on copper, taken at 16,000 fps. The copper is treated with a heated alkali solution, resulting in a highly wetting, nanoscale structured, cupric oxide layer with a static contact angle approaching 0° with water. In the top series an 11.5 µl water droplet impacts this surface from 60 mm. The interfacial forces are large compared with the inertia; the low advancing contact angle of the expanding front continues to pull the droplet outward and absorbs the droplet without any rebound. The droplet spreads to cover the entire 0.5x0.5 in2 surface in less than 500 ms. After the surface energy of the oxide layer is reduced with silane, this surface becomes highly non-wetting with a static contact angle of ~160° and a hysteresis <5°. The lower sequence shows the 11.5 µl water droplet dropped from the same height. The large advancing contact angle creates an inverted wedge at the triple line, and the advancing front quickly reaches a maximum diameter at 3 ms and begins to recede inward while the top of the droplet is still moving downward, creating a donut shape. The receding front collides at the center forcing a jet of liquid up and out. This jet pulls the remainder of the liquid upward at a decreasing velocity, relative to the head. This is apparent as the jet splits into secondary droplets at 16ms (which moves out of frame at 18 ms) and again at 22 ms, referred to as S-1 and S-2, respectively. As the S-2 splits off, surface tension force cause it to slow at 25 ms, while the parent droplet moves up to collide with, and impart momentum to S-2. They remain detached; S-2 moves out of view, the parent falls. This bouncing behavior continues until the energy is dissipated and the droplets come to rest. This can be seen as the parent drop rebounds again at 100ms, S-2 at 130 ms and S-1 in the final frame, forming a tertiary droplet. These surfaces are being studied for their effects on two phase heat transfer.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080910-080910-6. doi:10.1115/1.4033815.

Three different copper surfaces - bare, Al2O3 nano-coated, and Polytetrafluoroethylene (PTFE) coated - are prepared and tested to examine the effect of wettability on the pool boiling incipience in saturated water at 1 atm. A copper surface is coated with Al2O3 particles ranging 25~43 nm in diameter by immersing the surface in Al2O3/ethanol nanofluid (1g/l) and boiled for 3 min. SEM image in Fig. 1 shows the coated Al2O3 nanoparticles on the copper surface, together with the reference bare surface. PTFE coating is also applied to the copper surface using spin coating method with the mixture of Dupont AF 2400 particles and 3M FC-40 solvent. The final coating thickness of the PTFE coating is estimated to be 30 nm. The three surfaces exhibit different static contact angles, 78° (bare), 28° (nano-coated), and 120° (PTFE coated) in Fig. 2, respectively. Wettability affects the boiling incipience heat flux where initial bubble nucleation starts: 15 kW/m2 for the bare surface; 30 kW/m2 for the nano-coated surface; and 2.5 kW/m2 for the PTFE coated surface. Captured images from the high speed camera at 2,000 fps show significantly different bubble shapes and departure frequencies in Fig. 3. During the bubble growth, advancing contact angles are captured and shown qualitatively and found consistent with their static angle measurements for the sessile droplet observed at each surface. The larger bubble is generated on the nano-coated surface compared to that of the bare surface because improved wetting makes promising cavities flood and thus incipience is delayed, resulting in higher superheat. The single bubble life cycle appears to be much longer on the PTFE coated surface due to the increase of the contact angle which becomes hydrophobic (> 90°), resulting in lower bubble departure frequency. Successive tests at the same heat flux of 30 kW/m2 confirmed that life cycle on the PTFE coated surface (88.5 ms) is consistently longer than that on the bare surface (16.5 ms) and nano-coated surface (20 ms).

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):080911-080911-6. doi:10.1115/1.4033817.

A high thermal conductivity thermal paste can be developed by mixing the oxidized liquid metal alloy (OLMA) with graphene. Four kinds of graphene-OLMA pastes were synthesized at graphene concentrations of 0.25 wt%, 0.75 wt%, 1.5 wt%, and 2.0 wt%, respectively. The paste structures were characterized by MicroXCT-400, which can be used to readily measure the air pocket size, and their thermal conductivities measured by a laser flash analysis method. It is found that the OLMA structure is very different from the liquid metal alloy (LMA), and a small amount of air pockets were formed in the OLMA. The air pocket size significantly affected the thermal conductivity of the graphene-OLMA paste. When the graphene concentration increased, as shown in Fig. 1(c)-(e), the paste's thermal conductivity increased. However, more air pockets were formed around the graphene. In particular, when the graphene concentration increased to 2.0 wt%, clusters of graphene, as shown in Fig. 1(f), were formed resulting in the formation of big air pockets in the thermal paste, which directly affected the thermal conductivity as shown in Fig. 1(g). We thought that when the graphene concentration increases, the thermal conductivity should increases. But the results show that it was not and then we used MicroCT to see the internal structure of the thermal paste and found that the air pockets were formed and significantly affects the thermal performance.

Commentary by Dr. Valentin Fuster

### Research Papers: Conduction

J. Heat Transfer. 2016;138(8):081301-081301-11. doi:10.1115/1.4033326.

Thermoelectric coolers (TECs) are solid-state cooling devices which can be used in certain applications to reduce the operating temperature of electronics or increase their heat dissipation. However, the performance of the cooler is strongly influenced by the thermal system into which it is placed, and the cooler design should be optimized for a given system. In this work, the possible benefits of a TEC implemented within a realistic thermal system are quantified. Finite thermal conductances between the cooled device and the TEC and between the TEC and the heat sink are considered. The entire problem is treated using dimensionless parameters, which reduces the number of independent parameters and enables generalized performance maps which clearly show the maximum benefit (in terms of a reduced device temperature or increased device heat dissipation) that a prescribed TEC can deliver to a particular application. The use of these dimensionless parameters also allows for optimization of TEC parameters without considering the cooler detailed design geometry.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2016;138(8):081501-081501-9. doi:10.1115/1.4033347.

Falling film type condensers/reboilers applied to cryogenic air separation units (ASUs) have drawn more attentions in recent years. This paper presents and analyzes a mathematical model for the falling film plate-fin condensers/reboilers (FPCR). In the modeling, both the laminar falling film evaporation and condensation processes, incorporating with interference of mass transfer and interfacial shear stress, are considered, and related to a plate-fin heat exchanger (PHX). The liquid film flow and heat transfer characteristics of oxygen and nitrogen fluids in the PHX are analyzed under given conditions by solving the model with a numerical iteration method. The variations of liquid film thicknesses and local heat transfer coefficients of oxygen and nitrogen as well as the total local heat transfer coefficient have been obtained. Furthermore, the effects of the inlet mass flow rate allocation ratio (i.e., the ratio of inlet mass flow rate of oxygen liquid over the base plate to that over the fin surfaces) on the wetted length of the heat transfer surfaces, the heat transfer performance, and the oxygen liquid circulation ratio (i.e., the ratio of the inlet liquid mass flow rate to the generated vapor mass flow rate) are also discussed. A proper inlet mass flow rate allocation ratio of oxygen liquid is presented. The wave effects are further considered and analyzed through the inclusion of a model for the wave factor.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):081502-081502-7. doi:10.1115/1.4033210.

Smooth DLC (diamond-like carbon) coated surfaces can profoundly mitigate scaling during pool boiling of calcium sulphate solutions. Previous investigations though carried out mostly for the smooth surfaces rather than structured, i.e., finned tubes. This study compares experimental results of DLC coated smooth and finned tubes at clean and fouling conditions. Fouling runs were conducted during pool boiling of saturated CaSO4 solution of 1.6 g/L at 300 kW/m2. The substrate of the attempted tubes was stainless steel and finned tubes of 19 and 40 fins per inch were used. The DLC coated smooth tube showed an enhanced clean heat transfer up to 50% and reduced fouling resistance compared to the uncoated smooth tube. After a short operating time, though, the coated smooth tube reached an asymptotic fouling resistance of 0.00005 m2 K/W whereas for the uncoated smooth tube, it was 4.8 times higher. DLC coating of the finned tubes with the physical vapor deposition (PVD) technique implicated difficulties. The base surface of the finned tubes was defectively coated. The defectiveness of the coating was attributed to the limitation of the PVD for coating of structured surfaces.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):081503-081503-12. doi:10.1115/1.4033353.

Considering mass transfer and energy transfer between liquid phase and vapor phase, a mixture model for boiling heat transfer of nanofluid is established. In addition, an experimental installation of boiling heat transfer is built. The boiling heat transfer of TiO2–water nanofluid is investigated by numerical and experimental methods, respectively. Thermal conductivity, viscosity, and boiling bubble size of TiO2–water nanofluid are experimentally investigated, and the effects of different nanoparticle mass fractions, bubble sizes and superheat on boiling heat transfer are also discussed. It is found that the boiling bubble size in TiO2–water nanofluid is only one-third of that in de-ionized water. It is also found that there is a critical nanoparticle mass fraction (wt.% = 2%) between enhancement and degradation for TiO2–water nanofluid. Compared with water, nanofluid enhances the boiling heat transfer coefficient by 77.7% when the nanoparticle mass fraction is lower than 2%, while it reduces the boiling heat transfer by 30.3% when the nanoparticle mass fraction is higher than 2%. The boiling heat transfer coefficients increase with the superheat for water and nanofluid. A mathematic correlation between heat flux and superheat is obtained in this paper.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):081504-081504-8. doi:10.1115/1.4033354.

The thermal hydraulic performance of a miniature heat sink during flow boiling of distilled water is presented in this article. The unique design of the heat sink contains a number of microchannels of 1 mm × 1 mm cross section arranged in a regular hexagonal array. The design facilitates repeated division and joining of individual streams from different microchannels and thereby can enhance heat transfer. Individual slug bubble experiences a typical route of break up, coalescence, and growth. The randomness of these processes enhances the transport of heat. With the increase of vapor quality the heat transfer coefficient increases, reaches the maximum value, and then drops. The maximum heat transfer coefficient occurs at an exit vapor quality much higher than that observed in conventional parallel microchannel heat sinks. Repeated redistribution of the coolant in the interlinked channels and the restricted growth of the slug bubbles may be responsible for this trend.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2016;138(8):081701-081701-9. doi:10.1115/1.4033137.

Numerical results show that at supercritical pressures, once the buoyancy force increases, the effect of the turbulent Prandtl number, Prt, on convective heat transfer becomes considerable. This phenomenon has not been adequately addressed in the literature. In this study, the effect of the turbulent Prandtl number on the rate of heat transfer in both enhanced and deteriorated regimes of heat transfer to supercritical fluid flows has been extensively investigated. Having realized that variations of the turbulent Prandtl number can affect the model predictions so greatly, a new correlation to express the changes of Prt with respect to flow conditions in a supercritical environment is developed. Effects of various important parameters such as heat flux, mass flux, and fluid pressure are included in the proposed correlation. This correlation has been modified to be applicable for different supercritical fluids. The comparison with various experimental data shows that by implementing the new correlation of Prt in the numerical code, it is possible to significantly improve the simulation results. Such a correlation seems to be the first one introduced in the literature for a supercritical fluid flow.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2016;138(8):082001-082001-10. doi:10.1115/1.4033350.

In this work, convection heat transfer for combined electro-osmotic and pressure driven flow of power-law fluid through a microtube has been analyzed. Typical results for velocity and temperature distributions, friction coefficient, and Nusselt number are illustrated for various values of key parameters such as flow behavior index, length scale ratio (ratio of Debye length to tube radius), dimensionless pressure gradient, and dimensionless Joule heating parameter. The results reveal that friction coefficient decreases with increasing dimensionless pressure gradient, and classical Poiseuille solutions can be retrieved as the dimensionless pressure gradient approaches to infinite. To increase the length scale ratio has the effect to reduce Nusselt number, while the influence of this ratio on Nusselt number diminishes as the pressure gradient increases. With the same magnitude of dimensionless Joule heating parameter, Nusselt number can be increased by increasing both the flow behavior index and dimensionless pressure gradient for surface cooling, while the opposite behavior is observed for surface heating. Also, singularities occurs in the Nusselt number variations for surface cooling as the ratio of Joule heating to wall heat flux is sufficiently large with negative sign.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):082002-082002-9. doi:10.1115/1.4033351.
OPEN ACCESS

In this work, a discrete element model (DEM) is developed and implemented in the open source flow solver MFiX to simulate the effective thermal conductivity of powder beds for selective laser sintering (SLS) applications, considering scenarios common in SLS such as thin beds, high temperatures, and degrees of powder consolidation. Random particle packing structures of spherical particles are generated and heat transfer between the particles is calculated. A particle–particle contact conduction model, a particle–fluid–particle conduction model, and a view factor radiation model using ray-tracing for calculation of view factors and assuming optically thick particles are used. A nonlinear solver is used to solve for the particle temperatures that drive the net heat transfer to zero for a steady state solution. The effective thermal conductivity is then calculated from the steady state temperature distribution. Results are compared against previously published experimental measurements for powder beds and good agreement is obtained. Results are developed for the impacts of very high temperatures, finite bed depth, consolidation, Young's modulus, emissivity, gas conductivity, and polydispersity on effective thermal conductivity. Emphasis is placed on uncertainty quantification in the predicted thermal conductivity resulting from uncertain inputs. This allows SLS practitioners to control the inputs to which the thermal response of the process is most sensitive.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):082003-082003-8. doi:10.1115/1.4033388.

In the present work, a computational fluid dynamic (CFD) simulation has been performed to investigate single and two-phase vortex tube. Air in compressed form and partially condensed phase are used as working fluid, respectively. Simulation has been carried out using commercial CFD software package fluent 6.3.26. A detailed study has been performed to generate the profiles of velocity, pressure, and pathlines. These profiles provide an insight on how the process of energy separation as well as the flow field in the vortex tube gets affected on introduction of a liquid phase. The result shows that in case of cryogenic vortex tube, the flow reversal takes place closer to wall due to presence of a very thin wall adhering liquid film, while, in single-phase flow vortex tube, flow reversal is observed at the central portion. The model also predicts that presence of recirculation zone near warm end diminishes the refrigeration effect of vortex tube for two-phase flow.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2016;138(8):082101-082101-7. doi:10.1115/1.4033138.

A comprehensive analysis and optimization of a three-dimensional integrated circuit (3D IC) structure and its thermophysical attributes are presented in this work. The thermophysical and geometrical attributes studied in this paper include the die, device layer, heat sink, and heat spreader, which are critical structures within a 3D IC. The effect of the power density of the device layer which is the source of heat generation within the chip as well as the through silicon vias (TSV) and microbumps is also considered in our investigation. The thermophysical and geometrical parameters that have a significant impact on the thermal signature of the 3D IC as well as those that have an insignificant impact were established. The comprehensive analysis of different geometrical and thermophysical attributes can guide the design and optimization of a 3D IC structure and decrease the cost.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2016;138(8):082401-082401-6. doi:10.1115/1.4033328.

A laser propagating through a metal nanocolloid exhibits a far-field fringe pattern that is the signature of its optical and thermally induced response. Here, we directly exploit the sensitive far-field features to measure the thermo-optic coefficients of binary-solvent mixtures of ethanol and water. This study extends our fundamental understanding of the thermal self-diffraction toward future optical characterization of the nanocolloid fluid motion.

Commentary by Dr. Valentin Fuster

### Research Papers: Thermal Systems

J. Heat Transfer. 2016;138(8):082801-082801-9. doi:10.1115/1.4033346.

An entropic wall with circulating water inside could be a solution for acclimatizing a new building with high-energy efficiency and high levels of internal comfort. If circulating water is thermally stabilized by exchanging in the ground such has it happens in geothermal plants, a thermal shield could be realized keeping walls in comfort conditions and minimizing energy needs for further temperature regulations. This paper presents optimization guidelines of such a wall with the objective of maximizing the performances of the wall for reaching optimal internal wellness conditions. Optimization has been realized by a constructal law based method, which has been personalized by a step-by-step process and has been named constructal design for efficiency (CDE). The optimization of the system has been produced at different levels. It starts from a preliminary analysis at system levels, which allow defining the best objectives that could be reached. After this preliminary process, the system has been divided into modules, and the critical ones which have higher influence on the performances of the system have been evaluated. This analysis has been coupled also with an industrial analysis with the goal of defining an effective layout, which could be also manufactured with acceptable costs. The result has produced a final solution with a very good compromise between energetic performances and minimization of costs at industrial level. The results open interesting perspectives for the constructal law to become the core of an effective methodology of an industrial design which can couple perfectly with the modular approach which is currently the major part of industrial companies.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):082802-082802-12. doi:10.1115/1.4033209.

Optimal material utilization in metal-matrix heat sink is investigated using constructal design (CD) in combination with fin theory to develop a constructal tree of optimally shaped convective fins. The structure is developed through systematic growth of constructs, consisting initially of a single convective fin enveloped in a convective medium. Increasingly complex convective fin structures are created and optimized at each level of complexity to determine optimal fin shapes, aspect ratios, and fin-base thickness ratios. One result of the optimized structures is a functional grading of porosity. The porosity increases as a function of distance from the heated surface in a manner ranging from linear to a power function of distance with exponent of about 2. The degree of nonlinearity in this distribution varies depending on the volume of the heat sink and average packing density and approaches a parabolic shape for large volume. For small volume, porosity approaches a linear function of distance. Thus, a parabolic (or least-material) fin shape at each construct level would not necessarily be optimal. Significant improvements in total heat transfer, up to 55% for the cases considered in this work, were observed when the fin shape is part of the optimization in a constructal tree of convective fins. The results of this work will lead to better understanding of the role played by the porosity distribution in a metal-matrix heat sink and may be applied to the analysis, optimization, and design of more effective heat sinks for electronics cooling and related areas.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2016;138(8):084501-084501-7. doi:10.1115/1.4033164.

The present paper analyzes the problem of two-dimensional mixed convection boundary layer flow near the lower stagnation point of a cylinder embedded in a porous medium. It is assumed that the Darcy's law holds and that the solid and fluid phases of the medium are not in thermal equilibrium. Using an appropriate similarity transformation, the governing system of partial differential equations are transformed into a system of ordinary differential equations, before being solved numerically by a finite-difference method. We investigate the dependence of the Nusselt number on the solid–fluid parameters, thermal conductivity ratio and the mixed convection parameter. The results indicate that dual solutions exist for buoyancy opposing flow, while for the assisting flow, the solution is unique.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):084502-084502-4. doi:10.1115/1.4033377.

This study investigated the effect of air pressure on the freezing process of a water droplet on a cold surface. A common belief is that bulk liquid water is incompressible and air pressure does not affect the freezing point of the bulk liquid water over a wide range of pressure. However, our results demonstrated that, for a water droplet on a cold surface, its freezing process started early at lower ambient pressures. Such a phenomenon can be explained by the effects of the evaporative cooling.

Topics: Pressure , Freezing , Drops , Water
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):084503-084503-4. doi:10.1115/1.4033386.

A recent article presented axisymmetric numerical calculations showing substantial heat transfer enhancement in a laminar impinging flow with shear thinning inelastic fluids. This paper compares enhancement in planar and axisymmetric geometries and presents empirical dependencies correlating heat transfer rates to fluid rheology. The parametric correlation is expressed in the form $∼ ReGp$. ReG is a generalized Reynolds number based on the reference strain rate and fluid rheology, and it is larger than the Newtonian Reynolds number for the same mean nozzle velocity and flow geometry. The value of p > 0 is estimated from the numerical data for weak and strong shear thinning. Within the impinging zone spanning the nozzle cross section, the value of p is essentially similar for both geometries, but in the wall jet the planar flow shows a somewhat larger value. The total heat transfer rate in the planar wall jet may be two to ten times larger for a shear thinning fluid. This is because in shear thinning flow, the primary separation vortex is able to maintain the Nusselt number at a higher average value over a significantly longer length scale in the streamwise direction.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(8):084504-084504-5. doi:10.1115/1.4033390.

Electrohydrodynamic convective drying (EHD drying) is a novel drying method used to enhance forced convection drying (FC drying) by using a wire-electrode to create an electrostatic field. In a previous study, the efficiency of EHD drying (using three different wire-electrode configurations) was compared to classical FC drying by measuring the drying rate of methylcellulose gel. Efficiency was quantified in terms of exergy (transient exergetic efficiency) through the use of a proposed model. In that previous study, it was stated that methylcellulose gel can be used to simulate a food product and can be controlled to a predetermined moisture content. The purpose of this current work was to compare how methylcellulose gel compares to a real food product (mango fruit) in terms of drying kinetics for both EHD and FC drying. Drying kinetics were quantified in terms of a per unit area measurement of the exergetic efficiency, exergy supplied and used, drying rate, and total drying time to reach a moisture content of 50%. Initial results show that for both EHD and FC drying, methylcellulose gel and mango fruit exhibit similar drying kinetics.

Commentary by Dr. Valentin Fuster