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### Guest Editorial

J. Heat Transfer. 2016;139(2):020301-020301-1. doi:10.1115/1.4035402.
FREE TO VIEW

The 22nd Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for the 2016 Summer Heat Transfer Conference (SHTC) held in Washington, DC, on July 10–14, 2016. Twenty entries in total were submitted into the Photogallery session and the peer-reviewed evaluation conducted by both the participants and selected HTD K-22 Technical Committee members has identified the 13 final entries for publication in this ASME Journal of Heat Transfer February issue of 2017.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster

### Photogallery

J. Heat Transfer. 2017;139(2):020901-020901-1. doi:10.1115/1.4035571.

Despite the demand for high-performance, two-phase cooling systems, high-fidelity simulations of flow boiling in complex microchannel geometries remains a challenging numerical problem. We conduct a first-principles-based simulation of an evaporating two-phase flow in a high-aspect-ratio microchannel with bends using a volume of fluid-based numerical model. For the case shown, the lower horizontal section of the microchannel has a constant flux of 20 W/cm2 applied to the wetted wall area (heat flux at the base of 133 W/cm2); HFE-7100 vapor and liquid enter the channel at 2 m/s. The three-dimensional channel geometry requires a refined near-wall numerical mesh to resolve thin liquid film flow features. The recently developed saturated-interface-volume phase change model (Int J Heat Mass Trans 93:945-956, 2016) is implemented for prediction of mass and energy exchange across the liquid-vapor interface at a low computational cost (~80 hr; 6-core parallelization on Intel Xeon E3-1245V3). The model reveals transport details including the interface shape and fluid velocity and temperature fields. The interfacial temperature remains fixed at saturation with smooth velocity contours near the interface. The highest evaporation flux is located in the thin liquid film region near the heated wall.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020902-020902-1. doi:10.1115/1.4035572.

Droplet impact on rigid, superhydrophobic surfaces follows the well-known spreading, recoil, and lift-off behavior at lower impact speeds (a), and splashing at higher impact speeds (b). The contact time tc of these bouncing droplets is independent of the impact speed, and difficult to control. Using high speed imaging (9500 fps) of water droplets impacting superhydrophobic substrates with stiffness 0.5 to 7630 N/m (rigid), we were able to show that substrate flexibility can reduce contact times. Upon impact on a flexible substrate, the droplet excites the substrate to oscillate at the membrane or cantilever natural frequency (d). The oscillation accelerates the droplet upwards, initiating early droplet lift-off at the edges of the droplet close to the point of maximum spreading (c). Droplets fully lift off before fully recoiling, i.e. in a pancake shape. We call this phenomenon the springboard effect. Contact times are reduced by up to 50% compared to rigid substrates.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020903-020903-1. doi:10.1115/1.4035573.

High speed images of coalescence induced three-droplet jumping on a nanostructured superhydrophobic carbon nanotube (CNT) surface are presented ($θaapp$ ≈ 173º). When two or more droplets coalesce on a nanostructured superhydrophobic surface, the resulting droplet can jump away from the surface due to the release of excess surface energy. To more easily study the jumping phenomena, we have developed focal plane shift imaging (FPSI) to determine both jumping speed and direction. Figure 1(a) shows a schematic of the FPSI concept. A high speed camera was attached to an upright optical microscope, and samples were horizontally mounted on a cold stage. Initial conditions were obtained by moving the focal plane to be coincident with the middle of the droplets prior to coalescence (Figure 1b). Then the focal plane was shifted above the droplets by a known distance (Figure 1c), followed by measurement of the time taken for the jumping droplet to pass through the shifted focal plane (Figure 1d). By analyzing the initial and final conditions of the departing droplet for multiple three-droplet coalescence events, the three-droplet jumping droplet speed was determined (Figure 2b). Experimentally measured jumping speeds determined by the FPSI imaging technique show good agreement with three-droplet inertial capillary scaling (Figure 2b, dotted line, equation inset). The FPSI visualization technique provides a novel imaging platform for the study of complex multi-droplet jumping-droplet phenomena.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020904-020904-1. doi:10.1115/1.4035574.

Atomic force microscopy (AFM) is one of the most effective tools in nanotechnology researches. Using the raster scanning method with a soft cantilever, three dimensional sample surface topography can be obtained with a sub-nanometer resolution. In this study, AFM is employed to characterize the nanostructures of polymer film under two temperature treatments. 3-aminopropyltriethoxysilane (APTES) is selected as a model polymer molecule due to its wide applications for amino-functionalization of Si/SiOx based surfaces, such as silicon and mica. The APTES molecule has three ethoxy groups that can react with surface hydroxyl groups or with another APTES molecule through a siloxane bond. The self-assembled APTES self-assembly film has typically three configurations, namely uncrosslinked monolayer, crosslinked monolayer, and multilayer. Ten percent aqueous solution is used to cover the entire mica surfaces. Then the surfaces are rinsed with DI water and dried with N2 to remove left over solution. Two hours incubation was followed in 80°C or 140°C, respectively. At 80 °C (below the boiling point), the uniform APTES film is obtained. On the other hand, at 140 °C (above the boiling point), the film exhibits multilayer structure, as confirmed by AFM images. Contact angle measurement shows that the multilayer film is less wettable than the monolayer film by 20°. It seems that the evaporation rate of the water layer on the substrate is the key to various film configurations.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020905-020905-1. doi:10.1115/1.4035575.

A surface plasmon resonance (SPR) imaging microscopy coupled to a high-speed camera is used to visualize the frost propagation inside a subcooled liquid droplet. The SPR experimental setup consists of a 50 nm thick gold-coated cover glass placed on a BK7 dove prism and optically matched using index matching liquid. Collimated monochromatic light of 600 nm wavelength is incident on the gold-glass interface at 71.8°, which corresponds to the SPR minima angle for ice (RI 1.309). Images are captured using Photron APS-RS camera at 1000 fps with a shutter speed of 1 ms. The prism and the gold film are cooled using a thermo-electric cooler (TEC). A water droplet is placed on the gold film and the temperature of the droplet is decreased from room temperature (23.0 ± 1 °C) to below 0 °C. Adjacent to the water droplet, the vapor condensates to form tiny droplets. The tiny condensate droplets would freeze first and the frost propagates through the condensate region. During this period the central droplet is in a subcooled state. The speed of frost propagation through the condensates is slow and takes tens of seconds to cover the gold film with ice. Within a single condensate droplet, however, the frost propagation velocity is expected to be considerably higher. Eventually the frost line reaches the central droplet. There is a delay of few seconds between the frost line reaching the droplet and frost propagation inside the droplet. The point at which frost touches the subcooled droplet acts as a nucleation site for the droplet and the frost propagates in the droplet at high speed. The average velocities of frost propagations in the subcooled liquid droplet were calculated to be 5.2 ± 0.3 cm/s and 7.4 ± 0.5 cm/s, when the gold film temperature was -5.0 ± 1 °C and -7.8 ± 1 °C respectively.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020906-020906-1. doi:10.1115/1.4035576.

Copper HTCMC (High-temperature, Thermally Conductive Microporous Coating) with a coating thickness of ~300 µm was created by sintering 67 µm copper particles onto a flat copper surface. This was shown to be the optimum particle size and thickness combination, in terms of boiling heat transfer enhancement with water, during a prior pool boiling study conducted by Jun et al. [1]. The effects of orientation of pool boiling heat transfer in saturated distilled water at 1 atm were tested experimentally and compared with a plain copper surface. An SEM image (top left) shows the porous structure of HTCMC demonstrating reentrant cavities which promote nucleate boiling and lead to significant critical heat flux (CHF) enhancement compared to the plain copper surface (top right). The nucleate boiling incipience heat flux of HTCMC was demonstrated to be 5 kW/m2, which was an 8x reduction when compared to a plain copper surface which was found to have an incipience heat flux of 40 kW/m2. At this same 40 kW/m2 heat flux, the activated nucleation site density of HTCMC was extremely high, and each bubble appeared much smaller compared to a plain surface. This can be seen in the first row of images, captured with a high speed camera at 2,000 fps. The bubble growth times and departing bubble sizes of 0° and 90° are comparable for both HTCMC and plain surfaces with the order of 10 milliseconds and 100 micrometers. However, when oriented at 180°, the bubble growth time was the order of 100 milliseconds for both HTCMC and plain surface, and the departing bubble size was the order of 10 millimeters. This is due to the growth of a large bubble which coalesced with adjacent bubbles to become a relatively huge bubble which was stretched by buoyance forces before the bubble departed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020907-020907-1. doi:10.1115/1.4035577.

A visual experimental investigation was conducted to determine the effect of dual frequency ultrasound on the bubble formation and growth in a capillary quartz tube. Two piezoelectric ceramics were used in this experiment. They were made of Pb-based lanthanum-doped zirconate titanates (PLZTs). The PLZTs were placed on a quartz tube with an inner diameter of 2 mm and an outer diameter of 3 mm. The capillary tube was vacuumed first and then charged with water using a filling ratio of 70%. The ultrasonic sound was applied to the heating section of a capillary tube. The bubble formation and growth were recorded by a high speed camera. As shown in figures, when the ultrasound with a single frequency of either 154 kHz or 474 kHz was applied, only one bubble was generated. When the dual frequencies of 154 kHz and 474 kHz were applied, more bubbles were generated. The speed of the bubble growth with dual frequency ultrasound was much higher than that with a single frequency. When a dual frequency ultrasound (154 kHz and 474 kHz) was used, the nucleation sites for bubble formation were significantly increased and the bubble growth rate enhanced.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020908-020908-1. doi:10.1115/1.4035578.

The wetting condition effect of the condensation process on a hybrid superhydrophobic and superhydrophilic copper surface as shown in Fig. 1a was experimentally investigated. The superhydrophilic surface (Fig. 1b) consists of micro-flowers (CuO) and nanorods (Cu(OH)2) obtained by immersing the copper substrate into alkaline solution of 2.5 M sodium hydroxide and 0.1 M ammonium persulphate, and the superhydrophobic nanostructured surface (Fig. 1c) was formed by spin coating the Cytop on the hierarchically structured CuO / Cu(OH)2 surface. Experimental results show that the film condensation started on the superhydrophilic region while the dropwise condensation of tiny droplets with an average contact angle of 160° were formed on the superhydrophobic region. Because the film condensation was confined within the superhydrophilic region of 1 mm x 1 mm, the contact angle of this droplet became larger and larger. When a tiny droplet developed on the superhydrophobic area joins with the big droplet formed on the superhydrophilic surface (square region), the coalesced droplet obtains additional energy and jumps off from the condensing surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020909-020909-1. doi:10.1115/1.4035579.

The objective of this study was to visualize the droplet-induced crown splashing dynamics at high spatial and temporal resolutions. In this work, the effects of droplet train impingement on crown splashing dynamics were investigated experimentally and numerically. Experimentally, a HFE-7100 droplet train was produced using a piezo-electric droplet generator at a frequency ( f ) of 7500 Hz resulting in a droplet Weber number (We) of 489. Droplet-induced crown splashing dynamics was captured using a high-speed imaging system. It was observed that the free rim of the droplet-induced crown was smooth and axisymmetric during the early crown propagation phases (t*< 5, where t* = 2πft). However, development of cusps was observed on the free rim during the intermediate phases (5< t* <8.5). It was found that the sites of the spikes distributed almost uniformly along the periphery of the free rim. At late phases (t* > 8.5), fingering, detachment and secondary droplets (i.e. splashing) were observed on the free rim. Results show that the number of cusps (finally becoming fingers and spikes) based on experiments (ncups,exp) agrees well with the prediction (ncups,th) given by the Plateau-Rayleigh instability theory. Numerical simulations were carried out using a 3D transient coupled level set-volume of fluid (CLSVOF) solver with Courant number less than 1. A grid independence study was performed to ensure the results were independent of grid size. Reasonable agreement was reached between the numerical and experimental data in terms of crown morphology at different phases. This study should lead to a better understanding of the evolution of droplet-induced crown morphology at splashing conditions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020910-020910-1. doi:10.1115/1.4035580.

Ambiphilic surfaces have been used to support thin liquid films during condensation and imaged using environmental scanning electron microscopy (ESEM). Ambiphilic microstructures (a) are comprised of hydrophilic deep etched silicon micropillars with hydrophobic post tops made of PTFE deposited using iCVD. By restraining the growth of the liquid film using hydrophobic post tops (b), thermal resistance is reduced and heat transfer is increased. During condensation on ambiphilic microstructures the condensate initially fills the post array (b), but then bursts outward to accommodate continued production of liquid (c). This creates a low contact angle droplet on the surface (c), and could lead to complete flooding and decreased performance. With the addition of hydrophilic nanostructures to the micropost array (d), ambiphilic hierarchical structures have been fabricated with dedicated burst sites (e). During condensation the structures maintain a thin liquid film and excess liquid emerges from the burst sites as highly mobile spherical droplets (e). This maximizes the thin film area available for vapor-to-liquid phase change while minimizing thermal resistance across the condensate layer. Scale bars: (a) 5 µm, (b,c) 50 µm, (d) 2 µm, and (e) 25 µm.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020911-020911-1. doi:10.1115/1.4035581.

Heat pipe is a highly effective passive heat transfer device using phase change within small temperature difference. It is noted that heat pipe should be operated under heat transfer limit for practical heat pipe heat exchanger applications. The measurement in local and overall heat transfer coefficient is significant to anticipate the heat transfer limit. The wall temperatures and inner working fluid temperatures were measured to determine the heat transfer coefficient. The adiabatic part with transparent Pyrex glass was visualized to understand flow behaviors inside the thermosyphon. The dynamic behaviors of condensed working fluid were visualized for the specific tilted angle and power inputs at pseudo steady-state. At low heat input of 250W, the thin condensed liquid film is observed to be returned from condenser to evaporator. With increasing heat input of 500W, the nucleate boiling starts to occur in evaporator. More activated vapors turn to make wavy motion in free surface of the returned condensed liquid film which is thickened. In power input of 1,250W, the vigorous flow motion happens periodically and the interaction between vapor and liquid bursting reaches a maximum heat transfer which is led to the heat transfer limit in the thermosyphon. Over heat transfer limit (2,000 and 2,500W), the overall heat transfer is decreased when the degree of bursting motion between vapor and liquid is gradually reduced.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020912-020912-1. doi:10.1115/1.4035582.

The onset of nucleate boiling in water spray cooling on hot steel plate was investigated by a simultaneous boiling visualization and heat transfer measurement. The boiling phenomena were visualized with 4K video camera and the surface temperature of the hot steel plate was determined by solving 2-D inverse heat conduction during water spray cooling. The temperature was measured by a sampling rate of 10 data/sec. The hot steel plate was initially heated up to 900°C and the coolant temperature was kept at a constant temperature of 20°C. The spray nozzle with fullcone pattern was mounted with the three different heights (100, 200 and 300 mm). The more spray height was increased, the more scattered the spray pattern became, which could affect the partial spray intensity and overall cooling uniformity. The lower spray nozzle height of 100 mm shows the steep temperature gradient in inner zone. As the spray particles are more intense at inner zone which wets faster than outer zone. But the higher spray nozzle height of 300 mm, the temperature profile keeps constant within the 400 sec. After this time, the outer zone is wetted faster than inner zone. At the middle height of 200 mm, although the temperature gradient in inner zone is slightly higher than that in outer zone, the overall surface wetting is relatively uniform in the inner and outer zone. These results exhibit that the spray cooling uniformity can be controlled with optimized spray nozzle height. Furthermore the boiling visualization agrees well with the onset of nucleate boiling in surface temperature profiles.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(2):020913-020913-1. doi:10.1115/1.4035583.

Non-scanning 3D-CT(Computer Tomography) technique employing a multi-directional quantitative schlieren photographic system with flash light source, has been performed to obtain instantaneous density distributions of spark-ignited laminar / turbulent flame kernels. For simultaneous schlieren photography, the custom-made 20-directional schlieren camera was constructed and used. The concept of the multi-directional shclieren system is shown in top-right figure. Each quantitative schlieren optical system, indicated in top-left figure, is characterized by a rectangular-shaped right source with uniform luminosity. Middle-left picture gives the appearance of the multi-directional schlieren camera. The flame kernels are made by spark ignition for a fuel-rich propane-air premixed gas (flow velocity :1.0 m/s, equivalence ratio :1.4 ). Spark electrodes of 0.4 mm diameter with 1.0 mm gap are used. First, development of laminar flame kernel is indicated in high-speed images of middle-right figure. 3D printed model of the CT reconstruction result (left in bottom-left photograph) shows the spherical shape of flame kernel with a pair of deep wrinkles. The wrinkle is considered to be caused by spark electrodes. Next turbulent flame kernel behind turbulence promoting grid is selected (turbulence intensity 0.26 m/s). The high-speed images of bottom-right figures show corrugated flame shape. 3D model of CT result (right in bottom-left photo.) expresses the instantaneous 3D turbulent flame kernel shapes. These 3D solid models based on 3D-CT reconstructed data of 2 ms, are 3D-printed as 2 times large size for threshold density level of 0.7 kg/m3.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2016;139(2):021501-021501-9. doi:10.1115/1.4034812.

Heat transfer characteristics for condensation for R410A inside horizontal round (dh = 3.78 mm) and flattened tubes (aspect ratio (AR) = 3.07, 4.23, and 5.39) with larger horizontal than vertical dimensions at a saturation temperature of 320 K are investigated numerically. The flattened tube has flat upper and lower walls and circular end walls. The heat and mass transfer model for condensation is verified by comparing numerical heat transfer coefficients of round tubes with experimental data and empirical correlations. Liquid–vapor interfaces and local heat transfer coefficients are also presented to give a better understanding of the condensation process inside these tubes. The results indicate that local heat transfer coefficients increase with increasing mass flux, vapor quality, and aspect ratio. The enhancement of heat transfer coefficients for flattened tubes is more pronounced at higher mass flux and vapor quality values (about 1.5 times the heat transfer coefficients for round tubes when G = 1061 kg m−2 s−1, x ≥ 0.8). Unlike in the round tubes, the liquid film in the flattened tube accumulates at the sides of the bottom surface and at the middle of the top surface of the channels when vapor quality is low. Peak values of liquid film thickness in flattened tubes are obtained around angles about the centroid θ of 70 deg and 117 deg, where θ = 0 deg is upward.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(2):021502-021502-13. doi:10.1115/1.4034901.

Immersion cooling strategies often employ surface enhancements to improve the pool boiling heat transfer performance. Sintered particle/powder coatings have been commonly used on smooth surfaces to reduce the wall superheat and increase the critical heat flux (CHF). However, there is no unified understanding of the role of coating characteristics on pool boiling heat transfer enhancement. The morphology and size of the particles affect the pore geometry, permeability, thermal conductivity, and other characteristics of the sintered coating. In turn, these characteristics impact the heat transfer coefficient and CHF during boiling. In this study, pool boiling of FC-72 is experimentally investigated using copper surfaces coated with a layer of sintered copper particles of irregular and spherical morphologies for a range of porosities (∼40–80%). Particles of the same effective diameter (90–106 μm) are sintered to yield identical coating thicknesses (∼4 particle diameters). The porous structure formed by sintering is characterized using microcomputed tomography (μ-CT) scanning to study the geometric and effective thermophysical properties of the coatings. The boiling performance of the porous coatings is analyzed. Coating characteristics that influence the boiling heat transfer coefficient and CHF are identified and their relative strength of dependence analyzed using regression analysis. Irregular particles yield higher heat transfer coefficients compared to spherical particles at similar porosity. The coating porosity, pore diameter, unit necking area, unit interfacial area, effective thermal conductivity, and effective permeability are observed to be the most critical coating properties affecting the boiling heat transfer coefficient and CHF.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2016;139(2):021701-021701-7. doi:10.1115/1.4034774.

A turbulent convective flow of an incompressible fluid inside a staggered ribbed channel with high blockage at ReH ≈ 4200 is simulated with direct numerical simulation (DNS) and Reynolds-averaged Navier–Stokes (RANS) techniques. The DNS results provide the reference solution for comparison of the RANS turbulence models. The k–ε realizable, k–ω SST, and $v2¯–f$ model are accurately analyzed for their strengths and weaknesses in predicting the flow and temperature field for this geometry. These three models have been extensively used in literature to simulate this configuration and boundary conditions but with discordant conclusions upon their performance. The $v2¯–f$ model performs much better than the k–ε realizable while the k–ω SST model results to be inadequate.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2016;139(2):022001-022001-8. doi:10.1115/1.4034811.

A recent mathematical technique of homotopy perturbation method (HPM) for solving nonlinear differential equations has been applied in this paper for the analysis of steady-state heat transfer in an annular fin with temperature-dependent thermal conductivity and with the variation of thermogeometric fin parameters. Excellent benchmark agreement indicates that this method is a very simple but powerful technique and practical for solving nonlinear heat transfer equations and does not require large memory space that arises out of discretization of equations in numerical computations, particularly for multidimensional problems. Three conditions of heat transfer, namely, convection, radiation, and combined convection and radiation, are considered. Dimensionless parameters pertinent to design optimization are identified and their effects on fin heat transfer and efficiency are studied. Results indicate that the heat dissipation under combined mode from the fin surface is a convection-dominant phenomenon. However, it is also found that, at relatively high base temperature, radiation heat transfer becomes comparable to pure convection. It is worth noting that, for pure radiation condition, the dimensionless parameter of aspect ratio (AR) of a fin is a more desirable controlling parameter compared to other parameters in augmenting heat transfer rate without much compromise on fin efficiency.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(2):022002-022002-14. doi:10.1115/1.4034698.

The heat transfer coefficient (HTC) is often averaged spatially when designing heat exchangers. Since the HTC could vary appreciably about a heat transfer enhancement feature such as a pin fin or a rib, it is of interest to understand the effects of averaging the HTC on design. This computational study examines those effects via a unit problem—a flat plate of thickness H and length L, where L represents the distance between pin-fins or ribs. This flat plate is heated on one side, and cooled on the other. Variable HTC is imposed on the cooled side—a higher HTC (hH) over LH and a lower HTC (hL) over LL = L − LH. For this unit problem, the following parameters were studied: abrupt versus gradual transition between hH and hL, hH/hL, LH/L, and H/L. Results obtained show that if the averaged HTC is used, then the maximum temperature in the plate and the maximum temperature gradient in the plate can be severely underpredicted. The maximum temperature and the maximum temperature gradient can be underpredicted by as much as 36.3% and 542%, respectively, if the Biot number is less than 0.1 and as much as 13.0% and 570% if the Biot number is between 0.25 and 0.4. A reduced-order model was developed to estimate the underpredicted maximum temperature.

Topics: Temperature
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(2):022003-022003-8. doi:10.1115/1.4034756.

This paper presents performance definitions for calculating the overall effectiveness of three-fluid heat and moisture exchangers. The three-fluid heat and moisture exchanger considered in this paper is a combination of a liquid-to-liquid heat exchanger for heat transfer between a desiccant solution and a refrigerant and an energy exchanger for heat and moisture transfer between desiccant solution and air streams. The performance definitions presented in this paper are used to calculate the overall sensible and latent effectivenesses of a three-fluid heat and moisture exchanger, which has been tested under air cooling and dehumidifying operating conditions in a previous work (Abdel-Salam et al., 2016, “Design and Testing of a Novel 3-Fluid Liquid-to-Air Membrane Energy Exchanger (3-Fluid LAMEE),” Int. J. Heat Mass Transfer, 92, pp. 312–329). The effectiveness of this three-fluid heat and moisture exchanger is compared when calculated using the traditional energy exchanger effectiveness equations and the overall performance definitions. Results show that the overall performance definitions provide effectiveness values that are less sensitive to changes in the inlet refrigerant temperature and therefore are more generally applicable for energy exchanger design than the traditional effectiveness equations used in the literature.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(2):022004-022004-9. doi:10.1115/1.4034898.

This study investigates the unsteady heat transfer and entropy generation characteristics of a non-Newtonian fluid, squeezed and extruded between two parallel plates. In an effort to capture the underlying thermo-hydrodynamics, the power-law model is used here to describe the constitutive behavior of the non-Newtonian fluid. The results obtained from the present analysis reveal the intricate interplay between the fluid rheology and the squeezing dynamics, toward altering the Nusselt number and Bejan number characteristics. Findings from this study may be utilized to design optimal process parameters for enhanced thermodynamic performance of engineering systems handling complex fluids undergoing simultaneous extrusion and squeezing.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2016;139(2):022101-022101-9. doi:10.1115/1.4034855.

Squeeze casting of wrought aluminum 7075 was carried out on a 75-ton hydraulic press. Metal/die interface heat transfer phenomena in squeeze casting of the alloy were investigated. To facilitate experimental measurements, a five-step casting mold was designed for the experiments. The five-step casting consisted of five different section thicknesses of 2, 4, 8, 12, and 20 mm. Squeeze casing experiments were performed under the applied hydraulic pressures of 30, 60, and 90 MPa. Temperatures were measured at the casting surface and at various specific locations inside the die. At each step, thermocouples were placed at 2, 4, and 6 mm away from the inside die face. Based on the measured temperature results, the interfacial heat transfer coefficients (IHTCs) and heat fluxes were determined by solving the one-dimensional transient heat conduction equation with the inverse method. With increasing the casting section thicknesses from 2 to 20 mm, the peak IHTC values varied from 1683.46 W/m2 K to 9473.23 W/m2 K, 2174.78 W/m2 K to 13,494.05 W/m2 K, and 3873.45 W/m2 K to 15,483.01 W/m2 K for the applied hydraulic pressures of 30, 60, and 90 MPa, respectively.

Commentary by Dr. Valentin Fuster

### Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2016;139(2):022201-022201-12. doi:10.1115/1.4034773.

This study investigates the effects of sinusoidal pulsations externally imposed to an oblique round jet. The effectiveness of film coverage of an adiabatic wall onset for a thermally uniform bulk flow is presented in the perspective of gas turbine film cooling. For the injectant fluid, both the temperature and the mass flow rate are controlled prior to entrance to the periodic forcing system using a loudspeaker drive. The characteristic film cooling parameters including the blowing ratios and the temperature ratio are maintained at $M=ρiUi/ρ∞U∞$= 0.65, 1, and 1.25, and $Ti/T∞=2$ respectively. The injection fluid is pulsated to a nondimensionalized frequency of $St=f⋅d/U$= 0, 0.2, 0.3, and 0.5. In the present investigation, the impact of injectant film modulation is figured out by analyzing the velocity fields measured by a system of time-resolved particle image velocimetry (TR-PIV), as well as analyzing the adiabatic wall temperature and the convective heat transfer coefficient measured by a system of infrared thermography. The overall film-cooling effectiveness is revealed by the time-averaged analysis, in which altered time-averaged jet trajectories and wake behavior are focused. It is observed that the pulsations tend to result in lower effectiveness when the flow remained attached to the wall in steady blowing case. In steady blowing cases with jet liftoff, such as for $M=$ 1.25, rendering low-frequency pulsation helps in increasing film-cooling effectiveness due to the discharge of lower mass flow rate coolant during the significant time interval of the respective pulse cycle.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2016;139(2):022401-022401-5. doi:10.1115/1.4034757.

Metal nanoparticle has been a promising option for fillers in thermal interface materials due to its low cost and ease of fabrication. However, nanoparticle aggregation effect is not well understood because of its complexity. Theoretical models, like effective medium approximation model, barely cover aggregation effect. In this work, we have fabricated nickel–epoxy nanocomposites and observed higher thermal conductivity than effective medium theory predicts. Smaller particles are also found to show higher thermal conductivity, contrary to classical models indicate. A two-level effective medium approximation (EMA) model is developed to account for aggregation effect and to explain the size-dependent enhancement of thermal conductivity by introducing local concentration in aggregation structures.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2016;139(2):022501-022501-10. doi:10.1115/1.4034794.

In this paper, we found, by means of numerical simulations, a transition in the oscillatory character of the flow field for a particular combination of buoyancy and spacing in an array of six circular cylinders at a Reynolds number of 100 and Prandtl number of 0.7. The cylinders are isothermal and they are aligned with the earth acceleration (g). According to the array orientation, an aiding or an opposing buoyancy is considered. The effect of natural convection with respect to the forced convection is modulated with the Richardson number, Ri, ranging between −1 and 1. Two values of center-to-center spacing (s = 3.6d–4d) are considered. The effects of buoyancy and spacing on the flow pattern in the near and far field are described. Several transitions in the flow patterns are found, and a parametric analysis of the dependence of the force coefficients and Nusselt number with respect to the Richardson number is reported. For Ri=−1, the change of spacing ratio from 3.6 to 4 induces a transition in the standard deviation of the force coefficients and heat flux. In fact, the transition occurs due to rearrangement of the near-field flow in a more ordered wake pattern. Therefore, attention is focused on the influence of geometrical and buoyancy parameters on the heat and momentum exchange and their fluctuations. The available heat exchange models for cylinders array provide a not accurate prediction of the Nusselt number in the cases here studied.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(2):022502-022502-6. doi:10.1115/1.4034795.

The influence of gravitational modulation on natural convection in a horizontal porous annulus is investigated in this paper. The mathematical model describing the phenomenon consists of the heat equation coupled by the hydrodynamics equations under the Boussinesq approximation. The derived system of equations with the stream function–temperature formulation is obtained and solved numerically using the alternating direction implicit method. It is shown that the convective stability of the fluid can be gained for small amplitudes of the vibration, while it will be lost for large ones. It was also observed that increasing the frequency has a destabilizing effect.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(2):022503-022503-11. doi:10.1115/1.4034815.

This paper reports the numerical results for the natural convection flow of a two-phase dusty nanofluid along a vertical wavy frustum of a cone. The general governing equations are transformed into parabolic partial differential equations, which are then solved numerically with the help of implicit finite difference method. Comprehensive flow formations of carrier and dusty phases are given with the aim to predict the behavior of heat and mass transport across the heated wavy frustum of a cone. The effectiveness of utilizing the nanofluids to control skin friction and heat and mass transport is analyzed. The results clearly show that the shape of the waviness changes when nanofluid is considered. It is shown that the modified diffusivity ratio parameter, NA, extensively promotes rate of mass transfer near the vicinity of the cone, whereas heat transfer rate reduces.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2016;139(2):022701-022701-8. doi:10.1115/1.4034793.

Enhancing photon tunneling probability is the key to increasing the near-field radiative heat transfer between two objects. It has been shown that hexagonal boron nitride (hBN) and graphene heterostructures can enable plentiful phononic and plasmonic resonance modes. This work demonstrates that heterostructures consisting of a monolayer graphene on an hBN film can support surface plasmon–phonon polaritons that greatly enhance the photon tunneling and outperform individual structures made of either graphene or hBN. Both the thickness of the hBN films and the chemical potential of graphene can affect the tunneling probability, offering potential routes toward passive or active control of near-field heat transfer. The results presented here may facilitate the system design for near-field energy harvesting, thermal imaging, and radiative cooling applications based on two-dimensional materials.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;139(2):022702-022702-13. doi:10.1115/1.4034772.

The immersed boundary method (IBM) is gaining attention in the computational fluid dynamics but its applications in the field of a conjugated radiative–conductive or radiative–convective heat transfer seem limited. Therefore, the paper presents extension of this method to heat transfer problems dominated by thermal radiation in a nongray medium. The present model enables simulation of heat and fluid flows in a domain with complex stationary or moving internal and external boundaries on a fixed Cartesian grid (FCG) by applying the finite volume method. The special attention is paid to modeling thermal radiation and optical phenomena at highly curved, opaque, or transparent boundaries which confine the computational domain or separate zones of different thermophysical and optical properties, e.g., different values of a refractive index. The model is limited to a 2D planar or axisymmetric spaces. Detailed verification procedure proves accuracy and correctness of the developed model and shows its potential application field. The model may be used for simulations of a conjugated radiative–conductive or radiative–convective heat transfer in a nongray medium in a complex domain with opaque or transparent curved internal or external boundaries without unstructured or body fitted mesh generation.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2016;139(2):022901-022901-7. doi:10.1115/1.4034899.

The possibility of using a frost layer, created on the surface of a sample that undergoes cryogenic treatment, as a heat transfer enhancer was recently studied. This layer grows on the preliminary cooled sample surface as a result of its contact with moist air flow prior to its immersion into liquid nitrogen. A significant increase in the outflow heat flux (up to 12.8 times), or, alternatively, a cooling time shortening, in comparison with the bare sample was found. A detailed description of the frost layer development along with the influence of the thickness of the layer on the efficiency of the cooling process, as well as environmental parameters that affect the thickness itself is presented in the paper.

Commentary by Dr. Valentin Fuster

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