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### Guest Editorial

J. Heat Transfer. 2014;136(8):080301-080301-1. doi:10.1115/1.4027515.

The Seventeenth Heat Transfer Photogallery was sponsored by the K-22 Heat Transfer Visualization Committee for both the 2013 Summer Heat Transfer Conference (SHTC) held in Minneapolis, Minnesota on Jul. 14–19, 2013, and the 2013 International Mechanical Engineering Congress and Exhibition (IMECE) held in San Diego, California on Nov. 15–21, 2013. Both Photogallery sessions presented a total of 43 entries, and the peer-reviewed evaluation conducted by the participants has identified the 18 final entries for publication in this ASME Journal of Heat Transfer August issue of 2014.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster

### Photogallery

J. Heat Transfer. 2014;136(8):080901-080901-1. doi:10.1115/1.4027516.

Photogallery Entry 1

Topics: Freezing
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080902-080902-1. doi:10.1115/1.4027517.

Photogallery Entry 2

Topics: Drops , Evaporation , Bacteria
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080903-080903-1. doi:10.1115/1.4027518.

Photogallery Entry 3

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080904-080904-1. doi:10.1115/1.4027519.

Photogallery Entry 4

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080905-080905-1. doi:10.1115/1.4027520.

Photogallery Entry 5

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080906-080906-1. doi:10.1115/1.4027521.

Photogallery Entry 6

Topics: Drops
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080907-080907-1. doi:10.1115/1.4027522.

Photogallery Entry 7

Topics: Temperature , Drops , Water
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080908-080908-1. doi:10.1115/1.4027526.

Photogallery Entry 8

Topics: Drops , Water
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080909-080909-1. doi:10.1115/1.4027527.

Photogallery Entry 9

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080910-080910-1. doi:10.1115/1.4027528.

Photogallery Entry 10

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080911-080911-1. doi:10.1115/1.4027529.

Photogallery Entry 11

Topics: Condensation
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080912-080912-1. doi:10.1115/1.4027530.

Photogallery Entry 12

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080913-080913-1. doi:10.1115/1.4027531.

Photogallery Entry 13

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080914-080914-1. doi:10.1115/1.4027532.

Photogallery Entry 14

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080915-080915-1. doi:10.1115/1.4027533.

Photogallery Entry 15

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080916-080916-1. doi:10.1115/1.4027534.

Photogallery Entry 16

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080917-080917-1. doi:10.1115/1.4027535.

Photogallery Entry 17

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):080918-080918-1. doi:10.1115/1.4027536.

Photogallery Entry 18

Topics: Heat transfer
Commentary by Dr. Valentin Fuster

### Research Papers: Conduction

J. Heat Transfer. 2014;136(8):081301-081301-10. doi:10.1115/1.4027459.

In the last decade, various conductive networks for cooling heat generating bodies have been proposed, analyzed, and optimized. Nevertheless, many of these studies have not been based on an analytical or mathematical formulation of the effective parameters. In this trend, a new geometry is assumed and analyzed (by analytical or numerical methods) hoping to decrease the total thermal resistance of the system. Therefore, the objective of the present paper is to illustrate how to analyze a conductive cooling network and improve it using the analytical procedures based on the general formulation of thermal resistance. As an example, the conventional rectangular elemental volumes with I shaped conductive link is modified to V shaped and pencil shaped designs and optimized analytically. Moreover, general expressions for optimum local thickness and thermal resistance of the links with variable cross section in an arbitrary network are provided. It is shown that improvements up to 50% can be achieved easily by simple geometrical changes if the designer is equipped with a profound knowledge of the important governing parameters.

Commentary by Dr. Valentin Fuster

### Research Papers: Electronic Cooling

J. Heat Transfer. 2014;136(8):081401-081401-7. doi:10.1115/1.4027131.

Increasing trends toward integrated power electronic systems demand advancements in novel, efficient thermal management solutions to cope with the increasing the power density. This paper investigates the performance of a novel open loop pulsating heat pipe embedded in an FR4 organic substrate. The heat pipe is comprised of 26 parallel minichannels, 13 turns with an average hydraulic diameter of 1.7 mm and maximum surface roughness of 2.5 μm. The bulk thermal performance of three saturated working fluids—Novec 649, Novec 7200 and Ethanol (99.8%)—is investigated in terms of fill ratio, three angles of orientation, and applied heat fluxes ranging from 0.4 to 2.5 W/cm2 at subambient pressures. Novec 649 achieved quasi-stable pulsations at lower heat fluxes compared to Novec 7200 and Ethanol (99.8%). In addition, the dielectric Novec 649 fluid showed significant potential for integrated heat spreading applications demonstrating heat transfer of up to 176 W and thermal resistances as low as 0.25 °C/W for a filling ratio of 30%—16 times greater than that of a standard dry FR4 substrate

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2014;136(8):081501-081501-9. doi:10.1115/1.4027252.

It is well known that a phase transition from liquid to vapor occurs in the thermal boundary layer adjacent to a nanoparticle that has a high temperature upon irradiation with a high-power laser. In this study, the mechanism by which the evaporated layer adjacent to a laser-irradiated nanoparticle can grow as a bubble was investigated through detailed calculations. The pressure of the evaporated liquid volume due to heat diffusion from the irradiated nanoparticle was estimated using a bubble nucleation model based on molecular interactions. The bubble wall motion was obtained using the Keller-Miksis equation. The density and temperature inside the bubble were obtained by solving the continuity and energy equation for the vapor inside the bubble. The evaporation of water molecules or condensation of water vapor at the vapor–liquid interface and the homogeneous nucleation of vapor were also considered. The calculated bubble radius-time curve for the bubble formed on the surface of a gold particle with a diameter of 9 nm is close to the experimental result. Our study reveals that an appropriate size of the evaporated liquid volume and a large expansion velocity are important parameters for the formation of a transient nanosized bubble. The calculation result suggests that homogeneous condensation of vapor rather than condensation at the interface occurs.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):081502-081502-10. doi:10.1115/1.4027349.

In this study, experimental investigations regarding the heat transfer performance of an evaporator with capillary wick are presented. The capillary wick structure is composed of sintered multilayer copper mesh. The multilayer copper mesh was sintered on the copper plate. With different combinations of mesh screens, the wick thickness of mesh 140 ranged from 0.6 to 1.0 mm, and those of meshes 60 and 140/60 were both 1.0 mm. The operating pressures used in this study were 0.86 × 105, 0.91 × 105, 0.96 × 105, 1.01 × 105, and 2.0 × 105Pa. The experimental results indicate that the heat transfer performance was strongly dependent on the thickness of the sintered mesh structure and on the mesh size. The operating pressure also has a strong influence on the evaporation/boiling heat transfer performance of a mesh structure sintered using a single mesh size. However, it was also observed that the evaporation/boiling heat coefficient increased with an increase in the thickness of the capillary wick structure, which is less than 1.0 mm. The experimental results further illustrate that the composite sintered mesh structure was capable of properly enhancing the heat transfer performance, especially under high pressure. The maximum enhancement was 31.98%.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):081503-081503-10. doi:10.1115/1.4027365.

Saturation pool boiling experiments of degassed PF-5060 dielectric liquid investigated nucleate boiling on 13 Cu surfaces with average roughness, Ra, of 0.039 (smooth polished) to 1.79 μm at six inclination angles, θ, from 0 deg (upward facing) to 180 deg (downward facing). Values of the nucleate boiling heat transfer coefficient, hNB, in the upward facing orientation increase with increasing surface roughness and are correlated in terms of the applied heat flux, q: hNB = A qB. The exponent “B” decreases from 0.81 to 0.69 as Ra increases from 0.039 to 1.79 μm, while the coefficient “A” increases with Ra to the power 0.24. The values of the maximum heat transfer coefficient, hMNB, which occurs near the end of the fully developed nucleate boiling region, increase with increasing Ra and decreasing inclination angle. In the upward facing orientation, hNB increases by ∼58% with increasing Ra from 0.134 to 1.79 μm, while hMNB increases by more than 150% compared with that on smooth-polished Cu. Values of hMNB in the downward facing orientation are ∼40% of those in the upward facing orientation.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):081504-081504-11. doi:10.1115/1.4027352.

In this experimental study, flow boiling in mini/microtubes was investigated with surface enhancements provided by crosslinked polyhydroxyethylmethacrylate (pHEMA) coatings, which were used as a crosslinker coating type with different thicknesses (∼50 nm, 100 nm, and 150 nm) on inner microtube walls. Flow boiling heat transfer experiments were conducted on microtubes (with inner diameters of 249 μm, 507 μm, and 908 μm) coated with crosslinked pHEMA coatings. pHEMA nanofilms were deposited with initiated chemical vapor deposition (iCVD) technique. De-ionized water was utilized as the working fluid in this study. Experimental results obtained from coated microtubes were compared to their plain surface counterparts at two different mass fluxes (5000 kg/m2 s and 20,000 kg/m2 s), and significant enhancements in critical heat flux (up to 29.7%) and boiling heat transfer (up to 126.2%) were attained. The enhancement of boiling heat transfer was attributed to the increase in nucleation site density and incidence of bubbles departing from surface due to porous structure of crosslinked pHEMA coatings. The underlying mechanism was explained with suction-evaporation mode. Moreover, thicker pHEMA coatings resulted in larger enhancements in both CHF and boiling heat transfer.

Commentary by Dr. Valentin Fuster

### Research Papers: Experimental Techniques

J. Heat Transfer. 2014;136(8):081601-081601-9. doi:10.1115/1.4027551.

Thermal properties of thermal barrier coatings (TBCs) are important parameters for the safe and efficient operation of advanced turbine engines. This paper presents a new method, the pulsed thermal imaging–multilayer analysis (PTI–MLA) method, which can measure the coating thermal conductivity and heat capacity distributions over an entire engine component surface. This method utilizes a multilayer heat transfer model to analyze the surface temperature response acquired from a one-sided pulsed thermal imaging experiment. It was identified that several experimental system parameters and TBC material parameters may affect the coating surface temperature response. All of these parameters were evaluated and incorporated as necessary into the formulations. The PTI–MLA method was demonstrated by analyzing three TBC samples, and the experimental results were compared with those obtained from other methods.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2014;136(8):081701-081701-11. doi:10.1115/1.4027344.

In the present study, laminar pulsating flow over a backward-facing step in the presence of a square obstacle placed behind the step is numerically studied to control the heat transfer and fluid flow. The working fluid is air with a Prandtl number of 0.71 and the Reynolds number is varied from 10 and 200. The study is performed for three different vertical positions of the square obstacle and different forcing frequencies at the inlet position. Navier–Stokes and energy equation for a 2D laminar flow are solved using a finite-volume-based commercial code. It is observed that by properly locating the square obstacle the length and intensity of the recirculation zone behind the step are considerably affected, and hence, it can be used as a passive control element for heat transfer augmentation. Enhancements in the maximum values of the Nusselt number of 228% and 197% are obtained for two different vertical locations of the obstacle. On the other hand, in the pulsating flow case at Reynolds number of 200, two locations of the square obstacle are effective for heat transfer enhancement with pulsation compared to the case without obstacle.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):081702-081702-10. doi:10.1115/1.4027389.

This study reports on heat transfer characteristics on a curved surface subject to an inclined circular impinging jet whose impinging angle varies from a normal position θ = 0 deg to θ = 45 deg at a fixed jet Reynolds number of Rej = 20,000. Three curved surfaces having a diameter ratio (D/Dj) of 5.0, 10.0, and infinity (i.e., a flat plate) were selected, each positioned systematically inside and outside the potential core of jet flow where Dj is the circular jet diameter. Present results clarify similar and dissimilar local heat transfer characteristics on a target surface due to the convexity. The role of the potential core is identified to cause the transitional response of the stagnation heat transfer to the inclination of the circular jet. The inclination and convexity are demonstrated to thicken the boundary layer, reducing the local heat transfer (second peaks) as opposed to the enhanced local heat transfer on a flat plate resulting from the increased local Reynolds number.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):081703-081703-12. doi:10.1115/1.4027554.

A light and compact heat exchange system was realized using two air-to-refrigerant airfoil heat exchangers and a recirculated heat transport refrigerant. Its heat transfer performance was experimentally investigated. Carbon dioxide or water was used as a refrigerant up to a pressure of 30 MPa. Heat transfer coefficients on the outer air-contact and inner refrigerant-contact surfaces were calculated using an inverse heat transfer method. Correlations were developed for the Nusselt numbers of carbon dioxide and water on the inner refrigerant-contact surface. Furthermore, we proposed a method to evaluate a correction factor corresponding to the thermal resistance of the airfoil heat exchanger.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2014;136(8):082001-082001-13. doi:10.1115/1.4027390.

The effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of oscillating heat pipes (OHPs) were investigated in the paper. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Deionized water was used as the working fluid. Experimental results showed that superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the OHP with copper surface. A thin liquid film existed between the vapor plug and the wall of the OHP. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface contact line length in the hybrid surface OHP was longer than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by 5–20%. Comparing with the copper OHPs, the maximum amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by 0–127% and 0–185%, respectively. However, the maximum amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was reduced by 0–100% and 0–100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by 10–35% in the superhydrophobic surface OHPs.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2014;136(8):082501-082501-5. doi:10.1115/1.4027355.

A numerical study of the steady free convection flow in shallow and slender porous cavities filled by a nanofluid is presented. The nanofluid model takes into account the Brownian diffusion and the thermophoresis effects. The governing dimensional partial differential equations are transformed into a dimensionless form before being solved numerically using a finite difference method. Effort has been focused on the effects of four types of influential factors such as the aspect ratio, the Rayleigh and Lewis numbers, and the buoyancy-ratio parameter on the fluid flow and heat transfer characteristics.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):082502-082502-8. doi:10.1115/1.4027448.

Laminar natural convection of a nanofluid consists of water and copper in a differentially heated parallelogrammic enclosure has been studied numerically using the finite volume method (FVM). Governing equations are solved over a wide range of Rayleigh numbers (104≤ Ra ≤ 106), skew angles (−60 deg ≤ Φ ≤ +60 deg), aspect ratios (0.5 ≤ AR ≤ 4), and solid volume fractions (0 ≤ φ ≤ 0.2). Effects of all these parameters on flow and thermal fields are presented in form of streamline, isotherm contours and average Nusselt number. It is shown that the heat transfer rate increases remarkably by the addition of copper-water nanofluid and the shape of the convection vortices is sensitive to the skew angle variation.

Commentary by Dr. Valentin Fuster

### Research Papers: Thermal Systems

J. Heat Transfer. 2014;136(8):082801-082801-6. doi:10.1115/1.4027248.

An experimental study of a counter-flow Ranque–Hilsch vortex tube is reported here. Literature has been divided over the mechanism of energy transfer responsible for the temperature separation in the vortex tube. A black box approach is used to design experiments to infer the relative roles of heat transfer and shear work transfer in the counter-flow vortex tube. To this end, the stagnation temperature and the mass flow rates are measured at the inlet and the two outlets. In addition, pressure measurements at the stagnation condition and at the inlet section to the vortex tube were made. Based on these experiments, it is reasoned that the predominant mode of energy transfer responsible for temperature separation in a counter-flow vortex is the shear work transfer between the core and the periphery.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2014;136(8):084501-084501-4. doi:10.1115/1.4027354.

Within the framework of the potent lumped model, unsteady heat conduction takes place in a solid body whose space–mean temperature varies with time. Conceptually, the lumped model subscribes to the notion that the external convective resistance at the body surface dominates the internal conductive resistance inside the body. For forced convection heat exchange between a solid body and a neighboring fluid, the criterion entails to the lumped Biot number $Bil=(h¯/ks)(V/A)<0.1$, in which the mean convective coefficient $h¯$ depends on the impressed fluid velocity. However, for natural convection heat exchange between a solid body and a fluid, the mean convective coefficient $h¯$ depends on the solid-to-fluid temperature difference. As a consequence, the lumped Biot number must be modified to read $Bil=(h¯max/ks)(V/A)<0.1$, wherein $h¯max$ occurs at the initial temperature Ti for cooling or at a future temperature Tfut for heating. In this paper, the equivalence of the lumped Biot number criterion is deduced from the standpoint of the solid thermal conductivity through the solid-to-fluid thermal conductivity ratio.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2014;136(8):084502-084502-1. doi:10.1115/1.4027537.

Topics: Heat , Mass transfer
Commentary by Dr. Valentin Fuster