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# Current Issue

### Research Papers: Electronic Cooling

J. Heat Transfer 136, 081401 (2014) ();   Paper No: HT-13-1372;   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

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

J. Heat Transfer 136, 081501 (2014) ();   Paper No: HT-14-1010;   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.

J. Heat Transfer 136, 081502 (2014) ();   Paper No: HT-13-1335;   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%.

J. Heat Transfer 136, 081503 (2014) ();   Paper No: HT-13-1452;   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.

### Research Papers: Forced Convection

J. Heat Transfer 136, 081701 (2014) ();   Paper No: HT-12-1549;   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.

J. Heat Transfer 136, 081702 (2014) ();   Paper No: HT-13-1526;   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.

### Research Papers: Natural and Mixed Convection

J. Heat Transfer 136, 082501 (2014) ();   Paper No: HT-13-1559;   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.

### Research Papers: Thermal Systems

J. Heat Transfer 136, 082801 (2014) ();   Paper No: HT-12-1422;   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.

### Technical Brief

J. Heat Transfer 136, 084501 (2014) ();   Paper No: HT-13-1254;   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.