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Research Papers: Conduction

J. Heat Transfer. 2015;138(3):031301-031301-9. doi:10.1115/1.4031733.

The aim of this paper is to study the benefits of applying ultrashort pulsed lasers over nanosecond pulsed lasers for selective (i.e., superficial) heat treatment of materials in general and for selective heat treatment of thin films in particular. To this end, a background of the physics that govern the absorption of light and subsequent diffusion of heat in semiconductor and metallic materials is provided, when exposed to picosecond or nanosecond laser pulses, with a fluence below the ablation threshold. A numerical model was implemented using a commercial finite-element modeling package, to simulate the temperature fields in thin films induced by laser pulses. The results of the simulations provide insight in the temperature cycles and corresponding timescales, as function of the processing parameters, such as fluence, pulse duration, pulse repetition frequency, and laser wavelength. Numerical simulations were run for thin films of molybdenum (Mo) and zinc oxide (ZnO) on a glass substrate, which are materials commonly adopted as (back and front) electrodes in thin film solar cells.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2015;138(3):032401-032401-12. doi:10.1115/1.4031732.

Transient heat conduction in finite thin films subjected to time-varying surface heat flux incidences at both boundaries and internal heat generation is investigated via the dual-phase-lag (DPL) hyperbolic model. Analytical solution of the temperature profiles inside the solid is derived by using the superposition principle and the method of Fourier series expansion in conjunction with the solution structure theorems. For comparison purposes, the classical diffusion, Cattaneo–Vernotte (C–V) model, and simplified thermomass (TM) models are deduced from the generalized DPL model. This is made possible by adjusting the temperature and heat flux relaxation parameters, and offers the opportunity to examine various interconnected non-Fourier conduction heat transfer characteristics including wave and diffusion effects as well as their interrelationship. Details of this process are examined and results are explored in this study.

Commentary by Dr. Valentin Fuster

Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2015;138(3):032501-032501-9. doi:10.1115/1.4031788.

Airflow, heat, and contaminant transfer in a mechanically ventilated two-dimensional rectangular enclosure by discrete heat and contaminant sources as well as external forced convection at various inlet and outlet locations is numerically simulated. Two different enclosure configurations are considered. In configuration A, the cold air is injected at the top of the left vertical wall and exited at the bottom of the right vertical wall. In configuration B, the cold air is injected at the lower edge of the left vertical wall and exited at the top of the right vertical wall. The objective of the study is to find the relative locations of inlet and outlet in order to obtain more effective cooling in the core of the enclosure by maximizing the heat and contaminant removal rate and reducing the overall temperature. The developed mathematical model is governed by the two-dimensional continuity, momentum, energy, and concentration equations. The governing equations in Cartesian co-ordinates are solved by finite volume based semi-implicit method for pressure linked-equations (SIMPLE) algorithm based on a staggered grid system. Results are presented for different values of the Reynolds number, Grashof number, Sherwood number, and Buoyancy ratio in the laminar regime. A convective transport visualization technique is used to study the behavior of physical phenomena due to stream function, thermal, and solutal functions. The results indicate that cooling inside the core of the enclosure is most effective when the inlet is kept at the bottom of the left vertical wall.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2015;138(3):032701-032701-9. doi:10.1115/1.4031749.

Thermal management systems for space equipment commonly use static solutions that do not adapt to environmental changes. Dynamic control of radiative surface properties is one way to respond to environmental changes and to increase the capabilities of spacecraft thermal management systems. This paper documents an investigation of the extent to which origami-inspired surfaces may be used to control the apparent absorptivity of a reflective material. Models relating the apparent absorptivity of a radiation shield to time-dependent surface temperatures are presented. Results show that the apparent absorptivity increases with increasing fold density and indicate that origami-inspired designs may be used to control the apparent radiative properties of surfaces in thermal management systems.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;138(3):032702-032702-7. doi:10.1115/1.4031734.

Silica aerogels are porous ultralight materials with exceptional physical properties that are promising materials for thermal insulation applications. This paper theoretically and experimentally investigates the spectral scattering and absorption coefficients of a porous silica aerogel. Silica aerogel samples were prepared with the same compositions and various thicknesses using the sol-gel technique and supercritical drying. The spectral normal-hemispherical transmittances and reflectances of the silica aerogel samples with various thicknesses were measured for wavelengths of 0.38–15 μm. The reflectance and transmittance are higher at short wavelengths than in the infrared region due to the strong scattering and weak absorption at short wavelengths. The thicker samples strongly attenuate the spectral normal-hemispherical transmittance, but have little effect on the spectral normal-hemispherical reflectance. A modified two-flux radiative transfer model was used to analyze the radiation propagation in the silica aerogel with a rough surface morphology and millimeter thicknesses to develop theoretical expressions for the spectral directional-hemispherical reflectance and transmittance. Then, the optical constants, including the absorption coefficient and the scattering coefficient, were determined for wavelengths of 0.38–15 μm based on experimental data by the least-squares algorithm. The results show that when considering the radiation propagation inside the sample, the surface reflection at the air–aerogel interface can be neglected for aerogel thicker than 1.1 mm when the absorbing and scattering effects inside the sample are quite important. The analysis provides valuable data for the optical properties for silica aerogel applications.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2015;138(3):031901-031901-6. doi:10.1115/1.4031787.

Dimple structure is an effective heat transfer augmentation approach on coolant channel due to its advantage on pressure penalty. The implication of secondary protrusion, which indicates protrusion with smaller dimension than dimple, will intensify the Nusselt number Nu inside dimple cavity without obvious extra pressure penalty. The objective of this study is to numerically analyze the combination effect of dimples and secondary protrusion. Different protrusion–dimple configurations including protrusion print-diameter Dp, protrusion–dimple gap P, and staggered angle α are investigated. From the results, it is concluded that the implication of secondary protrusion will considerably increase the heat transfer rates inside dimple cavity. Cases 4 and 6 possess the highest Nusselt number enhancement ratio Nu/Nu0 reaching up to 2.1–2.2. The additional pressure penalty brought by the protrusion is within 15% resulting in total friction ratio f/f0 among the range of 1.9–2.1. Dimpled channels with secondary protrusions possess higher thermal performance factor TP, defined as (Nu/Nu0)/(f/f0)1/3, among which cases 4 and 6 are the optimal structures. Besides this, the TP of protrusion–dimple channels are comparable to the other typical heat transfer devices, and higher TP can be speculated after a more optimal dimple shape or combination with ribs and fins.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;138(3):031902-031902-7. doi:10.1115/1.4031920.

The third-generation enhanced heat transfer technologies, such as three-dimensional fin and dimple, are still important means of improving energy efficiency. This paper analyzes the condensation heat transfer performances of three edge-shaped finned tubes that were fabricated using the plowing–extruding process. Experimental results show that the shell-side heat transfer coefficient decreases with increases of heat flux and temperature difference between wall and vapor. The edge-shaped finned tubes exhibit better heat transfer performance than smooth tubes. At the identical temperature difference between the wall and the vapor, the shell-side heat transfer coefficient of the edge-shaped finned tubes is approximately 1.7–2.6 times larger than that of the smooth tubes. At the identical temperature difference between the wall and the vapor, the shell-side heat transfer coefficient of edge-shaped finned tubes is also higher than the reported value in the literature. The excellent performance of the edge-shaped finned tubes comes from the coordination of enhancement from the three-dimensional fins, dimples, and grooves. Finned tubes with grooves fabricated along the left direction have higher and thinner fins and therefore show better heat transfer performance. The shell-side heat transfer coefficients of edge-shaped finned tubes increase with plowing–extruding depth and feed increasing.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2015;138(3):031501-031501-9. doi:10.1115/1.4031882.

Flow-pattern based heat transfer correlations for elongated bubbly flow and annular flow in stable flow boiling micro/minichannels were developed separately based on a collected micro/minichannel heat-transfer database by using an improved conventional-to-micro/minichannel threshold Bo = 4 and BoRel0.5 = 200, where Bo is the Bond number and Rel is the liquid Reynolds number. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. The newly developed flow-pattern based predictive tool cannot only present a decent overall accuracy, but also estimate the parametric trends relatively well. Over 95% of the data points can be predicted by the proposed correlations within a ±50% error band for both elongated bubbly flow and annular flow. Therefore, the flow-pattern based correlations can be applied for heat exchanger design to improve the understanding of the underlying heat transfer mechanisms and to guide the development of further enhancement techniques for flow boiling in micro/minichannels.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;138(3):031502-031502-9. doi:10.1115/1.4031883.

In our efforts to improve the pool boiling heat transfer of water, three sets of experiments are carried out to investigate the best coolant for heat removal among alumina, silica, and zinc oxide as nanoparticles and water as base fluid: (a) pool boiling heat transfer of γ-alumina/water nanofluid with and without surfactant in both distilled water and treated water as base fluids, (b) pool boiling heat transfer of silica/water nanofluid with two different nanoparticle sizes, and (c) pool boiling heat transfer of zinc oxide/water nanofluid with surfactant. In all the above experiments, the effect of heater surface on boiling heat transfer coefficient has been investigated by repeating the experiment using pure water on the coated surface before cleaning it. Moreover, two effective thermophysical properties of fluids, dynamic viscosity and surface tension, are measured to explain the boiling behavior of the nanofluids. The experimental results indicate that (a) the presence of γ-alumina in the base fluid enhances the pool boiling heat transfer coefficient, but sodium dodecyl sulphate (SDS) as surfactant deteriorates the performance of pool boiling heat transfer of γ-alumina/water nanofluid and (b) silica nanoparticles reduce the boiling performance of pure water. Moreover, the larger particle size of silica nanoparticles shows less reduction in heat transfer coefficient, (c) zinc oxide/water nanofluid is the best coolant among all the above-mentioned nanoparticles for heat removal.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;138(3):031503-031503-10. doi:10.1115/1.4031884.

There is a clear need for cooling high heat flux generating electronic devices using a dielectric fluid without using a pump. This paper explores the feasibility of employing ethanol as a dielectric fluid in a horizontal, open microchannel heat sink configuration with a tapered gap manifold to yield very low pressure head requirements. The paper presents experimental results for such a system utilizing ethanol as a working fluid under gravity-driven flow. A heat flux of 217 W/cm2 was dissipated with a pressure drop of only 9 kPa. The paper further presents parametric trends regarding flow rate and pressure drop characteristics that provide basic insight into designing high heat flux systems under a given gravity head requirement. Based on the results, interrelationships and design guidelines are developed for the taper, ethanol flow rate and imposed heat flux on heat transfer coefficient and gravity head requirement for electronics cooling. Reducing flow instability, reducing pressure drop, and enhancing heat transfer performance for a dielectric fluid will enable the development of pumpless cooling solutions in a variety of electronics cooling applications.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2015;138(3):031504-031504-7. doi:10.1115/1.4031929.

In a heat pipe, operating fluid saturates wick structures system and establishes a capillary-driven circulation loop for heat transfer. Thus, the thermophysical properties of the operating fluid inevitably impact the transitions of phase-change mode and the capability of heat transfer, which determine both the design and development of the associated heat pipe systems. This article investigates the effect of liquid properties on phase-change heat transfer. Two different copper wick structures, cubic and cylindrical in cross section, 340 μm in height and 150 μm in diameter or width, are fabricated using an electroplating technique. The phase-change phenomena inside these wick structures are observed at various heat fluxes. The corresponding heat transfer characteristics are measured for three different working liquids: water, ethanol, and Novec 7200. Three distinct modes of the phase-change process are identified: (1) evaporation on liquid–vapor interface, (2) nucleate boiling with interfacial evaporation, and (3) boiling enhanced interface evaporation. Transitions between the three modes depend on heat flux and liquid properties. In addition to the mode transition, liquid properties also dictate the maximum heat flux and the heat transfer coefficient. A quantitative characterization shows that the maximum heat flux scales with Merit number, a dimensionless number connecting liquid density, surface tension, latent heat of vaporization, and viscosity. The heat transfer coefficient, on the other hand, is dictated by the thermal conductivity of the liquid. A complex interaction between the mode transition and liquid properties is reflected in Novec 7200. In spite of having the lowest thermal conductivity among the three liquids, an early transition to the mode of the boiling enhanced interface evaporation leads to a higher heat transfer coefficient at low heat flux.

Commentary by Dr. Valentin Fuster

Research Papers: Porous Media

J. Heat Transfer. 2015;138(3):032601-032601-10. doi:10.1115/1.4031868.

Fully developed and stationary forced convection in a plane-parallel porous channel is analyzed. The boundary walls are modeled as impermeable and subject to external heat transfer. Different Biot numbers are defined at the two boundary planes. It is shown that the combined effects of temperature-dependent viscosity and viscous heating may induce flow instability. The instability takes place at the lowest parametric singularity of the basic flow solution. The linear stability analysis is carried out analytically for the longitudinal modes and numerically for general oblique modes. It is shown that longitudinal modes with vanishingly small wave number are selected at the onset of instability.

Commentary by Dr. Valentin Fuster

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