Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2013"

Modeling and Optimization of Superhydrophobic Condensation

Abstract: Superhydrophobic micro/nanostructured surfaces for dropwise condensation have recently received significant attention due to their potential to enhance heat transfer performance by shedding water droplets via coalescence-induced droplet jumping at length scales below the capillary length. However, achieving optimal surface designs for such behavior requires capturing the details of transport processes that is currently lacking. While comprehensive models have been developed for flat hydrophobic surfaces, they cannot be directly applied for condensation on micro/nanostructured surfaces due to the dynamic droplet-structure interactions. In this work, we developed a unified model for dropwise condensation on superhydrophobic structured surfaces by incorporating individual droplet heat transfer, size distribution, and wetting morphology. Two droplet size distributions were developed, which are valid for droplets undergoing coalescence-induced droplet jumping, and exhibiting either a constant or variable contact angle droplet growth. Distinct emergent droplet wetting morphologies, Cassie jumping, Cassie nonjumping, or Wenzel, were determined by coupling of the structure geometry with the nucleation density and considering local energy barriers to wetting. The model results suggest a specific range of geometries (0.5–2 μm) allowing for the formation of coalescence-induced jumping droplets with a 190% overall surface heat flux enhancement over conventional flat dropwise condensing surfaces. Subsequently, the effects of four typical self-assembled monolayer promoter coatings on overall heat flux were investigated. Surfaces exhibiting coalescence-induced droplet jumping were not sensitive ( 2 μm). This work provides a unified model for dropwise condensation on micro/nanostructured superhydrophobic surfaces and offers guidelines for the design of structured surfaces to maximize heat transfer. Keywords: superhydrophobic condensation, jumping droplets, droplet coalescence, condensation optimization, environmental scanning electron microscopy; micro/nanoscale water condensation, condensation heat transfer.

Heat Transfer in Microchannels—2012 Status and Research Needs

Abstract: Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Heat Transfer in Thermoelectric Materials and Devices

Abstract: Solid-state thermoelectric devices are currently used in applications ranging from thermocouple sensors to power generators in space missions, to portable air-conditioners and refrigerators. With the ever-rising demand throughout the world for energy consumption and CO2 reduction, thermoelectric energy conversion has been receiving intensified attention as a potential candidate for waste-heat harvesting as well as for power generation from renewable sources. Efficient thermoelectric energy conversion critically depends on the performance of thermoelectric materials and devices. In this review, we discuss heat transfer in thermoelectric materials and devices, especially phonon engineering to reduce the lattice thermal conductivity of thermoelectric materials, which requires a fundamental understanding of nanoscale heat conduction physics.

Enhanced Specific Heat Capacity of Nanomaterials Synthesized by Dispersing Silica Nanoparticles in Eutectic Mixtures

Abstract: Anomalous enhancements in the specific heat capacity values of nanomaterials were measured in this study. Silica nanoparticles (∼2―20 nm) were dispersed into eutectic of lithium carbonate and potassium carbonate (62:38 by molar ratio) at 1.5% mass concentration. The specific heat capacity measurements were performed using a differential scanning calorimeter (DSC). The specific heat capacity of the silica nanocomposite (solid phase) was enhanced by 38―54% and the specific heat of the silica nanofluid (liquid phase) was enhanced by 118―124% over that of the pure eutectic. Electron microscopy of the samples shows that the nanoparticles induce phase change (forms a higher density "compressed phase") within the solvent material. Hence, a new model is proposed to account for the contribution of the compressed phase to the total specific heat capacity of the nanomaterials. The proposed model is found to be in good agreement with the experimental data. These results have wide ranging implications, such as for the development of efficient thermal storage systems that can enable significant reduction in the cost of solar thermal power.

Enhanced Flow Boiling Over Open Microchannels With Uniform and Tapered Gap Manifolds

Abstract: Flow boiling in microchannels has been extensively studied in the past decade. Instabilities, low critical heat flux (CHF) values, and low heat transfer coefficients have been identified as the major shortcomings preventing its implementation in practical high heat flux removal systems. A novel open microchannel design with uniform and tapered manifolds (OMM) is presented to provide stable and highly enhanced heat transfer performance. The effects of the gap height and flow rate on the heat transfer performance have been experimentally studied with water. The critical heat fluxes (CHFs) and heat transfer coefficients obtained with the OMM are significantly higher than the values reported by previous researchers for flow boiling with water in microchannels. A record heat flux of 506W/cm 2 with a wall superheat of 26.2 � C was obtained for a gap size of 0.127mm. The CHF was not reached due to heater power limitation in the current design. A maximum effective heat transfer coefficient of 290,000W/m 2 � C was obtained at an intermediate heat flux of 319W/cm 2 with a gap of 0.254mm at 225mL/min. The flow boiling heat transfer was found to be insensitive to flow rates between 40‐333mL/min and gap sizes between 0.127‐1.016mm, indicating the dominance of nucleate boiling. The OMM geometry is promising to provide exceptional performance that is particularly attractive in meeting the challenges of high heat flux removal in electronics cooling applications. [DOI: 10.1115/1.4023574]

From the Casimir Limit to Phononic Crystals: 20 Years of Phonon Transport Studies Using Silicon-on-Insulator Technology

Abstract: Silicon-on-insulator (SOI) technology has sparked advances in semiconductor and MEMs manufacturing and revolutionized our ability to study phonon transport phenomena by providing single-crystal silicon layers with thickness down to a few tens of nanometers. These nearly perfect crystalline silicon layers are an ideal platform for studying ballistic phonon transport and the coupling of boundary scattering with other mechanisms, including impurities and periodic pores. Early studies showed clear evidence of the size effect on thermal conduction due to phonon boundary scattering in films down to 20 nm thick and provided the first compelling room temperature evidence for the Casimir limit at room temperature. More recent studies on ultrathin films and periodically porous thin films are exploring the possibility of phonon dispersion modifications in confined geometries and porous films. [DOI: 10.1115/1.4023577]

On the Anomalous Convective Heat Transfer Enhancement in Nanofluids: A Theoretical Answer to the Nanofluids Controversy

Abstract: A theoretical answer to the controversial issue on the anomalous convective heat transfer in nanofluids has been provided, exploiting the Buongiorno model for convective heat transfer in nanofluids with modifications to fully account for the effects of nanoparticle volume fraction distributions on the continuity, momentum, and energy equations. A set of exact solutions have been obtained for hydrodynamically and thermally fully developed laminar nanofluid flows in channels and tubes, subject to constant heat flux. From the solutions, it has been concluded that the anomalous heat transfer rate, exceeding the rate expected from the increase in thermal conductivity, is possible in such cases as titania–water nanofluids in a channel, alumina–water nanofluids in a tube and also titania–water nanofluids in a tube. Moreover, the maximum Nusselt number based on the bulk mean nanofluid thermal conductivity is captured when the ratio of Brownian and thermophoretic diffusivities is around 0.5, which can be exploited for designing nanoparticles for high-energy carriers.

Molecular Dynamics Simulation of Normal and Explosive Boiling on Nanostructured Surface

Abstract: Molecular Dynamics (MD) simulation is carried out to investigate the normal and explosive boiling of thin film adsorbed on a metal substrate whose surface is structured by an array of nanoscale spherical particles. The molecular system is comprised of the liquid and vapor argon as well as a copper wall. The nanostructures have spherical shape with uniform diameters while the thickness of liquid film is constant. The effects of transvers and longitudinal distances as well as the diameter of nanoparticles are analyzed. The simulation is started from an initial configuration for three phases (liquid argon, vapor argon and solid wall); after equilibrating the system at 90 K, the wall is heated suddenly to a higher temperature that is well beyond the critical temperature of argon. Two different superheat degrees are selected: a moderately high temperature of 170 K for normal evaporation and much higher temperature 290 K for explosive boiling. By monitoring the space and time dependences of temperature and density as well as net evaporation rate, the normal and explosive boiling process on a flat surface with and without nanostructures are investigated. The results show that the nanostructure has significant effect on evaporation/boiling of thin film. The degrees of superheat and size of nanoparticles have significant effects on the trajectories of particles and net evaporation rate. For the cases with nanostructure, liquid responds very quickly and the number of evaporation molecules increases with increasing the size of particles from 1 to 2 nm while it decreases for d = 3 nm.

The Effectiveness of Secondary Flow Produced by Vortex Generators Mounted on Both Surfaces of the Fin to Enhance Heat Transfer in a Flat Tube Bank Fin Heat Exchanger

Abstract: Secondary flow is the flow in the cross section normal to the main flow. It plays an important role on the enhanced heat transfer and in the applications in other fields. Secondary flow can greatly enhance the convective heat transfer. In order to find the effectiveness of secondary flow for heat transfer enhancement, a nondimensional parameter, Se, based on the absolute vorticity flux is reported to specify the intensity of secondary flow. Its physical meaning is the ratio of inertial force to viscous force induced by secondary flow. As an example, the effectiveness of secondary flow was numerically studied for a flat tube bank fin heat exchanger with vortex generators (VGs) mounted on both surfaces of the fin. The contributions of VGs are investigated for the enhancements of secondary flow intensity, convective heat transfer, and pressure drop. The method is demonstrated using Se to find out the optimum configurations of VGs. The results reveal that close relationships exist not only between the span-average nondimensional intensity of secondary flow and the span-average Nusselt number but also between the volume average nondimensional intensity of secondary flow and the overall average Nusselt number. For the configuration studied, a ratio of Nusselt number enhancement to the friction factor enhancement increases with increasing the enhancement of secondary flow intensity. As a supplement to traditional criteria on a good performance heat transfer surface, the nondimensional intensity of secondary flow can be used clearly for an optimum value of VG parameter. [DOI: 10.1115/1.4023037]

The Treatment of Nongray Properties in Radiative Heat Transfer: From Past to Present

Abstract: Radiative heat transfer in high-temperature participating media displays very strong spectral, or “nongray,” behavior, which is both very difficult to characterize and to evaluate. This has led to very gradual development of nongray models, starting with primitive semigray and box models based on old experimental property data, to today's state-of-the-art k-distribution approaches with properties obtained from high-resolution spectroscopic databases. In this paper a brief review of the historical development of nongray models and property databases is given, culminating with a more detailed description of the most modern spectral tools.

Enhanced Pool Boiling With Ethanol at Subatmospheric Pressures for Electronics Cooling

Abstract: The growing trend in miniaturization of electronics has generated a need for efficient thermal management of these devices. Boiling has the ability to dissipate a high heat flux while maintaining a small temperature difference. A vapor chamber with pool boiling offers an effective way to provide cooling and to maintain temperature uniformity. The objective of the current work is to investigate pool boiling performance of ethanol on enhanced microchannel surfaces. Ethanol is an attractive working fluid due to its better heat transfer performance and higher heat of vaporization compared to refrigerants, and lower normal boiling point compared to water. The saturation temperature of ethanol can be further reduced to temperatures suitable for electronics cooling by lowering the pressure. Experiments were performed at four different absolute pressures, 101.3 kPa, 66.7 kPa, 33.3 kPa, and 16.7 kPa using different microchannel surface configurations. Heat dissipation in excess of 900 kW/m was obtained while maintaining the wall surface below 85 C at 33 kPa. Flammability, toxicity, and temperature overshoot issues need to be addressed before practical implementation of ethanol-based cooling systems can occur. [DOI: 10.1115/1.4024595]

Numerical Simulation of the Effect of the Size of Suspensions on the Solidification Process of Nanoparticle-Enhanced Phase Change Materials

Abstract: Nanostructure-enhanced phase change materials (NePCM) have been widely studied in recent years due to their enhanced thermal conductivity and improved charge/discharge in thermal energy storage applications. In this study, the effect of the size of the nanoparticles on the morphology of the solid–liquid interface and the evolving concentration field during solidification is reported. Combining a one-fluid-mixture approach with the single-domain enthalpy-porosity model for phase change and assuming a linear dependence of the liquidus and solidus temperatures of the mushy zone on the local concentration of the nanoparticles subject to a constant value of the segregation coefficient, thermal-solutal convection as well as the Brownian and thermophoretic effects are taken into account. A square cavity containing a suspension of copper nanoparticles (diameter of 5 and 2 nm) in water was the model NePCM considered. Subject to a 5 � C temperature difference between the hot (top) and cold (bottom) sides and with an initial loading of the nanoparticles equal to 10 wt. % (1.22 vol. %), the colloid was solidified from the bottom. The solid–liquid interface for the case of NePCM with 5 nm particle size was almost planar throughout the solidification process. However, for the case of the NePCM with particle size of 2 nm, the solid–liquid interface evolved from a stable planar shape to an unstable dendritic structure. This transition was attributed to the constitutional supercooling effect, whereby the rejected particles that are pushed away from the interface into the liquid zone form regions of high concentration thus leading to a lower solidus temperature. Moreover, for the smaller particle size of 2 nm, the ensuing solutal convection at the liquid–solid interface due to the concentration gradient is affected by the increased Brownian diffusivity. Due to size-dependent rejection of nanoparticles, the frozen layer that resulted from a dendritic growth contains regions of depleted concentration. Despite the higher thermal conductivity of the colloids, the amount of frozen phase during a fixed time period diminished as the particle size decreased. [DOI: 10.1115/1.4023542]

A fast hybrid fourier-boltzmann transport equation solver for nongray phonon transport

Abstract: Nongray phonon transport solvers based on the Boltzmann transport equation (BTE) are being increasingly employed to simulate submicron thermal transport in semiconductors and dielectrics. Typical sequential solution schemes encounter numerical difficulties because of the large spread in scattering rates. For frequency bands with very low Knudsen numbers, strong coupling between other BTE bands result in slow convergence of sequential solution procedures. This is due to the explicit treatment of the scattering kernel. In this paper, we present a hybrid BTE-Fourier model which addresses this issue. By establishing a phonon group cutoff Knc, phonon bands with low Knudsen numbers are solved using a modified Fourier equation which includes a scattering term as well as corrections to account for boundary temperature slip. Phonon bands with high Knudsen numbers are solved using the BTE. A low-memory iterative solution procedure employing a block-coupled solution of the modified Fourier equations and a sequential solution of BTEs is developed. The hybrid solver is shown to produce solutions well within 1% of an all-BTE solver (using Knc = 0.1), but with far less computational effort. Speedup factors between 2 and 200 are obtained for a range of steady-state heat transfer problems. The hybrid solver enables efficient and accurate simulation of thermal transport in semiconductors and dielectrics across the range of length scales from submicron to the macroscale.

Measurement of Coherent Thermal Emission Due to Magnetic Polaritons in Subwavelength Microstructures

Abstract: Spectral and directional control of thermal emission is critically important for applications such as space cooling and energy harvesting. The effect of magnetic polaritons (MPs) on spectral modulation has been analyzed in metallic grating structures with a dielectric spacer on a metallic film. It has been predicted that the spectral emission peaks exhibit omnidirectional characteristics when MPs are excited. The present work provides an experimental demonstration of coherent thermal emission from several microfabricated grating structures in the infrared region from room temperature to elevated temperatures. The emittance at elevated temperatures is directly measured using an emissometer, while the room-temperature emittance is indirectly obtained from the reflectance measurement. The rigorous coupled-wave analysis and an LC-circuit model are employed to elucidate the mechanisms of various resonant modes and their coupling effect, taking into consideration the temperature-dependent electron scattering rate of the metals. [DOI: 10.1115/1.4024469]

Mode-Wise Thermal Conductivity of Bismuth Telluride

Abstract: Thermal properties and transport control are important for many applications, for example, low thermal conductivity is desirable for thermoelectrics. Knowledge of modewise phonon properties is crucial to identify dominant phonon modes for thermal transport and to design effective phonon barriers for thermal transport control. In this paper, we adopt time-domain (TD) and frequency-domain (FD) normal-mode analyses to investigate mode-wise phonon properties and to calculate phonon dispersion relations and phonon relaxation times in bismuth telluride. Our simulation results agree with the previously reported data obtained from ultrafast time-resolved measurements. By combining frequency-dependent anharmonic phonon group velocities and lifetimes, mode-wise thermal conductivities are predicted to reveal the contributions of heat carriers with different wavelengths and polarizations. [DOI: 10.1115/1.4024356]

Magnetohydrodynamics Thermocapillary Marangoni Convection Heat Transfer of Power-Law Fluids Driven by Temperature Gradient

Abstract: This paper presents an investigation for magnetohydrodynamics (MHD) thermocapillary Marangoni convection heat transfer of an electrically conducting power-law fluid driven by temperature gradient. The surface tension is assumed to vary linearly with temperature and the effects of power-law viscosity on temperature fields are taken into account by modified Fourier law for power-law fluids (proposed by Pop). The governing partial differential equations are converted into ordinary differential equations and numerical solutions are presented. The effects of the Hartmann number, the power-law index and the Marangoni number on the velocity and temperature fields are discussed and analyzed in detail.

Thermal Processing of Biological Tissue at High Temperatures: Impact of Protein Denaturation and Water Loss on the Thermal Properties of Human and Porcine Liver in the Range 25–80 °C

Abstract: Biothermal engineering applications impose thermal excursions on tissues with an ensuing biological outcome (i.e., life or death) that is tied to the molecular state of water and protein in the system. The accuracy of heat transfer models used to predict these important processes in turn depends on the kinetics and energy absorption of molecular transitions for both water and protein and the underlying temperature dependence of the tissue thermal properties. Unfortunately, a lack of tissue thermal property data in the literature results in an overreliance on property estimates. This work addresses these thermal property limitations in liver, a platform tissue upon which hyperthermic engineering applications are routinely performed and a test bed that will allow extension to further tissue property measurement in the future. Specifically, we report on the thermal properties of cadaveric human and porcine liver in the suprazero range between 25 °C to 80 °C for thermal conductivity and 25 °C to 85 °C for apparent specific heat. Denaturation and water vaporization are shown to reduce thermal conductivity and apparent specific heat within the samples by up to 20% during heating. These changes in thermal properties significantly altered thermal histories during heating compared to conditions when properties were assumed to remain constant. These differences are expected to alter the biological outcome from heating as well.

A Model of Transient Heat and Mass Transfer in a Heterogeneous Medium of Ceria Undergoing Nonstoichiometric Reduction

Abstract: The redox chemistry of nonstoichiometric metal oxides can be used to produce chemical fuels by harnessing concentrated solar energy to split water and/or carbon dioxide. In such a process, it is desirable to use a porous reactive substrate for increased surface area and improved gas transport. The present study develops a macroscopic-scale model of porous ceria undergoing thermal reduction. The model captures the coupled interactions between the heat and mass transfer and the heterogeneous chemistry using a local thermal nonequilibrium (LTNE) formulation of the volume-averaged conservation of mass and energy equations in an axisymmetric cylindrical domain. The results of a representative test case simulation demonstrate strong coupling between gas phase mass transfer and the chemical kinetics as well as the pronounced impact of optical thickness on the temperature distribution and thus global solar-to-chemical energy conversion.