Showing papers in "Nanoscale and Microscale Thermophysical Engineering in 2014"
TL;DR: A review of recent developments in the area of surface-enhanced dropwise condensation can be found in this paper, where the development of fabrication techniques to create surface structures at the micro- and nanoscale using both bottom-up and top-down approaches has led to increased study of complex interfacial phenomena.
Abstract: In this review we cover recent developments in the area of surface-enhanced dropwise condensation against the background of earlier work. The development of fabrication techniques to create surface structures at the micro- and nanoscale using both bottom-up and top-down approaches has led to increased study of complex interfacial phenomena. In the heat transfer community, researchers have been extensively exploring the use of advanced surface structuring techniques to enhance phase-change heat transfer processes. In particular, the field of vapor-to-liquid condensation and especially that of water condensation has experienced a renaissance due to the promise of further optimizing this process at the micro- and nanoscale by exploiting advances in surface engineering developed over the last several decades.
215 citations
TL;DR: The 2013 International Workshop on Micro and Nanostructures for Phase-Change Heat Transfer brought together a group of experts to review the current state-of-the-art and discuss future research needs as discussed by the authors.
Abstract: Evaporation from thin films is a key feature of many processes, including energy conversion, microelectronics cooling, boiling, perspiration, and self-assembly operations. The phase change occurring in these systems is governed by transport processes at the contact line where liquid, vapor, and solid meet. Evidence suggests that altering the surface chemistry and surface topography on the micro- and the nanoscales can be used to dramatically enhance vaporization. The 2013 International Workshop on Micro- and Nanostructures for Phase-Change Heat Transfer brought together a group of experts to review the current state-of-the-art and discuss future research needs. This article is focused on the thin-film evaporation panel discussion and outlines some of the key principles and conclusions reached by that panel and the workshop attendees.
162 citations
TL;DR: In this paper, the mode-wise phonon contribution to the intrinsic thermal conductivity (κ) of graphene and the effects of extrinsic mechanisms (such as substrate, isotope, impurities, and defects) on κ are discussed.
Abstract: This article reviews recent numerical studies of thermal transport in graphene, with a focus on molecular dynamics simulation, the atomistic Green’s function method, and the phonon Boltzmann transport equation method. The mode-wise phonon contribution to the intrinsic thermal conductivity (κ) of graphene and the effects of extrinsic mechanisms—for example, substrate, isotope, impurities, and defects—on κ are discussed. We also highlight the insights from numerical studies aimed at bridging the gaps between 1D, 2D, and 3D thermal transport in carbon nanotubes/graphene nanoribbons, graphene, and graphite. Numerical studies on thermal transport across the interface between graphene and other materials and nonlinear thermal transport phenomena such as thermal rectification and negative differential thermal resistance are also reviewed.
59 citations
TL;DR: A review of the current state-of-the-art in boiling augmentation can be found in this article, where a brief historical perspective and primary findings are presented in this article.
Abstract: Advances in the development of micro- and nanostructured surfaces have enabled tremendous progress in delineation of mechanisms in boiling heat transfer and have propelled the rapid enhancement of heat transfer rates. This area of research is poised to make great strides toward tailoring surface features to produce dramatically improved thermal performance. A workshop was held in April 2013 to provide a review of the current state-of-the-art and to develop near-term and long-term goals for the boiling augmentation community. A brief historical perspective and primary findings are presented in this article. Though impressive gains have been made in enhancement of boiling heat transport, there still remain several unknowns such as the mechanisms that affect critical heat flux and optimization of surfaces for boiling heat transport. The promise of improved spatial resolution of optical techniques should improve knowledge of near-surface mechanisms. Standardization of experimental test sections and procedures...
59 citations
TL;DR: In this paper, the most prominent materials, fabrication methods, and manufacturing schemes for micro-and nanostructured surfaces that can be employed to enhance phase-change heat transfer phenomena are described.
Abstract: This article describes the most prominent materials, fabrication methods, and manufacturing schemes for micro- and nanostructured surfaces that can be employed to enhance phase-change heat transfer phenomena. The numerous processes include traditional microfabrication techniques such as thin-film deposition, lithography, and etching, as well as template-assisted and template-free nanofabrication techniques. The creation of complex, hierarchical, and heterogeneous surface structures using advanced techniques is also reviewed. Additionally, research needs in the field and future directions necessary to translate these approaches from the laboratory to high-performance applications are identified. Particular focus is placed on the extension of these techniques to the design of micro/nanostructures for increased performance, manufacturability, and reliability. The current research needs and goals are detailed, and potential pathways forward are suggested.
55 citations
TL;DR: In this article, a micro-Raman-based optical method was used to measure in-plane thermal conductivity of ultrathin films with the use of 20-nm-thick SiO2 substrates.
Abstract: We report a micro-Raman-based optical method to measure in-plane thermal conductivity of ultrathin films. With the use of 20-nm-thick SiO2 substrates that assure in-plane heat transfer, sub-100-nm Bi films and Al2O3 films as thin as 5 nm were successfully measured. The results of Bi films reveal that phonon boundary scattering, both at the surface/interface and at the grain boundaries, reduces in-plane lattice thermal conductivity. The measurements of amorphous Al2O3 films were accomplished using thin Bi film as a Raman temperature sensor, and the results agree with the minimum thermal conductivity models for dielectrics. Our work demonstrates that the micro-Raman method is promising for characterization of in-plane thermal conductivity and phonon behaviors of thin-film structures if the Raman temperature sensor material and substrate material are carefully selected.
30 citations
TL;DR: In this article, the effects of physical properties, concentration, Reynolds number, and geometrical configuration on the process of impingement cooling with nanofluids are investigated.
Abstract: Nanofluids exhibit interesting heat transfer enhancement capabilities that may render them good candidates for use in industrial applications. They can be used as a cooling medium to spray onto the bottom of a piston to reduce the overall heat load during the operation of an engine. However, the flow and temperature fields in an impinging jet of nanofluid are rather complex in the multiphase turbulent state. In order to accurately describe the flow and heat transfer processes in an impinging jet, the current article explores several different turbulence models and wall functions, discusses the velocity and temperature fields predicted by the single-phase model and various multiphase models, and analyzes the influence of nanoparticles on the base fluid. On this basis, the effects of physical properties, concentration, Reynolds number, and geometrical configuration on the process of impingement cooling with nanofluids are investigated. The results may provide a reliable reference for the transient spray coo...
26 citations
TL;DR: In this paper, two sets of open-loop pulsating heat pipes (OLPHPs) for two different magnetic nanofluids (with and without surfactant) were fabricated and their thermal performance was experimentally investigated.
Abstract: The present research proposes an effective method to enhance the heat transport capability of conventional electronic coolers and improve their thermal management. Pulsating heat pipes (PHPs) are outstanding heat transfer devices in the field of electronic cooling. In the present study, two sets of open-loop pulsating heat pipes (OLPHPs) for two different magnetic nanofluids (with and without surfactant) were fabricated and their thermal performance was experimentally investigated. Effects of working fluid (water and two types of magnetic nanofluids), heating power, charging ratio, nanofluid concentration, inclination angle, application of a magnetic field, and magnet location are described. Experimental results showed that magnetic nanofluids can improve the thermal performance of OLPHPs. Application of a magnetic field reduces the thermal resistance of OLPHPs charged with magnetic nanofluids. The optimum charging ratio and nanofluid concentration are reported. The best thermal performance of OLPHPs was ...
23 citations
TL;DR: In this paper, the authors present the first molecular dynamics simulations of classical thermosize effects for realistic molecular conditions and flows, which can be generated by applying a temperature gradient within the different-sized domains, and in this case the system investigated is a combination of a microchannel and a nanochannel.
Abstract: We present the first molecular dynamics simulations of classical thermosize effects for realistic molecular conditions and flows. The classical thermosize effect is the chemical potential difference induced between two different-sized channels that have different fluid transport processes. It can be generated by applying a temperature gradient within the different-sized domains, and in this article the system investigated is a combination of a microchannel and a nanochannel. Our molecular dynamics results are compared with a theoretical calculation of the induced chemical potential difference, and this yields useful new insight into diffusive transport in nonequilibrium gas flows.
13 citations
TL;DR: In this article, a numerical study on nanofluids turbulent flow in a horizontal tube was conducted to explore the mechanism of heat transfer enhancement in nano-fluids from the microflow aspect.
Abstract: A numerical study on nanofluids turbulent flow in a horizontal tube was conducted. The turbulent flow characteristics of nanofluids is revealed by investigating any differences between three models (Eulerian-Eulerian, Euler-Lagrange, and large eddy simulation (LES)-Lagrange) to explore the mechanism of heat transfer enhancement in nanofluids from the microflow aspect. The results indicated that the velocity slip between phases, growth of fluctuating velocity, increase of small-scale vortexes, and migration effect of nanoparticles may impose an important influence on heat transfer behavior of nanofluids. Therefore, the variation in fluid flow characteristics becomes the determining factor of pressure loss increment and heat transfer enhancement in nanofluids.
13 citations
TL;DR: In this article, a theoretical study that extends a classical method of treating the convection heat transfer in complex geometries to gaseous slip flow forced convection in microchannels with H1 thermal boundary condition is presented.
Abstract: This is a theoretical study that extends a classical method of treating the convection heat transfer in complex geometries to gaseous slip flow forced convection in microchannels with H1 thermal boundary condition. Through this line, the momentum and energy equations in cylindrical coordinates are made dimensionless. Afterward, solutions are presented that exactly satisfy the dimensionless differential equations along with the symmetry condition and finiteness of the flow parameter at the origin. The first-order slip boundary conditions are then applied to the solution utilizing the least squares matching method. Though the method is general enough to be applied to almost any arbitrary cross section, four different cross sections of polygonal, trapezoidal, rhombic, and elliptic shape are considered for presentation. The accuracy of the method is proven by comparing the results against available literature data. Finally, after a complete discussion of the results, the tabulated data of Nusselt number are p...
TL;DR: A survey of techniques for high-resolution measurements of temperature and heat flux at the solid surface and in the working fluid can be found in this article, where the authors discuss recent advances in experimental methods for characterizing phase change heat transfer.
Abstract: In this opinion piece, we discuss recent advances in experimental methods for characterizing phase change heat transfer. We begin with a survey of techniques for high-resolution measurements of temperature and heat flux at the solid surface and in the working fluid. Next, we focus on diagnostic tools for boiling heat transfer and describe techniques for visualizing the temperature and velocity fields, as well as measurements at the single bubble level. Finally, we discuss techniques to probe the kinetics of vapor formation within a few molecular layers of the interface. We conclude with our outlook for future progress in experimental methods for phase change heat transfer.
TL;DR: In this paper, the authors carried out two-dimensional numerical simulations of rarefied gas flow in convergent-divergent micronozzles and evaluated the impact of surface smoothness on the flowfield structure by including discontinuities on the divergent contour.
Abstract: This work carries out two-dimensional numerical simulations of rarefied gas flow in convergent–divergent micronozzles. Such a device is considered part of a micronozzle array. Although previous studies have investigated micronozzles with continuous divergent surfaces, the present work evaluates the impact of the surface smoothness on the flowfield structure by including discontinuities on the divergent contour. Such a scenario was not previously investigated. As a secondary goal, detailed physical explanations are given for those flow features reported but not extensively discussed in previous micronozzle studies. A direct simulation Monte Carlo method is employed to describe these flows due to the moderate rarefaction degree typically observed in microdevices. The results pointed out the sensitivity of the macroscopic properties—velocity, pressure, and temperature—due to the geometric variations on the divergent surface. In addition, Knudsen number distributions are computed to map nonequilibrium flow re...
TL;DR: In this article, a subcooled flow boiling is investigated over nanostructured plates at flow rates ranging from 69 mL/min to 145 mL/m. The nanorod arrays are deposited utilizing glancing angle deposition (GLAD) technique on the copper thin film (˜50 nm thick) coated on silicon wafer substrates.
Abstract: In this study, subcooled flow boiling is investigated over nanostructured plates at flow rates ranging from 69 mL/min to 145 mL/min. The first configuration of the nanostructured plate includes ˜600-nm-long, closely packed copper nanorod arrays distributed randomly upon the surface with an average nanorod diameter of ˜150 nm, and the second configuration consists of a periodic structure having ˜600-nm-long copper (Cu) nanorods with an average nanorod diameter of ˜550 nm and a center-to-center nanorod separation of ˜1 μm. The nanorod arrays are deposited utilizing glancing angle deposition (GLAD) technique on the copper thin film (˜50 nm thick) coated on silicon wafer substrates. Dimensions of the test section, heat flux values, and flow rates are chosen to ensure that nanostructured plates remain intact along with their nanorods in their original shape and position, so that the nanostructured plates could be used for many experiments. A consistent increase (up to 30%) in heat transfer coefficients is obse...
TL;DR: In this article, an analytical solution in forced convection of laminar fully developed flow and heat transfer of Al2O3-water nanofluid in a slit microchannel with constant wall heat flux in the presence of a viscous dissipation effect is presented.
Abstract: The present study deals with an analytical solution in forced convection of laminar fully developed flow and heat transfer of Al2O3–water nanofluid in a slit microchannel with constant wall heat flux in the presence of a viscous dissipation effect. The novelty in the solution is to include the magneto-hydrodynamic (MHD) effect, consider a slip condition for velocity at the wall, and use a temperature-dependent model for thermal conductivity that has not been applied to such a situation before. Closed-form solutions are obtained based on the Brinkman number, Hartman number, dimensionless slip coefficient, and nanoparticle volume fraction. The results are discussed in terms of dimensionless velocity and temperature distributions and the Nusselt number. It is observed that the Nusselt number diminishes with an increase in the dimensionless slip coefficient and Brinkman number and it increases with an increase in the Hartman number and nanoparticle volume fraction. In addition, it is shown that the classical ...
TL;DR: In this article, the catalytic combustion characteristics of a hydrogen-air mixture inside a microtube were investigated numerically and experimentally with different reaction models and the influence of wall thermal conductivity, inlet velocity, and tube diameter.
Abstract: The catalytic combustion characteristics of a hydrogen–air mixture inside a microtube were investigated numerically and experimentally. Numerical simulations with detailed gas-phase and surface catalytic reaction mechanisms of hydrogen–air combustion were investigated. Combustion characteristics for different reaction models and the influence of wall thermal conductivity, inlet velocity, and tube diameter, on surface catalytic combustion reactions are discussed. The computational results indicate that surface catalytic combustion restrains gas-phase combustion. The higher wall temperature gradient for low wall thermal conductivity promotes gas-phase combustion upstream and results in a higher temperature distribution. The microtube can be divided into two regions. The upstream region is dominated by the surface catalytic reaction and the downstream region is dominated by gas-phase combustion. With increasing inlet velocity, the region dominated by surface catalytic reactions expanded downstream and finall...
TL;DR: In this article, the effect of channel roughness was directly considered by modeling various shapes of wall roughness using second-order slip boundary condition, and a numerical scheme for simulation of flow in a microchannel was developed.
Abstract: A numerical scheme for simulation of flow in a microchannel was developed. The effect of channel roughness was directly considered by modeling various shapes of wall roughness using second-order slip boundary condition. Governing equations of compressible and incompressible flows with different Mach and Knudsen numbers were solved using the SIMPLE algorithm. Studying flows in smooth microchannels, unique slip and centerline velocities were found by applying either first- or second-order slip boundary condition, whereas in rough channels more reliable values were found by applying a second-order model. The same trend was observed for Poiseuille number and pressure gradient especially at higher Mach and Knudsen numbers.
TL;DR: In this paper, a linear array of finite-length nanoposts attached at the bottom wall of square microchannels under viscous flow conditions are investigated with lattice Boltzmann simulations.
Abstract: In this article, hydraulic responses to a linear array of finite-length nanoposts attached at the bottom wall of square microchannels under viscous flow conditions are investigated with lattice Boltzmann simulations. Different configurations of the array are considered because these changes can directly contribute to flow pattern deformation. Simulation results indicated that the flow structure strongly depends on nanopost height and space between two adjacent nanoposts in the nanopost line but not on the Reynolds number in the range examined. However, if nanoposts are grown far apart from each other, a fully developed velocity profile can be recovered at sufficiently long distance downstream and an empirical correlation for calculating the recovery length is proposed. In the studied low Reynolds number regime, energy loss due to friction drag was found to be inversely proportional to an appropriately defined Reynolds number that accounts for both the channel size and the nanopost aspect ratio.