Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2015"

A Note on the Correspondence Between Certain Nanofluid Flows and Standard Fluid Flows

Abstract: This note introduces a rescaling approach that greatly simplifies the evaluation of flow and physical parameters such as skin friction and heat transfer rate in recent single phase nanofluids research for which the nanofluids begin to create a non-Newtonian fluid after 5–6% volumetric concentration of nanoparticles. Much of the task is hence reduced to a particular case for a chosen fluid. By the help of specified transformations, the nanofluid results can be obtained from known properties of regular fluid. Such rescaling is shown to work well for the rotating disk boundary layer flow in nanofluid, and thus sheds light upon the future studies of this kind when further physical mechanism are taken into account.

Constructal Law: Optimization as Design Evolution

Abstract: Here, I review the physics meaning of optimization, knowledge and design evolution, and why these concepts and human activities are profoundly useful for human life. A law of physics is a concise statement that summarizes a phenomenon that occurs in nature. A phenomenon is a fact, circumstance, or experience that is apparent to the human senses and can be described. The design in nature phenomenon facilitates access for everything that flows, evolves, spreads, and is collected: river basins, atmospheric and ocean currents, animal life and migration, and technology (the evolution of the “human-and-machine species,” wealth, life). This phenomenon is summarized by the constructal law: the occurrence and evolution of designs in nature, its time direction. Based on its record, the constructal law accounts for the design phenomenon and also for all the phenomena that have been described individually (ad-hoc) with end-design (destiny) statements of “optimality” (min, max). Most notably, the constructal law accounts for contradictory end-design statements such as minimum entropy production and maximum entropy production, and minimum flow resistance and maximum flow resistance.

Optimization of Pin-Fins for a Heat Exchanger by Entropy Generation Minimization and Constructal Law

Abstract: Pin-fins are considered as one of the best elements for heat transfer enhancement in heat exchangers. In this study, the topology of pin-fins (length, diameter, and shape) is optimized based on the entropy generation minimization (EGM) theory coupled with the constructal law (CL). Such pin-fins are employed in a heat exchanger in a sensible thermal energy storage (TES) system so as to enhance the rate of heat transfer. First, the EGM method is used to obtain the optimal length of pin-fins, and then the CL is applied to get the optimal diameter and shape of pin-fins. Reliable computational fluid dynamics (CFD) simulations of various constructal pin-fin models are performed, and detailed flow and heat transfer characteristics are presented. The results show that by using the proposed system with optimized pin-fin heat exchanger the stored thermal energy can be increased by 10.2%.

Heat Transfer, Pressure Drop, and Entropy Generation in a Solar Collector Using SiO2/Water Nanofluids: Effects of Nanoparticle Size and pH

Abstract: In this paper, an analytical study is carried out on the heat transfer, pressure drop, and entropy generation in a flat-plate solar collector using SiO2/water nanofluid with volume concentration of 1%. In the study, the effects of two different values of pH, i.e., 5.8 and 6.5, and two different sizes of nanoparticles, i.e., 12nm and 16nm, on the entropy generation rate in turbulent flow are investigated. The results are compared with the results obtained for the case of water. The findings show that by using the Brinkman model to calculate the viscosity instead of experimental data one obtains a higher heat transfer coefficient and thermal efficiency than that in the case of water, while, when the experimental data are used, the heat transfer coefficient and thermal efficiency of water are found to be higher than that of nanofluids. The results reveal that using nanofluids increases the outlet temperature and reduces the entropy generation rate. It is also found that for nanofluids containing the particles with a size of 16nm, the increase in pH value would increase the entropy generation rate, while for nanoparticles with a size of 12nm the increase in pH would decrease the entropy generation. [DOI: 10.1115/1.4029870]

Natural Convection in a Wavy Porous Cavity With Sinusoidal Temperature Distributions on Both Side Walls Filled With a Nanofluid: Buongiorno's Mathematical Model

Abstract: Natural convection in a two-dimensional, square porous cavity filled with a nanofluid and with sinusoidal temperature distributions on both side walls and adiabatic conditions on the upper and lower walls is numerically investigated. The flow is assumed to be slow so that advective and Forchheimer quadratic terms are ignored in the momentum equation. The applied sinusoidal temperature is symmetric with respect to the midplane of the enclosure. Numerical calculations are produced for Rayleigh numbers in the range of 10–\(10^{4}\) in comparison with other authors. The present models, in the form of an in-house computational fluid dynamics code, have been validated successfully against the reported results from the open literature. It is found that the results are in very good agreement. Results are presented in the form of streamlines, isotherm contours, and distributions of the average Nusselt number.

Evolution in the Design of V-Shaped Highly Conductive Pathways Embedded in a Heat-Generating Piece

Abstract: This paper presents the evolution of architecture of high conductivity pathways embedded into a heat generating body on the basis of Constructal theory. The main objective is to introduce new geometries for the highly conductive pathways, precisely configurations shaped as V. Four types of V-shaped inserts, evolving from “V1” to “V4,” have been comparatively considered. Geometric optimization of design is conducted to minimize the peak temperature of the heat generating piece. Many ideas emerged from this work: first of all, the numerical results demonstrated that the V-shaped pathways remarkably surpass the performance of some basic configurations already mentioned in literature, i.e., “I and X-shaped” pathways. Furthermore, the evolution of configurations from V1 to V4 resulted in a gradual reduction of the hot spot temperature, according to the principle of “optimal distribution of imperfections” that characterizes the constructal law.

Characterizing Effects of the Shape of Screw Conveyors in Gas–Solid Fluidized Beds Using Advanced Numerical Models

Abstract: A numerical study of the effects of the shape of an enclosed screw conveyor on the mixing and heat transfer in a horizontal gas–solid fluidized bed was conducted using computational fluid dynamics (CFD) A two-fluid model (TFM) was employed to model the gas and solid phases as continua through mass, momentum, and energy conservations The motion of the screw conveyor was simulated by using a rotating reference frame (RRF) such that the computational mesh was free from dynamic reconstruction The diameters of the screw flight and shaft, the pitch, and the blade thickness were varied in the parametric study Under the operating conditions studied, it was found that the increase in the diameter of the screw flight results in the enhancement of the solid mixing and conveyance The increase in the diameters of the screw shaft and the screw blade thickness lead to the enhanced solid mixing but reduced conveyance The variation in the screw pitch gives rise to rather complex behaviors in the solid mixing and conveyance As the screw pitch is decreased, the solid mixing increases initially but then decreases before it increases eventually The solid conveyance capability was found to first increase and then decrease Explanations to the effects of the shape of the screw conveyor were discussed in this work

Enhanced Specific Heat Capacity of Molten Salt-Based Carbon Nanotubes Nanomaterials

Abstract: This study aims to investigate the specific heat capacity of a carbonate salt eutectic-based multiwalled carbon nanomaterial (or high temperature nanofluids). The specific heat capacity of the nanomaterials was measured both in solid and liquid phase using a differential scanning calorimetry (DSC). The effect of the carbon nanotube (CNT) concentrations on the specific heat capacity was examined in this study. The carbonate molten salt eutectic with a high melting point around 490 °C, which consists of lithium carbonate of 62% and potassium carbonate of 38% by the molar ratio, was used as a base material. Multiwalled CNTs were dispersed in the carbonate salt eutectic. A surfactant, sodium dodecyl sulfate (SDS) was utilized to obtain homogeneous dispersion of CNT into the eutectic. Four different concentrations (0.1, 0.5, 1, and 5 wt.%) of CNT were employed to explore the specific heat capacity enhancement of the nanomaterials as the concentrations of the nanotubes varies. In result, it was observed that the specific heat capacity was enhanced by doping with the nanotubes in both solid and liquid phase. Additionally, the enhancements in the specific heat capacity were increased with increase of the CNT concentration. In order to check the uniformity of dispersion of the nanotubes in the salt, scanning electron microscopy (SEM) images were obtained for pre-DSC and post-DSC samples. Finally, the specific heat capacity results measured in present study were compared with the theoretical prediction.

Heat Conduction in Nanostructured Materials Predicted by Phonon Bulk Mean Free Path Distribution

Abstract: We develop a computational framework, based on the Boltzmann transport equation (BTE), with the ability to compute thermal transport in nanostructured materials of any geometry using, as the only input, the bulk cumulative thermal conductivity. The main advantage of our method is twofold. First, while the scattering times and dispersion curves are unknown for most materials, the phonon mean free path (MFP) distribution can be directly obtained by experiments. As a consequence, a wider range of materials can be simulated than with the frequency-dependent (FD) approach. Second, when the MFP distribution is available from theoretical models, our approach allows one to include easily the material dispersion in the calculations without discretizing the phonon frequencies for all polarizations thereby reducing considerably computational effort. Furthermore, after deriving the ballistic and diffusive limits of our model, we develop a multiscale method that couples phonon transport across different scales, enabling efficient simulations of materials with wide phonon MFP distributions length. After validating our model against the FD approach, we apply the method to porous silicon membranes and find good agreement with experiments on mesoscale pores. By enabling the investigation of thermal transport in unexplored nanostructured materials, our method has the potential to advance high-efficiency thermoelectric devices.

Flow Boiling Heat Transfer and Two-Phase Flow Instability of Nanofluids in a Minichannel

Abstract: Single-phase convective heat transfer of nanofluids has been studied extensively, and different degrees of enhancement were observed over the base fluids, whereas there is still debate on the improvement in overall thermal performance when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclusive whether the same pessimistic outlook should be expected. In this work, an experimental study of forced convective flow boiling and two-phase flow was conducted for Al2O3–water nanofluids through a minichannel. General flow boiling heat transfer characteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-phase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFI). It was found that the presence of nanoparticles delays ONB and suppresses OFI, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the changes in available nucleation sites and surface wettability as well as thinning of thermal boundary layers in nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in two-phase flow of nanofluids, but with reduced amplitude in pressure, temperature, and mass flux oscillations. [DOI: 10.1115/1.4029647]

Analysis of Galinstan-Based Microgap Cooling Enhancement Using Structured Surfaces

Abstract: Analyses of microchannel and microgap cooling show that galinstan, a recently developed nontoxic liquid metal that melts at −19 °C, may be more effective than water for direct liquid cooling of electronics. The thermal conductivity of galinstan is nearly 28 times that of water. However, since the volumetric specific heat of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using structured surfaces (SSs) to reduce the overall thermal resistance of galinstan-based microgap cooling in the laminar flow regime. Significantly, the high surface tension of galinstan, i.e., 7 times that of water, implies that it can be stable in the nonwetting Cassie state at the requisite pressure differences for driving flow through microgaps. The flow over the SS encounters a limited liquid–solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance, but accompanying reductions in Nusselt number increase convective resistance. These are accounted for by expressions in the literature for apparent hydrodynamic and thermal slip. We develop a dimensionless expression to evaluate the tradeoff between the pressure stability of the liquid–solid–gas system and hydrodynamic slip. We also consider secondary effects including entrance effects and temperature dependence of thermophysical properties. Results show that the addition of SSs enhances heat transfer.

Effect of Particle Concentration on Shape Deformation and Secondary Atomization Characteristics of a Burning Nanotitania Dispersion Droplet

Abstract: Secondary atomization characteristics of burning bicomponent (ethanol-water) droplets containing titania nanoparticles (NPs) in dilute (0.5% and 1 wt.%) and dense concentrations (5% and 7.5 wt.%) are studied experimentally at atmospheric pressure under normal gravity. It is observed that both types of nanofuel droplets undergo distinct modes of secondary breakup, which are primarily responsible for transporting particles from the droplet domain to the flame zone. For dilute nanosuspensions, disruptive response is characterized by low intensity atomization modes that cause small-scale localized flame distortion. In contrast, the disruption behavior at dense concentrations is governed by high intensity bubble ejections, which result in severe disruption of the flame envelope.

Numerical Simulation of Forced Convective Condensation of Propane in a Spiral Tube

Abstract: A numerical simulation of forced convective condensation of propane in an upright spiral tube is presented. In the numerical simulations, the important models are used: implicit volume of fluid (VOF) multiphase model, Reynolds stress (RS) turbulence model, Lee's phase change model and Ishii's concentration model, and also the gravity and surface tension are taken into account. The mass flux and vapor quality are simulated from 150 to 350 kg·m−2·s−1 and from 0.1 to 0.9, respectively. The numerical results show that in all simulation cases, only the stratified flow, annular flow, and mist flow are observed. The heat transfer coefficient and frictional pressure drop increase with the increase of mass flux and vapor quality for all simulation cases. Under different flow patterns and mass flux, the numerical results of void fraction, heat transfer coefficient, and frictional pressure drop show good agreement with the experimental results and correlations from the existing references.

Exact Analytical Solution for Unsteady Heat Conduction in Fiber-Reinforced Spherical Composites Under the General Boundary Conditions

Abstract: The current study presents an exact analytical solution for unsteady conductive heat transfer in multilayer spherical fiber-reinforced composite laminates. The orthotropic heat conduction equation in spherical coordinate is introduced. The most generalized linear boundary conditions consisting of the conduction, convection, and radiation heat transfer is considered both inside and outside of spherical laminate. The fibers' angle and composite material in each lamina can be changed. Laplace transformation is employed to change the domain of the solutions from time into the frequency. In the frequency domain, the separation of variable method is used and the set of equations related to the coefficients of Fourier–Legendre series is solved. Meromorphic function technique is utilized to determine the complex inverse Laplace transformation. Two functional cases are presented to investigate the capability of current solution for solving the industrial unsteady problems in different arrangements of multilayer spherical laminates.

Experimental Investigation of Wall Nucleation Characteristics in Flow Boiling

Abstract: Wall nucleation experiments have been performed in a vertical annulus test section for investigation of the bubble departure diameter and bubble departure frequency. The experimental data in forced convective subcooled boiling flow is presented as a parametric study of the effect of wall heat flux, local bulk liquid subcooling, liquid flow rate, and system pressure. The data are shown to extend the database currently available in literature to a wider range of system conditions. Along with the current database in forced convective flow, the available models for bubble departure size and frequency are reviewed and compared with the existing database. The prediction of the bubble departure frequency is shown to require accurate modeling of the bubble departure diameter which has poor agreement with the experimental database.

Exact Multiple Solutions for the Slip Flow and Heat Transfer in a Converging Channel

Abstract: A special case of Falkner–Skan flows past stretching boundaries is considered when the momentum and thermal slip boundary conditions are allowed at the boundary. Exact analytical solutions are found for the converging channel (wedge nozzle). The solutions are shown to be unique, double, or triple depending on the slip parameter and wall moving parameter. The provided closed-form analytical solutions are rare class of exact solutions for the Falkner–Skan flow equations. Thresholds of existence of multiple solutions are determined. For each flow solutions, the corresponding energy equation is also exactly solved when the internal heat generated by viscous dissipation can be neglected or numerically integrated when the viscous dissipation is significant. Analytic and numeric values of the rate of heat transfer affected by the presence of a surface temperature jump are also worked out. The possibility of realistic physical solution out of multiple solutions is finally discussed.

Modeling the Optical and Radiative Properties of Vertically Aligned Carbon Nanotubes in the Infrared Region

Abstract: During the past decade, research on carbon nanotubes has revealed potential advances in thermal engineering applications. The present study investigates the radiative absorption and reflection of vertically aligned carbon nanotubes (VACNTs) in the broad spectrum from the near-infrared to far-infrared regions. The optical constants of VACNT are modeled based on the dielectric function of graphite and an effective medium approach that treats the CNT film as a homogenized medium. Calculated radiative properties show characteristics of near-unity index matching and high absorptance up to around 20 lm wavelength. The packing density and degree of alignment are shown to affect the predicted radiative properties. The Brewster angle and penetration depth of VACNTs are examined in the infrared spectrum. The radiative properties for VACNT thin films are also evaluated, showing some reduction of absorptance in the near-infrared due to transmission for film thicknesses less than 50 lm. This study provides a better understanding of the infrared behavior of VACNT and may guide the design for its applications in energy harvesting, space-borne detectors, and stealth technology. [DOI: 10.1115/1.4030222]

Effective Thermal Conductivities of a Novel Fuzzy Fiber-Reinforced Composite Containing Wavy Carbon Nanotubes

Abstract: This article deals with the investigation of the effect of carbon nanotube (CNT) waviness on the effective thermal conductivities of a novel fuzzy fiber-reinforced composite (FFRC). The distinctive feature of the construction of this novel FFRC is that wavy CNTs are radially grown on the circumferential surfaces of the carbon fibers. Effective thermal conductivities of the FFRC are determined by developing the method of cells (MOCs) approach in conjunction with the effective medium (EM) approach. The effect of CNT waviness is studied when wavy CNTs are coplanar with either of the two mutually orthogonal planes of the carbon fiber. The present study reveals that (i) if CNT waviness is parallel to the carbon fiber axis then the axial (K1) and the transverse (K2) thermal conductivities of the FFRC are improved by 86% and 640%, respectively, over those of the base composite when the CNT volume faction present in the FFRC is 16.5% and the temperature is 400 K, (ii) the effective value of K1 of the FFRC containing wavy CNTs being coplanar with the carbon fiber axis is enhanced by 75% over that of containing straight CNTs for the fixed CNT volume faction when the temperature is 400 K, and (iii) the CNT/polymer matrix interfacial thermal resistance does not affect the effective thermal conductivities of the FFRC. The present work also reveals that for a particular value of the CNT volume fraction, optimum values of the CNT waviness parameters, such as the amplitude and the wave frequency of the CNT for improving the effective thermal conductivities of the FFRC can be estimated. [DOI: 10.1115/1.4028762]

Effect of Evaporation and Condensation at Menisci on Apparent Thermal Slip

Abstract: We semi-analytically capture the effects of evaporation and condensation at menisci on apparent thermal slip lengths for liquids suspended in the Cassie state on ridge-type structured surfaces using a conformal map and convolution. An isoflux boundary condition is prescribed at solid–liquid interfaces and a constant heat transfer coefficient or isothermal one at menisci. We assume that the gaps between ridges, where the vapor phase resides, are closed systems; therefore, the net rates of heat and mass transfer across menisci are zero. The reduction in apparent thermal slip length due to evaporation and condensation relative to the limiting case of an adiabatic meniscus as a function of solid fraction and interfacial heat transfer coefficient is quantified in a single plot. The semi-analytical solution method is verified by numerical simulation. Results suggest that interfacial evaporation and condensation need to be considered in the design of microchannels lined with structured surfaces for direct liquid cooling of electronics applications and a quantitative means to do so is elucidated. The result is a decrease in thermal resistance relative to the predictions of existing analyses which neglect them.

Implementation of High-Order Spherical Harmonics Methods for Radiative Heat Transfer on openfoam

Abstract: Wenjun Ge School of Engineering, University of California, Merced, CA 95343 e-mail: wge@ucmerced.edu Ricardo Marquez School of Engineering, University of California, Merced, CA 95343 e-mail: rmarquez3@ucmerced.edu Michael F. Modest 1 Professor Fellow ASME School of Engineering, University of California, Merced, CA 95343 e-mail: mmodest@ucmerced.edu Somesh P. Roy School of Engineering, University of California, Merced, CA 95343 e-mail: sroy3@ucmerced.edu Implementation of High-Order Spherical Harmonics Methods for Radiative Heat Transfer on OPENFOAM A general formulation of the spherical harmonics (P N ) methods was developed recently to expand the method to high orders of P N . The set of N(N þ 1)/2 three-dimensional second-order elliptic PDEs formulation and their Marshak boundary conditions for arbi- trary geometries are implemented in the OPENFOAM finite volume based CFD software. The results are verified for four cases, including a 1D slab, a 2D square enclosure, a 3D cylindrical enclosure, and an axisymmetric flame. All cases have strongly varying radia- tive properties, and the results are compared with exact solutions and solutions from the photon Monte Carlo method (PMC). [DOI: 10.1115/1.4029546] Keywords: radiative heat transfer, RTE solvers, spherical harmonics, computer implementation Introduction The study of radiative heat transfer in a multidimensional ge- ometry with a strongly varying participating medium has become increasingly important in various practical applications like com- bustion, manufacturing, and environmental systems. The radiative transfer equation (RTE) is an integro-differential equation in five independent variables (three in space and two in direction), which is exceedingly difficult to solve. Many approximate methods have been developed over time. The most widely used approximate methods today are the discrete ordinates method (DOM) [1,2] or its finite volume version (FVM) [3], the PMC [4], and the spheri- cal harmonics method (SHM) [5]. The DOM/FVM method discre- tizes the entire solid angle by finite ordinate directions and is relatively simple to implement within modern CFD software. But an iterative solution is required for scattering media or reflecting surfaces, and computational cost is high for optically thick media. The method may also suffer from ray effects and false scattering due to the angular discretization [6]. The PMC method statisti- cally provides the exact solution with sufficient photon bundles, but accurate solutions are computationally expensive. The spheri- cal harmonics P N approximation is potentially more accurate than DOM/FVM at comparable computational cost, but higher order P N are mathematically very complex and difficult to implement. The P N method decouples spatial and directional dependencies by expanding the radiative intensity into a series of spherical har- monics. The lowest order of the P N family, the P 1 approximation, has been extensively applied to radiative transfer problems. Corresponding author. Contributed by the Heat Transfer Division of ASME for publication in the J OURNAL OF H EAT T RANSFER . Manuscript received August 19, 2014; final manuscript received December 29, 2014; published online February 10, 2015. Assoc. Editor: Zhuomin Zhang. Journal of Heat Transfer However, it loses accuracy when intensity is directionally very ani- sotropic [7], as is often the case in optically thin media. Applications of higher order SHM methods were limited to one-dimensional cases for a long time, because of the cumbersome mathematics. Recently, Modest and Yang [8,9] and Modest [10] have derived a general three-dimensional P N formulation consisting of N(N þ 1)/ 2 second-order elliptic partial differential equations (PDEs) and their Marshak boundary conditions for arbitrary geometries. The main purpose of this paper is to present the procedure of implementing high-order P N formulations within the OPENFOAM [11] open source libraries, and a preliminary version was pre- sented in Ref. [12]. The numerical methods used are summarized along with four example cases. The results of high-order P N meth- ods are found to be very close to the exact solution of the RTE and accurately predict the incident radiation and radiative heat source. Also, the time cost of the P N methods is summarized for different examples and orders. Formulation 2.1 Governing Equations. In the spherical harmonics approximation, the radiative intensity is expanded into a sum of spherical harmonics Iðs; ^ s Þ ¼ N X n X I n m ðsÞY n m ð^ s Þ n¼0 m¼n where s ¼ b r dr is an optical coordinate, and b r is the extinction coefficient. Y n m ð^ s Þ are the spherical harmonics and the upper limit N is the order of the approximation. The set of N(N þ 1)/2 elliptic PDEs [10] of the P N method for isotropic scattering in 3D Carte- sian coordinates are as follows: C 2015 by ASME Copyright V MAY 2015, Vol. 137 / 052701-1 Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 02/25/2015 Terms of Use: http://asme.org/terms