Heat Transfer Characteristics of Laminar Flow in Internally Finned Tubes under Various Boundary Conditions

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Heat and Monentum Transport in Micro—Channels

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Natural Convection Heat Transfer of Copper–Water Nanofluid in a Square Cavity With Time-Periodic Boundary Temperature

This paper presents a numerical study of natural convective heat transfer of copper–water nanofluid in a square enclosure where the temperature of the left vertical sidewall is sinusoidally oscillated with a constant average temperature, the right sidewall is cooled at a relatively low temperature, and the other walls are kept adiabatic. The influence of pertinent parameters such as Rayleigh number, solid volume fraction of copper nanoparticles, and dimensionless time period on the heat transfer characteristics is studied. The results show that the heat transfer rate increases using copper nanoparticles.     

Suitable Heat Transfer Model for Self-Similar Laminar Boundary Layer in Power Law Fluids

The classical power law non-Newtonian fluids energy boundary layer equation is proved improper to describe the self-similar heat transfer. A theoretical analysis for momentum and energy boundary layer transfer behavior is made and the full similarity heat boundary layer equation is developed, which may be characterized by a power law relationship between shear stress and velocity gradient with the Falkner-Skan equation as a special case. Both analytical and numerical solutions are presented for momentum and energy boundary layer equations by using the similarity transformation and shooting technique and the associated transfer characteristics are discussed.     

Freezing of Water in a Slab with Boundary Conditions of the Third Kind

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Heat and Mass Transport in Proton Exchange Membrane Fuel Cells—A Review

This review brings out those aspects of the development of proton exchange membrane (PEM) fuel cells over the last two to three decades that are of interest to the heat and mass transfer community. Because the heat transport and mass transport in proton exchange membrane fuel cells are very important from the efficiency point of view, an emphasis is given here to these transports and their influence on operating cell parameters. The works are classified as models with either isothermal or non-isothermal conditions of various assumed dimensionality and with either single-phase or two-phase flow. Along with modeling, a few experimental studies available are also reported here. Researchers in the area of PEM fuel cells are involved in activities such as development of new and low-cost materials, modeling the relevant physical processes, and electrochemical experimentation. These collective efforts may lead to making this technology viable to meet world needs for clean and cheap energy. This review brings out the fact that computational fluid dynamics (CFD) has become an inevitable tool in fuel cell analysis, as the detailed interactions between the flow structure geometry, fluid dynamics, multiphase flow, heat transfer, mass transfer, and electrochemical reaction can be modeled simultaneously, given the present state of the art in CFD. Through the predictive capability of CFD, it will be possible for fuel cell designers to better optimize the design and operating parameters of fuel cells before testing them in laboratory.     

Level Set–Based Topological Shape Optimization of Heat Conduction Problems Considering Design-Dependent Convection Boundary

A level set–based topological shape optimization method considering design-dependent convection boundaries is developed for steady-state heat conduction problems. We embed the level set function obtained from a Hamilton-Jacobi type of equation into a fixed initial domain to implicitly represent thermal boundaries. The effects of the implicit convection boundary obtained from topological shape variations are represented by numerical Dirac delta and Heaviside functions. The method minimizes the thermal compliance of systems by varying the implicit boundary, satisfying the constraint of allowable material volume. During design optimization, the boundary velocity to integrate the Hamilton-Jacobi equation is derived from an optimality condition.     

Heat conduction in nanofluids: Structure–property correlation

We examine numerically the effects of particle-fluid thermal conductivity ratio, particle volume fraction, and particle morphology on nanofluids effective thermal conductivity and phase lags of heat flux and temperature gradient, for six types of nanofluids containing sphere, cube, hollow sphere, hollow cube, slab-cross and column-cross nanoparticles, respectively. The particle’s radius of gyration and the non-dimensional particle-fluid interfacial area are found to be two characteristic parameters for the effect of particles’ geometrical structure on the effective thermal conductivity. The nanoparticles with larger values of these two parameters can change fluid conductivity more significantly. Due to the enhanced particle-fluid interfacial heat transfer, the nanofluid effective thermal conductivity can practically reach the Hashin–Shtrikman bounds when the particle-phase connects to form a network and separates the base fluid into a dispersed phase. The particle aggregation can effectively affect the effective thermal conductivity when the separation distance among particles is smaller than about one fifth of the particles’ dimension. For the nanofluids considered in the present work, the phase lags of heat flux and temperature gradient scale with the square of particle dimension and range from 10^?11 s to 10^?7 s; the effect of cross-coupling between the heat conduction in the fluid and particle phases is weak; the phase lag of temperature gradient is larger than that of heat flux such that the heat conduction in them is diffusion-dominant and their effective thermal conductivity can be well predicted by the predictive models developed in the present work based on the classical diffusion theory for two-phase systems.     

Inverse Radiation Problem of Boundary Incident Radiation Heat Flux in Semitransparent Planar Slab with Semitransparent Boundaries

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Analytical Solution of Melting in a Subcooled Semi-infinite Solid with Boundary Conditions of the Second Kind

The melting problem in a semi-infinite region with constant heat flux boundary condition is solved by a semi-expect method and an integral approximate method. Effect of subcooling on the transient solid-liquid interface location and surface temperature are also discussed in this paper.     

Measuring the Overall Volumetric Heat Transfer Coefficient in a Vapor-Liquid–Liquid Three-Phase Direct Contact Heat Exchanger

An experimental investigation of the volumetric heat transfer coefficient in a three-phase direct contact condenser was carried out. A 75-cm-long cylindrical Perspex column with a 4 cm diameter was used. Only 48 cm of the column was utilised as the active direct contact condensation height. Pentane vapor at three different initial temperatures (40°C, 43.5°C, and 47.5°C), with differing mass flow rates, and tap water at a constant initial temperature (19°C) with five different mass flow rates were employed as the dispersed phase and the continuous phases, respectively. The results showed that the volumetric heat transfer coefficient increased with increasing mass flow rate ratio (variable dispersed phase mass flow rate per constant continuous phase mass flow rate), the continuous phase mass flow rate and holdup ratio. An optimal value of the continuous phase mass flow rate is shown for an individual dispersed phase mass flow rates. This value increases with increasing vapor (dispersed) phase mass flow rate. Furthermore, it was observed that the initial driving temperature difference had no effect on the volumetric heat transfer coefficient. While, the temperature gained by the continuous phase has a considerable effect.     

Heat transfer of gas–liquid mixture in micro-channel heat sink

The main objective of the present investigation is to study heat transfer in parallel micro-channels of 0.1 mm in size. Comparison of the results of this study to the ones obtained for two-phase flow in “conventional” size channels provides information on the complex phenomena associated with heat transfer in micro-channel heat sinks. Two-phase flow in parallel micro-channels, feeding from a common manifold shows that different flow patterns occur simultaneously in the different micro-channels: liquid alone (or single-phase flow), bubbly flow, slug flow, and annular flow (gas core with a thin liquid film, and a gas core with a thick liquid film). Although the gas core may occupy almost the entire cross-section of the triangular channel, making the side walls partially dry, the liquid phase always remained continuous due to the liquid, which is drawn into the triangular corners by surface tension. With increasing superficial gas velocity, a gas core with a thin liquid film is observed. The visual observation showed that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. The probability of appearance of different flow patterns should be taken into account for developing flow pattern maps. The dependence of the Nusselt number, on liquid and gas Reynolds numbers, based on liquid and gas superficial velocity, respectively, was determined in the range of Re_LS = 4–56 and Re_GS = 4.7–270. It was shown that an increase in the superficial liquid velocity involves an increase in heat transfer (Nu_L). This effect is reduced with increasing superficial gas velocity, in contrast to the results reported on two-phase heat transfer in “conventional size” channels.     

Convective Heat and Mass Transfer in Water at Super—Critical Pressures under Heating or Cooling Conditions in Vertical Tubes

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Transport properties of Ar–Kr binary mixture in nanochannel Poiseuille flow

Transport properties, including thermal conductivity and shear viscosity, of the Ar–Kr binary mixture confined in a nanochannel under Poiseuille flow are calculated by equilibrium molecular dynamics (EMD) simulation through Green–Kubo formula. An external force is applied in the x-direction to drive the Poiseuille flow. Thermal conductivity of the confined mixture in the x- and y-direction is obviously higher than that in macroscale, as a result of the strong interacting potential between the fluid atoms and the wall atoms. Thermal conductivity of the flowing binary mixture is obviously anisotropic. With increasing the external driving force, in the x-direction the thermal conductivity increases, whereas in the y-direction it keeps constant. The xz- and yz-component of the shear viscosity of the confined mixture are enhanced comparing with the xy-component owing to the collisions between the fluid atoms and the wall atoms in the z-direction. They are higher than the results in macroscale and decrease with the external driving force increasing. For the binary mixture, thermal conductivity and shear viscosity vary with the mole fraction of the Kr atoms. The interactions between the fluid atoms and the wall atoms play a key role in the transport properties of the binary mixture confined in the nanochannel.     

Heat transfer in gas–solid fluidized bed with various heater inclinations

The study explored the heat transfer properties in an air-fluidized bed of sand, heated with an immersed heat transfer tube positioned at several angles of inclination. Operating with fluidizing velocity up to 0.5 m/s; and particles of 150–350 μm diameter, the effect of air velocity and particle size on the average and maximum achieved heat transfer coefficient was examined for the heat transfer tube at angles of inclination in the range 0–90°. Experimental results showed that the angle of inclination altered the bubble size and behavior close to the heat transfer tube hence the expected heat transfer coefficient, with the influence of tube inclination being less pronounced for smaller particles. The optimum angle of inclination was in the range of 10–15° relative to the direction of the flow, while the heat transfer coefficient had its lowest values at the angle of 45°, and thereafter improved upon transition to 90°. Upon comparison with existing correlations, a correction factor is proposed to account for the impact of the angle of inclination on the heat transfer coefficient calculated by the Molerus–Wirth semi-empirical correlation.     

Numerical Investigation of Heat Transfer by CuO–Water Nanofluid in Rectangular Enclosures

This paper analyzes heat transfer and fluid flow of natural convection in inclined cavity filled with CuO–water nanofluid and differentially heated. Conservation of mass, momentum, and energy equations are solved numerically by a control volume finite-element method using the SIMPLER algorithm for pressure–velocity coupling. The Prandtl number is fixed at 7.02, corresponding to water. Aspect ratio and solid volume fraction are varied from 0.5 to 4 and from 0% to 4%, respectively. The inclination angle is varied from 0° to 90° and used as a control parameter to investigate flow mode-transition and the accompanying hysteresis phenomenon (multi-steady solutions). It is found that the efficiency of heat transfer is improved by the addition of nanoparticles into base fluid; however, there is an optimum solid volume fraction that maximizes the heat transfer rate. Numerical results show also that the diameter of solid particle is an important parameter that affects the heat transfer efficiency; its impact is more important than the concentration itself. Effects of inclination angle on streamlines and on thermal boundary layer are presented. Combined effects of aspect ratio and inclination angle on heat transfer and hysteresis region are analyzed.     

Heat transfer in a reaction–diffusion system with a moving heat source

A general formulation is presented for a moving boundary problem in which heat is generated at the boundary due to an exothermic reaction involving a species which diffuses into a dispersed phase from an external medium of finite volume. The speed of the moving boundary is prescribed based on the solution of the mass diffusion problem and an analysis is presented of the thermal dynamics of the system. The set of equations describing heat transport leads to a Green’s function type problem with time dependent boundary conditions and the Galerkin finite element method is employed to develop a numerical solution. Transformations are introduced to freeze the moving boundary and partition the domain for ease of computation, and an iterative scheme is defined to satisfy the heat flux jump boundary condition and match the temperature field across the moving boundary. The numerical results are used to set the limits of applicability of an analytical perturbation solution. Essential aspects of thermal dynamics in the system are described and parametric regions resulting in a local temperature hot spot are delineated. Computed contour plots describing thermal evolution are presented for different combinations of parameter values. These may be of utility in the prediction of thermal development, for control and avoidance of hot spot formation, and in physical parameter estimation.     

An Iterative Technique for Coupled Conduction–Radiation Heat Transfer in Semitransparent Media

An iterative technique is developed to solve coupled conduction–radiation heat transfer in semitransparent media. Apart from a high convergence rate, the present algorithm preserves the conservation nature of the governing equation far better than other common methods and it is readily combined with other methods in solving radiative transfer. Using the technique described in this study, parametric studies are carried out for coupled heat transfer in a semitransparent slab and the results illustrate the “peak” effects of wall emissivity and scattering albedo on conductive and radiative heat fluxes, which are rarely mentioned in the existing research.     

Heat Transfer in Microchannels with Upstream–Downstream Regions Coupling and Wall Conjugation Effects

Heat transfer in microchannels is analyzed, including the coupling between the regions upstream and downstream of the heat transfer section and taking into account the wall conjugation and axial diffusion effects which are often of relevance in microchannels. The methodology is based on a recently proposed single-domain formulation for modeling the heat transfer phenomena simultaneously at the fluid stream and the channel walls, and applying the generalized integral transform technique (GITT) to find a hybrid numerical–analytical solution to the unified partial differential energy equation. The proposed mathematical model involves coefficients represented as space-dependent functions, with abrupt transitions at the fluid–wall interfaces, which carry the information concerning the transition of the two domains, unifying the model into a single-domain formulation with variable coefficients. Convergence of the proposed eigenfunction expansions is thoroughly investigated and the physical analysis is focused on the effects of the coupling between the downstream and the upstream flow regions.     

Heat and gas transport properties in pelletized hydride–graphite-composites for hydrogen storage applications

Metal hydrides are suitable for the compact, efficient and safe storage of hydrogen. Considering hydride-based hydrogen storage tanks, the enhancement of the heat and gas transport properties of the hydride bed is crucial for increased (un-)loading dynamics of the tank.