Flow condensation in parallel micro-channels – Part 2: Heat transfer results and correlation technique

This second part of a two-part study concerns heat transfer characteristics for FC-72 condensing along parallel, square micro-channels with a hydraulic diameter of 1 mm, which were formed in the top surface of a solid copper plate. Heat from the condensing flow was rejected to a counter flow of water through channels brazed to the underside of the copper plate. The FC-72 condensation heat transfer coefficient was highest near the channel inlet, where the annual liquid film is thinnest. The heat transfer coefficient decreased along the micro-channel because of the film thickening and eventual collapse of the annular regime. Notable heat transfer enhancement was observed for annular flow regions of the micro-channel associated with interfacial waves. Comparing the present data to predictions of previous annular condensation heat transfer correlations shows correlations intended for macro-channels generally provide better predictions than correlations intended specifically for mini/micro-channels. A new condensation heat transfer coefficient correlation is proposed for annular condensation heat transfer in mini/micro-channels. The new correlation shows excellent predictive capability based on both the present FC-72 data and a large database for mini/micro-channel flows amassed from eight previous sources.     

Heat transfer augmentation of ethylene glycol: water nanofluids and applications — A review

This paper introduces the historical background about the development of water based, ethylene glycol (EG) based and EG:water mixture nanofluids for the past 20 years. The primary consideration is to review the salient of research work related to EG:water mixture nanofluids and their applications. Nowadays, the fundamental studies of nanofluids are increasing rapidly for engineering applications. The determination of the forced convection heat transfer and pressure drop was reviewed for nanofluid flow in a tube. The experimental and numerical heat transfers of nanofluids were presented. A review of other relevant research studies is also provided. Substantial heat transfer literature has been studied on water based nanofluids used in the fundamental study for engineering applications. However, there are limited studies that use EG:water mixture nanofluids in evaluation of forced convection heat transfer. A number of research studies have been performed to investigate the transport properties of EG:water mixture nanofluids either in experimental or numerical approach. As the performance of EG:water mixture nanofluids could be verified through experimental studies, researchers have conducted the experimental works using several types of potential nanofluids. As a result, nanofluids have been used in certain engineering applications such as in automotive, transportation, cooling of electronics components, solar, and nuclear reactor coolant.     

Slip Boundary Conditions in Ballistic–Diffusive Heat Transport in Nanostructures

Ballistic–diffusive heat conduction, which is predominantly affected by boundaries and interfaces, will occur in nanostructures whose characteristic lengths are comparable to the phonon mean free path (MFP). Here, we demonstrated that interactions between phonons and boundaries (or interfaces) could lead to two kinds of slip boundary conditions in the ballistic–diffusive regime: boundary temperature jump and boundary heat flux slip. The phonon Boltzmann transport equation (BTE) with relaxation time approximation and the phonon tracing Monte Carlo (MC) method were used to investigate these two slip boundary conditions for the ballistic–diffusive heat conduction in nanofilms on a substrate. For cross-plane heat conduction where the boundary temperature jump is the dominant non-Fourier phenomenon, ballistic transport causes the temperature jumps and thus introduces a ballistic thermal resistance. Importantly, when considering the interface effect, the corresponding model was derived based on the phonon BTE and verified by comparing with the MC simulations. In addition, an interface–ballistic coupling effect was identified, which indicates inapplicability of the standard thermal resistance analysis. In contrast, for the in-plane case that is controlled by boundary heat flux slip, both phonon boundary scattering and perturbation of the phonon distribution function induced by the interface can cause heat flux slip, leading to a variation in in-plane thermal resistance. In addition, a model beyond the Fuchs-Sondheimer formula, which can address both the boundary scattering and the interface effects, was derived based on the phonon BTE. The good agreements with the MC simulations indicate its validity.     

Oxide ion and proton conduction in doped ceria–carbonate composite materials

The direct current four-probe method has been employed to investigate the conduction of oxide ion and proton in a doped ceria–carbonate composite electrolyte for fuel cells. The measurements are conducted in oxygen and in hydrogen atmospheres in the temperature range of 425–650 °C. The conductivities of both of O²⁻ and H⁺ increase with the increase of carbonate content above the melting point of the carbonate. The ionic conductivities of the composite electrolytes have also been simulated using the effective medium percolation theory. The deviations between experimental results and simulated values of O²⁻ conductivity are caused by the associating effect of ceramic and carbonate phases, which leads to a higher O²⁻ migration energy through the phase interface. According to the comparison of experimental data and simulated values, the conduction mechanisms of O²⁻ and H⁺ have been proposed.     

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.     

Thermal and hydraulic characteristics of turbulent nanofluids flow in a rib–groove channel

Thermal and hydraulic characteristics of turbulent nanofluids flow in a rib–groove channel are numerically investigated. The continuity, momentum and energy equations were solved by means of a finite volume method (FVM). The top and bottom walls of the channel are heated at a constant temperature. Nine different rib–groove shapes are considered in this study, which are three different rib shapes with three different groove shapes including rectangular, triangular and trapezoidal and they are interchanged with each other. Four different types of nanoparticles Al₂O₃, CuO, SiO₂, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 25 nm to 80 nm, are dispersed in different base fluids (water, glycerin, engine oil) are used. In this study, several parameters such as different Reynolds numbers in the range of 5000 < Re < 20000, and different rib–groove aspect ratios in the range of 0.5 ≤ AR ≤ 4 are also examined to identify their effects on the heat transfer and fluid flow characteristics. The results indicate that the rectangular rib–triangular groove has the highest Nusselt number among other rib–groove shapes. The SiO₂ nanofluid has the highest Nusselt number compared with other nanofluid types. The Nusselt number increased as the nanoparticle volume fraction, Reynolds number and aspect ratio increased; however, it decreased as the nanoparticle diameter increased. It is found that the glycerin–SiO₂ shows the best heat transfer enhancement compared with other tested base fluids.     

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.     

Fine structure of the price–demand relationship in the electricity market: Multi-scale correlation analysis

In this research we investigate the problems of dynamic relationship between electricity price and demand over different time scales for two largest price zones of the Russian wholesale electricity market. We use multi-scale correlation analysis based on a modified method of time-dependent intrinsic correlation and the complete ensemble empirical mode decomposition with adaptive noise for this purpose. Three hypotheses on the type and strength of correlations in the short-, medium- and long-runs were tested. It is shown that price zones significantly differ in internal price–demand correlation structure over the comparable time scales, and not each of the theoretically formulated hypotheses is true for each of them. We can conclude that the answer to the question whether it is necessary to take into account the influence of demand-side on electricity spot prices over different time scales, is significantly dependent on the structure of electricity generation and consumption on the corresponding market.     

Bubble generation in micro-volumes of “nanofluids”

We report experimental investigation of a transparent flat mini evaporator heated by laser beam. The influence of non-absorbing and absorbing nanoparticles immersed in pure water, and heat absorbing fluid on the heat transfer intensification was analyzed. Nanoparticles may initiate vaporization and boiling of fluid at low heat input. Providing specific task and conditions the nanoparticles may be used in passive or active modes. Passive mode assumes that nanoparticles do not generate thermal energy and improve bubble nucleation conditions due to additional nucleation of the fluid, thus decreasing boiling/vaporization temperature thresholds. Active mode assumes that nanoparticles act as converters of optical energy into thermal one.     

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.     

Flow Structure and Heat Transfer Between Two Disks Rotating Independently

In the present paper, fluid flow and convective heat transfer between two co-axial disks rotating independently are dealt with mainly based on the author's recent research on that topic. Three rotational modes, i.e. co-rotation, rotor-stator, and counter-rotation, are considered. Theory of rotating non-isothermal fluids with the presence of disk rotation and thermal effects is addressed. Rotational buoyancy effects on the flow structure development are highlighted. Results of flow visualization and heat transfer measurements are discussed to explore the thermal flow mechanisms involved in the two-disk flows at various rotational and geometric conditions. Potential issues open to the future investigation are also proposed.     

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.     

Advances of nanofluids in heat exchangers—A review

Recently, many researchers have focused on their studies on the analysis of nanofluid flows due to their participation in the enhancement of heat transfer rates in industrial processes. The ordinary fluids, such as water, mineral oils, and so on, are known for their low thermal conductivity in heat transfer processes. A significant enhancement in the thermal properties of ordinary fluid may be obtained by adding nanoparticles having a diameter of less than 100 nm or suspension of fibers. Better spreading, wetting, dispersion, and stability and with acceptable viscosity are the main advantageous properties of nanofluids on a solid surface. The nanofluids are encountered in various thermal engineering systems such as in heat exchangers, refrigeration, thermal management of fuel cells, cooling of nuclear reactors, microelectromechanical systems, and others. In particular, the thermal conversion is known as a great application of nanotechnology, and many studies have been achieved with such fluids in heat exchangers. Therefore, this paper aims to present a global insight into the different applications of nanofluids in various heat exchangers, that is, heat pipe and plate-fin heat exchangers. All research works have been summarized into three main parts: laminar, transition, and turbulent nanofluid flow regimes.     

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.     

An Unstructured Finite-Volume Method for Structure–Electrostatics Interactions in MEMS

Radio-frequency microelectromechanical systems (RF MEMS) are widely used for contact actuators and capacitive switches, and involve metal–dielectric contact. In these devices, the structure is activated by an electrostatic force, whose magnitude changes as the gap closes. It is advantageous to model fluid and structural mechanics and electrostatics within a single comprehensive numerical framework to facilitate coupling between them. In this article, we extend a cell-based finite-volume approach popularly used to simulate fluid flow to characterize structure–electrostatics interactions. The method employs fully implicit second-order finite-volume discretization of the integral conservation equations governing elastic solid mechanics and electrostatics, and uses arbitrary convex polyhedral meshes. Results are presented in this article for a fixed-fixed beam under electrostatic actuation.     

Retrieval of thermal properties in a transient conduction–radiation problem with variable thermal conductivity

This article reports an inverse analysis of a transient conduction–radiation problem with variable thermal conductivity. Simultaneous retrieval of parameters is accomplished by minimizing the objective function represented by the square of the difference between the measured and the assumed temperature fields. The measured temperature field is calculated from the direct method involving the lattice Boltzmann method (LBM) and the finite volume method (FVM). In the direct method, the FVM is used to obtain the radiative information and the LBM is used to solve the energy equation. With perturbations imposed on the measured temperature data, minimization of the objective function is achieved with the help of the genetic algorithm (GA). The accuracies of the retrieved parameters have been studied for the effects of the genetic parameters such as the crossover and the mutation rates, the population size, the number of generations and the effect of noise on the measured temperature data. A good estimation of parameters has been obtained.     

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.     

Critical Heat Flux of Boiling Heat Transfer in a Moderate Narrow Space

INTanDUCTI0NBoilingheattransferandcriticalheatflux(CHF)inaconfinednarrowspacehavebeenstudiedexperi-melltallybyanumberofinvestigatorsinthepastfewdecades.However,thereisnoanypopularlyacceptedmodelintheheattransferinnarrowspaceboiling,althoughsomepopularknowledgeabouttheboilingheattransferinthenarrowspacehavebeenacceptedbymanyresearchers.Theknowledgecanbecon-cludedasthatthenucleateboilingheattransferisenhancedatlowheatfluxregionanddeterioratedathighheatfiuxregi0nespeciallyatCHF.Theenhanceme…     

Effectiveness–NTU Relations for Counterflow Heat Exchangers: The Effect of Kinetic Energy Variation and Heat Leak From Outside

The effectiveness–number of transfer units (NTU) relations are useful data for designing and performance evaluation of heat exchangers with fluids having considerable variation in velocities in the presence of heat leak. In this article, the closed-form (benchmark) solutions for counterflow heat exchangers, when the heat leak is either on the hot or cold side of the heat exchanger in the presence of kinetic energy variation, are presented. It was found that the effectiveness depends on NTU and fluid capacity ratio along with six other dimensionless variables that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are also presented in a graphical form exhibiting the variation of effectiveness of the heat exchanger with the already-mentioned parameters. It was demonstrated that when the dimensionless heat leak and kinetic energy terms approach zero, the solution reduces to the classical effectiveness–NTU relations for counterflow heat exchangers.     

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.