Enhancement of heat transfer using nanofluids—An overview

A colloidal mixture of nano-sized particles in a base fluid, called nanofluids, tremendously enhances the heat transfer characteristics of the original fluid, and is ideally suited for practical applications due to its marvelous characteristics. This article addresses the unique features of nanofluids, such as enhancement of heat transfer, improvement in thermal conductivity, increase in surface volume ratio, Brownian motion, thermophoresis, etc. In addition, the article summarizes the recent research in experimental and theoretical studies on forced and free convective heat transfer in nanofluids, their thermo-physical properties and their applications, and identifies the challenges and opportunities for future research.     

Can segmented flow enhance heat transfer in microchannel heat sinks?

Liquid cooling is an efficient way to remove heat fluxes with magnitudes up to 10,000 W/cm². One limitation of current single-phase microchannel heat sinks is the relatively low Nusselt number, due to laminar flow. In this work, we experimentally investigate how to enhance the Nusselt number with the introduction of segmented flow. The segmented flow pattern was created by the periodic injection of air bubbles through a T-junction into water-filled channels. We designed a polycarbonate heat sink consisting of an array of seven parallel microchannels each with a square cross-section 500 μm wide. We show that segmented flow increases the Nusselt number of laminar flow by more than 100%, provided the mass velocity of the liquid is within the range 330–2000 kg/m² s.     

A lattice Boltzmann method for simulation of liquid–vapor phase-change heat transfer

A lattice Boltzmann model for the liquid–vapor phase change heat transfer is proposed in this paper. Two particle distribution functions, namely the density distribution function and the temperature distribution function, are used in this model. A new form of the source term in the energy equation is derived and the modified pseudo-potential model is used in the proposed model to improve its numerical stability. The commonly used Peng–Robinson equation of state is incorporated into the proposed model. The problem of bubble growth and departure from a horizontal surface is solved numerically based on the proposed model.     

Parametric effects on heat transfer in loop heat pipe’s wick

The capillary-driven flow with conductive, convective and evaporative heat transfer occurs in the capillary wick of a loop heat pipe, so-called the inverted meniscus evaporator. An axisymmetric two-dimensional mathematical model is developed to investigate the effects of heat flux and porous structure parameters on wick’s working states and performances. The full effects of the interaction between the flowfield and the liquid–vapor interface are adequately considered in this model, as well as the capillary effect of evaporation and the combined effect of conductive, convective and evaporative heat transfer. Wick’s performances are introduced and discussed while parametric effects are discussed in detail. Wick’s working states are classified into three operating modes while all operating modes are discussed in detail. Furthermore, the phase-diagrams on the q–k_t,l plane and the q–K plane are also obtained and discussed.     

Fluid flow and heat transfer characteristics of low temperature two-phase micro-channel heat sinks – Part 2. Subcooled boiling pressure drop and heat transfer

This second part of a two-part study explores the performance of a new cooling scheme in which the primary working fluid flowing through a micro-channel heat sink is indirectly cooled by a refrigeration cooling system. The objective of this part of study is to explore the pressure drop and heat transfer characteristics of the heat sink. During single-phase cooling, pressure drop decreased with increasing heat flux because of decreased liquid viscosity. However, pressure drop began increasing with increasing heat flux following bubble departure. These opposite trends produced a minimum in the variation of pressure drop with heat flux. Increasing liquid subcooling decreased two-phase pressure drop because of decreased void fraction caused by strong condensation at bubble interfaces as well as decreased likelihood of bubble coalescence. It is shown macro-channel subcooled boiling pressure drop and heat transfer correlations are unsuitable for micro-channel flows. However, two new modified correlations produced good predictions of the present heat transfer data.     

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.     

A numerical method to estimate temperature intervals for transient convection–diffusion heat transfer problems

This paper presents a numerical method to estimate intervals of temperature for transient convection–diffusion heat transfer problems when uncertainty of thermal parameters is characterized by the interval. A deterministic relationship of interval variables between temperature and thermal parameters is setup by Taylor series expansion and the interval analysis, and the lower and upper bounds of temperature can be estimated by a temporally piecewise adaptive algorithm and FEM. A prescribed computing accuracy at each discretized time interval can be achieved via an adaptive process for different size of time step, thus the computing accuracy over the whole time domain can be maintained. A 2D numerical example is provided to verify the proposed approach, and a good accordance can be observed in the comparison of results given by the combinatorial, probability and proposed approaches. The impact of the order of Taylor expansion and the size of time step on the result is discussed.     

A numerical study on heat transfer and MHD flow of a Newtonian through a porous channel—Local thermal nonequilibrium model

 Vast numbers of studies concentrate on the thermal equilibrium state whereas in many real-world applications the model exists in the nonequilibrium state. Also, local thermal non-equilibrium precisely represents the thermohydroflow characteristics. Therefore, the current study examines the heat transfer and fluid flow characteristics of the magnetohydrodynamic flow of a Newtonian fluid through a local thermal non-equilibrium (LTNE) porous channel in the presence of the induced magnetic field. The mathematical model of the prescribed flow encloses the coupled nonlinear equations which are difficult to approach analytically. Hence, they are solved numerically using the shooting method with the Newton–Raphson method. The implications of various physical parameters of the problem on fluid flow, induced magnetic field, current density, temperature profiles, and heat transfer are elucidated with the aid of plots and tables. From the examination, it is clear that the porous medium significantly influences the characteristics of the fluid flow. That is, the least value of the Darcy number is related to a higher momentum field. Another interesting phenomenon is that the induced magnetic field remarkably enhances when the Darcy number is high, whereas the process is contrary to the current density. The effect of LTNE on the flow characteristics and heat transfer ceases for higher values of inter-phase heat transfer coefficient and the ratio of thermal conductivities, which gives rise to the local thermal equilibrium (LTE) situation. Furthermore, the amount of heat transport is maximum in the LTE case compared to that of the LTNE case.     

Topology optimization of heat and mass transfer problems in two fluids—one solid domains

Abstract

Among various topology optimization methods used in fluid flow problems, density approach has gained more interest compared to other techniques as level set approach, topological derivative technique, and phase field method. The key part of density approach is the penalized interpolation function, which forces progressively porous cells made of fluid and solid simultaneously to belong discretely to fluid or solid sub-domains. However this type of problem was only solved in mono-fluid domains, in which the method accounts for the distribution of a single fluid and a single solid. The actual work aims to extend topology optimization in fluid flow problems to bi-fluid domain. A new interpolation function was developed for this purpose. Furthermore a penalization function was integrated in the multiobjective function, which ensure that each fluid takes its own path in the device, while maintaining a minimal required solid thickness between the channels of different fluids. The results showed the capacity of the proposed method to deal with multiple fluid phases in minimizing the pressure drop while maximizing heat exchange between different flows. The main conclusion is the potential of density approach to be applied on optimization of heat exchangers.     

Can pulsation unsteadiness increase the convective heat transfer in a pipe flow? A systematic study

Abstract

Unsteady laminar and turbulent pipe flows subjected to a constant temperature gradient in axial direction are investigated based on asymptotic considerations. The thermal boundary condition is that of a constant wall heat flux for a steady flow. The unsteadiness is induced by a sinusoidal pressure gradient with different amplitudes and frequencies. The asymptotic analysis is performed with the help of selected CFD simulations. The simulation model is a pipe which is assumed to be infinitely long with an ideal condition of fully developed flow at the pipe entrance. For laminar flows there is no effect with respect to the time averaged heat transfer performance. For turbulent flows appreciable heat transfer enhancement only occurs with large amplitudes and low frequencies. The asymptotic analysis with CFD results are in accordance with the conclusions according to full CFD simulations. Their computational costs, however, are several orders lower. The study shows that, prior to expensive CFD simulations, an asymptotic analysis with CFD simulations at ideal conditions can be considered to find the trend with respect to the parameters of interest.     

A three-dimensional enthalpic lattice Boltzmann formulation for convection–diffusion heat transfer problems in heterogeneous media

In this paper, an enthalpic lattice Boltzmann method formulation for 3D unsteady convection–diffusion heat transfer problems is used to overcome discontinuity issues in heterogeneous media. The new formulation is based on the appearance of a source term added to the collision step. The major achievement of the proposed enthalpic LB formulation is avoiding any interface treatments or geometry considerations even when dealing with complex geometries. The performance of the present method is tested for several three-dimensional convection–diffusion problems. Comparisons are made with the control volume method, and numerical results show excellent agreements.     

Numerical study of nanofluid heat transfer for different tube geometries – A comprehensive review on performance

The heat transfer performance of a system can be improved using a combination of passive methods, namely nanofluids and various types of tube geometries. These methods can help enhance the heat transfer coefficient and consequently reduce the weight of the system. In this paper, the effect of tube geometry and nanofluids towards the heat transfer performance in the numerical system is reviewed. The forced convective heat transfer performance, friction factor and wall shear stress are studied for nanofluid flow in different tube geometries. The thermo-physical properties such as density, specific heat, viscosity and thermal conductivity are reviewed for the determination of nanofluid heat transfer numerically. Various researchers had measured and modelled for the determination of thermal conductivity and viscosity of nanofluids. However, the density and specific heat of nanofluids can be estimated with the mixture relations. The different tube geometries in simulation work are analyzed namely circular tube, circular tube with insert, flat tube and horizontal tube. It was observed that the circular tube with insert provides the highest heat transfer coefficient and wall shear stress. Meanwhile, the flat tube has greater heat transfer coefficient with a higher friction factor compared to the circular tube.     

Generator absorber heat exchange based absorption cycle—A review

GAX based absorption cooling systems have been investigated in recent years by various groups across the world due to their advantage of offering a higher performance compared to that of the conventional ammonia–water absorption systems. In this paper, a comprehensive review of several different GAX cycle configurations has been explained in detail. The choice of working fluids and the performance of the GAX cycle in terms of coefficient of performance and temperature lift are also presented. The study reveals an improvement in the COP of about 10–20%, 20–30% and 30–40% in absorber heat recovery cycle, simple GAX and branched GAX cycle respectively, than that of a conventional single effect system for the same set of operating conditions. The importance of the GAX cycle with respect to the current energy scenario is also highlighted.     

Heat transfer in percolated fixed beds with temperature-dependent physical properties—compressive and dispersive effects

This study concerns the temperature waves generated in a packed bed by a change in the inlet temperature of a percolating fluid. The shape of these waves, and their deformation as they propagate, can be considerably affected by the variation of specific heats and specific volumes with temperature. Two qualitatively different behaviours are theoretically illustrated. For example when the temperature of air, flowing through a bed of glass beads, is raised from 20 to 100°C, the resulting temperature wave is ‘dispersive’ in the sense that it spreads more and more as it propagates, under the effect of the changes in densities and heat capacities of the two phases. A similar but weaker effect occurs for systems CO₂-glass and water-glass. On the other hand, a bed of lead beads percolated by carbon dioxide will exhibit a ‘compressive’ trend resulting in a steady and sharp ‘shock wave’. All trends are reversed when a cooling step is considered instead of a heating step. These effects are illustrated using a simplified model that neglects convective dispersion and heat transfer resistances between the two phases. The quantitative importance of the spreading is compared to that due to convective axial dispersion, by evaluating the corresponding Péclet numbers. Depending on the operating conditions the effect illustrated may be predominant (mostly in gas-solid systems and/or at low Reynolds number) or negligible (mostly in liquidsolid systems and/or at high Reynolds). Accounting for the spreading effect (or sharpening effect) due to property variations is thought to be important in the interpretation of fixed-bed experiments, for example when heat transfer coefficients are evaluated from such experiments.     

Wood as prospective materials for sustainable development—Evaluation of tactile warmth by heat transfer analysis

ABSTRACT

Wood can be a sustainable resource with the ability to fix the carbon dioxide by photosynthesis if the cycle of the felling, planting, and growing of trees would be controlled. The good tactile warmth of wood is treated by heat transfer analysis to sustain the cycle with increasing the demand for felled wood. The heat transfer analysis shows that the governing material property on the tactile warmth is the thermal effusivity. The contact surface temperature is proposed as the quantitative scale of the tactile warmth defined uniquely from physical quantities.     

Numerical simulation of heat transfer and fluid flow past a rotating isothermal cylinder – A LBM approach

In this paper, a viscous fluid flowing past a rotating isothermal cylinder with heat transfer is studied and simulated numerically by the lattice Boltzmann method (LBM). A numerical strategy for dealing with curved and moving boundaries of second-order accuracy for both velocity and temperature fields is proposed and presented. The numerical strategy and method are validated by comparing the present numerical results of flow without heat transfer with those of available previous theoretical, experimental and numerical studies, showing good agreements. On this basis, the convective heat transfer performance in such rotational boundary environments is further studied and validated; the numerical results are reported in the first time. The effects of the peripheral-to-translating-speed ratio, Reynolds number and Prandtl number on flow and heat transfer are discussed in details.     

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.     

A new approach for the prediction of the heat transfer rate of the wire-on-tube type heat exchanger––use of an artificial neural network model

This study presents an application of artificial neural networks (ANNs) to predict the heat transfer rate of the wire-on-tube type heat exchanger. A back propagation algorithm, the most common learning method for ANNs, is used in the training and testing of the network. To solve this algorithm, a computer program was developed by using C++ programming language. The consistence between experimental and ANNs approach results was achieved by a mean absolute relative error <3%. It is suggested that the ANNs model is an easy modeling tool for heat engineers to obtain a quick preliminary assessment of heat transfer rate in response to the engineering modifications to the exchanger.