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Porous media play important roles in a wide range of scientific and engineering problems. Recently, with their increasing application in energy conversion and storage devices, such as fuel cells, batteries and supercapacitors, it has been realized that transport processes and reactions occurring in the pores and at the interfaces of different constituents significantly affect the performance of the porous media, yet these pore-scale transport phenomena are not well described or even neglected in the conventional numerical models based on the representative element volume (REV). Pore-scale modeling is an efficient tool for the simulation of pore-scale transport and reactions in porous media because of its ability to accurately characterize these processes and to provide the distribution details of important variables which are challenging for current experimental techniques to provide either due to lack of in-situ measurement capability or due to the limited spatial and temporal resolution. In the present review, the advances and challenges of the state-of-the-art pore-scale modeling are summarized. The practical applications of pore-scale modeling in the fields of geoscience, polymer exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC) are discussed. Notable results from the pore-scale modeling are presented, and the challenges facing the pore-scale model development are discussed. This in-depth review is intended to give a well-rounded introduction of critical aspects on which the pore-scale modeling can shed light in the development of relevant scientific and engineering systems. ISSN:0360-1285 ISSN:0360-3202

Due to the complex microstructure in aerogels and the intricate heat transfer mechanism of solid-gas coupling heat conduction, modeling of the gas-contributed thermal conductivity of this type of material is quite difficult. The present work introduces a novel numerical methodology for computing the gas-contributed thermal conductivity of aerogels by analyzing their microstructural characteristics and heat transfer mechanism of the thermal coupling between the gas phase and the solid backbone of the system. Specifically, structures of aerogels are reconstructed by an improved three-dimensional diffusion-limited cluster-cluster aggregation (DLCA) method, and the contribution of the solid-gas coupling heat transfer to the gas-contributed thermal conductivity of aerogels is quantified. The present numerical model is fully validated by the available experimental data for different aerogels with porosity ranging from 78% to 97.7%. The proposed numerical method is flexible and versatile because it is capable to account for both the geometrical and topological details of the aerogel structure.

The two-dimensional steady-state heat conduction in irregular geometry is solved by a MLPG–FVM coupled method. The meshless local Petrove–Galerkin (MLPG) method is applied to the sub-region with skewed wall surface while the finite volume method (FVM) is used in the rest of the domain. The Dirichlet–Dirichlet method is adopted to couple the temperature between MLPG and FVM methods. In MLPG method, the Dirac's Delta function is taken as the test function to avoid the local domain integration which does not need the numerical integration and the solution is independent of the size of the test function. The proposed MLPG–FVM method is validated and proved to be an efficient numerical method for 2-D heat conduction in irregular geometry, which can exert their own advantages of MLPG and FVM.

Hollow silica nanosphere packings (HSNSPs) can significantly suppress heat conduction through solid and gas phases due to the voids, small interparticle contact areas, and nanosized pores, showing promising potentials towards energy-efficient building applications. The HSNSPs display a two-level structure, where the solid silica nanoparticles form the shells of hollow spheres, and the accretion of hollow spheres form the porous powder packing structures. Investigating thermal transport in HSNSPs helps to understand the fundamental thermal transport processes and to guide the design of their geometric structures. Herein, we developed a numerical model based on the two-level structure of HSNSPs to explore their effective thermal conductivities. The developed numerical model considers the geometric parameters such as sphere size, shell thickness, interparticle contact resistance, and the gas pressure inside and outside the hollow spheres. The developed numerical model was validated by the measured thermal conductivities of HSNSPs fabricated via the sacrificial template method. The results show that the effective thermal conductivity of HSNSPs can be reduced by decreasing sphere diameter, contact area and shell thickness. The influence of ratio of contact diameter to sphere diameter on the effective thermal conductivity becomes weaker as the hollow sphere size decreases (e.g.

Opacifiers are usually doped in the silica aerogels to reduce the radiative heat transfer at high temperature. However, the doped opacifiers will enhance the heat conduction in the solid phase and increase the density of silica aerogels dramatically. For developing lightweight and efficient opacifiers, in this paper, different types of core/shell opacifiers are designed, and their extinction performance is investigated, theoretically. Then the optimal temperature-dependent particle size and doping amount of these opacifiers are obtained by maximizing the Rosseland mean extinction coefficient and minimizing the total thermal conductivity of silica aerogel composite. The results show that the hollow carbon black opacifiers might greatly reduce the densities of the opacifier-doped silica aerogels without deteriorating appreciably their insulating capability, and the carbon/SiC, carbon/TiO2 and carbon/Al2O3 core/shell opacifiers exhibit excellent extinction performance and would be strong candidates for high temperature due to their high extinction performance, low density and high-temperature stability. Finally, a core/shell opacifier-gradient-doped silica aerogel based on the optimal opacifier types and doping amount is designed, which significantly improves the insulating performance and reduces the density of silica aerogel composite. The results of this paper present important references for the process design and improvement of comprehensive performance of opacifier-doped silica aerogels.

This paper provides an insight into the heat transfer in the solid backbone of aerogel and an effective approach to model the apparent solid thermal conductivity of aerogels. First, a model for the thermal conductivity of aerogel solid backbone is developed based on thermal constriction resistance between interconnected nano-particles by taking into account the size effect of a single particle, and validated by the numerical solutions of the gray Boltzmann transport equation (BTE). Then, combined with the analytical expression derived based on the Laplace heat conduction equation, the proposed model is used to predict the apparent solid thermal conductivity of aerogels, and the predictions are in good agreement with the available experimental data of different kinds of aerogels.

It is challengeable to obtain the stable and accurate solutions of convection-diffusion problems with large Peclet number (Pe) since the convection term may cause oscillation solutions at large Pe. In this paper, a unit operator (first level) and an orthogonal project operator (second level) are constructed to act as the stability terms for meshless local Petrov-Galerkin (MLPG) method, which is called a two-level variational multiscale MLPG (VMS-MLPG) method. The VMS-MLPG method is applied to eliminate oscillation, overshoots and undershoots of MLPG method at large Pe. The prediction accuracy and the numerical stability of the proposed method for the Smith-Hutton and the Brezzi problems are analyzed and validated by comparing with the MLPG method and the finite volume method (FVM) with various difference schemes. It is showed that the present VMS-MLPG method can guarantee the stable and reasonable solutions of convection-diffusion problems with large Peclet number.

This paper is aiming at numerically demonstrating the interrelationship and consistency between field synergy principle (FSP) via the field synergy number (Fc) and the entransy dissipation extremum principle (EDEP). Numerical simulation is conducted by using the FLUENT software and the user defined function programs (UDF) for fin-and-tube surfaces (plain plate and slotted fins) and composite porous materials. The thermal boundary conditions include given heat flux and given surface temperature. The flow includes laminar and turbulent. The air properties may be constant or vary with temperature. Based on the numerical data the analyzed results from the FSP via Fc are totally consistent with the results analyzed by the EDEP for all the cases studied. Such consistency between the FSP and the entransy theory can be regarded as a kind of demonstration of the reliability and correctness of both the FSP and the entransy theory.

Separation of particles and cells is an important function in many biological and biomedical protocols. Although a variety of microfluidic-based techniques have been developed so far, there is clearly still a demand for a precise, fast, and biocompatible method for separation of microparticles and cells. By combining acoustics and hydrodynamics, we have developed a method which we integrated into three-dimensional acoustofluidic tweezers (3D-AFT) to rapidly and efficiently separate microparticles and cells into multiple high-purity fractions. Compared with other acoustophoresis methods, this 3D-AFT method significantly increases the throughput by an order of magnitude, is label-free and gently handles the sorted cells. We demonstrate not only the separation of 10, 12, and 15 micron particles at a throughput up to 500 μl min-1 using this 3D-AFT method, but also the separation of erythrocytes, leukocytes, and cancer cells. This 3D-AFT method is able to meet various separation demands thus offering a viable alternative with potential for clinical applications.

Particle reinforced materials are widely used in many areas, including energy storage, energy saving, thermal protection, and thermal management. These applications tend to require an accurate foreknowledge of the effective thermal conductivity of these materials. Although there exist many different effective thermal conductivity models for this kind of materials, a general and accurate model for the composites reinforced by different types of spherical particles, including mono-particles, hybrid particles, and core-shell particles, is still lacking. This paper firstly collected some commonly used effective thermal conductivity models and tested their scope of use by numerical results. Then, a general and easy-to-use effective thermal conductivity model was provided by extending the model of Meredith and Tobias. This extended Meredith and Tobias's model was proven to be able to accurately predict the effective thermal conductivity of multi-phase composites reinforced by different types of spherical particles with large ranges of particle volume fraction and thermal conductivity ratio of particles to the matrix.

In this paper, a modified model for predicting the gaseous thermal conductivity in aerogel pores is proposed based on the kinetic theory of gases and the data from Direct Simulation Monte Carlo (DSMC) method. With the proposed modified model and the classical Effective Medium Theory (EMT) model, a gas-contributed thermal conductivity model which includes the equivalent gaseous thermal conductivity and the solid-gas coupling thermal conductivity is derived. The present gas-contributed model is validated by available experiment results for different types of aerogels. Comparisons between the present model and existing models show that the present gas-contributed model has a higher accuracy without complex calculations and assumptions.

This paper numerically investigated the coupled flow and heat transfer of a parabolic trough collector (PTC), with the non-uniform heat flux boundary condition on the absorber wall and the rarefied gas effects in the annular vacuum gap being taken into consideration. A fully coupled cross-sectional heat transfer model is established with Direct Simulation Monte Carlo (DSMC) method for the rarefied gas flow and heat transfer in the vacuum annual gap. The PTC tube efficiency can be obtained from the above simulation for a given HTF temperature. Such simulation is conducted for several specified HTF temperature and different efficiency data are obtained. These data are fitted by an equation. This equation is then used to advance the HTF temperature in the axial direction. In such a way a simplified 3D model for the design of a PTC receiver is obtained. Cross-sectional simulation results show that when the gas pressure is less than 0.1 Pa further decrease in pressure makes no further contribution to reduce the heat loss. The effects of periphery non-uniform distribution of heat flux, coating material emissivity, envelope diameter and HTF inlet velocity on the PTC efficiency are discussed. An operation variant is proposed by using the 3D model by which the total PTC tube length can be reduced for a given thermal load.

The present work focuses on the fully developed mixed turbulent flow and heat transfer in receiver tube heated by non-uniform heat flux, especially the effect of local buoyancy force induced by the non-uniform heat flux at Reynolds number of 2 × 104–105, Prandtl number of 1.5 and Grashof number of 0–1012. The friction factor and Nusselt number between forced convection and mixed convection under uniform heat flux and non-uniform heat flux are analyzed quantitatively. The effect of solar elevation angle on the fluid flow and heat transfer is also investigated. It is concluded that the mixed fluid flow and heat transfer under non-uniform heat flux is different from that under uniform heat flux. The solar elevation angle has strong influence on the mixed fluid flow and heat transfer characteristics. A criterion for the buoyancy free is proposed. It is not feasible to perform the heat transfer design and prediction for parabolic trough solar collector based on the experimental correlations for forced convection or conventional mixed convention.

Nanoparticle-based materials (NBMs), whose thermal insulating performance is dominated by the heat transfer between two neighboring nanoparticles, have attracted tremendous attention in recent decades due to their potential applications in thermal insulation. This work provides an insight into the conductive heat transfer between two touching solid or hollow nanospheres by solving the gray phonon Boltzmann transport equation (BTE). The influence of some factors, including particle size, contact ratio, and shell thickness on the temperature distribution and effective thermal conductivity of two touching particles, is examined. Based on the thermal constriction resistance theory and numerical results, a prediction model for the effective thermal conductivity of two touching solid nanospheres is then proposed and proved to have high accuracy with a relative error less than 3.0%. Finally, a brand new effective thermal conductivity model for two touching hollow nanospheres with the same form as that of two touching solid spheres and an acceptable prediction error of 6.5% is also proposed.

A fractal permeability model considering both the tortuous gas transport path and the gas slippage effect for nanoporous silica aerogel is developed. The fractal model is verified by experimental results and existing models. Then, a one-dimensional macro model, which combines the influence of heat conduction, thermal radiation and gas permeation, is developed to investigate the influence of gas permeation induced by pressure gradient on transient heat transfer within bulk silica aerogel with the acquired gas permeability by the present model. It is found that gas transport within silica aerogel exerts important effect on the temperature response performance of bulk silica aerogel, which is mainly reflected on changing the pressure distribution and inducing energy migration within the bulk material. For silica aerogel, energy migration has remarkable effect on unsteady heat transfer process when the gas permeability reaches the order of 10−14 m2. The dynamic temperature response of the cold wall will be strengthened when the directions of gas flow and heat transfer are the same, and vice versa.

Nanoporous insulating materials are of importance for applications such as energy-efficient buildings, energy storage and savings, and cryogenic engineering. Recent progress on manufacturing technology has enabled the fabrication of ordered nanostructures to be practical, providing a possibility to fabricate high-performance insulation materials. In this work, three kinds of nanoporous insulating materials with regular geometric structures and controllable thermal conductivities, including a simple cubic packing, a face-centered cubic packing, and a cubic array of intersecting spheres packing of uniform-sized hollow nanospheres, were designed. The effective thermal conductivity models of each packing structure were developed according to the assumption of one-dimensional heat transfer, in which the following factors including material types, size of the hollow nanosphere packing structure (e.g., sphere size, spherical shell thickness, contact ratio), gas pressure, the rarefaction effect of gas and the mean free path of phonons were considered. The developed models of the hollow nanosphere packing structures were validated by the experimental results from the literature. It is found that the effective thermal conductivity can be lowered to ~ 0.01 W/(m⋅K) by tuning the packing style and size of hollow nanosphere packing structures in the room environment, showing excellent thermal insulation performance. The work provides a new strategy for the design of super-insulation materials.

Radial graded porous volumetric solar receiver is designed to match the non-uniform solar flux distribution. Based on the computed tomography and image-processing techniques, uniform and radial graded porous volumetric solar receivers are reconstructed. The 3D printing technique and suitable post processing are implemented to fabricate complex porous samples using super-alloy Inconel 718 as material. Both experimental and numerical studies are conducted to investigate the fluid flow and heat transfer processes in porous volumetric solar receivers. The results present that the 3D printed porous samples are suitable for solar thermal energy absorption and high temperature utilization. As for uniform porous receivers, porous media with small pore diameter has larger thermal efficiency because of enhanced convective heat transfer. Compared with the uniform porous receiver with highest thermal efficiency, the radial graded porous volumetric solar receiver with large pore diameter inside could further relatively increase the thermal efficiency by 4.1% while relatively decreases the flow resistance by 8.6%. The reasonable distribution of pore diameter of porous media could regulate the mass flow distribution and direct more air to the high heat flux region. Moreover, local overheating phenomenon is observed in the uniform porous receiver using air as heat transfer fluid. By applying the coupled optimization method, an optimum pore diameter distribution is determined for the radial graded porous volumetric solar receiver.

Correction for ‘High-throughput cell focusing and separation via acoustofluidic tweezers’ by Mengxi Wu et al., Lab Chip, 2018, 18, 3003–3010, DOI: 10.1039/C8LC00434J.

A thermal conductivity test apparatus based on transient plane source method is built and developed to measure effective thermal conductivity of open porous materials at different gas pressures. The effective thermal conductivity of open nano-porous silica materials with porosity of 88.5% is measured under gas pressures ranging from 0.001 Pa to 1 MPa. The contribution of gaseous heat conduction to the effective thermal conductivity of materials is decomposed by subtracting the effective thermal conductivity at ultimate vacuum from that at different gas pressure. It is found that the contribution of gaseous heat conduction is much different with the gas thermal conductivity in nano-porous materials and that in free space. The result is also demonstrated by theoretical analysis and numerical simulation.

The present work focuses on the fully developed mixed turbulent flow and heat transfer in receiver tube heated by non-uniform heat flux, especially the effect of local buoyancy force induced by the non-uniform heat flux at Reynolds number of 2×104-105, Prandtl number of 1.5 and Grashof number of 0-1012. The friction factor and Nusselt number between forced convection and mixed convection under uniform heat flux and non-uniform heat flux are analyzed quantitatively. The effect of solar elevation angel on the fluid flow and heat transfer is also investigated. We found that it is not feasible to perform the heat transfer design and prediction for parabolic trough solar collector based on the experimental correlations for forced convection or traditional mixed convention.

Guyer-Krumhansl (G-K) equation is a promising macroscopic model to explore heat transport in nanoscale. In the present work, a new nonlocal characteristic length is proposed by considering the effects of heat carriers-boundaries interactions to modify the nonlocal term in G-K equation, and a slip heat flux boundary condition is developed based on the local mean free path of heat carriers. Then an analytical solution for heat flux across 2-D nanolayers and an in-plane thermal conductivity model are obtained based on the modified G-K equation and the slip heat flux boundary. The predictions of the present work are in good agreement with our numerical results of direct simulation Monte Carlo (DSMC) for argon gas nanolayer and the available experimental data for silicon thin layers. The results of this work may provide theoretical support for actual applications of G-K equation in predicting the thermal transport properties of nanolayers.

Continuous improvement of energy-efficient systems has led to various technical features such as miniaturization, integration, and portability. This development became the leading trend of the contemporary industry, with the evolvement of various new and miniaturized instruments such as micro-electro-mechanical systems and micro-satellite. Highly miniaturized and integrated electronic devices often release a large amount of heat within their micro scale inter-components areas, which under certain circumstances, may lead to the micro-device’s functional failure. This calls for techno-scientific efforts for efficient heat removal from the mentioned devices. Numerous innovatively scenarios have been so far explored to satisfy the greatly increasing heat removal demand. This paper explores two major types of heat removal technics for micro-scale single-phase flow, namely passive and active type. The passive type includes the geometry modification and working media change, where corresponding effects on heat transfer performance and fluid flow are addressed by Nusselt number and friction factor, respectively. Their most important advantages are stability and robustness in operation or integration with complex systems. For the active type, only the pulsating inlet flow and acoustic wave are considered in the present paper, which are less reviewed recently. Generally, the heat transfer enhancement is attributed to the effect of disturbed thermal boundary layers on mixing of fluids. The present review article is organized in a way that provides a historical perspective of the recent developments in this area, and advances comprehensive comments from overall situations; which may serve a substantial guidance for researchers in different background who intend to step into this area.

In order to reduce the thermal radiation and further deter the heat transfer in silica aerogel thermal insulation materials, the endothermic opacifier is designed with the shell of SiC and the core of phase change material (PCM) in this paper. Then the transient heat transfer characteristics of the endothermic opacifiers doped silica aerogel (EOSA) are numerically investigated and its short-term thermal insulation performance is estimated with a modified lattice Boltzmann method (LBM) model. The modified LBM scheme is established with a new source term to remedy the effect of the variation in the volumetric heat capacity of the specimen, and this scheme is fully validated based on the analytical results and available experimental data. The results demonstrate that adding the endothermic opacifier to silica aerogels results in a significant heat transfer delay and an improvement in short-term thermal insulation performance by suppressing radiation heat transfer and solid-liquid phase change heat absorption. With the same doping amount, the endothermic opacifiers doped silica aerogel exhibits more excellent short-term thermal insulation performance and a lower density than that of SiC opacifiers doped silica aerogels (SOSA). It indicates that the EOSA possesses great potential to be a candidate as the lightweight and efficient thermal insulation material at high temperature. Besides, exhaustively parametric studies based on the EOSA, including doping amount, latent heat of PCM, phase change temperature and endothermic opacifier particle size are conducted, and several design rules are established. This analysis could serve as a framework to develop and design silica aerogel composites with low-density and excellent short-term thermal insulation performance.

In this paper, the heat transfer characteristics of porous material adopted in the receiver of a concentrated solar power (CSP) with different structure parameters are numerically investigated. The commercial software FLUENT and the user defined function program (UDF) are adopted to implement the simulation. The porous material geometry is represented by periodic structures formed with packed tetrakaidecahedron. The air flow and heat transfer characteristics under the boundary conditions of constant heat flux and constant wall temperature are studied. The field synergy principle (FSP) and the entransy dissipation extremum principle (EDEP) are used to analyze the flow and heat transfer performance of the composite porous material. From the numerical results the best composite of the porous material is obtained. The effects of different boundary conditions are revealed. It is also demonstrated that the FSP and the EDEP are inherently consistent.

In this paper, large eddy simulations with the dynamic Smagorinsky eddy viscosity model are performed for a fully developed turbulent flow and heat transfer in a square duct with unstable natural convection on the cross section at a turbulent Reynolds number of 400, a Prandtl number of 0.7 and different Grashof numbers from 105 to 5 × 107. The MPI parallel programming language is implemented to speed up the calculations. The influences of the buoyancy force on the mean flow and heat transfer, turbulent intensity and Reynolds stresses are analyzed. The results show that the mean velocity decreases while the subgrid viscosity, turbulent intensity and heat transfer increase obviously with the increase of Grashof number. The turbulent intensity in the streamwise direction decreases considerably in the near-wall region while the turbulent intensity in the spanwise direction increases clearly in almost whole regions with an increase in Grashof number. The spatial distribution of Reynolds stresses are mainly influenced by the thermal buoyancy force. The turbulence production rate and the buoyancy force production term of Reynolds stress component 〈v′v′〉 increase with the increase of Grashof number.

The research on the near-field thermal radiation between two nanoparticles separated by nanoscale distance is of great significance and is closely related with the materials design. The remarkably increased radiation energy transfer may be harmful to the performance of super-insulating materials such as silica aerogels with nanoporous structures. Based on the fluctuation–dissipation theorem, this article analytically investigates the influences of important parameters including the distance, particle radius, dielectric function and doping levels on the near-field heat transfer between two spherical nanoparticles. By tuning these parameters, it is expected to control the near-field thermal radiation. This study will help in investigating the heat transfer of group nanoparticles and developing novel insulating materials with special properties.

The pool boiling heat transfer performance of refrigerant R134a on single horizontal tube surfaces sintered with open-celled copper foam was studied at T s = 6 °C and P s = 3.62 bar. Three pore density values: 40, 80, and 130 PPI; two porosity values: 90% and 97%; and two thickness values: 1.6 and 2.5 mm, characterize the 12 studied tubes. The nucleate boiling data and heat transfer coefficients were obtained. The influences of pore density, porosity, and foam thickness on heat transfer performance were investigated as well. The foam provides activated nucleation sites; however, it also causes transport resistance for the bubbles within the foam to move from the active nucleation sites to the surface. Performance is dependent on whichever of these two prevails in the heat transfer process. The foam-coated tubes show significant performance enhancement compared with plain tubes at heat fluxes below 30 kW/m 2 . However, a sharp reduction in heat transfer coefficient is encountered for 130 PPI at larger heat fluxes. Tubes coated with thin foam and high porosity, e.g., pore density: 80 PPI, porosity: 97%, and thickness: 1.6 mm, exhibit a comparatively superior heat transfer performance. The current experimental data provide useful information for the design of highly effective boiling surfaces.

Direct numerical simulations (DNS) of turbulent flow and convective heat transfer in a square duct with axial rotation were carried out. The pressure-driven flow is assumed to be hydrodynamically and thermally fully developed, for which the Reynolds number based on the friction velocity and hydraulic diameter is kept at constant (Reτ = 400). In the finite length duct, two opposite walls are perfectly insulated and another two opposite walls are kept at constant but different temperatures. Four thermal boundary conditions were chosen in combination with axial rotation to study the effects of rotation and Grashof number on mean flow, turbulent quantities and momentum budget. The results show that thermal boundary conditions have significant effects on the topology of secondary flows, profiles of streamwise velocity, distribution of temperature and other turbulent statistic quantities but have marginal effects on the bulk-averaged quantities; Coriolis force affects the statistical results very slightly because it exerts on the plane normal to main flow direction and the rotation rate is low; Buoyancy effects on the turbulent flow and heat transfer increase with the increase of Grashof number (Gr), and become the major mechanism of the development of secondary flow, turbulence increase, and momentum and energy transport at high Grashof number.

A comprehensive numerical study of the equivalent thermal conductivity of a multi-holed clay brick with the size of 240 × 115 × 90 (in mm) has been conducted, and 50 kinds of combination of holes and arrangements are examined. The indoor–outdoor temperature difference varies from 50 °C to 20 °C. The effects of following factors are studied in details: the hole surface radiation, the width-wise and length-wise hole numbers, and the indoor–outdoor temperature difference. The major findings are as follows: (1) the radiation between hole surfaces has some effect on the equivalent thermal conductivity, thus it should be taken into account; (2) the hole number and arrangement affect the thermal conductivity in a rather complicated manner. Analysis shows that depending on the relative importance of natural convection, surface radiation and heat conduction through the clay solid, the thermal conductivity may decrease with the hole number or increase with the hole number and (3) among the 50 kinds of combination, the optimum configuration is found which has five length-wise holes, four width-wise holes and all the holes are from bottom to top in the depth direction of a brick. Its equivalent thermal conductivity is 0.419 W/(m K), which is only 53.1% of solid clay of which it is made, showing great energy-saving possibility if it is adopted in the construction of building wall. Detailed discussion of the simulated results is conducted and flow field and temperature distributions are also provided for some typical configurations.

In this paper, the basic idea of the field synergy principle (FSP) is briefly reviewed and is validated experimentally by incompressible flow through a square duct with an imposed temperature difference between vertical walls and perfectly insulated on the horizontal walls. This creates a situation where the steamwise flow velocity is normal to the cross section temperature gradient. The experimental results show the independency of crosswise heat transfer rate on the steamwise flow velocity. Detailed discussion is provided to account for some minor deviation from the expected results of FSP.

In the finite volume approach for incompressible flow and heat transfer, the discretization of the convection term and the treatment of the coupling between velocity and pressure are the two major issues affecting solution stability, accuracy and convergence rate. In this review paper, some recent advances made in the CFD/NHT & Enhanced Heat Transfer Center of Xi'an Jiaotong University, China, are presented and their applications in the study of heat transfer enhancement are also briefly summarized.

Experimental and numerical studies were conducted to investigate the forced convection and flow friction of a turbulent airflow in a horizontal air-cooled rectangular duct, with square-sectioned cross-ribs mounted on its bottom surface. Cross-sectional dimensions of the cross-ribs were 6.37 mm × 6.37 mm. Reynolds number of the fully turbulent flow was maintained constant at 12,380. Heat was supplied uniformly to the airflow via bottom surface of the duct only. Effects of varying the angle formed by the cross-ribs between 30° and 120° on the forced convection and flow friction were studied. It was found that an optimum angle corresponding to the highest heat transfer coefficient occurred between 60° and 70°. Computational predictions of forced convection and flow friction of the same rectangular duct mounted with cross-ribs were performed with the zonal k– e model, the stability-guaranteed second-order difference scheme and the block implicit method. A comparison with the experimental results indicated that a reasonably good agreement had been achieved. Existence of an optimum angle formed by the cross-ribs between 60° and 70° was also indicated, except the numerical predictions were lower than the experimental findings by approximately 2–10%. However, the flow friction had been slightly over-estimated by 2–11% with the numerical model. It was shown that forced convection could be enhanced sufficiently by mounting cross-ribs on the internal surfaces of a rectangular duct, especially when the optimum angle was used.

In this paper, the Diffusion Limited Cluster Aggregation (DLCA) method is employed to reconstruct the three-dimensional network of silica aerogel. Then, simulation of nitrogen adsorption at 77 K in silica aerogel is conducted by the Grand Canonical Monte Carlo (GCMC) method. To reduce the computational cost and guarantee accuracy, a continuous-discrete hybrid potential model, as well as an adsorbed layer thickness estimation method, is employed. Four different structures are generated to investigate impacts of specific surface area and porosity on adsorptive capacity. Good agreement with experimental results is found over a wide range of relative pressures, which proves the validity of the model. Specific surface area and porosity mainly affect nitrogen uptake under low pressure and high pressure, respectively.

Vacuum tubular receiver is the core component of parabolic trough solar collectors (PTCs). To enhance the heat transfer rate from solar radiation to heat transfer fluid (HTF), one of the effective techniques is to improve the convective heat transfer inside the inner tube. In this study, we focus the heat transfer of the HTF side. A numerical simulation on the fully developed turbulent flow and heat transfer in the inner tube with and without helical fins, protrusions and dimples has been investigated. The results show that the receiver tubes with dimples have superior performance of heat transfer augmentation compared with that with protrusions or helical fins. Then, the effects of the geometry sizes and arrangements of dimples on the convective heat transfer performance are further studied. The performance evaluation plot of heat transfer enhancement techniques show that dimples with deeper depth, narrower pitch and more numbers in the circumference direction is benefit for improving the performance of heat transfer enhancement while the varied arrangements have no obvious influence.