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Hydrogen storage plays a fundamental role in the future hydrogen energy system, and carbon aerogel is one of the most potential hydrogen storage materials because of its high gravimetric and volumetric density on hydrogen adsorption. In this paper, the amorphous structure of carbon, obtained by a numerical simulation process by using the molecular dynamic and Monte Carlo methods, as well as the primary unit method, was intercepted as a sphere structure for numerical annealing to build a carbon nanosphere, which serves as the basic unit to reconstruct the carbon aerogel's skeleton by the Diffusion Limited Cluster Aggregation (DLCA) method. The hydrogen adsorption in carbon aerogel was simulated by using the self-coding parallel grand canonical Monte Carlo (GCMC) method. The influences of particle diameter, density, temperature, pressure, and specific surface area on the hydrogen adsorbing capacity in carbon aerogel were analyzed in detail. The results showed that the carbon aerogel's hydrogen storage capacity with a specific surface area of 2680 m2/g could reach 4.52 wt % at 77 K and 3.0 MPa.

A voltage-driven cathode sheath model in an atmospheric-pressure argon arc discharge is developed in the framework of an implicit particle-in-cell Monte Carlo collision (PIC–MCC) method. Plasma transport processes are solved numerically in one dimension without any local-equilibrium hypotheses, in particular, without explicitly dividing sheath and a quasi-neutral plasma region. The right boundary of the computational domain located at the pre-sheath is determined first by observing the variation in typical parameters. A comparison of results is given with different positions of the right boundary to study the plasma transport processes in the cathode sheath. Number densities, spatially averaged energies, electric field and potential, collision frequency, heating rate of electrons, as well as the spatially averaged electron energy probability function inside the sheath, are predicted self-consistently based on this newly developed kinetic model. It is shown that both excitation collisions and ionization collisions occur inside the sheath, and collision frequency of the former is larger than the latter. The collision frequency of charge exchange is higher than that of elastic collision for ions. In addition, the effects of different electron emission processes are described. It is indicated that the thermionic emission on the hot cathode surface is not the only significant emission mechanism to sustain the arc discharges.

A two-level variational multiscale meshless local Petrov-Galerkin (VMS-MLPG) method is presented for incompressible Navier-Stokes equations based on two local Gauss integrations which effectively r...

A gradually-varied porous structure is designed to increase the thermal performance of the porous volumetric solar receiver. Based on the replica method and multilayer recoating technique, the silicon carbide porous ceramic with linear-changed geometrical parameters is fabricated. The performances of the uniform and gradually-varied porous volumetric solar receivers are studied by both experiment and numerical simulation. An optimization method combining genetic algorithm and computational fluid dynamics analysis is applied to determine the optimum porosity distribution. The results present that porous volumetric solar receiver with linear-changed geometrical parameters exhibits better thermal performance than the uniform porous volumetric solar receivers, especially when the thickness of the receiver is small. Larger porosity in the front is beneficial for increasing the solar radiation penetration depth, which limits the reflectance and thermal radiative losses. Smaller porosity in the rear traps more solar radiation and increases the convective heat transfer. When the receiver’s thickness is larger, the performance of the gradually-varied volumetric solar receiver is nearly identical to that of the uniform receiver with largest porosity. The double-layer configuration is found to be the optimized structure of the gradually-varied porous volumetric solar receiver. The thermal efficiency could be further improved using genetic algorithm with an 11 K increase of the outlet temperature.

The method of successive under-relaxation (SUR) is an effective way for numerically solving a nonlinear system of equations governing the fluid flow and heat transfer, resulting in stable convergen...

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.

It is an important and challenging work to measure the energy flux density distribution of the concentrated radiation during the concentrating solar power applications. In order to evaluate the performance of a multi-lamps high-flux solar simulator, a novel flux mapping system based on the indirect method has been developed. It features two Lambertian targets. One is a stationary water-cooled Lambertian target where there is a circular hole in the center used to install a flux sensor. The other is a movable Lambertian target used to cover the flux sensor when shooting the concentrated light beam image. This kind of design can obtain the gray value of flux sensor region directly and does not require the interpolation in the sensor-influencing area. The design theory and principle, the hardware implementation and the experimental validation of this flux mapping system have been presented in detail. The repeatability experiments and the error analyses showed that the total relative errors of this flux mapping system were ±8.1% with a repeatability of 1.1%, and ±8.5% with a repeatability of 2.7%, for evaluating the flux and the total radiant power, respectively.

The near-cathode region plays a crucial role in exploring the transport characteristics of the transition from arc column to the hot cathode in atmospheric-pressure arc discharges because of the existing non-equilibrium phenomena. A one-dimensional unified model, including the near-cathode region and the cathode body, is developed for an argon arc discharge with the tungsten cathode at atmospheric pressure in this paper. The electrostatic model coupled with an external circuit in the near-cathode region is solved based on the implicit particle-in-cell coupled Monte Carlo collision method without any assumptions of thermal or ionization equilibrium or quasi-neutrality. A detailed description of the arc plasma–cathode and cathode–gas interactions is obtained by calculating the nonlinear heat conduction equation in the cathode. It is shown that the space-charge sheath strongly affects particle transport in the near-cathode region and energy transport from arc plasma to the thermionic cathode. The total current density has significant effects on the kinetic characteristics of arc plasma by feedback-like mechanisms. The Joule heating by the external circuit and charged particles deposited into the cathode are dominating mechanisms of energy transfer from the near-cathode region to the cathode, while energy loss by radiation is more significant compared with natural convection.

To improve the spatial uniformity of the concentrating solar simulators, a concept of non-coaxial deflection angle was introduced to the typical ellipsoidal reflectors. Based on this idea, a new-type 42 kWe high-flux non-coaxial concentrating solar simulator was designed and built. The Monte Carlo ray-tracing technique was applied to optimize the value of the non-coaxial deflection angle and simulate the flux distribution of this new-type solar simulator. A flux mapping system based on the indirect method was used to characterize the solar simulator optically. The relative deviation for the measured and the simulated results of this new-type solar simulator, as well as the simulated results of a conventional concentrating solar simulator, were compared and analyzed. The results show that the spatial nonuniformity of this new-type solar simulator over a circular target of 50 mm in diameter, improved to 7.2% from 40.3% of a conventional one. This non-coaxial concentrating solar simulator is considered very suitable for high-temperature solar thermal, thermochemical and high-concentration photovoltaic applications, especially where there are strict requirements for spatial uniformity.

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.

Hollow fiber-based porous structures (HFPSs) demonstrate promising potentials in thermal insulations. However, the thermal transport in HFPSs was not explored yet. Herein, we developed a unit cell model to explore the effect of both geometric and thermophysical parameters on the effective thermal conductivity of HFPSs. The predictions from the developed model agree with the experimental data in the literature. The modeling results show that decreasing external hollow fiber diameter, shell thickness, the thermal conductivity of solid backbone, and gas pressure can reduce the effective thermal conductivity of the HFPSs. The effective thermal conductivity of the HFPSs trends to that of stationary air (0.026 W/(m⋅K), 300 K, 1.0 atm) as porosity increases. Reducing the pore size to nanometer-scale or decreasing the gas pressure is the most effective way to achieve a thermal conductivity below that of stationary air. This work guides the structural design and optimization of HFPSs as lightweight super-thermal insulating materials.

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.

A meshless local Petrov–Galerkin (MLPG) approach based on the streamline upwind (SU) idea and the variational multiscale (VMS) concept, called as VMS-SUMLPG method, is herein proposed to solve the ...

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.

An experimental investigation on the pool boiling heat transfer of refrigerant R134a outside three enhanced tubes is conducted. The heat flux is from 10,000 to 370,000 W / m 2 . The heat transfer is substantially enhanced at the heat flux less than 200 kW/m2. An increase of heat transfer coefficient up to 330% above the plain tube is observed. However, at the heat flux higher than 200 kW / m 2 , it is found that the heat transfer coefficient of enhanced tubes is even lower than the plain tube. The same features are also observed for other enhanced tubes in the literature.

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.

Fiber-reinforced composites are attractive for many applications in energy fields, such as thermal energy storage and building energy-saving. In these applications, their effective thermal conductivity is extremely important; however, research addressing the effect of various parameters on effective thermal conductivity is scarce. In this paper, the influences of different parameters, including volume fraction, aspect ratio, and orientation of fibers, and the thickness of coating layers on the effective thermal conductivity of fiber-reinforced composites, are numerically investigated by the Lattice Boltzmann method. Based on numerous numerical results, a correlation of the effective thermal conductivity is proposed for the composites with fibers randomly distributed in space. It is found that the thermal conductivity of fiber and coating layers are the two most dominant factors which influence the effective thermal conductivity of fiber-reinforced composites. The thickness of the coating layer affects the effective thermal conductivity of composites with fibers randomly distributed in space remarkably, while its effect on the effective thermal conductivity of composites with fibers arranged perpendicular to the heat transfer is negligible. The results of this work could provide important references for the process design and improvement of thermal performance of fiber-reinforced composites.

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.

A physically consistent difference scheme is proposed to discretize the convection–diffusion equation in this paper. The interface variables of control volume are calculated by the interpolation of the discretized convection–diffusion equation rather than the direct interpolation with neighbor nodes in the direction perpendicular to the interface. In the new scheme, all the nodes of parallel and normal to the interface are involved, which means that the influence of cross-stream fluxes is considered. Obviously, the proposed interpolation scheme is full of physical meaning and better than the traditional convective interpolation schemes. Two simulations of typical benchmark problems, lid-driven cavity flow in square cavity and flow over a backward-facing step, are carried out with this scheme and some traditional schemes to prove the advantages of the proposed scheme.

The fully-developed mixed turbulent convective heat transfer characteristics in dimpled tubes of parabolic trough receiver are numerically studied at a certain Reynolds number of 2 × 104 and different Grashof numbers ranged from 0 to 3.2 × 1010 to produce substantial surface heat transfer augmentations with relatively small pressure drop penalties. The Boussinesq approximation is applied, in which variations in fluid properties other than density are ignored. The Realizable k-e two-equation turbulence model with enhancement wall treatment is adopted. The influences of outer wall heat flux distributions and dimple depth on flow resistance and heat transfer rate are illustrated and analyzed. The results indicate that the average friction factor and Nusselt number in dimpled receiver tubes under non-uniform heat flux (NUHF) are larger than those under uniform heat flux (UHF). In most cases, the comprehensive performance of dimpled receiver tube under NUHF is also better than that under UHF. The deep dimples (d/Di = 0.875) are far superior to the shallow dimples (d/Di = 0.125) at a same Grashof number.

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.

This study combines both concepts of solar ventilation technology and solar air collector. This is a quite innovative and potential facility to effectively use thermal energy and reduce the accumulation of heat in the indoor space simultaneously. The purpose of this study is to create a prototype and implement the experiments. Computational fluid dynamics (CFD) approach is employed to validate the characteristics of the flow and heat transfer. For the accuracy of numerical predictions, the method of Solar Ray Tracing was used for thermal radiation flux as boundary condition on the wall. The local heat transfer correlation was investigated to predict surrounding wind speed upon device cover. Three sorts of glasses and several aspect ratios of flow channels have been compared to conclude the optimal configuration. In addition, four important factors, such as the stagnant layer thickness, emissivity on the illustrated surface, mass flow rate and the height of the device, are also considered and discussed in detail. The result showed that the optimal design is dominated by the combination of an aspect ratio of 50 mm:10 mm, and appropriate mass flow rate to the height of the device. The present work on thermal energy collection can assist us in designing a powerful solar air collector in some potential applications.

The intermediate heat exchanger (IHX), which consists of thousands of tubes assembled along concentric circles, has a complex geometry that makes it extremely difficult to analyze through the 3-D CFD approach, especially when there exists coaxial expansion bends. The motivation of this work is to study its thermal hydraulic characteristics and figure out the influence of the coaxial bends on its performance. In addition, both of the 3-D detail simulation method and the distributed parameter method are employed in this research. And the respective results by these two methods show that the shell-side main flow direction is generally consistent with the tube direction and the rear straight tube section is the main working section with the heat transfer rate of nearly 45% of the total heat transfer rate, and that the bending section plays an important role in controlling the non-uniformity of temperature field.

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.

In this paper, a numerical investigation is presented, aiming at the effect of the buoyancy force induced by the non-uniform heat flux on the laminar flow and heat transfer characteristics in the solar receiver tube of parabolic trough collector. The flow and heat transfer performances are analyzed for forced and mixed laminar convection in receiver tube heated by uniform and non-uniform heat fluxes with different Grashof numbers, Reynolds numbers and solar elevation angles. The results show that the natural convection can increase heat transfer rate of laminar forced convection by more than 10% when the Grashof number is greater than a threshold value. The mixed fluid flow and heat transfer characteristics vary with solar elevation angle. Heat transfer deterioration occurs when the Richardson number is greater than 12.8.

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.

The intermediate heat exchanger (IHX) is a type of shell-and-tube heat exchanger. It is assembled with thousands of straight or expansion bend tubes in concentric circles and extremely huge and complex in geometry which leads to the non-uniform flow in the shell-side. The motivation of this work is to design a 6 m long, 290 MW IHX and clarify the influence of the tube specification on its performance. Traditional thermal design method is employed here for the design process, while 2-D porous medium model with anisotropic properties is adopted for simulation to support the design process adjusting the tube specification. The results show that the tube specification mainly affects the flow uniformity in the inlet region but has little effects on outlet region flow distribution. Comparing the IHX thermal-hydraulic performance under different tube specifications, it is found that the specification with 19 mm outer diameter, 25 mm radial pitch and 0.8 mm wall thickness is the best choice.

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.

Silica aerogel has attracted great interest in thermal insulation applications due to its ultralow thermal conductivity. However, silica aerogel is transparent to the infrared radiation in the range of 3-8 µm, making them be not suitable for high temperature thermal insulation applications. Here, we developed an optimization method considering both thermal radiation and heat conduction to design the geometric structures of aerogel composites with minimized thermal conductivity. The results show that when the ambient temperature is lower than ~ 600 K, the additives with low thermal conductivity are preferred. When the ambient temperature is higher than ~ 600 K, the additives with high extinction coefficients are needed. The additives with a broad size distribution could enable the aerogel composites to have an optimal thermal insulation performance in the environment with a changing temperature. The work provides a guideline for the geometric design of aerogel composites for high temperature thermal insulation applications.

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.

Recent studies have shown that thermo-field emission is a dominating electron source in microdischarges at the cathode temperature far above room temperature. However, few researches focus on the post-breakdown nature of microdischarges. In order to explore the post-breakdown characteristics of thermo-field emission driven microplasma, a one-dimensional implicit Particle-In-Cell with Monte Carlo Collision method is adopted and updated by considering the thermo-field electron emission to investigate the kinetic characteristics of direct-current argon microdischarges at atmospheric pressure. The fundamental properties of the microplasmas, such as electric field, particle number density, averaged particle temperature and current density, are analyzed in the post-breakdown regime. In addition, the sheath behavior is investigated to further observe how the space charge affects the thermo-field emission. The results indicate that thermo-filed emission driven micro-scale discharges can produce high current density and high-energy particles with low applied voltage of 20V. The impacts of cathode temperature on enhancing the thermo-field emission are more pronounced compared to the applied voltage and the electrode spacing. The electron energy probability function shows a multi-peak distribution.

The reflectance losses are the predominate energy losses in solar radiation transfer process for the porous volumetric solar receiver. However, few studies pay attention to the influence of geometrical parameters of porous media on the reflectance losses, and the data of reflectance losses of porous volumetric solar receiver in solar spectrum is not complete. In this paper, both experimental and numerical simulation methods are applied to comprehensively investigate the reflectance losses of the porous volumetric solar receiver. A series of silicon carbide reticulated porous ceramics (SiC RPCs) is fabricated by replica method and recoating technique. The reflectance losses are measured based on UV-VIS-NIR spectrophotometer. Meanwhile, porous models with different geometrical parameters are artificially reconstructed. Monte Carlo Ray Tracing method is used to calculate the total reflectance. The results present that the SiC RPCs exhibit small reflectance losses in the solar spectrum with a peak at about 420 nm. The geometrical parameters, such as porosity and pore diameter do not change the spectral behavior but only influence the magnitude of the reflectance. Larger porosity and larger pore diameter are beneficial for reducing the reflectance losses. The correlation of the reflectance losses as functions of pore density and porosity has been developed. Furthermore, the influence of the incident angle of radiation on reflectance losses is studied. Relatively large increase is observed as the incident angle is larger than 30°. This phenomenon becomes more obvious for the porous media with larger porosity or larger pore diameter.

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.

In this work, the condensation of refrigerants on a single, high-density, low-fin tube and full-sized shell and tube condensers were investigated experimentally. The low-fin tube had an external fin density of 56 fins per inch (fpi) and fin height 1.023 mm. Another three-dimensional (3D) finned tube was also tested for comparison. The condensing heat transfer coefficient of the refrigerant R134a was first investigated outside a single horizontal tube at saturation temperature of 40 °C. The overall heat transfer coefficients of the two tubes were similar in magnitude. The condensing heat transfer coefficient of the low-fin tube was 16.3–25.2% higher than that of 3D enhanced tube. The experiments of the two condensers mounted with low-fin and 3D enhanced tubes were then conducted in centrifugal and screw chiller test rigs. It was found that chillers with the two different condensers generally had the same refrigeration capacity under the same experiment conditions. The refrigeration capacity of the screw chiller was smaller. It had fewer tube rows and elicited fewer inundation effects owing to the falling condensate. The heat transfer coefficients of the condensers with R134a in centrifugal chillers equipped with high-density low-finned tubes were higher than those in the screw chillers. The total number of tubes for low-fin tube condensers, in the two chillers, was reduced by approximately 15% compared with the use of domestic advanced condensers equipped with the 3D enhanced tubes.

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.