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In order to improve the thermal conductivity of pure paraffin (RT4) which is regarded as cool storage of phase change material (PCM), four kinds of paraffin-carbon nanotubes cool storage of composite PCMs are prepared by adding different amount of carbon nanotube (CNT) into paraffin, and their thermo-physical properties are tested by using differential scanning calorimeter (DSC) and laser thermal conductivity meter. The results show that, with the increasing of carbon nanotubes, the phase transition temperature of the cool storage of composite PCMs diminishes, phase change latent heat dwindles while thermal conductivity gradually increases. Compared with the pure paraffin, increase in mass fraction of carbon nanotube 3% can provide an increase in thermal conductivity of the solid and liquid 30.3% and 28.5%, and a reduce in the melting and solidification phase change latent heat of composite PCM 8.9% and 9.3%, respectively. Further investigation applies the composite PCMs into the shelf of vertical open -type refrigerated display cabinet to improve the performance. The results show that the shelf filled with the composite PCMs can decrease the internal temperature of the VORDC and temperature fluctuation during the defrosting period. Compared with the ordinary shelf, the lengthways and depthwise temperature different of composite shelf filled into only RT4 reduce by 80.0% and 7.6%, respectively; the lengthways and depthwise temperature different of composite shelf filled into 3%-CNT composite PCMs reduce by 92.0% and 12.2%, respectively. It indicates that the composite PCMs further increases the thermal conductivity of composite shelf and improve the performance of VORDC.

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.

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.

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.

Pseudomonas fluorescens 2P24 produces 2,4-diacetylphloroglucinol (2,4-DAPG) as the major antibiotic compound that protects plants from soil-borne diseases. Expression of the 2,4-DAPG biosynthesis enzymes, which are encoded by the phlACBD locus, is under the control of a delicate regulatory network. In this study, we identified a novel role for the outer membrane protein gene oprF, in negatively regulating the 2,4-DAPG production by using random mini-Tn5 mutagentsis. A sigma factor gene sigX was located immediately upstream of the oprF gene and shown to be a positive regulator for oprF transcription and 2,4-DAPG production. Genetic analysis of an oprF and sigX double-mutant indicated that the 2,4-DAPG regulation by oprF was dependent on SigX. The sigX gene did not affect PhlA and PhlD expression, but positively regulated the level of malonyl-CoA, the substrate of 2,4-DAPG synthesis, by influencing the expression of acetyl-CoA carboxylases. Further investigations revealed that sigX transcription was induced under conditions of salt starvation or glycine addition. All these findings indicate that SigX is a novel regulator of substrate supplements for 2,4-DAPG production.

In order to improve the performance of refrigerated display cabinet, decrease the food temperature, improve the uniformity of food temperature and restrain the temperature fluctuation during the defrosting time, a new kind of composite shelf is designed for the food vertical open refrigerated display cabinet (VORDC), which is based on the heat pipe technology and the cool storage characteristics of phase change materials (PCMs) among the heat pipes. Experimental results show that the novel composite shelf can enhance the heat transfer between food and shelf, reduce the core temperature of food, decrease temperature fluctuation in defrosting process and reduce the temperature difference along height, width and depth direction. Compared with the normal shelf, the temperature differences of food packages on both sides of the composite shelves (RT3, RT4 and RT5) are reduced by 40%, 80% and 40%, respectively. The average temperature of food packages is reduced by 13.7–32% and the temperature fluctuation of food packages is reduced by 53.3–83.3% during defrosting. For these kinds of composite shelves, the temperature of food packages of RT4 composite shelf is lowest, and the average temperature of food packages at each layers and rows are controlled in 5 °C. This novel composite shelve can effectively reduce the temperature rise during defrosting, and improve the performance of VORDC obviously.

Paraffin is widely used in medium- and low-temperature storage because of its excellent properties. To improve thermal conductivity of paraffin, carbon nanomaterials filler with high thermal conductivity was gradually added to. In this paper, composite phase change materials (PCMs) composed of cold storage paraffin with a phase transition temperature of 4°C and two types of high thermal conductivity nanoparticles, graphene nano-platelets (GNPs) and carbon nanotubes (CNTs), were selected and prepared by mechanical separation with a suitable dispersant. The relationship between thermal conductivity and standing time of two composite PCMs revealed that the thermal conductivity of the composite PCMs of GNPs/paraffin and CNTs/paraffin remained stable at room temperature for 20 days and 40 days, respectively. When the mass fraction of GNPs or CNTs is 5%, the thermal conductivities of the composites, which are 0.266 W / ( m·K ) and 0.250 W / ( m·K ) , respectively, increase the most. Compared to those of pure paraffin and the CNTs/paraffin composites, the DSC curves of the GNPs/paraffin composites moved forward due to the higher thermal conductivity of the composite PCMs, which was more accurate for the experimental operation procedure. Due to the different structure and arrangement of particles, the latent heat of CNTs/paraffin was slightly lower than that of GNPs/paraffin with the same mass fraction of paraffin. The composites exhibited stable and excellent phase transition characteristics after 100 melting and solidification cycle tests.

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.

The uneven distribution of refrigerants in tube of a microchannel heat exchanger has a serious impact on the performance of the microchannel heat exchanger. The addition of a deflector in the microchannel heat exchanger can play a role in the secondary distribution. The difference in structure significantly affects the distribution uniformity of the refrigerant. In this paper, air and water are used as two-phase working fluids, and the effect of inserting a deflector on the distribution uniformity of the fluid in the header of a parallel flow heat exchanger with horizontal round tubes and 24 parallel flat tubes is studied. The superficial velocity ranges of the gas and liquid were 2.47–3.04 m/s and 0.67–0.86 m/s, respectively. Six kinds of deflector structures were studied in the experiment, in which the total cross-sectional area of the deflector and the total flow area of the flat tubes were tested as both different and the same. The experimental results show that under the experimental conditions, the uniformity of the gas and liquid distributions in the three-symmetrical deflector structure is better, followed by the two-symmetrical deflector structure, and then the uniform aperture deflector structure is the worst. Through a comparative analysis of these six deflector structures, the distribution uniformity of the gas and liquid two-phase fluid in the A1 variable-aperture deflector is the best. The open-area of the deflector is 670.39 mm2, which is the best value and is different from the total flow area of the flat tubes.

A multi-scale approach has been formulated in a previous work. The influence of HVAC system in supermarket on the performance of vertical open refrigerated display cabinet (VORDC) is considered. The influence is evaluated by varying the supply air temperature and velocity at the diffusers of HVAC. The computational results show that there is a direct interaction between the HVAC system and VORDC in supermarket. When the supply air temperature of HVAC system decreases from 19 °C to 16 °C, the electricity energy input in VORDC system decreases by 6.36%, and the highest products temperature is reduced by 0.18 °C. When the diffuser supply air velocity of HVAC system increases from 2.0 m/s to 3.5 m/s, the energy input in VORDC system decreases by 23.4%, and the highest product temperature reduces by 0.4 °C.

The composite fin is presented based on the advantage of longitudinal vortex generator and slit fin, respectively. The performance of air-side heat transfer and fluid flow is investigated by numerical simulation for Reynolds number ranging from Re = 304 to 2130. Stepwise approximation method is applied on the mesh generation for the irregular domains of delta winglets and slit fins. The mechanism for augmenting heat transfer is also analyzed based on the local fluid field, field synergy principle and entransy dissipation principle. The computational results show that some eddies are developed behind the X-shaped slit and delta winglet, which produce some disruptions to fluid flow and enhance heat transfer; compared with plain fin and slit fin, it shows the composite fin has better heat transfer performance. By applying on the field synergy principle and entransy dissipation principle to analyze the composite fin, the computational results show that composite fin can improve the synergy of temperature gradient and velocity fields, and its equivalent thermal resistance is smaller and its irreversibility of heat transfer is lower.

In this paper, the effect of the back panel structure on the performance of fluid flow and heat transfer of vertical open refrigerated display cabinets (VORDC) is presented by experiments and numerical simulation. Experimental tests are performed to validate the accuracy of numerical predictions. The characteristics of heat transfer and fluid flow of VORDC are investigated at the different locations of the perforations of back panel at the same porosity and the different flow ratios between back panel and air curtain at different porosities. By comparing numerical results with experimental results, the predictive abilities of the computational model have been revealed. Further computational results have also shown that less than 3% porosities can provide a better performance in the VORDC; the location of perforations has a minor influence on the temperature distribution of products. Furthermore, the suitable porosities in the back panel among the shelves are more likely to improve the uniformity of products temperature in the VORDC. As a result, the overall uniformity of product temperature inside the refrigerated display cabinet and the maximum deviation values of product temperature have been improve to 41% and 49%, respectively. The present study can provide the theoretical guide for the design of the narrow VORDC.

The numerical solution of the convection–diffusion equation represents a very important issue in many numerical methods that need some artificial methods to obtain stable and accurate solutions. In this article, a meshless method based on the local Petrov–Galerkin method is applied to solve this equation. The essential boundary condition is enforced by the transformation method, and the MLS method is used for the interpolation schemes. The streamline upwind Petrov–Galerkin (SUPG) scheme is developed to employ on the present meshless method to overcome the influence of false diffusion. In order to validate the stability and accuracy of the present method, the model is used to solve two different cases and the results of the present method are compared with the results of the upwind scheme of the MLPG method and the high order upwind scheme (QUICK) of the finite volume method. The computational results show that fairly accurate solutions can be obtained for high Peclet number and the SUPG scheme can very well eliminate the influence of false diffusion.

In this paper, the meshless local Petrov-Galerkin (MLPG) method is extended to solve the incompressible fluid flow problems. The streamline upwind Petrov-Galerkin (SUPG) method is applied to overcome oscillations in convection-dominated problems, and the pressure-stabilizing Petrov-Galerkin (PSPG) method is applied to satisfy the so-called Babuska-Brezzi condition. The same stabilization parameter @t(@t"S"U"P"[email protected]"P"S"P"G) is used in the present method. The circle domain of support, linear basis, and fourth-order spline weight function are applied to compute the shape function, and Bubnov-Galerkin method is applied to discretize the PDEs. The lid-driven cavity flow, backward facing step flow and natural convection in the square cavity are applied to validate the accuracy and feasibility of the present method. The results show that the stability of the present method is very good and convergent solutions can be obtained at high Reynolds number. The results of the present method are in good agreement with the classical results. It also seems that the present method (which is a truly meshless) is very promising in dealing with the convection- dominated problems.

In this article, the meshless local Petrov–Galerkin (MLPG) method is applied to compute two steady-state heat conduction problems of irregular complex domain in 2D space. The essential boundary condition is enforced by the transformation method, and the MLS method is used for interpolation schemes. A numerical example that has analytical solution shows the present method can obtain desired accuracy and efficiency. Two cases in engineering with irregular boundary are computed to validate the approach by comparing the present method with the finite volume method (FVM) solutions obtained from a commercial CFD package FLUENT 6.3. The results show that the present method is in good agreement with FVM. It is expected that MLPG method (which is a truly meshless) is very promising in solving engineering heat conduction problems within irregular domains.