Abstract ：

Understanding the coupling mechanism between multi-material pollution sources and sinks is key to predicting the pollution load. Indoor fabric materials strongly adsorb volatile organic compounds (VOCs) owing to their high loading rates and large specific surface areas. The secondary source effects generated by their desorption easily aggravates indoor air pollution and prolongs the pollution period. The existing research conclusions on the VOC mass-transfer properties of building materials are difficult to apply directly to fabrics due to their multilayered anisotropic fiber-interlaced structure. In this study, the triple porous structure of the fabrics was characterized, and the mass-transfer network were analyzed. Moreover, a multistage fractal-like tree network model was proposed to characterize the fabric's pore structure and establish a theoretical prediction model of the VOC diffusion coefficient. Subsequently, the mass-transfer characteristic parameters of the fabrics were measured at different ambient temperatures through loading and emission experiments of formaldehyde, benzene, toluene, ethylbenzene, and xylene (BTEX) on typical indoor fabrics. A comparison of the experimentally determined and theoretically predicted values revealed that the proposed model could accurately predict the diffusion coefficient of fabrics. This study can help understand the dynamic source and sink characteristics of fabrics in an indoor environment. © 2022 Elsevier B.V.

Keyword ：

Air quality Diffusion Forecasting Fractals Indoor air pollution Pore structure Volatile organic compounds

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Abstract ：

Direct lightning strike seriously threatens the safety of power systems. With the development of lightning model and fractal theory, the lightning fractal model has been gradually applied to the shielding analysis of power systems, yet the reliability and accuracy of the model still lack experimental verification. In this paper, a series of lightning simulation tests were conducted based on rod-rod air gaps. By changing the configuration of the test gap, the attachment probabilities were obtained and the influence factors of the attachment probabilities were investigated. Upward discharges were also recorded and analyzed. The comparison of the simulation results and the experimental data shows that the overall trends of the attachment probability are basically consistent, and the average probability differences are within 10%. The lightning fractal model was therefore proved to be of good reliability in prediction of lightning strike probability. © 2022 IEEE.

Keyword ：

Fractals Lightning Probability Reliability analysis Reliability theory

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Abstract ：

Silicon-based fractal tree-shaped microchannel heat sinks with different bifurcation angles were designed and manufactured. The heat transfer and pressure drop characteristics of the fractal microchannel network were experimentally studied under an inlet Reynolds number of Re0 = 500–1700. The results showed that compared with the traditional linear microchannel heat sink, the fractal microchannel heat sinks could achieve considerable flow drag reduction and heat transfer enhancement. Maintaining the chip temperature below 85 °C, the pressure drop can be reduced by up to 92%, and the heat transfer coefficient can be enhanced by 1.5 times of the fractal microchannel network at the same flow rate. The optimized bifurcation angle of the fractal network was recommended to be 30°. The COP of fractal microchannel heat sinks with a bifurcation angle of 30° can reach 12–13 times that of the linear microchannel. © 2022 Elsevier Masson SAS

Keyword ：

Bifurcation (mathematics) Drops Electronic cooling Fractals Heat resistance Heat sinks Heat transfer coefficients Microchannels Pressure drop Reynolds number

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Abstract ：

Graphite foams with excellent effective thermal conductivity and large surface area have primarily constituted a new area for the emerging fields of energy conversion, conservation, and management, potentially crucial for reaching the goals of carbon neutrality and emission peak. For such energy applications, graphite foam's thermal conductivity and permeability pave the physical foundation for understanding, designing, and operating thermofluidic flows inside the porous medium. However, previous prediction models of conductivity and permeability seldomly considered the effects of random distributions of pore shape and size intrinsically induced during processing of the graphite foam. To rectify this problem, analytical models of permeability and effective thermal conductivity for graphite foam are derived based on fractal theory, being duly accounted for random distributions of pore shape and size. In parallel, pore-scale numerical simulations are carried out, providing cross-validation and shedding light on transport mechanisms at pore level. Analytical model predictions and numerical simulation results are compared with existing experimental data. Results revealed that fractal analytical models accurately predicted the permeability and conductivity of graphite foams in a porosity range from 0.686 to 0.918, with different parent ligament materials and filling fluids (e.g., air and paraffin wax). © 2022 Elsevier Masson SAS

Keyword ：

Air Analytical models Energy conversion Foams Fractals Graphite Numerical models

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Abstract ：

In order to improve the heat transfer performance of the gas cooler in transcritical CO2 heat pump system, a novel biomimetic honeycomb fractal heat exchanger, with the advantages of great heat transfer efficiency and high compactness, is proposed for the transcritical CO2 heat pump in this study. Also, the conjugate heat transfer performance between supercritical CO2 and water in the novel gas cooler is investigated by the computational fluid dynamics method. The results firstly indicate that the difference of peak heat transfer coefficient in various flow channels reaches to 20.4% when the inlet flow flux of CO2 is 30 kg/(m2·s), significantly influenced by the uneven distribution of density and the secondary flow caused by buoyancy force. Secondly, when the flow flux of CO2 is 30 kg/(m2·s), compared with the horizontal flow, the heat transfer coefficients decrease by 11.4% and 40.7%, respectively, as CO2 flows vertically upward and downward. Finally, the heat transfer coefficient and comprehensive performance of the biomimetic honeycomb fractal heat exchanger are 144.6% and 20.4% higher than those of the printed circuit heat exchanger. The comprehensive performance of the 2nd and 3rd level biomimetic honeycomb fractal heat exchangers are 59.7% and 92.4% higher than that of the 1st level biomimetic honeycomb fractal heat exchanger. © 2022 Elsevier Ltd

Keyword ：

Biomimetics Carbon dioxide Computational fluid dynamics Cooling systems Fractals Heat transfer coefficients Honeycomb structures Pumps

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Abstract ：

To ensure the safe and reliable operation of wind turbines, a model based on Convolutional Neural Network (CNN) convolution neural network with fractal features for wind turbine operation state recognition was proposed. The initial sample feature data set was established based on fractal and other statistical features. Density-Based Spatial Clustering of Applications with Noise (DBSCAN) unsupervised clustering method was used to mark the sample states, and regularization feature selection method was used to determine the final sample feature data set. The state recognition model was established based on CNN. The experiment results showed that the accuracy of the model was 98.925%, which provided scientific reference and theoretical guidance for 'prior maintenance' mode of wind turbines. It could be effectively applied for state recognition of complex electromechanical systems and provide a basis for data mining and pattern recognition of wind turbines and other complex electromechanical systems. © 2022 CIMS. All rights reserved.

Keyword ：

Complex networks Convolution Convolutional neural networks Data mining Feature extraction Fractals State estimation Wind turbines

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Abstract ：

Electrical contact is an important part of electromechanical equipment. The performance of electrical contact has an important impact on the overall reliability of the system. Based on the study of the topological characteristics of fractal rough surfaces and the elastic–plastic deformation behavior of rough bodies, a theoretical model of electric contact is established by introducing a mathematical description of contact resistance which takes into account the effect of nano-scale. The corresponding relationship among contact area, contact pressure and compression displacement of asperities is obtained. The accuracy of fractal electrical contact model has been greatly improved. It is satisfactory in the field of precision instruments and analysis of the microscopic mechanism of electrical contacts. The model is used to qualitatively analyze the influence of different factors on contact resistance. The results show that the fractal parameters of rough surfaces influence the electrical contact according to some general rules. The mechanism of influencing factors is analyzed. © 2021, Beijing Oriental Sun Cult. Comm. CO Ltd.

Keyword ：

Contact resistance Electric contacts Fractals Nanotechnology Surface measurement Topology

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Abstract ：

The asymmetry of heat conduction in thermal diodes is attributed to the thermal regulation in thermal logic circuits, thermal control devices, etc. Currently, thermal rectification devices, however, are constrained by either specific on/off switching thresholds, or low sensitivity to manufacturing errors, as well as the restrictions for specific roughness and material characteristics. Here, a thermal diode based on fractal structures (inverted tapered branch channel, 5.7 mm branch length, and 4 branches) with liquid metal/insulator interface is proposed to avoid these existing constraints. The thermal rectification coefficient reaches 0.4704 (16.43 times larger than that of the basic structures) when the filling fraction only increases 1.33 times. The maximum temperature difference for the optimized thermal diodes is 18.31 °C when the heating temperature is 80 °C. By different arrangements of two thermal diodes mounted on chambers’ surfaces, the interior temperatures range from 16.06 to 52.34 °C in the same ambient conditions (23.49 °C), indicating the excellent thermal regulation effect of the thermal diodes. These controlled temperature chambers can be applied in greenhouses or cold chain storage devices, more combinations with higher degree of thermal-threshold tunability offer possibilities in solving complicated thermal management problems and promote the energy utilization efficiency. © 2021 Wiley-VCH GmbH.

Keyword ：

Computer circuits Diodes Energy utilization Fractals Heat conduction Liquid metals Temperature control Thermal management (electronics) Virtual storage

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Abstract ：

Building upon fractal theory and relying exclusively on analytical models, we develop models for predicting the permeability and effective thermal conductivity of two-directional (2-D) fibrous porous materials. In contrast to previous permeability and conductivity models, two hypothetical parameters (β and ζ) with physical meaning are introduced to consider the contact thermal resistance at the fiber interfaces, and analytical models for both in-plane and out-of-plane directions are developed. Relevant geometrical and physical parameters, including porosity, average tortuosity, and thermal resistance, are obtained by modeling the representative structure (RS) of the fibrous porous material. Good agreement with existing experimental data for fibrous materials over a wide range of porosity (from 0.50 to 0.99) validates the developed models, for both the permeability and effective thermal conductivity. It is demonstrated that, compared with previous models based on simplified geometries relying on periodic distribution assumptions, the current fractal model can better characterize the randomness of pore size and distribution commonly found in commercial fibrous materials. © 2021 Elsevier Masson SAS

Keyword ：

Analytical models Fractals Pore size Porous materials Thermal conductivity

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Abstract ：

Due to its outstanding heat transfer performance, flow boiling has a wide range of applications in many fields, especially for cooling of electronic devices. Previous studies have shown that the liquid replenishment on the downstream of the heating surface is the critical restriction of the increase of the critical heat flux (CHF). In this work, we designed a series of heterogeneous surfaces with fractal treelike hydrophilic networks for flow boiling enhancement. The micro-pin-finned surface structures are expected to increase the CHF and reduce the superheat by its high wickability. Moreover, by virtue of the efficient transport capacity of treelike networks, the fractal hydrophilic paths are designed to serve as the liquid delivery channels for the liquid replenishment on the downstream of the heating surface. The heterogeneous surfaces improve the comprehensive boiling heat transfer performance, especially the CHF, which is 82.2% higher than that of the smooth surface and 5.4% higher than the surface homogeneously covered by the microstructure with twice of the extended surface area. This work provides reference for the design of heterogeneous surfaces with both smooth and structured parts to increase the flow boiling CHF to some extent. © 2021 American Chemical Society.

Keyword ：

Electronic cooling Forestry Fractals Heat flux Hydrophilicity Liquids

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