Convective Heat Transfer in Porous Materials

Thermal convection stands out as an exceptionally efficient thermal transport mechanism, distinctly separate from conduction and radiation. Yet, the inherently elusive nature of fluid motion poses challenges in accurately controlling convective heat flow. While recent innovations have harnessed thermal convection to achieve effective thermal conductivity, fusing thermal convection in liquids and thermal conduction in solids together to form hybrid thermal metamaterials is still challenging. In this review, we introduce the latest progress in convective heat transfer. Leveraging the right porous materials as a medium allows for a harmonious balance and synergy between convection and conduction, establishing stable heat and fluid flows. This paves the way for the innovative advancements in transformation thermotics. These findings demonstrate the remarkable tunability of convective heat transport in complex multicomponent thermal metamaterials

Controlling mass and energy diffusion with metamaterials

Diffusion driven by temperature or concentration gradients is a fundamental mechanism of energy and mass transport, which inherently differs from wave propagation in both physical foundations and application prospects. Compared with conventional schemes, metamaterials provide an unprecedented potential for governing diffusion processes, based on emerging theories like the transformation and the scattering cancellation theory, which enormously expanded the original concepts and suggest innovative metamaterial-based devices. We hereby use the term ``diffusionics'' to generalize these remarkable achievements in various energy (e.g., heat) and mass (e.g., particles and plasmas) diffusion systems. For clarity, we categorize the numerous studies appeared during the last decade by diffusion field (i.e., heat, particles, and plasmas) and discuss them from three different perspectives: the theoretical perspective, to detail how the transformation principle is applied to each diffusion field; the application perspective, to introduce various intriguing metamaterial-based devices, such as cloaks and radiative coolers; and the physics perspective, to connect with concepts of recent concern, such as non-Hermitian topology, nonreciprocal transport, and spatiotemporal modulation. We also discuss the possibility of controlling diffusion processes beyond metamaterials. Finally, we point out several future directions for diffusion metamaterial research, including the integration with artificial intelligence and topology concepts.Comment: This review article has been accepted for publication in Rev. Mod. Phy

Pattern Recognition of Development Stage of Creepage Discharge of Oil–Paper Insulation under AC–DC Combined Voltage Based on OS-ELM

The recognition of the creepage discharge development process of oil–paper insulation under AC–DC combined voltage is the basis for fault monitoring and diagnosis of converter transformers; however, only a few related studies are available. In this study, the AC–DC combined voltage with a ratio of 1:1 was used to develop a recognition method for the creepage discharge development process of an oil–paper insulation under a cylinder–plate electrode structure. First, the pulse current method was used to collect the discharge signals in the creepage discharge development process. Then, 24 characteristic parameters were extracted from four types of creepage discharge characteristic spectra after dimensionality reduction. Finally, based on the online sequential extreme learning machine (OS-ELM) algorithm, these characteristic parameters were used to recognize the development stage of the creepage discharge of the oil–paper insulation. The results showed that when the size of the sample training set used in the OS-ELM algorithm is close to the number of hidden layer neurons, a high recognition accuracy can be obtained, and the type of activation function has little influence on the recognition accuracy. Four stages of the creepage discharge development process were recognized using the OS-ELM algorithm; the trend was the same as that of the characteristic parameters of the entire creepage discharge development process. The recognition accuracy was 91.4%. The algorithm has a high computing speed and high accuracy and can train data in batches. Therefore, it can be widely used in the field of online monitoring and evaluation of electrical equipment status

Pattern Recognition of Development Stage of Creepage Discharge of Oil–Paper Insulation under AC–DC Combined Voltage Based on OS-ELM

The recognition of the creepage discharge development process of oil–paper insulation under AC–DC combined voltage is the basis for fault monitoring and diagnosis of converter transformers; however, only a few related studies are available. In this study, the AC–DC combined voltage with a ratio of 1:1 was used to develop a recognition method for the creepage discharge development process of an oil–paper insulation under a cylinder–plate electrode structure. First, the pulse current method was used to collect the discharge signals in the creepage discharge development process. Then, 24 characteristic parameters were extracted from four types of creepage discharge characteristic spectra after dimensionality reduction. Finally, based on the online sequential extreme learning machine (OS-ELM) algorithm, these characteristic parameters were used to recognize the development stage of the creepage discharge of the oil–paper insulation. The results showed that when the size of the sample training set used in the OS-ELM algorithm is close to the number of hidden layer neurons, a high recognition accuracy can be obtained, and the type of activation function has little influence on the recognition accuracy. Four stages of the creepage discharge development process were recognized using the OS-ELM algorithm; the trend was the same as that of the characteristic parameters of the entire creepage discharge development process. The recognition accuracy was 91.4%. The algorithm has a high computing speed and high accuracy and can train data in batches. Therefore, it can be widely used in the field of online monitoring and evaluation of electrical equipment status

Effects of needle-plate corona plasma on improving the degradation of agricultural mulch film

Mulch film has provided significant convenience for agricultural development. However, its stable physical and chemical properties make it challenging to manage under natural conditions, potentially posing a serious environmental threat if mishandled. This study investigates the influence of various experimental factors on the degradation of agricultural film using low-temperature plasma degradation technology. A self-made needle-plate electrode structure was employed to treat polyethylene film with a high-frequency AC voltage. The study analyzesd the effects of input power, discharge time, electrode number, and air circulation on mulch film degradation. Changes in mulch quality, Raman spectrum, scanning electron microscope observations, and relative molecular mass were compared before and after treatment. The results revealed a 2.6-fold increase in mulch film degradation efficiency when the input power rised from 26 W to 80 W. While increasing input power enhanced high-energy electrons and active substances in the reaction space, energy efficiency did not proportionally increase with the degradation rate. At 51 W input power, the energy efficiency reached a maximum of 58.82 μg/(W·h). The number of needle tips influences plasma uniformity and input power, impacting film degradation efficiency. As the number of needle electrodes increases, film degradation efficiency initially decreased from 3.49% to 2.1%, then rised to 3.57%. Prolonged discharge time made the molecular chain structure more vulnerable to plasma attack and breakage. Additionally, air fluidity affects ozone concentration in the reactor, with higher concentrations in a closed environment at low input power. Increasing the input power to 80 W results in higher ozone concentration with good air fluidity, aiding in improving mulch film degradation efficiency

In situ Simulation of Thermal Reality

Simulated reality encompasses virtual, augmented, and mixed realities—each characterized by different degrees of truthfulness in the visual perception: “all false,” “coexistence of true and false,” and “difficult distinction between true and false,” respectively. In all these technologies, however, the temperature rendering of virtual objects is still an unsolved problem. Undoubtedly, the lack of thermal tactile functions substantially reduces the quality of the user’s real-experience perception. To address this challenge, we propose theoretically and realize experimentally a technological platform for the in situ simulation of thermal reality. To this purpose, we design a thermal metadevice consisting of a reconfigurable array of radiating units, capable of generating the thermal image of any virtual object, and thus rendering it in situ together with its thermal signature. This is a substantial technological advance, which opens up new possibilities for simulated reality and its applications to human activities