Institute of Thermal Physics

Loop heat pipes (LHPs) are two-phase heat transfer devices with capillary pumping of the working fluid inside a sealed, closed loop, including an evaporator and a condenser connected by smooth-walled pipelines for separate movement of vapor and liquid, the diameter of which usually varies from 2 to 8 mm. This design feature enables easy integration of LHPs into densely packed systems. The LHP evaporator is equipped with a fine-pored capillary structure (wick) (pore radius from 1 to 20 μm), which creates high capillary pressure. Loop heat pipes are used in energy-efficient systems for utilizing low-potential heat, heating or cooling remote objects, and for uniform heat distribution over a large surface area of ​​heat sinks. Currently, the most popular area of ​​application for LHPs is electronic cooling systems operating under a wide variety of conditions. Various liquids can be used as working fluids in LHPs, depending on their operating temperature range, chemical compatibility with structural elements, and the required operating characteristics of the LHPs. This paper presents the theoretical foundations and operational conditions for LHPs. It also discusses the issue of selecting a working fluid to enhance the heat transfer capacity of LHPs. Quality criteria are proposed for assessing working fluids based on their efficiency in these devices. The results of a comparison of six different working fluids using these criteria are presented, showing that ammonia is the most effective working fluid in the operating temperature range relevant for electronics, from 40 to 70°C. Based on the p, t diagram of the working cycle of the working fluid, the thermal resistance of the “heat source–LHP–heat sink” system is visualized, as well as the thermal resistance of the main structural elements.

This study explores the development of new room-temperature NO2 sensors utilizing carbon nanofibers (CNFs), single-walled carbon nanotubes (SWCNTs), and their hybrids with reduced graphite oxide (rGO), fabricated via a facile drop casting method with varying concentrations of carbon/ethanol mixtures. The concentration-dependent relation of sensor response to NO2 has been found. Comprehensive characterization techniques, including electron microscopy, Raman spectroscopy, optical microscopy, and X-ray diffraction were employed to analyze the sensing materials. Our results reveal that CNFs exhibit superior sensitivity, reaching −1.32%/ppm at an optimal suspension concentration of 1.5 mg/mL, outperforming SWCNTs. The creation of hybrid composites, specifically CNFs/rGO and SWCNTs/rGO, further enhances sensing performance due to synergistic effects. Molecular dynamics simulations revealed increased adsorption behavior of the CNFs/rGO hybrid sensing material. The fabricated devices, based on all-carbon composites, are effective and energy-efficient platforms for NO2 detection, offering promising solutions for environmental monitoring, the chemical industry, and industrial safety applications.

Abstract Electrothermal purification is an effective method for achieving over 99.9 pct purity for graphite particles with minimal environmental impact. However, the lack of a suitable heat and electric resistivity model has hindered the scaling up of electrically heated fluidized bed (EHFB) reactors for graphite purification. In this study, three commercial natural graphite flake populations were tested in a bench-scale fluidized bed reactor at temperatures of up to 1000 °C. The experiments varied key parameters, including the graphite particle size, particle bed temperature, fluidization index, and electrode depth within the particle bed. Controlling the fluidized bed reactor at high temperatures requires an understanding of bed resistivity and how current flows throughout the EHFB system. The results show that fluidized bed resistivity decreases with temperature, with a diminished effect at higher temperatures. Smaller particles exhibit a higher resistance, likely due to a larger number of contact points required to pass current between the electrodes. In this study, a Finite Difference Method (FDM) model was developed using Visual Basic for Applications (VBA) in Excel®. Additionally, a Finite Element Method (FEM) model was created using COMSOL Multiphysics®. The FDM model assumes the current flows only radially, whereas the FEM model accounts for both radial and vertical current flow. The FDM model was validated against experimental data. Additionally, the FDM model was verified through a comparison with the FEM model. The FDM model showed good agreement with experimental resistance data and moderate agreement with power consumption, while the FEM model provided more accurate predictions by accounting for a detailed geometry and heat loss mechanisms. Achieving a uniform temperature distribution within the fluidized bed is influenced by the electrode’s contact area. Deeper immersion of the electrode enhances thermal uniformity and provides results that more closely match experimental observations.

This paper presents the results of an experimental study on the formation of carbon dioxide hydrate in a colloidal aqueous solution of aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) and the anionic surfactant sodium dodecyl sulfate (SDS), which acts simultaneously as a kinetic promoter of hydrate formation and a stabilizing agent for the colloidal solution. Oxide nanoparticles were selected for the study due to their high specific surface area, which potentially facilitates the nucleation of gas hydrate. The presence of a large number of aluminum oxide particles increases the number of nucleation sites during crystallization. Additionally, these particles may participate in the process of gas transfer from the liquid-gas interface to the bulk liquid. The dependencies of the released energy and the conversion of the solution and carbon dioxide into the gas hydrate state were obtained.

Abstract The article analyses the degree of water superheating with respect to the liquid-vapour equilibrium line in experiments on the micro-explosion of a composite droplet comprised of two immiscible liquids. The analyses were carried out for water-in-fuel drops under conditions of high-power heating. This degree is compared with the mechanical effect of droplet decay, involving the formation of daughter droplets. Our attention was drawn to the smallness of the degree of superheating preceding the decay. A model of the boiling up of such a droplet is constructed taking into account the sources of premature boiling up of water inherent in micro-explosive experiments. The dependencies of the boiling up temperature of water on the heating rate obtained in the model turned out to be in accordance with the experimental data across a wide range of heating rates. A hypothesis about the local superheating of the transition layer, which is not detected in the experiment, is formulated. Thus, a step has been taken to clarify the essence of the mismatch of the degree of superheating of water recorded by macroscopic equipment along with a completely satisfactory generation of daughter droplets serving as the basis for advanced fuel technology.

We studied the structure and phases in porous samples of 316L austenitic steel manufactured by laser 3D printing. Transmission electron microscopy revealed the presence of residual δ-ferrite along with austenite in the sample. A high density of dislocations is also observed in the sample. EBSD analysis revealed a lack of texture.