Inside a closed, thin-walled hollow cylinder, there is a solid state of phase change material (NePCM) that has been nano-enhanced. This NePCM is heated at its bottom, with nanoparticles (Al2O3) inserted and homogenized within the PCM (sodium acetate trihydrate, C2H3O2Na) to create the NePCM. The hollow cylinder is thermally insulated from the outside ambient temperature, while the heat supplied is sufficient to cause a phase change. Once the entire NePCM has converted from a solid to a liquid due to heating, it is then cooled, and the thermal insulation is removed. The cylindrical liquefied NePCM bar is cooled in this manner. Thermal entropy, entransy dissipation rate, and bar efficiency during the heating and cooling of the NePCM bar were analyzed by changing variables. The volume fraction ratio of nanoparticles, inlet heat flux, and liquefied bar height were the variables considered. The results indicate a significant impact on the NePCM bar during liquefaction and convective cooling when the values of these variables are altered. For instance, with an increase in the volume fraction ratio from 3% to 9%, at a constant heat flux of 104 Wm−2 and a liquefied bar height of 0.02 m, the NePCM bar efficiency decreases to 99%. The thermal entropy from heat conduction through the liquefied NePCM bar is significantly lower compared to the thermal entropy from convective air cooling on its surface. The thermal entropy of the liquefied NePCM bar increases on average by 110% without any cooling. With a volume fraction ratio of 6%, there is an 80% increase in heat flux as the bar height increases to 0.02 m.

The numerical simulation of friction stir welded T-joints made of AA2024 T3is investigated. Analysis of heat generation due to friction and plastic workis performed, as well as of the reaction force in the normal direction duringthe plunge stage of the friction stir welding. The effect of joint geometry isstudied for butt joints and T-joints produced from the same material.Different tool rotation speeds and tool pin lengths were considered forT-joint FSW welding. It was shown that the temperature at the root of theweld below the tool pin is lower in the T-joint than in the butt joint, due tothe efficient conduction of the heat produced through the normal plate. Also,the reaction force was higher for the T-joint than for the butt joint; so, heatproduction by friction was more intense in comparison with the heatproduced by plastic deformation. The reaction force was moderatelyincreased for the tool with a shorter pin, increasing both components of theheat produced. An increase in the tool rotation speed decreased theresistance to the tool plunging into the T-joint, increasing the frictional heatand decreasing the amount of heat generated by plastic deformation.

Today’s accelerated construction of buildings generates a enormous number of reinforced concrete rooms in which people live and in which an increasing number of various electrical devices are installed. Since buildings are characteristic of urban areas, mostly polluted air, consisting of particulate and gaseous pollutants, gets inside them. The electrical devices generate an electromagnetic field in their environment that multiplies with the number of these devices. The electromagnetic field cannot leave the reinforced concrete construction of buildings because of the so-called Faraday cage. The electromagnetic field generator in this analysis is focused on the electric foil heating floor. In addition, polluted air has a deficit of negative oxygen ions, which is further reduced near electronic devices since they generate positive ions. Due to their extremely high mobility, ultrafine and fine particles quickly reach from the streets even to the highest floors of buildings. The triple synergistic impact caused by the generation of electromagnetic fields, positive ions and fine particles inside closed spaces is the subject of experimental analysis carried out in this paper. The conducted analysis is carried out when varying the working parameters within one room as a research polygon.

Short-wave infrared radiation allows for efficient heating of a body (target) with minimal thermal interaction with the gaseous medium through which it passes. The mutual geometric relationship between the infrared heating source and the target impacts the spatial resistances of radiation heat transfer. Therefore, a significant portion of the short-wave infrared radiation emitted by the heat source does not reach the target, thereby reducing its efficiency. To maximize the use of thermal radiation, this study analyzes a profiled nanofluid collector on which a heated target is placed. Nanofluid with Al2O3 nanoparticles flows through internal arrays of round nozzles and a profiled housing, being heated by the inner surface of the collector. The paper establishes a methodology based on the thermal irreversibility of the heat source, collector and nanofluid. The established methodology allows for minimizing thermal entropy in order to optimize the geometric and process parameters of the described system. The results of the conducted analysis are based on the cross-influence of the Reynolds number of the nanofluid, target and collector emissivity, short-wave heating time and nanoparticle volumetric ratio. The results obtained indicate that as the target emissivity, heater temperature and heating time increase, the thermal entropy of the mutual interaction between the heater and the target also increases significantly. Similarly, the thermal entropy of the mutual interaction between the collector and the nanofluid is greatly influenced by factors such as the Reynolds number (from 2000 to 4000), volumetric ratio (3% and 5%) and type of nanoparticles used (Al2O3, TiO2, and CuO). In this way, the specially designed collector allows for the utilization of captured heat, while the established methodology offers the opportunity to optimize the process-geometric parameters of the heating system being analyzed.

A technical issue with fluid flow heating is the relatively small temperature increase as the fluid passes through the heating surface. The fluid does not spend enough time inside the heating source to significantly raise its temperature, despite the heating source itself experiencing a substantial increase. To address this challenge, the concept of the multiple circular heating of air was developed, forming the basis of this work. Two PTC heaters with longitudinal fins are located within a closed channel inside housing composed of a thermal insulation material. Air flows circularly from one finned surface to another. Analytical modeling and experimental testing were used in the analysis, with established restrictions and boundary conditions. An important outcome of the analysis was the methodology established for the optimization of the geometric and process parameters based on minimizing the transient thermal entropy. In conducting the analytical modeling, the temperature of the PTC heater was assumed to be constant at 150 °C and 200 °C. By removing the restrictions and adjusting the boundary conditions, the established methodology for the analysis and optimization of various thermally transient industrial processes can be applied more widely. The experimental determination of the transient thermal entropy was performed at a much higher air flow rate of 0.005 m3s−1 inside the closed channel. The minimum transient entropy also indicates the optimal time for the opening of the channel, allowing the heated air to exit. The novelty of this work lies in the controlled circular heating of the fluid and the establishment of the minimum transient thermal entropy as an optimization criterion.

The heating of a body (heat target, HT) by thermal radiation is often accompanied by heat losses, caused by the scattering of thermal rays and by not hitting its surface. These losses occur in infrared heating of different rooms. The heat source, i.e. modular infrared heater, can change the output intensity of thermal radiation within various wavelength intervals. Although there are different combinations of modular infrared heaters with variations in power, and geometric position in relation to HT, in this paper one characteristic combination, is analyzed. By setting the HT on the surface of the nanofluid collector with nano-enhanced phase change material (NePCM), it enables the increase in the overall efficiency of this heating process. The nanofluid collector consists of a complex pipe element through which the nanofluid flows, and a collector inside which the thermal-accumulating NePCM is placed. According to their characteristics, infrared thermal rays heat only HT, while the heating of the ambient air through which they pass is negligible. Based on this fact, the accumulated heat inside the NePCM can be used for convective heating of the ambient air around the HT surface. This process reduces the convective heat dissipation from HT to the ambient air and increases the efficiency of the modular infrared heat source. Furthermore, the accumulated heat inside the NePCM can be used for various technical applications. In this study, a mathematical model of the unsteady thermal entropy generation of the described heating system is established. By finding the unsteady thermal entropy, the next process of minimizing thermal irreversibility and maximizing the energy efficiency of the analyzed system is enabled. The volume fraction ratio of Al2O3 nanoparticles varies within the base fluid (water). Furthermore, the temperature of the infrared heaters varies as well as the volume fraction ratio of Al2O3 within the NePCM

: The intensity of convective electric heating of the fluid is mainly determined by its volumetric flow, the installed power of the heater and the geometric characteristics of the channel through which it flows. The temperature of the surface of the heating source, and its power is limited by the maximum allowed value. The constant convective surface of the electric heating source, with the above limitations, results in a wide range of electric convective heaters. The thermal efficiency of these heaters depends on a case-by-case basis, while the temperature of the fluid varies in some intervals in relation to the required temperature that needs to be achieved. During fast transient fluid heating processes, convective electric heaters are thermally inert, low efficiency, while in some cases their application is unjustified. Therefore, the thermally generated entropy of the described convective heaters and fluids increases, from case to case, while their energy efficiency is minimized.