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.

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.

The novel segment electric in-line process electric heaters heater designed to heat various fluids analyzed in this work. The complete electric heater consists of several hollow cylindrical segment heating elements. The segment heating elements can vary positioned in relation to the fluid flow. The total power of the segment process heater is equal to the sum of the power of all heating segments and is 0.756kW. Volumetric air flow variations in the amount of 0.001m3s-1, 0.002m3s-1 and 0.003m3s-1. The heating elements are positioned in the three combinations in relation to the direction of the fluid flow. The comparative numeric analysis, conducted for this work, has the goal to determine the influence of the arrangement of segment heaters on the overall energy efficiency of the segment electric heater. In order to verify the results of the numeric simulations carried out and experimental investigations of the segment electric heater. Keywords: In-line heater Energy efficiency Segment elements Fluid flow, Convective heating

Various viruses can hide within fluid and solid structures and thus successfully cross different distances, causing the spread of viral infections. Analytical modeling of the triple treatment of virus within a small liquid droplet and within a solid porous particle is the basic research polygon of this paper. The three-stage treatment aims to maximize the efficacy of deactivating viruses indoors. In order to achieve this, viruses undergo treatment by infrared heating, ultraviolet deactivation and ionization–electrostatic deactivation by negative ions. When the droplets are treated with infrared heating, incomplete evaporation occurs, reducing their initial diameter by 10 times; an initial diameter of droplets is 0.01 mm, 0.03 mm and 0.05 mm. Thermal inactivation of viruses inside the droplets is almost negligible, due to short exposure time and a maximum temperature of 100 °C. On the other hand, when solid porous particles are heated to a much higher temperature at the same exposure time, this causes significant thermal inactivation of viruses inside them. Reducing the diameter of the droplet (due to evaporation) by 10 times causes a multiple increase in UV-C deactivation of viruses inside the droplets. The effect of UV-C radiation on viruses within solid porous particles is not included in this paper.

A hollow electric heating cylinder is inserted inside a thermo-insulating cylindrical body of larger diameter, together representing a single cylindrical heating element. Three cylindrical heating elements, with an independent electrical source, are arranged alternately one after the other to form a heating duct. The internal diameters of the hollow heating cylinders are different, and the cylinders are arranged from the largest to the smallest in the nanofluid’s flow direction. Through these hollow heating cylinders passes nanofluid, which is thereby heated. The material of the hollow heating cylinders is a PTC (positive temperature coefficient) heating source, which allows maintaining approximately constant temperatures of the cylinders’ surfaces. The analytical analysis used three temperatures of the hollow heating cylinders of 400 K, 500 K, and 600 K. The temperatures of the heating cylinders are varied for each of the three cylindrical heating elements. In the same arrangement, the inner diameters of the hollow cylinders are set to 15 mm, 11 mm, and 7 mm in the nanofluid’s flow direction. The basis of the analytical model is the entransy flow dissipation rate. Furthermore, a new dimension irreversibility ratio is introduced as the ratio between entransy flow dissipation and thermal-generated entropy. This paper provides a suitable basis for optimizing the geometric and process parameters of cylindrical heating elements. An optimization criterion can be maximizing the new dimensionless irreversibility ratio, which implies minimizing thermal entropy and maximizing entransy flow dissipation.