## 1 Introduction  
In the era of the energy crisis, in connection with the introduction of closed-loop technology all over the world, and to protect our natural environment, an important issue is the intensification of heat exchange, including convective exchange. Each improvement of this process may have a positive impact on the environment and bring measurable economic benefits, e.g., by shortening energy-intensive operations, such as drying. The methods of intensifying convective heat exchange can be divided into passive and active [1]. Passive methods include the use of liquids with enhanced thermal properties [2; 3] or a profile with a specially shaped flow geometry [1; 4]. In turn, active methods include the use of an external electric field [5], a magnetic field [6], or forcing the flow pulsation of the flow [7]. Passive air cooling of metal components in the air environment is widely used in technology, for example in electric vehicles, which are to dominate the future transport sector where many components, such as inverters, batteries, computers, and drive systems require dedicated cooling systems [8]. Thermal management is also crucial for electronic devices, where excessive heating during the operation of the device results in the most damage. In addition, the modern trend of the miniaturizing of electronic devices with better performance and features further increases the need for a suitable cooling system [9]. Furthermore, high temperature (especially in some environments) significantly affects metal corrosion; therefore, methods are sought to reduce the susceptibility to corrosion [10]. Passive air cooling is also frequently used in the food industry. Thermally processed food products usually need to be cooled for further processing or packaging. Cooling is used, among other things, in the manufacture of soybean products that are produced by the steam cooking of soybeans, then draining the seeds and cooling them to 40 \({}^{\circ}\)C and mashing, and then further processing them into a specific product. Furthermore, shapes and molds in the confectionery industry, such as candies and jelly, are made by havingheated products deposited in molds to cool and harden. Cooling is also used as a basic step in the production of dried ham as a production step between smoking at around 60 or 80 \({}^{\circ}\)C and 60% humidity for 12-36 h and vacuum packaging. In the production of bread, the baked bread is cooled and then packaged and stored. Generally, the production of bread and bakery products consists of several common steps, including cooling [11]. A manufacturing process in which the cooling stage is also extremely important is heat blanching, which when required in the preparation of food should be carried out by heating the food to the required temperature, holding it at this temperature for the required time, and then rapidly cooling the food or passing it to subsequent manufacturing without delay [11]. Another place where the positive effect of air cooling on the technological process can be observed is the use of cooling in intermittent drying. Intermittent drying involves the use of periodically varying drying conditions during the process [12]. The dried material can be, for example, alternately heated and cooled down. The use of drying in such conditions significantly shortens drying time, reduces electricity consumption, and improves the quality of dried products [13]. However, passive cooling has many disadvantages, including a long cooling time and the fact that cooling may be insufficient in the case of highly heated elements. A long cooling time can be a key issue because prolonged overheating of a given material can irreversibly destroy the structure of the material or the device. Another problem in which shortening the cooling time is important is that it can slow down the technological process of manufacturing products, which can be a \"bottleneck\" of the entire production time. Techniques are sought to support passive cooling, which will significantly improve the kinetics of heat exchange.