###### Abstract
The heat pipe consists of a hermetically sealed circuit with a working medium at a precisely defined pressure. The heat transfer is based on a two-phase self-circulation of the heat transfer working medium. There is a condenser at the top and an evaporator at the bottom of the heat pipe. The distance between the evaporator and the condenser must be chosen with an adequate height difference. Inside the heat pipe, the working medium circulates from the evaporation to the condensation section in one direction in a closed loop. The liquid boils in the evaporator depending on the amount of supplied heat from the environment in which the heat pipe is located. The gaseous state of the working substance flows to the condenser, where it condenses to a liquid and releases the heat of evaporation. The working substance is cooled and, under the action of gravitational force, flows down into the evaporator again [11]. The entire heat transfer process in a heat pipe is constantly repeated. Gravity loop heat pipes have better thermal properties than active cooling systems because they use phase changes to transfer heat. The advantage is that the phase changes also occur at low mass flow rates, which increases the heat transfer coefficient. The selection of the working medium is flexible because there is no direct contact of the medium with the cooled component. The working medium is able to flow over relatively long distances without needing an external power supply, e.g., pump. Heat transfer over longer distances depends mainly on the thermodynamic properties of the working substance. The service life of the heat pipe is almost unlimited. This is due to the hermetically sealed design of the heat pipe without moving elements and the phase changes that are constantly repeating. The simplicity and uniqueness of loop thermosiphons in relation to phase-change heat transfer encourage many kinds of research of new prototypes and applications of these exceptional devices. Scientists [PERSON] and [PERSON] were among the first to focus on the choice of working substance and its effect on performance and design in closed heat pipes. The research results were demonstrated by simulating heat transfer [12]. Further research on two-phase heat transfer equipment has focused on various working substances used in the heat pipe system. [PERSON] and his scientific team used the azeotropic working substance water-ethanol in a thermosiphon closed heat tube. The experiments were focused on the thermal properties of this mixture, the heat transfer performance, and the total thermal resistance. The results of the research proved the functionality and practical use of the proposed device in various applications even with an evaporator inclination from 0\({}^{\circ}\) to 90\({}^{\circ}\)[13]. [PERSON] and [PERSON] presented work focused on an experimental investigation of a carbon dioxide thermosiphon loop designed to fulfill the geometric and temperature requirements of a specific FPSC (free-piston Stirling cooler). Experiments were carried out varying the temperature difference between the heat source, i.e. air at the entrance of the evaporator and heat sink and the internal surface of the condenser, refrigerant charge, and evaporator airflow rate. The experimental results were explored using the thermal conductance concept applied to each heat exchanger and also to the whole loop. It was found that the loop was able to carry a maximum heat transfer rate of 514 W with a heat source-sink temperature difference of 11 \({}^{\circ}\)C [14]. Specific applications of the use of heat pipes can be seen, e.g., in the industrial power industry for the cooling of nuclear reactors, gas turbine blades, rotors, and transformers, in the electronics for the cooling of power components of electronic equipment, and in the avionics, medicine and automotive industries [15; 16; 17; 18; 19]. Heat pipes can also be used to reduce fine gas temperature in the flue gas tract of the heat source described by [PERSON] et al. [20].