###### Abstract
Consequently, for the traditional air supply method, three cross-sectional shapes were used, namely circle, square, and equilateral triangle, the geometric dimensions of which are shown in Figure 3. The geometric dimensions of the supply channels were determined based on the equality of the equivalent hydraulic diameter for all configurations (circle diameter--32 mm; square side--32 mm; and triangle side--55 mm). Accordingly, the cross-sectional area of the square was 21% larger than the circular cross-section. The area of the triangle was 39.5% larger than the circle. It should be noted that the experiments were performed for the same volume flow through the system for all configurations of the supply channels. The length of the supply channel in all cases was approximately 100 mm. All supply channels had a technically smooth surface (average roughness was 6.3 \(\upmu\)m). Photographs of the supply channels with different cross-sections and vertical diffusers are shown in Figure 4. It should be noted that the visualising grid did not pass through the centre of the triangle (Figure 4c). This is due to the geometric limitations of the vertical diffuser and the dimensions of the triangular cross-section. It can be assumed that the thermograms will show the flow structure in one of the corners of the triangle. A general view of another method for supplying air to a vertical diffuser with nozzle tubes is shown in Figure 5a. Figure 5b also illustrates the geometric dimensions of nozzle tubes with cross-sections of different shapes. The geometric dimensions of the nozzle tube cross-sections were also determined based on the equality of the equivalent hydraulic diameter for all configurations (circle diameter--5 mm; square side--5 mm; and triangle side--8.6 mm). The total areas of square and triangular tubes are 21 and 39.5% larger than those for round tubes. The total length of the nozzle tubes in all cases was approximately 100 mm. The length of the tube sections with square and triangular cross-sectional shapes was 60 mm. All nozzle tubes had a technically smooth surface (average roughness was 6.3 \(\upmu\)m). It should be noted that the experiments were carried out in modes with the same volumetric air flow through a system with different configurations of supply tubes. Figure 3: Sketches of the designs of supply channels with different cross-sectional shapes: (**a**) circle; (**b**) square; (**c**) triangle. Nozzle tubes (four pieces) were installed symmetrically on a cylindrical base at an angle of 45\({}^{\circ}\) relative to the vertical axis. Photographs of air supply nozzles with different tube designs and vertical diffusers are shown in Figure 6.  
Therefore, in this research, six air supply designs in a vertical diffuser and their influence on the gas dynamic structure was studied with thermal imaging. It should be noted that half of the diffuser was manufactured for experimental studies. The use of a half vertical diffuser assumes that the flow in the vertical diffuser is completely symmetrical for a stationary mode of air movement. This study is aimed at experimentally obtaining a qualitative physical picture of the flow structure in a vertical diffuser for different air supply methods. Quantitative data and more detailed information about the gas dynamics of flows will be obtained using the PIV method in the future. Figure 4: Photographs of the studied vertical diffuser structures with different cross-sectional shapes of the supply channel: (**a**) circle; (**b**) square; (**c**) triangle. Figure 5: Overall view of the design for nozzle air supply into a vertical diffuser (**a**) and geometric dimensions of supply tubes with different cross-sectional shapes (**b**). Analysis of the Gas-Dynamic Structure of Flow in a Vertical Diffuser with Different Air Supply Methods  
First, experimental data (thermograms) on the flow structure in a vertical diffuser were compared with the results of other authors (Figure 7). This is necessary because half of the diffuser was used to obtain the thermograms in this study.