###### Abstract
Description of the Test Bench, Experimental Methods, and Studied Designs for Supplying Air to a Vertical Diffuser  
The flow distribution inside a vertical diffuser when air is supplied with different supply channel designs was studied on the test bench shown in Figure 1. The test bench consisted of a forcing compressor (with the ability to change the air flow \(q\) through the system in the range from 0.015 to 0.06 m\({}^{3}\)/s), a duct air heater (with the ability to control air heating temperature up to 65 \(\pm\) 1 \({}^{\circ}\)C), supply systems of various configurations (the designs are presented below), and a transparent vertical diffuser (geometric dimensions are also given below) with a mesh inside (the mesh cell size was 1 \(\times\) 1 mm). This mesh served to record the flow structure with thermal imaging. A reservoir with a honeycomb was installed between the compressor and the duct heater to stabilise the flow (not shown in Figure 1). The experiments were carried out under stationary air flow conditions in the system under consideration for different air flow rates. The Reynolds number Re for the air flow at the outlet of the supply channel ranged from 42,500 to 150,000. The Re calculation was performed for a diffuser configuration with air supplied through a channel with a circular cross-section. The thermal imaging method used in this study is described in more detail in [34]. This method consists of placing a thermal imaging grid of polymer threads in the flow and is based on the analysis of temperature dispersion fields obtained through a thermal imager. Accordingly, as stated above, it was necessary to preheat the air to a temperature of 65 \(\pm\) 1 \({}^{\circ}\)C to obtain thermograms (temperature dispersion fields). Thus, a qualitative physical picture of the flow structure in a vertical diffuser was obtained using this method. It is necessary to utilise other experimental methods to obtain quantitative characteristics of the flow. When carrying out the experiments, the following equipment provided the measuring base:  
* A thermal imager (model Testo 890-2, Titisee-Neustadt, Germany), with which thermograms of the flow distribution inside the vertical diffuser (measurement error \(\pm\) 0.2 \({}^{\circ}\)C) were obtained (calibration of the device was carried out in a specialised measuring centre);
* A constant-temperature hot-wire anemometer (model Irvis TA-5.1, Kazan, Russia) through which air flow through the system was determined (the relative standard measurement uncertainty \(q\) was 5.1%);This study studied the following two design methods for supplying air to a vertical diffuser:  
1. The conventional method for supplying air through a straight channel from below (Figure 2a);
2. Nozzle air supply through four tubes at an angle of 45\({}^{\circ}\) (Figure 2b). Figure 1: Three-dimensional model of a test bench for studying flow structure in a vertical diffuser: 1—compressor; 2—channel heater; 3—supply channel; 4—vertical diffuser; 5—mesh; 6—thermal imager; 7—heating-level control unit; 8—air-flow control unit (note: the three-dimensional solid-state model was created using Russian KOMPAS-3D (v.18) software). Figure 2: Geometric dimensions of the vertical diffuser under research and air supply methods: (**a**) conventional air supply from below through a channel; (**b**) nozzle air supply at an angle. The main geometric dimensions of the diffuser under study are presented in Figure 2. Figure 2 also shows schematic diagrams of both design methods for supplying air. This study further examined the influence of the cross-sectional shape of the channel and nozzle tubes on the gas-dynamic flow structure in a vertical diffuser. Stable, longitudinal, and vortex structures arise in square and triangular channels, which have a significant impact on the gas-dynamic behaviour of air flow [35; 36].