## 2 Statement of the Research Problem and Measurement Tools  
The experimental setup was designed and manufactured to study the gas-dynamic characteristics of stationary flows in a vertical flat diffuser (Figure 1). The main elements of the experimental setup were as follows: (1) a slower for creating air movement in the gas-dynamic system with the ability to control consumable characteristics by changing the rotation frequency of the blower rotor; (2) a receiver for equalizing and stabilizing the flow after the centrifugal blower; (3) a vertical conical diffuser with holes for hot-wire anemometer sensors; and (4) an automated measuring system based on a constant-temperature hot-wire anemometer, an analog-to-digital converter, and specialized software. The receiver had a volume of about 0.04 m\({}^{3}\) with a leveling grid inside. The receiver served to reduce the amplitudes of the pulsations of the speed and pressure of the flow after the centrifugal blower. The stabilized air flow entered the vertical diffuser. The geometry of the vertical diffuser under study and its photograph are shown in Figure 2a,b. Figure 1: Experimental setup diagram: 1—air blower; 2—receiver with equalizing grate; 3—vertical conical diffuser; 4—frequency converter (air flow control through the system); 5—hot-wire anemometer sensor for measuring air flow velocity; 6—constant-temperature hot-wire anemometer; 7—analog-to-digital converter; 8—computer with specialized software. Figure 2: The main geometric dimensions of the diffuser under study (**a**), its photograph (**b**) and the approximate direction of air flow in the diffuser (**c**). The main geometric dimensions of the diffuser were as follows: the height of the diffuser was \(h=645\) mm, the height of the straight section was 1280 mm (the total height of the installation was \(H=1925\) mm), the width of the inlet section was 50 mm, the width of the straight section was \(L=500\) mm, and the thickness of the installation was 20 mm. Accordingly, the cross-section of the air inlet channel into the diffuser was a rectangle with sides of 50 mm and 20 mm. The main elements of the stand (diffuser) were made of transparent polystyrene with a wall thickness of 2 mm. The approximate movement of the air flow in the diffuser is shown in Figure 2c. It should be noted that the air exited the diffuser from the right side of the straight section of the installation and not vertically upwards. The size of the outlet section was 200 mm (the thickness was also 20 mm). The design and geometric dimensions of the experimental setup were chosen based on the potential application of the results of this study to a flow gasifier, i.e., a device for producing synthetic gas from wood waste. This device has the same geometric dimensions but is made in a three-dimensional (volumetric) configuration. The air flow velocity at the diffuser inlet varied from 0.4 m/s to 2.22 m/s, which corresponded to Reynolds' numbers from 750 to 4160. These flow regimes also correspond to the flow velocities in the designed in-line gasifier for synthetic gas production. The industrial gasifier has similar geometric, speed and regime characteristics to air flow during operation [33]. Therefore, it is impractical to investigate higher initial flow velocities in the diffuser. Accordingly, the laboratory results can be transferred to a real-life sawdust gasification plant. Air with a temperature of 22 \({}^{\circ}\)C was used as the working medium in these studies. The problem statement was isothermal, i.e., the air temperature was constant during the experiments. The main components of the measuring system are shown in Figure 3. The gas-dynamic characteristics of the air flow in the diffuser were assessed using the thermal anemometry method [34; 35]. Constant-temperature hot-wire anemometer IRVIS TA-5.1 (Kazan, Russia) with a single-filament sensor was used for these purposes.