## 3 Experimental Research Results, Analysis and Discussion
In turn, an increase in the initial velocity at the diffuser inlet causes a significant increase in TI in the range of 14 to 48% for \(h>350\) mm. The greatest differences are characteristic of \(h=450\) mm. The obtained data can be used to find technical solutions for controlling the gas-dynamic characteristics of air flow to improve the heat and mass exchange of working media in a vertical diffuser (for example, as applied to a flow gasifier). For example, it is known that the level of air flow turbulence affects the position and parameters (length, opening angle, etc.) of the flame in diffuser-type combustion chambers [39]. The obtained data are also used to improve the design of a flow gasifier with similar geometric dimensions and similar boundary conditions [33]. Moreover, the revealed patterns are applicable to other technical devices with similar diffuser designs. Spectral analysis of the functions \(w_{x}=f\) (\(\tau\)) along the height and along the diffuser axis was performed to clarify the gas-dynamic characteristics of the air flow (Figure 11). The dominant frequencies and their amplitudes were identified based on the obtained spectrograms (Figure 12). Figure 11: Graphs of the amplitudes of air flow velocity spectrum \(A_{w}\) at different heights \(h\) of diffuser (along diffuser axis) for initial velocities \(w_{\rm o}=2.22\) m/s: (**a**) \(h=50\) mm; (**b**) \(h=150\) mm; (**c**) \(h=250\) mm; (**d**) \(h=350\) mm; (**e**) \(h=450\) mm; (**f**) \(h=550\) mm; and (**g**) \(h=630\) mm. The graphs of the spectrum amplitudes of the function \(w_{x}=f\) (\(\tau\)) at all heights have a similar pattern (except for \(h=50\) mm), with a prevalence of frequencies of 5-20 Hz in the spectrum (Figure 11). This indicates the prevalence of large-scale turbulent structures, the sizes of which are comparable with the characteristic sizes of the flow or channel. This also indicates the presence of reverse flow or recirculation zones in the area of the diffuser axis. Indeed, the peaks of the flow velocity profiles are located to the right of the diffuser axis, which means that the flow core also passes to the right of the axis (according to Figure 8b). The spectrogram for the height \(h=50\) mm is notable for the fact that the spectrum of the predominant frequencies is much wider and lies in the range of 5-200 Hz, as well as in the vicinity of 500 Hz. This indicates the mutual coexistence of large- and small-scale structures in the flow with a clear predominance of the latter. Indeed, the point under consideration is located at the inlet section of the diffuser. Therefore, large-scale turbulence has not yet formed in this area, but small vortices have already been generated. Accordingly, the turbulence intensity has decreased, and the frequency spectrum has shifted to a higher-frequency region. The flow core partially passes through the point under consideration at a height of \(h=50\) mm (according to Figure 8b). Figure 12 shows that the dominant frequency \(f\) decreases from 52 Hz at \(h=50\) mm to 9 Hz at \(h=630\) mm (except for the point at \(h=350\) mm with the dominant frequency \(f=16\) Hz, which is out of the general trend). This indicates that there is an increase in the length scale of the turbulent structures downstream, which can contribute to the intensification of the heat and mass transfer processes in machines and devices with a conical diffuser. The amplitude of the velocity pulsations \(A_{w}\) increases from 0.021 m/s at \(h=50\) mm to a maximum of 0.095 m/s at \(h=350\) mm. The peak value of the amplitude may be due to the development of hydrodynamic instabilities associated with the strongest recirculation. Indeed, the highest velocities \(w\) and turbulence intensities TI are observed at a height of \(h=350\) mm at the extreme left points (except for the extreme left points at a height of \(h=50\) mm) according to Figures 8b and 9b.