Experimental study of hydrodynamics in a cold model of a conical reactor for sawdust gasification using random process theory

During the work, a flat cold model of a gas generator was studied, including an air inlet area, a conical section with an opening angle of $38^\circ$, a rectangular section measuring $500\times20$ mm, and an outlet connected to a filter. The unit is equipped with a video surveillance system for dispersed flow and pressure sensors, with digital recording for subsequent statistical analysis. The experiments were carried out with different sawdust fractions ($0.1$–$0.25$ mm; $0.25$–$0.5$ mm; $0.5$–$0.8$ mm; $0.8$–$1.0$ mm; $1.0$–$1.25$ mm; $1.25$–$1.6$ mm) and their mixtures, with an initial layer height of $50$ mm. The air velocity at the inlet varied from $0.4$ to $5.4$ m/s, with each experiment lasting $40$ seconds. The results showed that at low velocities ($0.4$–$1.9$ m/s), the material is in a stable flowing mode with main pressure pulsation frequencies of $3$–$5$ Hz. An increase in speed leads to a transition to unstable pseudo-fluidisation: the fluctuations range expands to $0.5$–$8$ Hz, and at even higher air flow rates, the layer transitions to a rarefied mode with a predominance of low frequencies (about $2$ Hz). Smaller fractions ($0.1$–$0.8$ mm) lose stability more quickly, while larger particles ($0.8$–$1.6$ mm) remain stable for longer. Spectral analysis of pressure pulsations at heights of $30$ and $200$ mm shows a noticeable change in the root-mean-square deviation of fluctuations in height, with the upper level reacting more strongly to an increase in speed. According to the distribution of static pressure by height, it has been established that the volume concentration of particles in the zone $30$–$50$ mm above the grid reaches $30\%$. With a further increase in air flow, the layer is washed upward, particle clusters form in areas distant from the diffuser inlet, and the volumetric concentration of particles decreases significantly. The obtained results allow to predict the dynamic behaviour of beds of different dispersibility and contribute to the development of effective gasification processes.