Simulation of air flow distribution for fixed bed combustion in an underfeed stoker using fluent

The study of air flow distribution in a packed bed of wood is a significant area of research in lump fuel wood combustion since the mechanisms involved in this process are dependent not only on fuel properties and temperature but also on air flow distributions. The main objective of this research involves the simulation of air flow distribution of isothermal packed bed of wood chips with various particle sizes and shapes. This is achieved by creating a geometric model of the retort in GAMBIT and then subsequently computing the air flow distribution of the bed in FLUENT at different primary to preoxidizing air ratios (5:1, 2:1, 1:1, 1:2, 1:0 - primary air only and 0:1 - pre-oxidizing air only) for a total air flow rate equivalent to 0.01066 kg/s. Ergun, Mehta and Hawley and Reichelt Correlations were tested to model the inertial and viscous resistance coefficients of the packed bed. Ergun equation with χ2 equivalent to 0.021442 gave the most accurate values among the three equations when compared with the experimental results and thus, was used for the simulation of the rest of the runs of the experiment. Sensitivity analysis of the type of packing mode (loose, normal or dense) was also performed to determine whether if this parameter greatly affects the velocity of air flowing in the bed. Results showed that this parameter does not greatly affect the values of air velocity in the bed for all shapes as the χ2 values are relatively small with a maximum value of 0.00298. Axial, radial and air velocity patterns were also simulated at different primary to pre-oxidizing air ratios for cubes and rectangular prisms. For radial velocity profile (near the primary air inlet), a bimodal curve was formed for a ratio of 1:1 and 1:2. On the other hand, the presence of bimodal curves were not detected for ratios 5:1 and 1:2. The maximum velocity occurs at the sides for ratios 5:1 and 1:2 for both shapes with velocities of 1.25m/s and 1.00m/s, respectively. For the axial velocity, it was shown that for primary to pre-oxidizing air ratios of 1:2 and 0:1, the air velocity increases from the bottom to the top of the bed and for air flow rate of ratios of 5:1, 2:1, 1:1 and 1:2, the velocity of air decreases