Fluid dynamics and thermal characteristics of a conical bubbling fluidized bed riser

Conical fluidized beds show a unique inherent dynamic feature due to a lower pressure drop than conventional fluidized beds. Although the literature deliberating on the hydrodynamics of this type of riser is rich, the thermal characteristics of this type of riser are yet to be investigated. Moreover, the interphase heat transfer has not been deeply understood in previous literatures by considering the effect of air velocity. As in real fluidized bed industries, hot air is supplied through the inlet of a riser to heat the particles in the drying process and other phenomena. As a result, the interphase heat transfer characteristics are more important than the core-to-wall thermal characteristics. The thermal phenomena are ineluctably directed by the two-phase flow hydrodynamics. The present investigation is conducted to research fluid dynamics and interphase heat transfer phenomena of the air-sand mixture in a conical bubbling fluidized bed riser of cone angle 10 degrees with varying inlet air velocity. Air velocity is the main parameter which leads to the fluidization in fluidized beds. It is this parameter that materializes the elutriation of particles and determines the nature of the bed, whether bubbling or recirculating. This, in turn, affects the heat transfer and overall efficiency of the bed. Experiments along with corresponding three-dimensional (3-D) numerical simulations are performed at five superficial air velocity conditions (1, 1.25, 1.5, 1.75 and 2 m/s) and a 30 cm height of bed materials. The results are then compared between experimental and simulation conditions. Both the experimental and 3-D numerical results indicate that a velocity of 2 m/s results in the lowest pressure drop of 2830 Pa. Similarly, the average bed temperature in this velocity is 444 K which is the highest, whereas the interphase heat transfer coefficient is 320 W/m2K. The bed-towall heat transfer coefficient is found to increase by 47.2% and 45.8% at the central axial locations of 10 and 20 cm, respectively. Hence, it can be concluded that air velocity plays an important role in the operation of a fluidized bed in terms of pressure drop, heat transfer and thermal efficiency. Air velocity of 2 m/s is found to be the best value for the present study because the bed pressure drop decreases and heat transfer increases.