Studies On Hydrodynamics & Heat Transfer In The Bottom Zone In A Circulating Fluidized Bed

The investigation on hydrodynamics & heat transfer in the bottom zone in circulating fluidized beds (CFBs) is of importance for engineering applications. In this work, the flow structure in bottom zone in a rectangular cross-section CFB was investigated by using hot particles as tracer, the characteristic on heat transfer was also studied by using a miniature heat transfer probe, in addition, visualization techniques was employed to investigate the flow structure in the bottom zone. Based on the experimental results, a 3-D dynamic core-annulus model describing the solids flow pattern in the bottom zone of the CFB riser is proposed.The experiments were carried out in a cold model CFB of rectangular cross-section. The riser has an inner cross-section of 0.3 m ( 0.5 m and a height of 5.8 m. The solids were returned into the riser at a height of 0.75 m above the air distributor at an angle of 40 degree. Quartz sand was used as the bed material. The hot particles were also quartz sand. The temperature responses in the bed were measured with four copper-constantan thermocouples, and were used to analyze the flowing direction of the solids.Local instantaneous temperature contains resourceful information relating to hydrodynamic behaviors in fluidized beds. Local instantaneous temperature and time-averaged heat transfer coefficient were measured using a miniature heat transfer probe. The results show that the heat transfer coefficient is higher near the walls, and become lower near the central region, and that the heat transfer coefficient decreases with increment of the air velocity due to the associated reduction of solids holdup in the bottom zone. In addition, it is found that the power spectrum density functions of the local instantaneous temperature signal can be characterized by the 1/f-like distribution.In addition, the characteristic of solids and bubble motion in the wall region in the bottom zone were investigated by using visualization techniques. Based on the experimental results, a modified core-annulus model is proposed. The simulation results are presented.