Local heat transfer to surfaces immersed in the freeboard region of a fluidized bed.

摘要

This work describes the local heat transfer in the freeboard region of a fluidized bed. Plate surface to particle suspension heat transfer coefficients in the freeboard of a fluidized bed (30 cm i.d.) have been measured by a newly designed miniature heating plate. Four types of plate orientations have been employed to investigate the directional effect of gas-solids flow to plate surface on the heat transfer rate. Local solids concentrations in the freeboard were also obtained by a nozzle-type sampling probe.; Results indicate that the heat transfer coefficient varies with axial and radial plate location, plate orientation, and gas velocities. Variations of heat transfer coefficients with radial position are more significant at lower freeboard heights than at higher ones for all the gas velocities (0.28-0.64 m/s) examined. For plates parallel to both the axial and radial directions of the column, the heat transfer coefficient (h) is higher at the center than at wall. The plate with exposed surface parallel to the axial and perpendicular to the radial directions gives nearly the same value of h at the center and the wall. At lower freeboard levels, the center-facing surface exhibits higher heat transfer rate than the wall-facing one, which can be explained from the higher solids concentration and center to wall movement of particles. Plate with downward exposed surface exhibits the highest heat transfer rate at high gas flow rates because of direct bombardment of solid particles splashed from the bed. Variations of heat transfer coefficients with elevation in the freeboard are as much as an order of magnitude. However, these variations with elevation are relatively insensitive to plate orientations investigated.; A surface-to-suspension heat transfer model has also been developed to elucidate the surface to particle heat transfer mechanism in this lean phase system. The proposed model can explain reasonably well the inter-dependence of the measured heat transfer coefficient, solids concentration, heat capacity, particle size, and solids velocity in the freeboard.