Experimental studies of heat transfer in circulating fluidized beds

This project was carried out to study heat transfer processes in circulating fluidized beds (CFB's). The heat transfer coefficients between a cylinder and the bed material in cross-flow and between the enclosing walls and the bed material were measured in a small-scale CFB operating at room temperature and the mathematical models based on the experimental data are proposed. The furnace temperature and the heat transfer coefficient within a large-scale CFB combustor were also determined and the radiative contribution to heat transfer in CFB combustors was theoretically studied.;The solid flow patterns in the cold model CFB were mostly in the dilute phase flow regime with higher solids concentration in the centre of the column than in the vicinity of the wall. In this flow regime, the up flow solid flux and the movement of particles along the underside of a horizontal cylinder immersed in the bed have a strong effect on the cross flow heat transfer coefficient while the heated section length and the gas velocity are the main variables affecting the wall-to-bed heat transfer coefficient.;Under the operating conditions of the cold model CFB, the values of the heat transfer coefficient over the horizontal cylinder placed at the centre of the column varied from 79.1 to 192.8 W/m;The temperature measured in the Chatham CFB combustor at various heights and depths varied from 770;It is demonstrated using theoretical arguments that transfer of heat by radiation in CFB combustion chambers is a local phenomenon and that the unidimensional temperature field method may be applicable in the estimation of the radiative heat transfer at the wall.