ON THE RADIATIVE HEAT-TRANSFER IN BUBBLIN G FLUIDIZED-BEDS

A model was developed to determine the radiative heat transfer in bubb ling fluidized beds taking the bubble and suspension contact at the wa ll into account. The bubble radiation can be regarded as the heat exch ange between two isothermal surfaces. The suspension radiation is desc ribed between single particles of the near wall particle layer and the wall during suspension contact at the wall. The two mechanisms partic le convection and suspension radiation are simultaneous effects which both results in a decrease of the temperature at the particle surface. The time dependent particle surface temperature can be determined fro m heat balances of the particle and at the particle surface which are simultaneously influenced by convection and radiation. From this infor mation the suspension radiation can be evaluated. The model results an d experimental data were compared and a relatively good agreement was found. The discrepancies are small and no systematic deviations were o bserved. The model results show not only the expected influence of the bed temperature on the radiative heat transfer but also a strong infl uence of the particle diameter. Increasing particle diameter increases the radiative heat exchange. The fluidizing velocity influences the r adiation via the bubble volume fraction within the bed. Furthermore th e radiation is effected by the wall temperature. The system pressure h as a minor influence on the radiation by the pressure dependency of th e particle convection. Since the application of the radiation model pr esented here is rather complicated an empirical equation was developed to determine the radiative heat exchange. This equation is easy to ha ndle and represents the results of the complete model as well as all i nfluences which are discussed here. This equation fits the experimenta l results with a maximum deviation of 20%. The radiation model is veri fied in a wide range of different parameters so that it includes not o nly applications of pressurized fluidized bed combustion but includes also other applications for example fluidized beds with metallic bed m aterial.