Heat flow rates from the gas to the wall and to the bed have been derived from temperature profiles measured in the UBC pilot rotary kiln. The experimental gas-to-wall heat flux has been found to agree closely with theoretical predictions based on a simple radiative model consisting of a grey gas surrounded by a grey surface at uniform temperature. Under identical conditions, the gas-to-solids heat flux is up to ten fold greater than the heat flux between the gas and wall. The gas-to-solids heat flux is a function of the solids feed rate at low throughputs but is constant at higher throughputs. At the lowest feed rates and corresponding rotation speeds heat flow to the bed is limited by mixing of the solids. Heat transfer control changes to the gas side of the bed at higher feed rates and rotation speeds. Under the latter conditions convection appears to account for the major fraction of heat flow to the bed. The heat flow measurements relate directly to observations of the bed motion which can be conveniently characterized by a bed behavior diagram. The low heat flux to the bed at low feed rates is due to a slumping action at the surface and concomitant poor mixing. The high heat flux obtained at high feed rates coincides with the observation of a rolling bed. Burden-side and gas-side convective heat transfer coefficients, calculated from the heat flux data, have values ranging from 700 to 1200 and 120 to 240 w/m2K respectively.
Watkinson, A.P., Brimacombe, J.K. Heat transfer in a direct-fired rotary kiln: II. Heat flow results and their interpretation. Metall Mater Trans B 9, 209219 (1978). https://doi.org/10.1007/BF02653686