The heat transfer coefficient between a particle and a bed (packed or fluidised) of much larger particles

Abstract

The heat transfer coefficient has been measured for a heated phosphor-bronze sphere (diam. 2.0, 3.0 or 5.56 mm) added to a bed of larger particles, through which air at room temperature was passed. The bronze heat transfer sphere was attached to a very thin, flexible thermocouple and was heated in a flame to ∼ 140 ∘ C before being immersed in the bed. The cooling of the bronze sphere enabled the heat transfer coefficient, h , to be measured for a variety of U / U mf , as well as diameters of both the particles in the bed and the heat transfer sphere. It was found that before the onset of fluidisation, h rose with U , but h reached a constant value for U ⩾ U mf . These measurements indicate that in this situation (of a relatively small particle in a bed of larger particles) all the heat transfer is between the hot bronze sphere and the gas flowing over it. Consequently, a Nusselt number, based on the thermal conductivity of the gas, is easy to define and for U ⩽ U mf (i.e. a packed bed), Nu is given by Nu = 2 + 0.90 Re 0.62 ( d s / d b ) 0.2 for air flowing through the bed. When the bed was fluidised and U > U mf , the heat transfer sphere moved freely. The constant value of Nu for a fluidised bed can be calculated by putting the superficial velocity in the particulate phase equal to U mf in the above equation. Thus, even in a fluidised bed all the heat transfer is to the gas flowing over the hot sphere, so particle–particle contacts are unimportant when d s < d b . The measurements were made for turbulent flows of air over the heat transfer sphere. The above correlation gives Nu to within 20%.