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

The heat transfer coefficient has been measured for a heated phosphor-bronze sphere (diam. 2.0, 3.0 or 5.56mm) 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 similar to 140degreesC 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 Ugreater than or equal toU(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 Uless than or equal toU(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 Umf 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)