Wall-to-Bed Heat Transfer at Minimum Gas-Solid Fluidization

The heat transfer from a fluidized bed to the cooling jacket of the vessel has been studied for various powders at minimum fluidization conditions, by both convection and conduction approaches. These heat transfer characteristics are important as the point of transition between packed and fluidized bed operations and are needed in designing heat transfer operations where bubble-flow is not permitted. The effective thermal conductivity of the emulsion moreover determines the contact resistance at the heating or cooling surface, as used in packet renewal models to predict the wall-to-bed heat transfer. In expressing the overall heat transfer phenomenon as a convective heat transfer coefficient, it was found that the results could be fitted by . 1. Introduction A powder is a heterogeneous system in which solid particles are surrounded by gas. There are an unlimited number of solid-gas systems possible ranging from the single solid in single gas system to the more complex fluidized bed. The specific reasons for investigating the heat transfer at minimum fluidization are fourfold: (i) it is an important design value for operations where bubble-flow is not permitted, for example, cooling of safety glass or slow and controlled cooling/hardening of metal-alloy wire; (ii) it is the point of transition between packed and fluidized bed operations; (iii) it defines the extent of the thermal gradient within the bed close to the heat exchanging wall; and (iv) it provides data of the effective thermal conductivity of the bed at minimum fluidization: data on the effective thermal conductivity are essential to the estimation of the contact resistance at the heating or cooling surface, as used in packet renewal models to predict the wall-to-bed heat transfer and further discussed in Section 3.4. Attempts to understand how heat is transferred through the system usually devolve into attempts to determine its “convective heat transfer coefficient,” (W/m2K), as defined in the standard equation for heat transfer by convection: Experiments allow the determination of the temperature difference ( , in ) for a known heat flow rate, , and a known surface area of the heat exchanger (m2). The heat exchange surfaces are either an “outside-wall,” that is, a heat transfer jacket, or an “internal” surface with an immersed tubular heat exchanger. Both experimental and industrial equipment commonly use cylindrical configurations to contain the bed, often with immersed tubular heat exchanger and/or heat transfer jacket. The heat transfer coefficient in bubbling fluidized beds has been