Heat Transfer to Flat Strips Immersed in a Fluidized Bed

Heat transfer to objects immersed in a fluidized bed has been studied extensively across a relatively large range of geometries, though most work has looked at cylinders, a geometry important in power generation fluidized bed applications. As the power generation industry has been the primary stimulant to fluidized bed heat transfer research, very little information is available regarding geometries significant in heat treating applications. In this work, heat transfer to thin flat strips immersed in a fluidized bed is examined. This geometry is important in the steel strap manufacturing industry where many manufacturers use environmentally damaging molten lead baths to heat-treat their product. In order to determine the feasibility of a fluidized bed heat treating system as an alternative to the more hazardous lead-based process, an experimental investigation has been conducted in which Nusselt number data for flat strips with widths in the range of 6.35-25.4 mm are obtained using a laboratory-scale fluidized bed (310 mm diameter). Aluminum oxide sand particles in the range of d(p) = 145-330 mu m (50-90 grit) are used as the fluidized media within the fluidized operating range from 0.15G(mf) to approximately 10G(mf). The strip orientation angle theta(o) was also varied to establish the position from which maximum heat transfer is obtained. It was found that a decrease in particle diameter, an increase in fluidizing rate, and an increase in sample diameter resulted in an increase in Nusselt number. It was also observed that for the smaller samples tested, a maximum Nusselt number plateau was reached, at approximately G/G(mf) = 2.5. Finally, it was shown that an increase in theta(o) (from 0 deg to 90 deg) resulted in an increase in Nusselt number. A correlation for the maximum Nusselt number was developed, providing excellent agreement within +/- 15%. [DOI: 10.1115/1.4003530]