Heat transfer to small cylinders and flat strips immersed in a fluidized bed

Fluidized bed heat treating systems have been used to heat treat low carbon steel wires for a number of years. Extending this application to high carbon steel wires and metal straps has been implemented with very little success due to the lack of knowledge of heat transfer coefficients or, alternatively, Nusselt number for small cylinders and flat strips. The objective of this study was to provide reliable data for predicting a suitable Nusselt number for small horizontal cylinders and flat strips at various orientations under conditions typically encountered in heat treating fluidized bed systems. In this study, resistively heated small cylinders and flat strips ranging in diameter from 1.27 to 9.53mm and in width from 6.25 to 25.4mm respectively were immersed in a 311mm in diameter lab-scale fluidized bed. The bed consisted of fine alumina oxide sand of mean particle size ranging from 145 to 330[micro]m fluidized by air at ambient temperatures. The fluidized bed unit was capable of fluidizing rates ranging from 0.14 to 23 G/Gmf. The cylinder and flat strip samples were positioned horizontally in the bed. The flat strip samples were rotated around the length's center axis in 15° increments from a 0° horizontal position to a 90° vertical position. The results showed that published correlations over-predict small cylinder Nusselt numbers over the entire fluidizing range; furthermore, their trends did not agree. Flat strip results demonstrated highest heat transfer rates at a vertical position. A correlation that predicts the mean Nusselt number within ±15% for both geometries was developed for operating conditions covered by the experiments.