Heat transfer characteristics in a pressurized fluidized bed of fine particles with immersed horizontal tube bundle

Abstract

The effect of pressure on the average and local heat transfer coefficients between a submerged horizontal tube (25.4 mm-O.D.) and a fluidized bed has been determined in a fluidized-bed-heat-exchanger (FBHE; 0.20 x 0.26 x 0.58 m-high) of silica sand particles. The heat transfer coefficients were measured around the tube circumference by thermocouples. The average heat transfer coefficient (h(avg)) exhibits a maximum value with variation of gas velocity (U-g) irrespective of pressure. The h(avg) increases with increasing pressure at a given fluidizing number (U-g/U-mf) due to increase of gas density and improvement of fluidizing quality with indication of low standard deviation of solid holdup fluctuation. Variation of instantaneous local heat transfer coefficient (h(i)) is a function of bubble behavior around the tube surface. The h(i) exhibits higher values at the bottom than top location of the tube, and the highest value at the side of the tube (0 degrees) at the given bed pressure. Instantanous h(i) variations at the top regions (+45 degrees, +90 degrees) becomes much uniform with high peak frequency at higher pressures. The obtained maximum heat transfer coefficients (h(max)) in terms of the maximum Nusselt numbers (Nu(max)) have been correlated with Archimedes, Prandtl and Froude numbers. (C) 2013 Elsevier Ltd. All rights reserved.