The heat transfer coefficient for a freely moving sphere in a bubbling fluidised bed

Abstract

Heat transfer coefficients have been measured for phosphor bronze spheres freely moving around a bed of hot sand fluidised by air. The experiments involved inserting such a sphere (diam. 2– 8 mm ) attached to a very thin (o.d. 0.2 mm ), flexible, light thermocouple into an electrically heated fluidised bed of sand, held at 750° C, 825° C or 900°C. Measurements were made of the rate at which the phosphor bronze sphere was heated up for various fluidising velocities, sizes of sand, etc. The heat transfer coefficient was measured in such a way as to exclude any contribution from radiation, which was established as operating quite independently of the fluidising conditions. The heat transfer coefficient due solely to convection and conduction was found to increase only slightly with the temperature of the bed; also there were small decreases when U/ U mf was increased. However, there were significant decreases in heat transfer coefficient when the size of the sand particles was increased; in addition, the heat transfer coefficient was slightly smaller for larger phosphor bronze spheres. A model was developed to predict such heat transfer coefficients; the dominant contribution usually turns out to be heat transfer from the hot sand particles to the immersed sphere, with heat transfer from the fluidising gas being less important. The model predicted heat transfer coefficients in tolerably good agreement with the values measured. However, the heat transfer coefficients measured here are somewhat lower than the differing values obtained by previous workers; also they do not depend on the fraction of the fluidised bed occupied by bubbles. Furthermore, the coefficient for heat transfer from the fluidised particles to the phosphor bronze sphere does not depend on the diameter of that sphere. This contrasts with heat transfer from the fluidising gas, for which the heat transfer coefficient decreases, when the diameter of the phosphor bronze sphere is increased.