Local instantaneous and time-averaged heat transfer in a pressurized fluidized bed with horizontal tubes: Influence of pressure, fluidization velocity and tube-bank geometry

Abstract

Measurements of local instantaneous bed-to-tube heat transfer were carried out in a cold pressurized bed with horizontal tubes. The influence of fluidization velocity and pressure was studied at different circumferential positions around a horizontal tube within a tube bank for three different tube-bank geometries. The signal from a heat transfer sensor was compared with capacitance probe signals sampled simultaneously in an adjacent position. The capacitance probe registers the alternating passage of bubbles and dense phase at the probe, and the respective contributions to the local instantaneous heat transfer from the gas and particle convection were thus identified by comparison with the heat transfer signal. The local time-averaged heat transfer coefficients at the different circumferential positions were determined from the instantaneous heat transfer signals, for the different operating conditions, as were the spatial-averaged heat transfer coefficient for the tube. The time-averaged heat transfer results were correlated with the hydrodynamic properties of the bed obtained in a previous investigation under the same operating conditions, and a strong coupling between the local mean bubble frequency and the local time-averaged heat transfer coefficient was obtained. The heat transfer results show a significant increase of the bed-to-tube heat transfer coefficient with increasing pressure, while results from a previous erosion study in the same bed show that, at high pressures, the tube erosion decreases with increasing pressure. Thus, it should be favourable to operate a bed at high pressure levels. The heat flux was measured using flush-mounted sensors on a heated copper cylinder replacing one of the tubes in the horizontal tube banks. The bed has a cross-section of 0.2m × 0.3 m, and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s. Of the three tube-bank geometries used, one had a fairly dense pitch and two had more sparse configurations. The bed material was silica sand with a mean particle diameter of 0.7 mm.