Evaluation of bed-to-tube surface heat transfer coefficient for a horizontal tube in bubbling fluidized bed at high temperature

Abstract

Experiments have been conducted in a bubbling fluidized bed to determine the bed-to-tube surface heat transfer coefficient to a horizontal tube at high bed temperatures (400–950 °C). The aim was to study the influence of the bed temperature, the particle size, the bed material and the superficial gas velocity. A tube (do = 6 mm) through which water is flowing was placed close to the middle of the bed with a fixed bed height of 17 cm. Silica sand, crushed and beneficiated ilmenite and ground steel converter slag were used as bed materials. Air was used as fluidization gas. The mean particle diameter of the used bed materials was in the range 123–327 μm. The estimated heat transfer coefficients were compared with correlations from literature describing the convective bed-to-tube heat transfer coefficient (all of which have been derived at temperatures below 400 °C), with the addition of the radiative heat transfer contribution. The heat transfer coefficient increased with increasing bed temperature, decreasing particle size and increasing superficial gas velocity up to a certain value. These trends were observed for all bed materials although significant differences were still observed in the estimated heat transfer coefficients. Ilmenite was the material for which the highest heat transfer coefficient was estimated. The experiments verified high heat transfer coefficients (768–1858 W/(m2K)) to the outside tube surface operating at a superficial gas velocity of 0.15 m/s, which aligned very well with two of the heat transfer correlations and reasonably well with a third. The remaining three correlations were not as accurate at the examined conditions. The contribution from the convective heat transfer is significantly higher than the estimated radiative heat transfer to the tube.