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Forces on tubes immersed in a fluidized bed Hosny, Nasr. M.


Heat transfer tubes and other surfaces fixed inside fluidized beds are subjected to buffetting forces due to solids and gas motion. Coupled with erosion and corrosion, these forces can lead to tube failure. The objective of the present work was to investigate the nature and origin of the forces acting on tubes immersed horizontally in gas fluidized beds, and to provide information to be used in structural design of heat exchanger tube bundles. Experiments were carried out in a column of cross-section 215 x 200 mm and height 1.5 m. The fluidizing gas was air and the bed was operating at ambient temperature. The parameters varied in the experiments were superficial gas velocity (from U[sub=mf] to U[sub=mf] + 1.4 m/s), static bed height (from 0.30 m to 0.45 m), mean particle size (from 185 to 430 μm), particle density (from 920 to 4100 kg/m³), tube diameter (from 15 to 32 mm) and tube shape (finned vs. unfinned). All tubes were mounted horizontally at a height of 0.30 m above the orifice plate distributor. Both vertical and horizontal forces were measured on tubes in isolation and on tubes within arrays of different configurations. For some runs, local pressure variations at the surface of the test tubes were also measured simultaneously with the forces. The relation between force characteristics and bubble properties was examined by separate experiments involving injection of single bubbles. Statistical parameters of the measured forces have been calculated. Instantaneous forces on each tube within a freely bubbling bed consisted of a series of pulses whose magnitudes, durations and rate of occurrence depend on the operating parameters. The magnitude of the forces is strongly influenced by superficial gas velocity, slightly dependent on particle size, and moderately affected by bed depth and particle density. Horizontal forces are generally of significantly lower magnitude than vertical components and oscillate from side to side with a zero mean. Forces can be characterized as containing both periodic and random components and are statistically stationary. The primary frequency content of the forces is in the range 0-20 HZ, with the major frequencies being almost independent of the operating conditions and always below 10 HZ at the height of measurement, 300 mm above the distributor. The trends are consistent with the hydrodynamics of bubbling fluidized beds. Force pulses have been found to be related to bubble motion, and pulse maxima correspond to arrival of bubbles at the tube surface. The intensity of forces on individual tubes within an array have been found to depend on the position of the tube in the array. The intensity of forces can be reduced significantly by using reasonably tight inter-tube spacings and by reducing the level of the tube-bundle above the gas distributor.

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