Graduate Thesis Or Dissertation
 

Experimental studies of heat transfer with an array of horizontal tubes immersed in a high-temerature fluidized bed

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  • An experimental study is described in which time-average local heat transfer coefficients were obtained for arrays of horizontal tubes immersed in a hot fluidized bed. Spatial-averaged heat transfer coefficients, computed from local values, are also reported. Refractory particles with surface mean diameters of 2.14 mm and 3.23 mm were fluidized by combustion products of propane, which produced bed temperatures of 810 K, 922 K, and 1005 K. The superficial gas velocity ranged from the packed-bed condition through approximately twice the minimum fluidization level, which was limited by air blower capacity. An array of nine tubes arranged in three horizontal rows was employed. The 50.8 mm diameter tubes were arranged in an equilateral triangular configuration with 15.24 cm spacing between centers. The center tube in each of the three rows in the array was instrumented with thin thermopile-type transducers for direct measurements of local heat fluxes and surface temperatures at intervals of 30° from the bottom to the top--a total of seven sets of values for each of the center tubes. The three sets of data are representative of the heat transfer behavior of tubes at the bottom, top, and the interior of a typical array. Data were also obtained for a single horizontal tube to compare with the results of tube bundle performance. Existing correlations for the prediction of overall heat transfer coefficients were evaluated using the results of this study, as well as other data available for large particles. Higher rates of heat transfer were obtained with smaller particles and/or higher bed temperatures. Superficial gas velocity had a significant effect on heat transfer coefficients. For local coefficients, the tops of the tubes were affected the most due to the presence of the lee stack. For spatially averaged values, a sharp increase in the coefficient was observed when the gas velocity reached that for minimum fluidization; relatively little variation was noted with further increase in gas flow rate. Comparison with results for a single tube in a bubbling bed indicated that bed-to-tube heat transfer in the presence of adjacent tubes was affected only a small amount. Tubes in the bottom, top, and interior rows exhibited different heat transfer performance. The tube array position in the bed had rather slight influence on heat transfer; the static bed height showed practically no effect on the coefficients.
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