Heat transfer at Ar > 108
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From the viewpoint of engineering practice, Archimedes numbers above Ar = 108 are rather exotic because they are attained for particle sizes of perhaps dp> 5 mm at elevated pressures of say 20 bar (2 MPa), even higher in the case of hot systems, owing to increased gas viscosity. These systems, however, reveal an interesting feature, which is different to those discussed in the preceding chapters. As visualized in Fig. 9.1, with increasing superficial gas velocity, a fixed bed shows a gradual increase in the heat transfer coefficient with increasing superficial gas velocity (curve 1). However, in sharp contrast to the behaviour observed for Ar < 108, a fluidized bed that exceeds its minimum fluidization velocity umf deviates sidewards, so to speak, from the fixed bed line, i. e. fixed bed heat transfer coefficients are higher than those observed with fluidized beds.
KeywordsHeat Transfer Heat Transfer Coefficient Minimum Fluidization Heat Transfer Measurement Minimum Fluidization Velocity
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