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Predicting Freeboard Heat Transfer by Using Empirical Correlations in High Temperature Fluidized Beds (23)

This article is suggesting some empirical correlations to calculate the freeboard entrainment heights and the average heat transfer coefficients for a horizontal tube over the freeboard region of high temperature fluidized bed combustors. The correlations are tested against some published experimental measurements for silica sand 465 μm and limestone 1,400 μm diameters in fluidized beds for temperatures up to 750°C. The height of a fluidized bed over the static bed is defined as the entrainment height; first a correlation is used to calculate the entrainment heights in order to determine the extent of the freeboard region at different bed temperatures and gas velocities. The heat transfer occurs by radiation, gas convection, and particle contact mechanisms in a high temperature fluidized bed combustor. Contribution of these heat transfer mechanisms to the overall heat transfer is described in detail over the freeboard region of fluidized beds by using empirical correlations. These correlations can be used to calculate the heat transfer characteristics in the design of fluidized bed combustors. The relative contribution of radiation increases with increasing freeboard elevation and approaches a constant fraction of the total average heat transfer.