An investigation of fluidized-bed scaling: heat transfer measurements in a pressurized fluidized-bed combustor and a cold model bed

Abstract

A method is proposed for predicting heat transfer coefficients in hot fluidized-bed combustors by translating results measured in a scaled-down, cold model bed. Provided the beds are scaled to hydrodynamic similarity, local heat transfer coefficients measured in the cold model bed can be translated into local hot-bed convective coefficients with the aid of existing correlations for the gas and particle convective components. To obtain the total hot-bed-to-surface coefficients, a radiative component is then added. The chief advantages of the proposed method are that existing convective heat transfer correlations can be applied locally in a bed, and that no a priori knowledge of the voidage close to the transfer surface is required. In a previous paper by Almstedt and Zakkay, measurements of the bubble activity in a pressurized fluidized-bed burning coal and in a scale-down pressurized model bed operating at room temperature showed that a good hydrodynamic similarity can be obtained between the beds. The heat transfer translation method proposed here has been validated by comparing heat transfer coefficients measured in the same two beds, operating under scaled conditions. Average heat transfer coefficients for four different horizontal tube bundles, as well as local coefficients measured with probes in four different positions were compared. The results indicate good agreement between the hot-bed results measured and the results translated from the model bed measurements employing the proposed method. Furthermore, the present paper presents heat transfer measurements from the cold model bed for three different bed materials, at pressures of 0.1, 0.24 and 0.5 MPa and fluidization velocities ranging from 0.15 to 1.3 m/s. The results are in good accordance with existing theory, but indicate that the gas convective component (as well as the particle convective component) is significantly dependent on the fluidization velocity.