Evaluation of conductive heat transfer mechanisms between an immersed surface and the adjacent layer of particles in bubbling fluidized beds

GERMÁN DÉLFOR MAZZA; NÉSTOR JAVIER MARIANI; GUILLERMO FERNANDO BARRETO

CHEMICAL ENGINEERING COMMUNICATIONS

Gordon and Breach Science Publishers

Lugar: Londres, Inglaterra; Año: 1997 vol. 162 p. 92 - 123

0098-6445

The evaluation of the heat transfer coefficient hwp between a heat exchanging surface immersed in a gas fluidized bed and the adjacent layer of dense phase particles is analyzed in this contribution. Gas convective and radiant effects are not included in the present analysis. The inclusion of hwp, or an equivalent formulation, in mechanistic  models describing heat transfer has been necessary because the sudden voidage variation close to the immersed wall restrains significantly the heat transfer rate. However, there is not at present a widely accepted expression to evaluate hwp. A precise formulation for hwp accounting for transient conduction inside spherical particles, the Smoluchowski effect, the concentration of particles in the adjacent layer (Np) and an effective separation gap (l0) is developed here. Although Np can be estimated, in principle, from experimental evidence  in packed beds, and it is reasonably expected that l0=0, the analysis of experimental heat transfer rates in moving beds, packed beds, and bubbling fluidized beds indicate that values of hwp are, in general, smaller than expected from these assumptions. Appropriate values of l0 and Np are then estimated by fitting the experimental data. The probable effect of surface asperities is also discussed by analyzing a simplified geometrical model. It is concluded that the parameter l0 can be also effective to account for particle roughness, independently of thermal properties.