Fluidized beds time-averaged heat transfer

These authors numerically solved the system of equations with appropriate boundary conditions to derive the time-averaged radiant and conductive heat fluxes between the fluidized bed and the heat transfer surface. Using... 

The time-averaged heat-transfer coefficientbetween the vertical wall and fluidized beds, using fine particles, is controlled by the unsteady heat transfer due to packet renewal at the heat-transfer surface (M14, M16, Y18). According to this mechanism, is expressed as ... 

Olssen SE, Almstedt AE. Local instantaneous and time-averaged heat transfer in a pressurized fluidized bed with horizontal tubes influence of pressure, fluidization velocity and tube-bank geometry. Chem Eng Sci 50 3231-3245, 1995. 

The parameter C in Eq. (25) is a dimensionless parameter inversely proportional to the average residence time of single particles on the heat transfer surface. It is suggested that this parameter be treated as an empirical constant to be determined by comparison with actual data in fast fluidized beds. The lower two dash lines in Fig. 17 represent predictions by Martin s model, with C taken as 2.0 and 2.6. It is seen that an appropriate adjustment of this constant would achieve reasonable agreement between prediction and data. 

Two-fluid simulations have also been performed to predict void profiles (Kuipers et al, 1992b) and local wall-to-bed heat transfer coefficients in gas fluidized beds (Kuipers et al., 1992c). In Fig. 18 a comparison is shown between experimental (a) and theoretical (b) time-averaged porosity distributions obtained for a 2D air fluidized bed with a central jet (air injection velocity through the orifice 10.0 m/s which corresponds to 40u ). The experimental porosity distributions were obtained with the aid of a nonintrusive light transmission technique where the principles of liquid-solid fluidization and vibrofluidization were employed to perform the necessary calibration. The principal differences between theory and experiment can be attributed to the simplified solids rheology assumed in the hydrodynamic model and to asymmetries present in the experiment. 

The above is a very condensed discussion of the hydrodynamic characteristics of fast fluidized beds. As presented below, heat transfer is strongly dependent on the time-averaged local concentration of solid particles and is therefore influenced by these hydrodynamic characteristics. Almost all the heat transfer models require information on solid concentration. The reader is referred to Chapter 19 of this book for more detailed discussion of the hydrodynamics, and... 

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