Gas convective component

Ackeskog et al. (1993) made the first heat transfer measurements in a scale model of a pressurized bubbling bed combustor. These results shed light on the influence of particle size, density and pressure levels on the fundamental mechanism of heat transfer, e.g., the increased importance of the gas convective component with increased pressure. 

The heat transfer rate for the gas convective component can be regarded as comparable to that at incipient fluidizing conditions. Thus, assuming hgc = hmf, Xavier and Davidson (1985) simulated the system by considering a pseudofluid with the apparent thermal conductivity Kd of the gas-solid medium flowing at the same superficial velocity and the same inlet and outlet temperatures as the gas. Therefore, the heat conduction of the fluid flowing... 

At high temperatures, the decreased gas density can decrease the gas convective component /tgc. On the other hand, the increased gas conductivity at high temperature can increase /tgc, Ke, and hpc- For a bed with small particles, the latter is dominant. Thus, a net increase in hc with increasing temperature can be observed before radiation becomes significant. For Group D particles, hc decreases with increasing temperature [Knowlton, 1992]. The effects of temperature and pressure on hpc, /igc, and Amax are illustrated by Fig. 12.13. 

In circulating fluidized beds, the clusters move randomly. Some clusters are swept from the surface, while others stay on the surface. Thus, the heat transfer between the surface and clusters occurs via unsteady heat conduction with a variable contact time. This part of heat transfer due to cluster movement represents the main part of particle convective heat transfer. Heat transfer is also due to gas flow which covers the surface (or a part of surface). This part of heat transfer corresponds to the gas convective component. 

Figure 4 plots, against suspension density, the heat transfer coefficients measured by Basu (1990) over a wide range of bed temperature for 296 pm sand, by Kobro and Brereton (1986) at a temperature of 850°C for 250 pm sand and by Grace and Lim (1989) at 880°C for 250-300 pm sand. The overall heat transfer coefficient is shown to increase with bed temperature. Before radiation becomes dominant in heat transfer, the observed rise in heat transfer coefficient with bed temperature may be explained as follows. The gas convective component is expected to decrease mainly because of the inverse dependence of gas density on temperature. On the other hand, the particles convective component will increase with temperature, thus leading to an increase in gas conductivity, because the latter is dominant for... 

In addition to direct contact with clusters, the wall of a fast bed is constantly exposed to the up-flowing gas, which contains dispersed solids (Li et ai, 1988). The gas convective component can be estimated on the basis of correlations for gas flow alone through the column, at the same superficial gas velocity and with the same physical properties. When a tall heat transfer surface is used or the bed is operated at high solids concentrations, errors caused by using different approaches will usually be small since htc is generally much less than hpc, provided the solids concentration is low and temperature high. 

Gas Convective Component. The gas convective component is caused by the gas percolating through the particulate phase and the gas bubbles coming in contact with the heat transfer surface. For small particles, though the contribution of gas convective component is small in the in-bed region, it could be important in the freeboard region. The gas convective... 

The gas convective component of mass transfer coefficient, k, is, therefore, obtained by substituting 0 = 0, in Equation 26. 

For larger particles, the effect of pressure is to increase the gas convective component of the transfer coefficient, since in the case of these materials... 

Increasing temperature has two effects (1) by decreasing gas density, the gas convective component of heat transfer is decreased slightly, and (2) by increasing the thermal conductivity of the gas, the effectivness of packets of emulsion phase in contact with the transfer surface is increased. The overall effect for Group A and B powders is to increase the convective transfer coefficient as was shown by Botterill and Teoman (1980). In the case of Group D powders where the gas convective... 

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