Cluster residence time

The particle convection is in general important in the overall bed-to-surface heat transfer. When particles or particle clusters contact the surface, relatively large local temperature gradients are developed. This rate of heat transfer can be enhanced with increased surface renewal rate or decreased cluster residence time in the convective flow of particles in contact with the surface. The particle-convective component hpc can be expressed by the following equation, which is an alternative form of Eq. (12.39) ... 

Subbarao and Basu (1986), Basu and Nag (1987) and Basu (1990) derived the expression of cluster residence time lc on the heat transfer surface based on Subbarao s (1986) cluster model, although the model is not widely accepted. Lu et ai (1990) and Zhang et al. (1987) have also obtained empirical correlations independently for predicting cluster residence time based on their heat transfer experiments. However, because of the lack of available and reliable information about the residence time of clusters at the surface and the fraction of the clusters in solids suspension, a significant discrepancy between the results predicted by the different approaches mentioned earlier has been observed. Besides, it should be pointed out that the major shortcoming in the earlier models is that they all take no account of the heat transfer surface length. 

If the reverse of Reaction 1 is slow compared to 2 ( the colli sional stabilization step) then overall cluster growth will not depend strongly upon the total helium pressure. This is found to be the case using RRK estimates for k n and hard sphere collision cross sections for ksn for all clusters larger than the tetramer. The absence of a dependence on the total pressure implies that the product of [M] and residence time should govern cluster growth. Therefore, a lower pressure can be compensated for by increasing the residence time (slower flow velocities). 

The above behavior of narrow-pore supported cobalt catalysts toward co-fed water can also be explained in terms of relative size of cobalt clusters, pore network of support, expected location of cobalt clusters within the pore network, and relative differences in the residence time of water vapor within and outside the... 

Figure 8 shows the results now the NO scattered pulse shows clearly a demodulation that proves that the residence time of the NO molecules is larger than the rise time of the pulse. In addition N2 is present but no other products (like N20, N02, 02) were detected. The measurable stay time of NO comes from the chemisorption of NO on the Pd clusters. The angular distribution (Fig. 8b, solid circles) shows a clear increase of the cosine component due to chemisorption of NO on the Pd particles. From the pulse shape (Fig. 8a) we see that when the NO beam is turned on, the NO signal increases abruptly then more slowly. The first part called fast component corresponds to NO scattered or desorbed from the clean MgO, while the slow component is associated to NO desorbing (from a chemisorbed state) from the Pd clusters. Then, it is possible to measure the intensity of the two components as a function... 

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