Cluster layers

Fig. 6.7 (a) Perspective of Ti2Nb5Cli404 structure in the [001] direction, (b) Cluster layer in Ti2Nb6Cli404. (c) Zigzag chain of edge-sharing [TiCl402] octahedra in Ti2Nb6Cli404. 

The contact of adsorbed ethanol layers should bring about the long-range attraction observed between glass surfaces in ethanol-cyclohexane mixtures. The attraction starts to decrease at -0.5 mol% ethanol, where ethanol starts to form clusters in the bulk phase. It is conceivable that the cluster formation in the bulk influences the structure of the adsorbed alcohol cluster layer, thus modulating the attraction. We think that the decrease in the attraction is due to the exchange of alcohol molecules between the surface and the bulk clusters. 

FePt C nanocluster films were prepared with a multilayer method in which FePt cluster layers and C layers were alternately deposited onto a Si substrate. C served to isolate the clusters and to avoid cluster-aggregation during thermal annealing. The average FePt cluster size is about 4.5 nm. The nominal thickness of each cluster layer is about 1 nm, so that the FePt clusters can be well separated with C layer. The thickness of C layer was adjusted with C volume fraction changing from 7 to 45 % [48],... 

The local heat transfer coefficients measured by both probe B and probe A are compared in Fig. 19. The local heat transfer coefficient for probe B is evidently larger than that for probe A. This difference in local heat transfer coefficients measured by the two probes gradually decreases from top to bottom of the probes at any radial position in the bed. Such enhancement in heat transfer is attributed to the promotion of cluster layer renewal surrounding the probe, as has been well recognized (Glicksman, 1988 Wu et al, 1989, 1990 Basu, 1990 Leckner, 1991 and Bai et al., 1990). 

To account for the influence of the length of the heat transfer surface on heat transfer, several cluster-layer type models have been reported in the literature (Glicksman, 1988 Wu et al., 1990 Mahalingam et al., 1991 Basu, 1990). However, all these models can only be applied to heat transfer between the suspension and the walls. 

When the heat transfer surface is immersed in the bed at any radial position, the solids particles form a downflowing cluster layer surrounding the surface. The thickness of the cluster layer grows up to reach a Anal constant value. 

Heat transfer between the cluster layer and the surface is mainly caused by unsteady particle convection. The cluster layer is renewed from time to time by the moving particles and fluidizing gas. 

Equation (7) shows that the local heat transfer coefficient is a function of the thickness and the renewal frequency of the cluster layer. If the gas-solids flow is fully developed and rb dp, Eq. (7) can be simplified to... 

The renewal frequency of the cluster layer at the heat transfer surface is mainly dependent on the solids exchange rate between the layer and the other suspension, and usually assumed to be proportional to the ratio of the characteristic velocity to the characteristic length. Here we define... 

In order to evaluate the descending velocity of the cluster layer, an alternative approach proposed originally by Glicksman (1988) is adopted. As shown in Fig. 22, clusters are assumed to sweep along the surface and then accelerate downwards under the action of gravity, until they reach a maximum velocity, Fcma,. According to an analysis of force balance, the cluster layer descending velocity can be expressed as... 

The parameter C represents the probability of contact between the cluster and the heat transfer surface when the cluster layer velocity remains constant. 

T time-averaged temperature within cluster layer, K... 

The mass and heat balance equations are the same for any type of dryer, but the particle transport equation is completely different, and the heat- and mass-transfer correlations are also somewhat different as they depend on the environment of the particle in the gas (i.e., single isolated particles, agglomerates, clusters, layers, fluidized beds, or packed beds The mass-transfer rate from the particle is regulated by the drying kinetics and is thus obviously material-dependent (at least in falling-rate drying). 

Durston P J ef a/1997 Scanning tunnelling microscopy of ordered coated cluster layers on graphite Appi. Phys. Lett. 71 2940... 

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