Bubbles circulation induced

For the bed-to-surface heat transfer in a dense-phase fluidized bed, the particle circulation induced by bubble motion plays an important role. This can be seen in a study of heat transfer properties around a single bubble rising in a gas-solid suspension conducted... 

BOILING HEAT TRANSFER IN CONDITION OF LIQUID CIRCULATION INDUCED BY RISING BUBBLES... 

Solids Circulation Pattern. Yang et al. (1986) have shown that, based on the traversing force probe responses, three separate axial solids flow patterns can be identified. In the central core of the bed, the solid flow direction is all upward, induced primarily by the action of the jets and the rising bubbles. In the outer regions, close to the vessel walls, the solid flow is all downward. A transition zone, in which the solids move alternately upward and downward, depending on the approach and departure of the large bubbles, was detected in between these two regions. 

If pnliL is large, Q approaches unity. If ////// is small, Q approaches a value of 1.5. Thus the effect of circulation is small when a liquid drop falls in a gas although is large when a gas bubble rises in a liquid. If the fluid within the drop is very viscous, the amount of energy which has to be transferred in order to induce circulation is large and circulation effects are therefore small. 

In the emulsion phase/packet model, it is perceived that the resistance to heat transfer lies in a relatively thick emulsion layer adjacent to the heating surface. This approach employs an analogy between a fluidized bed and a liquid medium, which considers the emulsion phase/packets to be the continuous phase. Differences in the various emulsion phase models primarily depend on the way the packet is defined. The presence of the maxima in the h-U curve is attributed to the simultaneous effect of an increase in the frequency of packet replacement and an increase in the fraction of time for which the heat transfer surface is covered by bubbles/voids. This unsteady-state model reaches its limit when the particle thermal time constant is smaller than the particle contact time determined by the replacement rate for small particles. In this case, the heat transfer process can be approximated by a steady-state process. Mickley and Fairbanks (1955) treated the packet as a continuum phase and first recognized the significant role of particle heat transfer since the volumetric heat capacity of the particle is 1,000-fold that of the gas at atmospheric conditions. The transient heat conduction equations are solved for a packet of emulsion swept up to the wall by bubble-induced circulation. The model of Mickley and Fairbanks (1955) is introduced in the following discussion. 

Description The unique reactor design uses a simple vertical U-shaped leg connected to a horizontal gas-liquid separation vessel. Reactant gases are fed to the bottom of the U where they dissolve and combine under sufficient static pressure to prevent boiling in the reaction zone. Above this zone, the heat of reaction produces vapor bubbles that flow upwards into the horizontal vessel. A natural circulation of EDC is induced by the density difference in the two legs of the U. ... 

The lateral distribution of gas bubbles induces bulk recirculation of the emulsion phase. For teeter beds, the flow pattern of solid particles has been studied by Werther (W7, W8), Bui ess and Calderbank (B17), Oki and Shirai (03) and Whiteheade/a/. (W12), in experiments carried out with alumina particles, quartz sand, and petroleum coke under conditions of dp s 83 /Ltm and (f/, - Umd/Umf — 14. The mode of bulk circulation was centrally descending and peripherally ascending for If/Dr 1. while the ascending flow moved toward the center with increasing Lf/Dx. Werther (W8) showed that this circulation flow is caused entirely by bubbles which carry solid particles upward in their wakes. 

In contrast to the teeter bed, less work has been carried out on the bulk flow pattern in fluid beds (Lll, M29). Measurements at the relatively high gas velocities of practical interest (C/f > 10 cm/sec, Uf/Umt 1) show that the rate of circulation exceeds that of solid particles conveyed by the bubble wake, and results from the buoyant force induced between the centrally ascending bubble-rich phase and the peripherally descending emulsion phase of low bubble content (see Fig. 2). The behavior greatly resembles that in bubble columns, as will be discussed quantitatively in a later section (H9, K15, M31, P3, P6, T23, U3, Y2, Y22, and Kato and Morooka, unpublished data). For a bubble column, Shyu and Miyauchi... 

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