Jet penetration and bubble formatio n

The physics underlying bubble formation is not well understood . Some plausible speculations are advanced in section 2 .8 . However, literature abounds in phenomenological descriptions of the process .The fluidizing gas is introduced into the bed through the distributor, located at the bottom of the bed . In a comprehensive study on the subject of jet penetration and bubble formation, Massimilla (1985) has identified five different flow patterns leading to the formation of gas bubbles from the gas discharge at the distributor . However, he suggests that qualitative differences may in part be attributed to diffiiculties associated to the photographic techniques employed.Rowe et al. (1979) have suggested the use of only two modes of gas discharge:a stable jet and a succession of bubbles . Massimilla (1985) indicates that the flow pattern evolves from the chain-of-bubbles type to the permanent-jet type as the particle size increases.Hirsan et al. (1980) have defined three different jet penetration lengths (depicted in Figure 2 .5):

LB , the penetration of bubbles formed at the jet tip finto the bed before losing their momentum, such loss being evinced by the significan t deviation of the bubble from the vertical direction.

LMAX, the maximum length of the succession of cavities attached to the jet . LMIN , the jet penetration length .

Several correlations have been published for the jet penetration lengths (see Massimilla (1985) for a comprehensive listing) . Inspection of these correlations reveals that jet penetration :(1) decreases as particle density and size increases;(2) increases with bed pressure .

The effect of orifice diameter Do on L/Do is, however, controversial; while some correlations show no influence, others show dependence ; and, furthermore , this dependence does not always display the same trends. For PFBC, the correlation by Hirsan et al . (1980) has the merit of using the complete fluidizing velocity uef (i.e. the velocity at which the whole bed is fluidized), rather than the minimum fluidizing velocity u,,, f , as independent variable . For widely distributed particle sizes (as is the case in FBC) uc f is more significant than and it is also more sensitive to the effect of pressure (u, f is further discussed below in section 2 .6.1). This correlation reads

Figure 2.5 Jet penetration lengths .