KURENAI : Kyoto University Research Information Repository

V. 5 DROPLET VELOCITY AT INTERFACE
When the gas flux jg is small, the flow regime in a pool is a bubbly
flow. In this regime, discrete bubbles rise up to the surface of the pool and
collapse there. This mechanism of the bubble burst and subsequent entrainment
have been studied previously and an expression for the velocity of entrained
droplets has been developed empirically or theoretically [51-57]. According
to Newitt et al. [54], the initial velocity of entrained droplet due to bubble
burst is given by
t
vi =2D
BPfBDB'Po '(41)
where tB, DB, and Po are bubble burst time, bubble diameter, and pressure
around the bubble.
However, for the pool entrainment, the bubbly flow regime is limited to a
very small gas velocity. For example, in an air-water system at the
atmospheric pressure, the flow regime transition from bubbly to churn
turbulent flow occurs at jg in the ,order of 10 cm/sec [58]. Applying the
drift flux model [59] to a bubbling system and using the transition criterion
from bubble to churn turbulent flow regime [60] given by a = 0.3, the
transition gas flux becomes
j9=
p 1/2 0.325(--9-142)
f These indicate that the churn turbulent flow may be the most dominant flow
regime in a bubbling pool. In case of the churn turbulent flow, the initial
velocity of entrained droplets is not determined by the bubble burst
mechanism, but by a momentum exchange mechanism suggested by Nielsen et al.
[61].
This momentum exchange mechanism is shown schematically in Fig. 2. The
equation of motion for an element of the liquid ligament is given by,
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