KURENAI : Kyoto University Research Information Repository

]]I. 3 DROPLET GENERATION BY ENTRAINMENT
Ishii and Grolmes [14] have developed a criterion for droplet entrain-
ment based on the shearing off of roll-wave for film Reynolds number > 160.
At the gas velocity beyond the inception of entrainment, it can be considered
that droplets are generated by the shearing off of roll-waves. Combined ef-
fects of entrainment'and subsequent disintegration and coalescence of drop-
lets produce a droplet size distribution. However, it is expected that drop-
let size distributions are mainly characterized by sizes of droplets which
are entrained by the shearing off of roll-waves in annular flow.
Theoretical estimation of a droplet size based on this mechanism will be
discussed below. In Fig. 1, the mechanism of the shearing off of roll-wave
is illustrated. The force balance on the ligament, which is about to be torn
off, is given by
TrDa =4D2CD1
gj2•(9)
Here D is the characteristic diameter and CD is the drag coefficient at the
wave crest. The term irDa is a surface tension force and the right hand side
of Eq. (9) is a drag force acting on the ligament. Introducing Weber number,
2D
We -Pgog, Eq. (9) can be rewritten as
We = C
D
Now in what follows it will be shown that the drag coefficient,
to interfacial shear stress, Ti. In a droplet annular two-phase
facial force per unit volume acting on dispersed phase, Mid, is
and Chawla [27]) by
FDad adAd 1
Mid =V d=Vd-2CDpgvIv
=rI ,
where FD, ad, Vd and Ad are drag force, volume fraction, volume
area of a typical particle. Assuming that droplets are spheres,
160
CD, is
flow,
given
(10)
related
inter-
(Ishii
(11)
and projected
one obtains