KURENAI : Kyoto University Research Information Repository

M. 1 INTRODUCTION
A loss of coolant accident (LOCA) and subsequent rewetting and ref-
looding phenomena in a nuclear reactor constitute another important phases
of the nuclear reactor accident. In this case, large amount of vapor
is generated and high quality two-phase flow often encountered.
Powerlevels under these accidents are relatively low (decay heat level)
and the transient process is slow. Under these circumstances, the boil-
ing and two-phase flow phenomena are not localized , but depend on the
upstream conditions. In the nuclear reactor core, the heated length is
considerably long extending to several meters. Therefore, the boiling and two-phase
flow phenomena involved are those for large length to diameter ratio, 2/dhe'
The heat and mass transfer mechanisms are considered to be quite different
from those for small 2/dhe which have been studied in the previous chapters.
The transient and steady state boiling phenomena under these large
2/dhe conditions are basically determined by the mass, momentum and energy
transfer processes of annular or annular dispersed flowswhich are usually
observed in high quality two-phase flow.
Therefore, in this chapter and succeeding two chapters (Chapter 1V
and V), the hydrodynamic characteristics' of annular or annular dispersed
flow are studied. Several practical correlations which are ne-
cessary to the analyses of nuclear reactor accidents have been developed.
Firstly, the droplet size and its distribution are analysed in this chapter.
An accurate knowledge of the mean droplet size and droplet size distri-
bution is essential to the detailed analyses of droplet or annular dispersed
flow. The inception of droplet entrainment from the liquid film significantly
changes the mechanisms of mass, momentum and energy transfer [1,2]. The avail-
able Interfacial area and droplet transport depend on the amount of entrain-
ment and droplet size. Therefore, in order to accurately model and predict a
number-of important physical phenomena in annular dispersed flow, an under-
standing of mechanisms of entrainment, generation of droplets and size dis-
tribution are prerequisite. In particular, the relative velocity and droplet
carryover, the dryout and post dryout heat transfer [3-7] and the effective-
ness of the emergency core cooling in light water reactors [8-11] are signif-
155