KURENAI : Kyoto University Research Information Repository

On the other hand, in a later stage of the LOCA and reflooding where
liquid flow rate is extremely low, the pool entrainment phenomena play im-
portant roles in heat and mass transfer processes. Therefore, these phenomena
were studied. Droplets which are suspended from a free surface are
partly carried away by streaming gas and partly returned back to the free
surface by gravity. A correlation is developed for the pool
entrainment amount based on simple mechanistic modeling and a number of
data. This analysis reveals that there exist three regions of entrainment
in the axial direction from a pool surface. .In the first region (near
surface region), entrainment is independent of height and gas velocity.
In the second region (momentum controlled region), the amount of
entrainment decreases with increasing height from the free surface and
increases with increasing gas velocity. . In the third region (deposition
controlled region), the entrainment increases with increasing gas velocity
and decreases with increasing height due to deposition of droplets. The
present correlation agrees well with a large number of experimental data
over a wide range of pressure for air-water and steam-water systems.
Along with the separate effects of the transient boiling and two-phase
flow on the thermohydraulic characteristics of a nuclear reactor, the over-
rall characteristics throughout the entire loop of a nuclear reactor com-
prise another set of important problems. The phenomena correspond to the
transient boiling and two-phase flow under extremely large t/dh e where t
covers the entire loop. One of the most important applications of the
above problems is the natural circulation coolability. The phenomena are
particularly important associated with small break LOCA and subsequent pump
trip in a nuclear reactor. A model facility is often used in evaluating
the natural circulation coolability. In designing the model facility.
which' simulates a prototype reactor, , scaling laws are impotant. Therefore,
scaling criteria for a natural circulation loop under single phase and
two-phase flow conditions have been derived. For a single phase case the
continuity, integral momentum, and energy equations in one-dimensional area
average forms have been used. From this, the geometrical similarity groups,
friction number, Richardson number, characteristic time constant ratio, Biot
number, and heat source number are obtained. The Biot number involves the
heat transfer coefficient which may cause some difficulties in simulating the
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