KURENAI : Kyoto University Research Information Repository

Visual observations were made of steady state boiling to provide a
better understanding of burnout phenomen .
Photo 1 shows some boiling aspects at pressure 0.143 MPa, velocity
1.35 m/s, subcooling 10 K and nearly maximum heat flux. It can be seen
that small bubbles generated on the heater coalesce to form larger bubbles
as flow goes downstream (upper side of this photo). It is also observed
that these larger bubbles further coalesce and vapor blankets appear ,
around the heater surface.
1.3.2. Transient Performances
I.3.2.a. Transient Non-Boiling Heat Transfer
During a period from the initiation of heating to the boiling inception ,
heat is transferred to water by transient non-boiling heat transfer . Analy-
tical or experimental workshave been presented by Chambre et al. [25] and
Soliman et al. [26] for transient non-boiling heat transfer under forced
convection with exponential heat input to flat plates. Chambre et al.
assumed a slug flow model where the velocity profile is uniform , while
Soliman et al. took into consideration the boundary layer over a flat
plate and compared the result with experiments. Both analyses predicted
that the transient non-boiling heat transfer coefficient had
initially a decreasing tendency with time , t, and attained an asymptotic value
for T>TUwhere t is the time elapsed from the initiation of heating .
The present experiment showed a trend similar to those predicted by
the above analyses. Figure 14 shows the variation of transient non-boil-
ing heat transfer coefficients with time for velocity 1 .35 m/s, water
temperature 316 K and period from 10 ms to 5 s using 1 .2 mm diam and 7 cm
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