A visual study of film boiling was carried out to determine the flow r
egime transition in the post-CHF region for a transient bottom reflood
ing of a hot transparent test section. The effect of test liquid subco
oling and inlet velocity on flow transition as well as on the quench f
ront propagation was investigated. The respective ranges for liquid ve
locity and subcooling were 1.8-26.8 cm/s, and 20-45-degrees-C, respect
ively. The test liquid was Freon 113 which was introduced into the bot
tom of the quartz test section whose walls were maintained well above
the film boiling temperature of the test liquid, via a transparent hea
t transfer fluid. The flow regimes observed down stream of the upward
moving quench front were the rough wavy, the agitated, and the dispers
ed droplet/ligaments in agreement with a steady state, two-phase core
injection study carried on recently by one of the authors. A correlati
on for the flow regime transition between the inverted annular and the
dispersed droplet/ligament flow patterns was developed. The correlati
on showed a marked dependence on the void fraction at the CHF location
and hence on the flow regime encountered in the pre-CHF region.