The behaviour of a two-phase mixed convection loop with subcooled boiling h
as been investigated in order to design a passive cooling system. In a test
facility with a L-shaped slab geometry subcooled water is pumped into the
horizontal lower duct and heated up by the hot bottom plate. Vapour bubbles
generated at the heated wall establish a disperse two-phase region with co
nsiderable void, measured by a gamma densitometer. The structure and stabil
ity of the two-phase mixed convection can be characterised by the Phase Cha
nge Number and the Subcooling Number. Operation regimes with flashing induc
ed instabilities in the chimney are indicated in the stability diagram. For
the numerical simulation the Eulerian two-fluid-model in the commercial so
ftware package CFX4 has been used. The two-fluid-model had to be extended w
ith physical models for bubble generation under subcooled boiling in the lo
wer duct and for water-bubble interfacial forces in the chimney, which have
been validated by data of a bubbly flow experiment and of a boiling experi
ment.