Steam-condensate countercurrent two-phase flow was studied in a vertic
al steam pipe with a cooling pipe. The lower plenum pressure and distr
ibution of coolant temperature in axial direction were measured. The p
lenum pressure increases as the steam velocity increases, and attains
a maximum when condensed water started to overflow from the top of pip
e. For a further increase of steam velocity, the plenum pressure begin
s to decrease and shows periodic oscillation in the case of low flow r
ate of coolant. During overflow of the condensate, no steam emission w
as observed from the pipe. The heat balance indicates the existence of
a region in which the condensate might stay as a single phase.