Reflux Condensation and Transition to Natural Circulation in a Vertical U-Tube

Reflux and natural circulation condensation in vertical inverted U-tube steam generators form an important heat removal mechanism for nuclear reactors in certain accidents. As a first step in understanding the behavior of such steam generators, condensation was studied in a single vertical tube with a cooling jacket. Steam was fed into the tube from an inlet plenum and condensed in the jacketed region. The inlet and outlet pressures and cooling jacket conditions were controlled to give well-defined boundary conditions. The amount of steam condensed and the flow patterns obtained were determined. The steam flow rate into the tube initially increased with pressure differences between the inlet and outlet plenums. The condensate ran back to the inlet plenum countercurrent to the steam flow (reflux flow). At a certain pressure difference, no further increase in steam inlet flow rate was observed though pure refluxing was maintained. Instead, a column of liquid formed above the two-phase condensing region. The length of this column increased as the pressure difference was increased. At a sufficiently large pressure difference the liquid column carried over the top of the vertical U-bend and there was a dramatic change in flow regime to natural circulation condensation in which the bulk of the condensate flowed cocurrently with the steam. The behavior of the system was explained by postulating that “flooding” conditions were reached at the inlet when the pressure difference became large enough for a liquid column to form above the condensing region. A small perturbation analysis of the stability of the condensing and liquid column regions was done using a lumped parameter approach and constant pressure boundary conditions. Experimental results on the frequency of oscillations in a single tube followed the qualitative trends predicted by the linear analysis, but the predicted frequencies were about twice as high as those observed.