HEATING LIMITS AND INSTABILITY OF DOWNWARD TWO-PHASE FLOW

Flow characteristics and heating limits of downward two-phase flow in single or parallel multi-channels were investigated experimentally and analytically. The heated channel used in the experiments was made of a glass tube with a heater rod inserted, and the flow regime inside was observed.
In single channel experiments under low flow rate conditions, it was found that the gas phase which flows upward against the downward liquid phase flow condenses and diminishes as it rises, being cooled by inflowing liquid. But as the heating power was increased, a portion of the gas phase reached the top and accumulated to form a liquid level, which eventually caused the flow excursion as well as the dryout. On the other hand, under high flow rate conditions, the flooding initiated at the bottom of the heated section was the cause of the dryout and the flow excursion.
In parallel multi-channel experiments, reversed (upward) flow leading to the dryout was observed in some of these channels with low flow rates. The situation was the same in the single-channel case under high flow rate conditions.
Analyses were carried out to predict the onset of the dryout using the drift flux model and the Wallis'(1969) flooding correlation. The analytical results agree well with the experimental results.