Incipience of two-phase flow from a stratified gas-liquid region in multiple discharging branches : experimental investigation including PIV measurements

The importance of predicting the incipience of two-phase flow from a stratified gas-liquid region in discharging branches has strong implications in industries where safe operation is of primary concern. This is particularly true in the nuclear reactor industry, where two-phase flow in the reactor cooling channels occurs due to a loss-of-coolant accident (LOCA). The incipience of two-phase flow can be characterized by the location of the gas-liquid interface relative to the discharging branch. If the gas-liquid interface is above the discharging branch, gas can entrain into the branch liquid flow by either vortex formation or vortex-free gas pull through. If the gas-liquid interface is below the discharging branch, liquid can entrain into the branch gas flow. A semi-circular test section geometry, with three discharging branches, was used to simulate a typical CANDU header-feeder. The experimental investigation consisted of a two part study. In the first, the onset of liquid entrainment was investigated in multiple discharge scenarios. In the second, the liquid velocity flow field was investigated at the onset of gas entrainment in a single discharging bottom branch.