UBC Theses and Dissertations
Cavitation and entrainment in a downcomer entrance Ryan, James Arthur
Two-phase instabilities have been observed at a downcomer entrance in an experimental rig that drains near-saturated Freon-11 from a vessel. The regimes have been characterized using dimensionless parameters describing the pool depth, downcomer flowrate and the subcooling at the liquid interface in the pool. The two primary mechanisms involved in the instability were entrainment and cavitation. The entrainment regimes observed in this study can be correlated with the previously documented occurrence of air-water incipient drawdown. Drawdown was observed at higher pool depths than expected (for a given flowrate) when vapour bubbles were present at the entrance. Cavitation, the mechanism responsible for bubble formation and growth, has been found to be very susceptible to the presence of nucleation sites and to the amount of subcooling at the pool interface. Severe instability may occur at a flowrate where the local velocity at the entrance is equal to the rise velocity of a particular vapour bubble. Due to the generation of vapour through entrainment and cavitation, bubbles conglomerate at the entrance, with the vapour having no mechanism for escape. Some segregation occurs, with small bubbles discharging downwards and large bubbles rising upwards. Downcomer entrance geometry was varied as an independent parameter. In general, the observed regimes in the re-entrant and sharp-edged geometries were similar. The flow regimes in the rounded geometry were unique due to the effect of the streamlined curvature reducing cavitation at the entrance.
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