The mechanism of violent condensation shocks

In direct-contact steam condensers, violent condensation shocks (VCSs) occur at low steam flow rates. This phenomenon was also observed in the pressure-suppression system of nuclear boiling water reactors. Thus, the phenomenology of condensation in this type of condenser has been investigated. The main design feature of the Plexiglas test apparatus used here is that the events of rapid steam-water condensation are observed in a sectional view instead of an external view. This allows the phenomena at the phase interfaces to be observed in detail. In our experiments we found that a characteristic feature of VCSs is the appearance of considerable entrainment inside so-called steam pockets, which is characterized by atomization and is correlated to the extremely high rate of condensation. This avalanche-like increase in water atomization is induced by the Kelvin—Helmholtz instability and occurs because of the increasing steam flow velocity along the freely movable surface of water, primarily in the bottle-neck of the steam pocket. Detailed examination of the experimental data shows that the entrainment causes the temporary high condensation rates, which were previously observed but not understood. This is in agreement with the conditions of a sonic steam jet blowing into a subcooled pool of water where entrainment stimulates the condensation process in a similar way. The extraordinarily high condensation rates in the steam pocket induce very high steam velocities in the vent pipe, so that the entrainment often propagates outside the steam pocket in the vent pipe. We conclude that the initiating mechanism of VCSs is this self-amplifying feedback process which lasts until the initiating steam pocket has disappeared. The induced state of rapid condensation outside the steam pocket decays with a time constant in the range of 0.1 s.