Combined heat and mass transfer in gas-liquid two-phase systems

Two important Processes of current interest to the safety of nuclear reactors, which involve change-of-phase in the presence of innert species in two-phase systems, were investigated.; The steady-state condensation in the presence of a noncondensable gas in a two-phase channel flow was mechanistically modeled using the two-fluid modeling technique. A flow regime map consisting of bubbly, churn-slug, and annular patterns was used, flow regime-dependent correlations for gas-liquid heat, mass and momentum transfer processes were utilized, and the effect of the noncondensable gas was accounted for using the stagnant film model. Model predictions were compared with available relevant data and indicated that the two-fluid technique along with the stagnant film model are adequate for modeling condensing two-phase channel flows with various flow patterns.; The combined heat and mass transfer during the evaporation of a superheated spherical droplet subject to an external flow field composed of its own saturated vapor, when the droplet and its surrounding vapor contain a tracer amount of a volatile transferred species, was numerically modeled. The droplet is assumed to have internal circulation similar to Hill's vortex flow. The problem is relevant to the transport of volatile radioactive material following certain nuclear reactor accidents, where the desorption rate of dissolved radionuclides need to be calculated. The droplet mass transfer model was an extension of the Kronig and Brink method to variable droplet radius, in which the transferred mass conservation equation is formulated, and numerically solved, using a coordinate system based on the streamlines defining the droplet internal circulation. Gas-side heat and mass transfer were treated using the quasi-steady film theory. Calculated parametric results, demonstrating the effects of important parameters when the partition coefficient of the transferred species is much larger than one, were performed. It was shown that the evaporation process strongly affects the transient concentration profile inside the droplet, and depending on the droplet initial superheat the average droplet concentration can increase significantly due to evaporation, before falling to the equilibrium level. Within the parameter range important to the transport of volatile radionuclides, furthermore, the species transfer process can be gas side-controlled.