Numerical investigations on the component separation phenomenon and transport behavior of steam-air-hydrogen mixture induced by condensation.

The relative content and transport behavior of steam-air-hydrogen are of vital importance for nuclear power plants under severe accidents. Since there are many cold walls in the nuclear reactor containment building, the condensation of steam can directly change the mixture gas component and affect the air-hydrogen transport characteristics. The local accumulation of air-hydrogen increases the risk of hydrogen combustion, and then threatening the containment structural integrity. To reveal the coupling effect between steam condensation and multicomponent gas transportation, the present work established a mechanism CFD model which can locally evaluate multi-component gas heat and mass transfer. Numerical simulations were performed in pressures of 0.2–1.3 MPa, steam volume fractions of 10–90%, and hydrogen volume fractions of 10–70% in non-condensationable gas. In the whole parameter range, three flow patterns were found and classified: homogeneous downward flow, separation flow, and homogeneous upward flow. The reasons for the formation of the flow patterns were revealed and the margins amongst the three flow patterns were quantitatively marked on a Shapiro-Moffette ternary diagram. Moreover, a key component separation phenomenon induced by steam condensation was firstly discovered. [ABSTRACT FROM AUTHOR]