• experiments
• condensation
• CFD
• helium

This PhD research aims at improving the understanding of condensation phenomena
of interest for the safety analysis of nuclear reactor containments. Particular
attention if focused on the effect related to the possible presence of
hydrogen, which could be released in the containment atmosphere during a loss
of coolant accident with loss of core cooling and fuel pin clad oxidation.
The way steam condensation phenomena have been investigated in this work
is multiple: theoretical, experimental and numerical analyses have been carried
out.
A theoretical analysis of steam condensation is firstly proposed to clarify
fundamental issues related to the modelling of diffusion phenomena in multicomponent
mixtures.
Experimental data available by the CONAN and COPAIN separate effect
test facilities were analyzed. Further experimental activities have been performed,
aimed at collecting specific data concerning the effect induced by the
presence of a noncondensable gas lighter than steam .
Three different condensation models are proposed, basing on the two main
strategies adopted for wall condensation modelling in CFD codes: a fine approach
based on the detailed resolution of the concentration, temperature and
velocity gradients near the condensing wall and a less expensive approach adopting
coarser discretization in the proximity of the condensing wall, based on the
heat and mass transfer analogy. The HMTDMMSD and the HMTDMEBD approaches
are proposed based on the first strategy, but different diffusion models.
The HMTAM model is a fast running model based on the heat and mass transfer
analogy.
The capabilities of several turbulence models in reproducing turbulent transpiration
phenomena have been formerly evaluated. A first stage consisted in
analyzing the capabilities of turbulence models to reproduce nondimensional velocity
profiles in the presence of pure turbulent mass and momentum transfer.
A second stage consisted in a numerical analysis of suction effects induced by
condensation.
The condensation models have been finally applied to the modelling of the
CONAN and COPAIN steam-air and steam-air-helium tests. An extensive comparison
with local and integral experimental data is proposed.
A theoretical and numerical analysis is finally proposed, aimed at assessing
to what extent helium is an appropriate substitute for hydrogen.
Concluding remarks are drawn and future activities are suggested, either
under the experimental than the numerical point of view.