Numerical analysis of steam condensation at sub-atmospheric pressure in water suppression tank

A preliminary analysis has been performed with ANSYS© Fluent code to support the experimental campaign planned to be carried out at DICI-University of Pisa in the Experimental Test Tank (ETT), aiming at studying and characterizing Direct Contact Condensation (DCC) in relevant configurations (sub-atmospheric pressure) for International Thermonuclear Experimental Reactor (ITER) Vacuum Vessel Pressure Suppression System (VVPSS). The selected accidental scenarios subject to the investigation are a Loss Of Coolant Accident (LOCA)/Ingress of Coolant Event (ICE) and/or Loss Of Vacuum Accident (LOVA). The numerical analysis is focused on the scheduled test n. 21, injecting 0.488 kg/s of superheated steam at 150°C through a sparger into the ETT water pool at 50°C and sub-atmospheric pressure (PSAT about 12.4 kPa), representing the last part of the ICE transient scenario. Two simulations have been performed: the first considering the full domain of the ETT and the second a reduced portion of it. In this latter, part of water and cover gas regions were modelled as solid domains aiming at reducing the calculation time of the transient analysis. The DCC scenario was simulated by 2D axisymmetric Fluent models, assuming the injection of equivalent liquid water, conserving mass, momentum and energy of the actual steam flow. In order to take into account the released latent heat of condensation, an equivalent energy source was set in the expected transition phase region, close the sparger injection points. The simulations show that the simplified model provides results very close to those obtained with the full model, in terms of temperature and velocity profiles in the time domain, while allowing for a significant saving in computational time by a factor of five. The analysis also showed that the volume of water below the sparger holes is only slightly affected by the water dynamics in the upper part of the tank.