Performance assessment and improvement of mixing in ventilation outlets using CFD simulation

Nuclear equipment in the nuclear industry workshops are subject to provisions for controlling their compliance and aging. As part of this monitoring, it is requested that the ventilation discharges from the facilities through the roof outlets be monitored by radionuclide measurements through a sampling equipment. In order to ensure the relevance of this measurement, in particular to check if the good mixing between flows coming from the workshops is achieved, the height of the sampling plane in the outlet is defined by ISO standards at a certain distance from the last tapping. However, this distance cannot always be achieved depending on the installations. Through numerical simulation, it is then possible to determine, for the desired locations, the homogeneity of the mixture of the various flows coming from the ventilation extractions as well as the concentrations of particles potentially released. The homogeneity is checked by the evaluation of several mixing coefficients. In the case presented in this article, several geometries, involving mixers and/or deflectors, have been tested in order to identify potential improvements to the installation. The results of the fluid and particle simulations are thus presented, compared and commented.