Flow behaviour of the exchange flow through a ceiling vent in natural convection: A numerical approach using CALIF3S-ISIS CFD software

Abstract

In the fire safety context, the exchange flow through a horizontal opening connecting two superposed zones receives a particular interest for developing predictive models and validation of computational codes. Its random behaviour over time could influence the fire source and the vertical spread of the fire smoke. This work is part of a research program developed at the French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN), and its main objective is to bring new knowledge on this subject by carrying out a numerical study with the computational open-source software named CALIF3S-ISIS developed by IRSN. The configuration of the simulations is that of an experimental reduced-scale set-up, on which data have been obtained and could be used for validation. Large Eddy Simulation (LES) were performed (variation of wall boundary conditions, air injection temperatures, vent diameter and vent mesh refinement), and the bidirectional flow is properly estimated in comparison with the experiment. Particularly, it was determined that the increment of the geometric ratio L/D of the vent of diameter D and thickness L, also leads to an increment of a dimensionless exchange flow rate Froude number at the vent. However, a discrepancy was observed with the model proposed by Epstein based on his experiments in natural convection. The disagreement between the simulation results and the experiments is attributed principally to differences on the vertical temperature stratification of the lower room. Finally, it was investigated the influence of the vertical location within the vent over the mean flow behaviour through different levels of the vent, specifically an exchange of the downward and upward flow organization within the opening.