Assessment of an insulating air layer model of façade external system: contribution to fire simulation of facade performance fire test

Abstract

Ventilated façades as a part of external thermal insulation are useful systems when applied to building design, especially in bioclimatic construction. In the context of the fire safety engineering, well taking into account the air layer arranged in such systems is of importance to assess the fire behavior of a building. In fire engineering studies, CFD codes as FDS (Fire Dynamics Simulator) are widely used. However, mesh sizes used in simulation are generally greater than 100 mm. Thus, the air layer cannot be modeled accurately because of typical thickness lower than mesh size.In the present study, a method based on an analytical formulation of the thermal resistance for building components is evaluated to approach the local air layer behavior. Radiative, convective and conductive mechanisms in the layer are then accounted. The proposed method could be implemented in a numerical model when a ventilated layer must be considered through an equivalent thermal conductivity for the air layer.CFD simulations are performed with FDS for fire exposed façade during LEPIR II performance tests where the ventilated façades with the larger air layer thicknesses are modeled. Ambient conditions in terms of flow temperature and velocity inside the air layer are evaluated in several locations of the façade.These façade models are then modified to replace the air layer by an equivalent thermal conductivity analytically assessed. Its local internal temperature is then compared with the one predicted with CFD, using 2D thermal calculations with the FEM code SAFIR. Extrapolations for smaller thicknesses of air layers that cannot be model with FDS can then be assessed numerically.This preliminary work shows encouraging application for air layer reserved in façades during a fire. Further development and validation are in progress to apply this method to other configurations of air layering.