# Fire Risk Analysis in Large Multi-Compartment Structures Using a Hybrid Multiscale Approach
## 2 The Hybrid Multiscale Approach
### The CFD 1D Model  
A one-dimensional version of the software SAFIR, a three-dimensional CFD fire compartment model developed jointly by DGA and the University of Aix-Marseille (for example [10]), is used to model the gas flow and heat transfer in a ventilation duct. It is based on the conservation laws for mass, momentum, and energy. Two different duct flow configurations are considered. In the first configuration, ventilation air is flowing through the duct, which is heated along its circumference in the fire compartment, while in the second, the hot combustion products produced in the fire compartment enter the duct and propagate heat. In the second configuration, 2D axisymmetric gas radiation (including the contribution of radiatively participating gaseous species and soot) is considered.  
For model validation, eight experiments, plus replications, were conducted in the ventilation network of the DIAMAN device described in [14]. The stainless-steel duct connected to the fire compartment is 21 m long, 20 cm in diameter, and 1 mm thick. It was instrumented by placing 16 thermocouples on the outer surface of the duct, at about 1 m intervals for the measurement of the skin temperature, noted Tp1 to Tp16, and six Pitot tubes with integrated thermocouple for the measurement of the velocity and temperature of the gas in the duct, noted (V1,T1) to (V6,T6) (Figure 6).  
Figure 5: Flow chart of the network model.  
Figure 6: Schematic of the DIAMAN device, showing the instrumented ventilation duct.  
Due to the limitations in heat release rate of the fires conducted (approximately 1.25 MW), the criteria for fire transmission through a ventilation duct, namely 340 and 550 \({}^{\circ}\)C, could not be met. Therefore, in the present study, we assumed that fire transmission in the compartments that are crossed by the duct occurs when the temperature of its outer face exceeds 180 \({}^{\circ}\)C or when the temperature of the combustion products flowing through exceeds 250 \({}^{\circ}\)C. A detailed description of the model and validation is given in [32].  
The study case presented here corresponds to a 70 cm diameter heptane pool fire with a maximum heat release rate of approximately 1.2 MW (Figure 7a). The initial extraction rate was 36 m\({}^{3}\)/h. Experimental measurements at the inlet of the duct are used as boundary conditions (temperature, volume flow rate, and species concentrations). Figure 7b shows the predicted and measured time evolutions of the distances traveled by the 180 and 250 \({}^{\circ}\)C isotherms. A good agreement was obtained, as also observed for the other experiments (not shown). The model results can therefore be used to estimate the time of duct failure. For example, in the presented study case, fire transmission occurs in a target compartment located 15 m away from the fire compartment after 148 or 507 s depending on whether the ventilation duct passes through the target compartment or the hot gases flow into it (Figure 7b). The CFD 1D model is used here to determine the propensity of a fire compartment, modeled by the zone model, to transmit fire through the ventilation network.