Fire Growth Rate Strategies in FDS

Abstract

The HRR curve is characterized by four phases: the incipient phase, the growth phase, the fully developed phase and the decay phase. A central parameter in the design of buildings and the provision of fire protection measures, is the rate at which the fire develops. Therefore the speed at which a space becomes untenable is mainly dependent on the fire growth rate, which is also closely related to the risk profile selection. To reproduce the fire growth phase, it is generally recommended the use of the t-squared fires [1].Fire Dynamics Simulator (FDS) 6.6.0 [2] enables us to reproduce the t-squared fire in several ways, by using different “fire growth functions”, i.e. RAMP-function and SPREAD_RATE function. These functions use different approaches. While the RAMP function associates the time history of the HRR curve with the entire surface, the SPREAD_RATE function is similar to a cellular automaton approach and it mimics a radially growing fire along the surface with a constant speed.In this paper the behavior of the dimensionless heat release rate Q*, during the fire growth phase, has been studied for both the fire growth rate functions. Furthermore, a numerical study of the “growth function dependency” of some important outputs has been carried out, in order to quantify the implications on safety design. If on one hand, in accordance with the Swedish Best Practice [3], the SPREAD RATE should be preferred, seeing that the burning surface is kept small when the HRR is low and therefore it is possible to maintain applicable values of the ∗during the fire growth, on the other the case studies performed, using the RAMP function, have shown to be the worst case scenario analysing the gas temperature and the visibility in order to evaluate the Available Safe Egress Time.