Assessing the current live fire training structure environment in Ethekwini using CFD

Abstract

eThekwini Fire and Emergency Services currently uses a repurposed structure to train their firefighters. This study identifies fire related hazards for trainee firefighters when using the ground and first floor of the existing structure. The purpose is to prevent shortcomings being repeated in the design of future firefighter training structures. The fire related hazards have been identified by using Computational Fluid Dynamics (CFD) to simulate fires in the existing structure. The fundamentals of CFD, as well as the selected CFD based fire model, have been summarized. CFD is selected due to its flexibility, accuracy, and cost effectiveness [1]. The Fire Dynamics Simulator (FDS) is the selected CFD based fire model as it has been extensively validated in the past decade [2], which is an important factor should the CFD code claim any credibility [3]. It was developed by the National Institute of Standards and Technology (NIST) to model fire driven fluid flow. It does this by numerically solving a form of the Navier Stokes equations appropriate for thermally driven low Mach flow [4]. Appropriate inputs required for FDS were investigated specifically for live fire training structures. A unique heat release rate (HRR) was investigated and subsequently proposed for a fire on both the ground floor and first floor. The HRR was assessed to find a rate that will be safe from inducing ventilation-controlled conditions and therefore preventing the occurrence of an explosive backdraught. This was investigated by monitoring the effect of the existing structure on a t-squared fire. A t-squared fire uses a selected growth coefficient to estimate the fire’s HRR when the data on the actual fire is not available. Also, the suitability of selecting the emissivity of soot for surfaces was investigated. This was done because it is expected that there would be residual soot deposits in the existing structure. The investigation used the soot modelling capabilities of FDS. This identified the soot density on exposed surfaces and provided an indication on the number of fires required to cover the majority of the exposed surfaces with soot. The simulations performed in this study were within the required validation range. This included using a selected numerical grid size that was within the validation range for the plume resolution index. There is a range of grid sizes that are valid for the plume resolution index and so to assist in the selection of a suitable grid size from the range of valid grid sizes, the implications of time constraints to complete a simulation were investigated. The investigation compared the accuracy of FDS results when having to restart the simulation multiple times due to limited computer access time, with the accuracy of FDS when using a coarser grid. From the fire induced environment, the heat flux and gas temperature were estimated to assess the safety of training firefighters. After examining past firefighter deaths, it was considered necessary to include normal civilian tenable limits in the study to identify the time to incapacitation should mistakes occur during training. The structure’s surface temperature was also measured to assess possible structural damage due to the concern that the existing structure has been damaged from repeated heating and cooling.