Digital simulation for fire safety
Simulation now offers a range of tools to engineers specialized in building fire safety. This modeling is particularly appropriate in Reunion Island to complete the regulations which are not always adapted to local specificities. Typically, the principles of natural ventilation (favoring open buildings) are sometimes in conflict with fire safety interests. Numerical tools can then help to validate the relevance of a fire safety strategy.
Example of a fire simulation result [1].
Spread of a fire in a room open on a corridor.
Visualization of smoke and temperature at different times.
Fire safety: a strategy considered at the design stage of a building
Fire risk is one of the major concerns when designing a building. In the event of a fire, it is essential to be able to protect the lives of the occupants.
From this point of view, smoke control is a central element of the fire safety strategy. Indeed, from the first phases of an uncontrolled fire, smoke constitutes a major risk because it hinders visibility and is particularly toxic for the body. Smoke poisoning is the leading cause of death among fire victims.
A national regulation sets the requirements for fire safety. Thus, the design offices accompany the building projects to ensure a satisfactory level of safety.
Towards a performance-based safety design
Although current regulations are mostly prescriptive, they are increasingly open to a performance-based approach. This approach allows more flexibility in the strategy employed, but the level of safety must be rigorously justified based on Fire Safety Engineering.
It is in this context that numerical simulation is a valuable asset. Simulation tools can be used to quantify the development and consequences of fires. Performance objectives are established and fire scenarios are selected in order to best size the prevention measures to be adopted.
Fire modeling
The available calculation codes can be divided into two families: zone models and field models. As an example, the CFAST and FDS software are respectively references in terms of implementation of zone and field models.
- Zone models
This approach consists in roughly dividing the domain of interest into sub-zones assumed to be uniform in order to facilitate the calculations. The assumptions greatly simplify the reality but the method has the advantage of being fast. Moreover, the approximations remain reasonable in many basic situations. Thus, these models are frequently used in the building industry.
- Field models
These models are much better because they calculate the detailed evolution of the fire at any point in space. The major drawback of this approach is the computation time required. Typically, the simulation of a fire scenario in a building can take several days. Nevertheless, their use tends to become more widespread with the continuous improvement of computing capacities.
Schematic diagram of a zone model for a room [1].
Two zones are defined (hot smoke layer and cold air layer) for which physical quantities are calculated in order to respect the mass and energy balances. Interactions with the exterior are also modeled (air or smoke inlets/outlets, heat exchanges at the walls).
Schematic diagram of a field model [1].
The domain is divided into fine meshes and the physics is solved at each point of the defined grid. It is then possible to extract values, such as temperature or smoke concentration, calculated for each mesh.
Let's remain cautious and critical of the numerical results
The representation of a fire by a calculation model is very complex because of the different physical and chemical phenomena intermingled. It is therefore necessary to keep in mind that simulations are never perfectly faithful to reality. It is the responsibility of the numerical simulation engineer to know the limits and uncertainties of the models used, to adapt the resolution method to each specific case, and to be critical of the results.
Simulations are nevertheless advantageous in order to evaluate the effectiveness of prevention measures, regardless of the specificities of a project. This approach makes it possible to optimize the means implemented while guaranteeing the targeted safety objectives. It also complements the regulations imposed, particularly in the event of conflicts or gaps in the standards.
In Reunion Island, simulation can help to respond to local specificities
The regulations concerning fire safety are not adapted for the French Overseas Departments and Regions, despite the particularities of the local climate and construction. The simulation is then all the more relevant to justify the strategies implemented.
INTEGRALE Ingénierie actively participates in the evolution of the fire safety sector in La Réunion through research projects. In 2019, a guide for fire safety related to smoke removal in naturally ventilated buildings was published. Currently, a thesis project aims to reduce the calculation time of numerical simulations of smoke removal.
[1] Guillaume, E. Fire Modeling - Numerical modeling tools for fire development. Sécurité et gestion des risques (2012).
