SAFETY/FIRE | TECHNICAL
DESIGNING IN FIRE SAFETY THROUGH SIMULATION
Florian Michelin, Ventsim Manager, Howden, discusses approaches to simulating the complex variables at play in fire incidents in tunnels, and so help to design effective ventilation systems as part of fire safety measures
Fire safety is an integral part of tunnel ventilation design. The ventilation system design comes about through
experienced practitioners applying their engineering knowledge to each specific tunnel project. This knowledge includes modelling to understand the potential behaviour of fire, which will determine system and procedural responses. Howden’s VentsimTM
software is an enabling tool to
design complete ventilation systems by developing a 3D model, but it is the testing component of the software that is key to dealing with design for potential fire risks.
SIMULATION OBJECTIVES Fire can be a complex event to simulate accurately. For tunnels, one of the desired outcomes of a fire simulation is to predict what will happen to the ventilation circuits underground. Smoke and gases may flow within an expected route, but fire can also change some of the fundamentals of the ventilation system given the amount of heat, change of air density, and buoyancy of the hot gas that can result in quite marked changes of airflow and direction.
If the dynamics of a fire and its effect on the
ventilation circuit are radically different from what is expected then the risk of exposing people to gas and smoke is substantially increased, as well as the way in which escape and rescue is designed. This is at the core of the need for fire simulation, to help designers investigate the variables and seek to minimise, or eliminate, the unexpected and achieve the best possible outcome for tunnel safety. Fire simulation can be a great planning tool also
when used in conjunction with the design of evacuation exercises. The fire simulator can enable plausible scenarios to be tested and then it can be used within actual training exercises to see how people respond, especially in realistic underground settings. In principal, a fire simulator could be used during an actual operational emergency, based on pre-developed likely simulations, where expected outcomes can support and guide emergency responders.
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Development of Ventsim fire simulation Network ventilation system fire simulators have been developed for many decades. The fire simulator in Ventsim builds on industry experience to deliver accurate insight, while also maintaining the high usability, rapid results and visual aspect that is core to the broader Ventsim design software. Developing any model benefits from validation.
Actual Ventsim
In the case of fire simulation, there is a famous set of underground fire experiments based on real fires called the Waldo fire experiments, which attempted to validate some of the outcomes of what happens in fires underground. There are three basic validations sought, temperature
changes, the transport of gases and fumes and changes of airflow. The most important part of a fire simulator is to get
6.1 30.5 61 91.4 121.9 Distance from fire (m) Above, figure 1: Waldo fire experiment: actual -v- simulated results SOURCE: HOWDEN 152.4 182.9 213.4
the temperatures coming off the fire correct, because these temperatures are what causes the air density changes and it also causes the buoyancy effects, which can result in airflow directions changing. As we go further away from the fire, the air
temperature rapidly drops, because the ground will rapidly cool any sudden change of temperature and
August 2023 | 15
Temperature (°C)
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