TECHNICAL | SAFETY/FIRE
1
2
Left, figure 2: 1 Column of CO simulation graphics 2 Column of wet bulb simulation graphics
absorb the heat from the fire very quickly. This is
represented on the chart as ‘actual’. It is this zone of air temperature change that can cause a whole ventilation system in the tunnel to radically change. If this air is located somewhere where there is a vertical separation distance the buoyancy can actually result in quite significant pressures and that can push airflow into other directions that you would not normally consider. The test using Ventsim maps very close to the actual
measurements. Numerous other validations have been done to ensure accurate results as well as fast simulation speeds.
UNDERSTANDING FIRE The actual nature of fires can be difficult to assess and so knowledge from actual incidents that have occurred or experimental data can be used within the simulator to increase understanding of outcomes. We want to know, in particular, how hot fires will get
and how quickly they release the heat during the event. All fires will start small over some time, growing until the fuel becomes exhausted and then decay and go out. It is this curve of change that we have to understand, because this is called the heat release rate and is really what drives a lot of the ventilation conditions and changes that you see during a fire. Unfortunately, fire simulators do not actually try to
simulate the fire itself. Information addressing what is going to happen to the fire needs to be inputted and then the simulators will take that information, convert it to heat and apply that heat to the ventilation model.
DEVELOPING A SIMULATION A lot of the development of fire simulation, such as in Ventsim, is not necessarily running simulations rather it is about doing some planning first to work out what is going to happen to the different types of fires in the tunnel. One of the first steps is to establish the fuel source for
the fire. What will burn, where will the fuel come from and what type of fuels are involved. This may differ between road, rail and metro tunnels as the core fuel base comes from the vehicles within the tunnel. In the case of a road tunnel, it would be vehicles fuelled by petrol, diesel or batteries and tyres, plastics and seating. We can accumulate knowledge on the specifications associated with each vehicle type from manufacturer’s data (e.g., fuel tank capacity, tyre mass). Once fuels are established, the data can be entered
into the software to build a library of fire sources and have them available for simulation. The driving goal is to understand how fire can change ventilation systems
16 | August 2023
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49
