A truck and a tunnel

A new way of studying and researching the effects of fire is to use models of fire on a computer. Sometimes, an actual fire is replicated on the computer, and scientists can study it to find out how it started and how the fire behaved. They can then make recommendations that might prevent that situation happening again.

Scientists may make a computer model of a building or structure that is going to be built and then analyse it to see how fire-safe it would be. They might model a fire in the structure to see how it might behave under certain conditions. They test it many times under different conditions. Then they make fire safety recommendations for the building or structure.

The truck in a tunnel

Mun Kit Cheong is a PhD student who studies fire using computers. He was wondering what would happen if a truck caught fire in a tunnel:

• How bad would the fire be?
• What would the effect be on the other motorists in the tunnel?
• How big would the flames be?
• How much smoke would there be?
• How close might other vehicles be and when would they ignite.

In the news there has been some discussion about putting a tunnel under Auckland harbour. Fire engineers would need to think very carefully about the fire risks, and engineers would have to plan for fire in their design.

It can happen. A truck caught fire in the Mont Blanc tunnel between France and Italy in 1999, and 39 people died. We don’t want that to happen again.

There are lots of things to think about when designing for fire risk. How long will the tunnel be? Can you design for a big fire? How much will it cost? How quickly will a fire develop? How fast will the fans blow? How severely will a truck burn? No one can predict what is on a truck or how the contents are arranged.

A model New Zealand could use

Mun Kit Cheong made a computer model of a tunnel. This computer model could be very useful for us in New Zealand if we do build a traffic tunnel under a harbour. We would want to know that the tunnel is as fire-safe as possible.

It took 3 years for Mun Kit Cheong to make his model. The truck was made from block shapes and put into the tunnel. Then he caused a simulated fire in the truck.

The model had to be repeated with lots of different situations to study various fire behaviour. For example, he varied the truck dimensions (heavy goods vehicle and light goods vehicle), the tunnel cross-sectional area (a wide 3-lane tunnel or a narrower 2-lane tunnel) and whether the fire started at the front or the rear of the truck.

Airflow from the tunnel fans would also have an effect on the fire behaviour. For example, what is the airflow needed to push the smoke in a particular direction away from people in the tunnel, and how would increasing the airflow affect the fire?

The different models show how big the fires get, how far the flames can reach, how quickly they burn and where the smoke goes.

Some of the demonstrations might show a number of cars in the tunnel to see what effects the fire might have. Will the heat ignite the vehicles close to the truck? People might think they can drive out of a tunnel when there’s a fire, but what if there are too many cars? When there’s a fire, oxygen is used up, so cars will stop because engines need oxygen to run.

Designing for safety

Tunnels need to be carefully planned with fans to blow away smoke and lots of exits to help people get out safely in the case of a fire. Models can help with this kind of planning. They help engineers decide how to design a tunnel for fire safety.

Computational modelling is very helpful for including fire safety in all kinds of buildings and structures such as hospitals, schools, big shopping malls, bridges and tunnels – anywhere there are lots of people.