The LWF Blog

Fire Safety Engineering for Design – Fire Dynamics – Part 79

May 9, 2022 12:13 pm

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 78, LWF began to discuss fire behaviour or fire dynamics. In part 79, we look at fire growth and compartment fire behaviour.

For a fire to combust and burn, three elements must be present – oxygen, heat and fuel. If any one of the elements is removed, the reaction terminates.

A part of the burning process is called pyrolysis, which is the thermal decomposition of fresh fuel (when heat is applied to a fuel source). The process produces volatiles from the fuel surface which oxidise and generate combustion products, which releases heat.

If oxygen is available and there is sufficient fuel, a fire will grow in a manner which may be predictable, based on previous data. Calculating fire growth or flame spread from first principles is not a simple matter, however.

The ways in which a fire may develop are dependent upon various circumstances, both in terms of the elements making up the fire composition, and in terms of the environment in which the fire ignites.

As a theoretical experiment, two identical fires could be set with one fire situated in an outdoor environment and the other within a compartment, and the outcomes would be quite different.

In the open air, radiated heat generated by a fire is lost to the surroundings. In a compartment, the heat from a fire is radiated into the compartment walls, which are also heated by convection from the hot gases accumulating in the area.

If a compartment is not air-tight, oxygen will enter and provided there is sufficient fuel, the fire will continue to grow. The layer of hot gases at ceiling level will increase in temperature through fire growth and may reach a point where the downward radiation heat is so intense that all the fuel in the compartment will burn spontaneously. This reaction happens when layer temperatures reach 500-600 oC and is commonly referred to as ‘flashover’.

When flashover has occurred, the heat release rate of the fire increases rapidly and oxygen content decreases. Should any person be left in the compartment post-flashover, they would be unlikely to survive. Additionally, the risk of fire spreading from the original compartment to adjacent areas increases greatly and the structure becomes heated.

In part 80 of LWF’s series on fire engineering, we will continue to look at the development of compartment fires. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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