The LWF Blog
Fire Safety Engineering for Design – Fire Dynamics – Part 78May 3, 2022 11:41 am
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 77, LWF looked at the impact of a risk assessment which does not consider the needs of vulnerable building occupants in a fire situation. In part 78, we will begin to discuss fire behaviour or fire dynamics.
Fire dynamics is the study of fire behaviour through chemistry, physics, heat transfer and fluid dynamics. Although an in-depth knowledge is only required by fire specialists, such as fire engineers and firefighters, an overall understanding of the basics can be of use in understanding fire safety engineering solutions, or to respond to a potential fire hazard appropriately.
A fire is started when a chemical reaction happens between a combustible material and oxygen. This produces heat. The mode of burning will depend more upon the physical type of fuel and the environment than on chemistry. For instance, when making a fire, kindling is used as this will ignite much more easily than a log or a lump of coal. Once a quantity of kindling is lit, small sticks can be burned and once the fire has grown, larger quantities of fuel can be added.
When calculations are presented by a fire engineer, they are based upon experiment and testing. The results are, therefore, limited to the criteria applied and are not suitable for extrapolation beyond the conclusion gained.
The ignition of a fire takes place when a material (which will become fuel) changes from a usually inert state to one where a reaction takes place which can produce temperatures in excess of ambient. Ignition of most materials requires an external heat source which causes the surface temperature of the fuel to rise.
In the case of a liquid fuel, or flammable liquid, heat applied to the surface releases vapour. With solid materials, sufficient heat causes decomposition and releases flammable volatiles.
Combustion then takes place in the gaseous phase above the fuel surface.
Whether there is ignition and whether the reaction becomes self-propagating depends upon a complex heat balance between the incident heat flux, the convective and radiative heat gains by the fuel and the heat losses to the surroundings.
For the types of materials commonly found in a construction environment, critical radiant heat flux for ignition (where there is already a flame present) is in the range 10-30 kW . m-2.
For spontaneous ignition without a flame present, critical heat fluxes are higher, around 40 kW . m-2.
Of course, the actual values will depend upon the fuel in question.
In part 79 of LWF’s series on fire engineering, we will discuss fire growth before looking at 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.