By Stefan Svensson
Fire is a fascinating phenomenon that has attracted people for millennia. It catches the eye, and many times the fire is described as a living thing, an unpredictable force of nature, or even magic. The effects of fire and the consequences of its force are often described with superlatives. It happens that we express the fire as “extreme.” But, our way of describing fire is based on what we see, feel, or perceive. However, if we consider fire from another perspective, from a “fire point of view,” it’s no longer alive or unpredictable, magical, or extreme. It may very well be hard to predict how fire will develop or to predict the consequences of fire. The consequences might be horrifying and way beyond any imagination, but this still doesn’t make the fire extreme in any way, not if we look at it as a physical phenomenon.
Laws of Nature
Fire is a process of combustion, and it is driven by laws of nature. Such laws describe how various phenomena in nature are related and affect each other. Among such laws, relevant in the case of fire in buildings, are the following:
- The first law of thermodynamics, which states that energy cannot be destroyed or created, only transformed. From a very strict scientific perspective, fire is energy given off by some fuel. The energy stored in a fuel, in terms of bonding energy between molecules, is transformed into heat and light.
- Newton’s general law of gravitation. Gravity is what makes things fall to the ground, but differences in density created by differences in temperature of fire gases (combustion products) will result in buoyancy of hot gases.
- Bernoulli’s law describes the relation between pressure and flow. In addition to buoyancy, differences in temperature created by the energy given off by fuels will create pressure differentials, resulting in the flow of hot gases from higher pressure to lower pressure.
- The general gas law describes the relation between pressure, volume, temperature, and mass. This in some sense ties it all together. Energy is given off in the fire, resulting in an increase in temperature and pressure, increasing the volume of the hot gases, and creating a mass flow inside buildings and between a building and its surroundings.
Fire Characteristics
The fire simply follows the conditions given, whether we know them or not. The fire requires certain conditions, and it is controlled and influenced by some basic laws and conditions. They include the fuel (its characteristics, location, and geometry); the volume, such as the building (its characteristics and geometry); and the fire service (its tactics). All these influences on the fire soon make the situation complex. That we do not always understand the fundamental laws and how they affect the behavior of a fire does not make the fire extreme in any way. It reacts and develops as it is supposed to, given the circumstances.
A fire can be described by the fact that fuel evaporates and, if it’s a solid, decomposes (pyrolysis). The gaseous (and possibly pyrolysed) fuel reacts with oxygen in the air during a process of combustion and produces a wide range of simple and complex products of combustion. In many cases, some of the fuel remains more or less unaffected by the combustion process (but can react some time later). Meanwhile, energy is released, some of which is used to keep the process going. The process is self-sustaining, something that is very significant for a fire. More energy is released than what is needed to keep the fire going. This released energy helps to keep the fire going.
Emissions of heat and light are other significant characteristics of fire. Note that a very large part of the energy delivered from the fire is emitted in the form of radiation outside the visible spectrum—although we cannot see flames, the fire (or hot surfaces, objects, or particles) might very well radiate, thereby influencing its surroundings.
How a fire develops is determined, among other things, by the surroundings and the presence and type of combustible material. The heat developed by the fire is spread through radiation, conduction, and convection.
It is worth noting that fire gases, objects, and construction elements that initially are heated by the fire in turn spread heat by radiation, conduction, and convection, thus further affecting the development of fire (and various parts of a building). Since the surroundings and the presence and type of combustible materials are not always known, the fire and its development are difficult to assess and predict. But even if the course of the fire was unexpected (we never experienced anything like this before), the outcome of the fire was more or less unpredictable (as a result of the lack of knowledge) and if the course of the fire otherwise was difficult to determine in advance, it does not make the fire extreme in any way. From a physical-chemical point of view, or from a “fire point of view,” the development of a fire always is expected and in some sense predictable, as it follows the conditions given and the laws of nature. However, the consequences of a fire can be severe and the risks during a fire in a building are often high simply because the circumstances and the factors influencing the fire behavior are not known.
To understand fire behavior, you must know a number of basic thermal properties of the fuel as well as of the surroundings. Among these properties are heat of combustion, heat of evaporation, ignition temperature, vapor pressure, specific heat capacity, thermal conductivity, and geometry of the building. On top of this, the available amount of air can have a huge impact on the course of the fire.
The effect of firefighting tactics on the fire makes this even more complex. We all know that water is a very good extinguishing agent. As water is heated, it absorbs heat, and it even blocks radiation to some degree. In many cases, putting out fire by the use of water is a matter of flow rate and possibly a matter of droplet size. Or, to put it simply, a large fire requires more water for extinguishment than a small fire. The fire service does a number of things on a fireground. And it’s fairly simple to understand how each individual thing we do interacts with fire. But, when we start to do several things at the same time, the situation soon becomes very complex, and this adds to our lack of understanding of fire development.
And even if our knowledge regarding properties of fire or effects of our tactics is good, it is not always possible to predetermine or assess development of fires. We cannot always retrospectively explain or understand why something happened. But this is not the same as the development of the fire being extreme in any way. The fire is still behaving as expected from a physical-chemical perspective.
There is no such thing as extreme fire behavior.
Additional Links
Fire Commentary: Picking up the Gauntlet
The Only Way to Firefighter Safety Is Through Knowledge
The Science of Firefighter Safety
STEFAN SVENSSON, Ph.D., is an associate professor at Lund University, responsible for the fire laboratory and the experimental work at the Department of Fire Safety Engineering. Svensson started his career as a firefighter in the Swedish Air Force in 1986. In 1989, he earned a bachelor’s degree in fire safety engineering and, in 2002, a Ph.D. at Lund University, Sweden. Since 1994, he has been involved in experimental and theoretical investigations on firefighting tactics, including firefighting methods as well as problems of command and control. The safe and effective use of firefighting resources is a particularly important feature of his work, especially in relation to fire safety design of buildings. He is the author of several books, scientific articles, and reports. He is involved at the local fire brigade as a firefighter/crew commander.
Stefan Svensson will present “There Is No Such Thing as Extreme Fire Behavior” on April 24, 10:30 a.m.-12:15 p.m., at FDIC International 2015 in Indianapolis.
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