The tragedy in Crans-Montana, Switzerland, is particularly affecting public opinion in light of an apparently very simple question: how can a small localized fire become so large and so quickly as to cause dozens of victims in just a few minutes? In a film that is making the rounds a lot at the moment, and which we attach to this article, we see a fire that is all in all contained in the ceiling of the local scene of the tragedy, which does not seem to particularly worry the kids in the room. Within a matter of minutes, however, there would have been 47 confirmed deaths and over 100 injured. In reality, the development of a fire in a closed environment is much more complex than it may seem at first glance, and knowing its dynamics can help us deal with an emergency situation and even save our lives in the worst cases.
From a purely scientific point of view, there are various models that describe the evolution of a fire in a delimited environment in the absence of any intervention. Conceptually, however, a contained fire involves 5 phases:
- ignition;
- growth;
- flashovers;
- complete development;
- decay.
Quantitatively, the phases are delimited by the behavior of temperature as a function of time, as you can see in the graph below.
Ignition can be caused by a spark, a flame, an ember, lightning or in general any element capable of providing a large, very concentrated quantity of heat. The ignition surface must contain materials capable of acting as combustibles, generally in solid form. Initially, however, no combustion occurs: the heat decomposes part of this material without producing flames, in a process called pyrolysis which does not involve oxygen and produces volatile gases which dissolve in the air.
At this point there is the growth phase, in which the flames spread and the temperature increases. This happens because the heat produced by the flames reaches (also thanks to the hot gases developed during the ignition phase) surrounding surfaces which, once heated sufficiently, begin to catch fire. The growth rate of a fire depends on many variables, including the type of combustible material, the amount of oxygen needed for combustion to occur, and so on. The hot gases, and therefore not very dense, rise by buoyancy and accumulate in the upper part of the environment. Smoke is also produced due to incomplete combustion of various materials.
The flashover phase is the “turning point” of a fire of this type, and often also the point of no return. The hot gases, further increasing their temperature (up to around 600 °C), emit increasingly greater quantities of thermal radiation, which end up hitting all the flammable surfaces in the room. At a certain point these materials can no longer accumulate heat: the growth phase ends and all the surfaces of combustible material are affected by the flames. The fire has become widespread. This is perhaps the least intuitive part of the process: thermal radiation causes the entire combustible surface of the environment to be enveloped in flames simultaneously, in times that can range from a few seconds to a few minutes depending on the specific situations. If this had happened in the bar in Crans-Montana, it would explain – at least in part – how the relatively contained fire we see in the footage led to the deaths of almost fifty people. In short, in a closed and isolated environment – such as the basement room of the Swiss bar – even a small, apparently not too threatening fire can become a deadly blaze at any moment.
In the fully developed stage everything that can burn is burning. The temperature reaches its maximum value (typically between 700 °C and 1200 °C) and the heat emission rate also reaches its maximum. Both values generally depend on the availability of oxygen present in the environment as well as on the quantity and type of materials undergoing combustion. For example, a closed but well-ventilated room can always count on new oxygen entering from the external environment, thus allowing the complete development phase to extend considerably over time.
Finally we have the decay phase, in which the temperature and the rate of heat emission decrease until they are completely zero. This phase begins when one or more of the three “ingredients” necessary to sustain a fire begin to run out: the oxidizer (i.e. oxygen), the fuel (what is burning) and the heat. In a well-insulated room, for example, the concentration of oxygen progressively decreases, as this element reacts with fuels. The percentage of oxygen in the air is 21% under normal conditions; when it drops below 16%, the flame tends to be no longer able to sustain itself. Ultimately, firefighting devices are tools that anticipate or accelerate the decline phase, removing heat and/or fuel and/or oxidizer from the fire site.
