Fire is a rapid, exothermic oxidation process that releases heat, light, and various reaction products. The intensity of this process is measured by the Heat Release Rate, which indicates the fire’s power. Fires progress through four stages: ignition, growth, fully developed, and decay. This article focuses on the requirements a fire must meet to sustain and accelerate its development during the growth stage.
The Basic Requirements for Fire
Any self-sustaining fire requires a continuous supply of three components, often visualized as the Fire Triangle: heat, fuel, and an oxidizer, typically oxygen. If any one of these elements is removed, the combustion process will stop. For example, water extinguishes a fire by removing heat, while a fire blanket removes the oxygen supply.
This model is more accurately represented by the Fire Tetrahedron, which adds a fourth component: the uninhibited chemical chain reaction. The energy released by the fire itself must be sufficient to maintain the combustion cycle. This self-sustaining chain reaction is what allows the fire to continue converting fuel and oxygen into heat and light without an external ignition source. The growth stage is defined by the fire intensifying this chain reaction to consume an increasing amount of material.
Fuel Load and Continuity
The fuel component involves not just the presence of material, but its quantity and arrangement, known as the fuel load and continuity. A fire can only grow if it can access and convert adjacent unburnt materials into a consumable form. The speed and intensity of the growth stage are determined by the characteristics of the available fuel.
Solid fuels, like wood or fabric, do not burn directly; they must first undergo pyrolysis. Pyrolysis is the thermal decomposition of a material caused by the fire’s heat, which releases volatile, flammable gases. These gases then mix with oxygen and ignite, producing the visible flames that sustain the fire.
The fuel load is the total amount of burnable material available, while fuel continuity refers to how closely packed and uniformly arranged that material is. Closely packed fuel with a high surface area-to-volume ratio, like kindling or dry grass, promotes rapid flame spread and a fast growth stage. A continuous path of fuel allows the fire to spread easily, ensuring a constant supply of new gases needed to accelerate the chemical reaction.
Thermal Feedback and Sustained Heat
The fire’s ability to grow hinges on its capacity to generate and transfer heat to adjacent, unburnt materials. This mechanism, known as thermal feedback, drives the growth stage. Heat is transferred in three ways: conduction, convection, and radiation.
Conduction, the transfer of heat through direct contact within a solid, is the least significant mechanism for fire spread. Convection involves the movement of hot gases and smoke, which rise and transfer heat to materials above the fire, often along the ceiling of an enclosure. This convective heating pre-conditions the upper atmosphere, allowing gases to accumulate and potentially cause a phenomenon like “rollover.”
As the fire grows, heat transfer by radiation becomes the dominant mechanism for lateral spread. Radiant heat, which travels as electromagnetic waves, is projected outward from the flames and hot surfaces, preheating nearby fuel sources. When this radiant heat flux reaches a threshold, typically around 20 kilowatts per square meter, the unburnt material begins to pyrolyze, ensuring rapid ignition and continuous growth. This positive feedback loop drives the fire toward the fully developed stage.
Airflow and Oxygen Supply
The growth stage requires a continuous supply of oxygen to act as the oxidizer for combustion. Ambient air contains approximately 21% oxygen, and the fire must draw in this oxygen to react with the pyrolytic gases. The intense heat creates a strong convective column, which pulls in fresh air from below and around the fire base, ensuring a constant flow of oxygen to the reaction zone.
If the fire is in a confined space, the growth stage can quickly transition from fuel-controlled to ventilation-controlled. As the fire consumes oxygen, the concentration drops, and the fire’s intensity decreases, slowing or halting growth. If the oxygen level falls below about 15%, visible flames may disappear, forcing the fire into a smoldering state or the decay stage.
Restricted ventilation, such as in a tightly closed room, limits fresh air intake, which in turn limits the Heat Release Rate. Conversely, the sudden introduction of fresh air, such as opening a door or window, can rapidly supply the oxygen needed to reignite the accumulated hot gases, causing a sudden acceleration of the growth stage. This dynamic interaction makes airflow a factor in managing fire growth.