Key Factors in Fires

Many other factors also influence how materials will ignite and burn. For solids, these include:

  • Fuel orientation and geometry
  • Surface area-to-mass ratio
  • Moisture content
  • Fire retardants

Fuel Orientation

The fuel orientation and ignition location (for example, whether a vertical fuel is ignited from the top or bottom) both have a significant impact on flame spread and HRR. In a real fire, the surface of a solid fuel can be in any orientation: vertical, horizontal or at any other angle.

Figure 47: Fuel orientation and its effect on flame spread

Figure 47 shows the effect of orientation on flame spread. The most dramatic difference is shown in the upper and lower right quadrants. In the upper frame, the flame spreads more quickly because the hot gases from the flame directly contact and preheat the solid in their path. This leads to the formation of more fuel vapour and more vigorous burning. The air travelling to the burning regions is also travelling in a direction parallel to the hot gases, again helping to increase the rate at which the flame spreads up the stick.

In the middle, downward flame spread can be several times slower because the hot gases from the burning zone move upwards and away from the unburned fuel, making it much harder for the flame to continue burning. The flow of air into the combustion region is also travelling in the opposite direction to the spreading flame (flame spread is counter-current).

When a flame spreads horizontally across a surface, as in the image on the right, there is some preheating of the fuel by radiation from the flames. The flow of air entrained into the combustion reaction is also moving in the opposite direction of the spreading flame. Horizontal flame spread is therefore much slower due to the limited heat transfer imposed back to the virgin fuel. This results in an intermediate rate of flame spread.

Any orientation of fuel between horizontal and vertical is called inclined. The angle of inclination will also influence the flame spread. It has been reported that a shift from counter-flow flame spread to concurrent flame spread will occur when the angle of the incline is increased past 25o.

The shape of the fuel surface will also affect the heat transfer into the fuel. Flat and concave surfaces reflect much more energy than do convex surfaces. Convex surfaces therefore absorb heat into the fuel much more easily, resulting in easier ignition and higher rates of flame spread.

Surface Area-to-Mass Ratio

As the surface area-to-mass ratio of a fuel increases (surface area increases and mass decreases), the fuel particles become smaller or more finely divided. This makes the material easier to ignite. As previously mentioned, this is the principle behind why atomized liquids can ignite when the same pool of that liquid cannot, or why sawdust can ignite while a wood log may not.

Figure 48: Surface area-to-mass ratio in dust explosions

Once ignited, a larger surface area-to-mass ratio allows the fuel to absorb heat and vaporize faster. Therefore, the rate at which the fire can grow and spread, as well as the HRR of the fire, will increase-as long as enough oxygen is available-even though temperatures will remain the same. For example, take a wooden log and apply a match. Now cut or sand the log, collect the wood shavings and fine particles, and light them with a match. The shavings will ignite faster, and burn quicker. The same principle applies when trying to ignite the bulk surface of a material versus igniting a material at the edge.

Materials that are generally considered benign can be explosive when they are finely divided and suspended before ignition or during flame propagation. This is because the surface area-to-mass ratio has an impact on flame spread and HRR. When there is less material behind or beneath the surface of a fuel, the flame and heat move through it more quickly. This is why it is easier to burn a single sheet than a thick stack of paper. In Figure 48, dust, which has minimal mass behind its surface area, reacts quickly when ignited. The same concept applies in fire suppression by water mist or finely divided water droplets from a hose stream compared to large droplets from a sprinkler. With the larger surface area-to-mass ratio of the water droplets in a water mist system (or finely divided hose stream), the droplets absorb heat, vaporize, expand and cool a hot fire environment very quickly, whereas a larger unbroken stream will not cool the environment as effectively.

Fuel Moisture Content

The amount of moisture in a fuel is another factor that is critical to both the rate of burning and flame spread characteristics of solids. Moisture changes the burning characteristics because water trapped within the fuel has to evaporate and form steam as the fuel heats. Since water absorbs a lot of energy when it is evaporating, fuels with a higher moisture content will take longer to reach their peak burning modes (lower burning rate)-at least until the moisture is driven out of the fuel. There is also a chance they may self-extinguish.

Figure 49: The effect of moisture content on fire development

Fire Retardants

Figure 50 shows two sofas at the same time after ignition. The sofa on the left was built without fire-retardant additives, while the one on the right contains fire retardants. The difference in burning characteristics between these sofas is obvious, but it is important to note that if different quantities of fire retardant or strategies for fire retarding the sofa were used, the sofa with additives might still burn. It is also important to be aware that the definitions and methods for 'fire retarding' materials and products differ significantly by jurisdiction.

The fire-retardant chemicals in the sofa on the right quickly interfere with the chemical chain reaction taking place in the fire. This cools down the reaction so that not enough energy is produced by the oxidation reactions. The fire is unable to sustain itself and burns out. The fire on the left, however, continues to release more and more heat as it grows, generating more flammable fuel vapours which mix with air, and the fire spreads up the back of the sofa. Without the retardant to slow its progress, the fire rapidly spreads to involve the entire sofa, reaching peak HRR.

Figure 50: Comparison of furniture burn with and without fire retardant

As was shown in Figure 47, if fuel that is oriented vertically (as in the back of the couch in Figure 51) is ignited at the bottom, its virgin fuel heated by the flowing heated gases through convective and conductive heat transfer. In addition to this, there is a much greater radiant heat factor associated with the parallel fuel and flame source. The fire plume also entrains air easily, as the entrained air moves in the same direction as the heated gases as it enters the fire plume. This is termed "concurrent flame spread."

Figure 51: Effect of fuel orientation on fire development

As expected, the flame spreads up the back of the seat faster than it spreads across the flat seat cushion due to fuel orientation and flame spread.