Extreme Fire Behavior: Backdraft

A backdraft involves deflagration or rapid combustion of hot pyrolysis products and flammable products of combustion upon mixing with air. Several conditions are necessary in order for a backdraft to occur within a compartment. The fire must have...


A backdraft involves deflagration or rapid combustion of hot pyrolysis products and flammable products of combustion upon mixing with air. Several conditions are necessary in order for a backdraft to occur within a compartment. The fire must have progressed into a ventilation-controlled state with a high concentration of pyrolysis products and flammable products of combustion. Oxygen concentration in the compartment is low, generally to the point where flaming combustion is limited. In addition, there must be sufficient temperature to ignite the fuel when mixed with air

This is the second of three articles dealing with the extreme fire behavior phenomena, flashover, backdraft, and smoke explosion. The first article used a case study involving the deaths of three firefighters to examine flashover. While flashover and backdraft are quite different phenomena, both involve rapid fire progress and are frequently confused when encountered on the fireground.

A backdraft involves deflagration (explosion) or rapid combustion of hot pyrolysis products and flammable products of combustion (such as carbon monoxide) upon mixing with air. Several conditions are necessary in order for a backdraft to occur within a compartment. The fire must have progressed into a ventilation-controlled state with a high concentration of pyrolysis products and flammable products of combustion. Oxygen concentration in the compartment is low, generally to the point where flaming combustion is limited. In addition, there must be sufficient temperature to ignite the fuel when mixed with air (Grimwood, Hartin, McDonough, & Raffel, 2005; Karlsson & Quintiere, 2000).

As illustrated in Figure 2, the energy release from a backdraft is extremely rapid and is generally transient, lasting only a short time. However, as illustrated in Figure 1, the fire often transitions to a fully developed state due to changes in ventilation resulting from the overpressure and heat release caused by the backdraft (Karlsson & Quintiere).

A Firefighters View Of Gas Laws

Gases and aerosols are fluids because they flow and to not have a definite shape. Gases and aerosols expand to fill the entire volume of their container. Understanding fluid dynamics (movement of gases and aerosols such as smoke) is important in understanding many aspects of fire development. However, this topic has particular importance in understanding how backdraft conditions develop and the way in which a backdraft occurs.

Charles Law: Gases expand in direct proportion to the absolute temperature (temperature in degrees Kelvin, Ko = Co + 273) applied to them. If the absolute temperature of a given quantity of gas is doubled its volume will double.

Gay-Lussac's Law: When the volume of a gas remains the same and temperature is increased, pressure increases in proportion to the absolute temperature of the gas.

Key Points for Firefighters:

  • Gases expand when heated
  • Gases become less dense and will rise when heated
  • When gases are confined and heated, pressure increases
  • Increased pressure indicates higher temperatures

When a fire develops in compartment, smoke rises to the ceiling and spreads horizontally through the compartment. Increased temperature reduces gas density. Less dense gases will rise. The difference in density between hot smoke and cooler air below causes them to separate into two distinct layers. The boundary between these two layers called the neutral plane. This is because the hot gas layer is trying to expand (Charles Law), but if it cannot, pressure will rise (Gay-Lussac's Law). Fluid pressure is exerted in all directions in an attempt to reach equilibrium.

Gravity Current and Air Track

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