Backdraft

What is a Backdraft?

A backdraft occurs when a fire has depleted the oxygen in an enclosed space, and opening a door or breaking a window allows a sudden influx of oxygen, causing an explosive burst of flames.

Difference Between Flashover and Backdrafts

The key difference between a flashover and a backdraft lies in the oxygen levels within the fire room. A flashover happens when there is enough oxygen for the fire to burn intensely. In contrast, a backdraft occurs when the fire is smoldering due to a lack of oxygen.

Dangers of a Backdraft

A backdraft is difficult to predict because it depends on conditions like the room’s temperature and oxygen levels.
When a backdraft occurs, the room is quickly engulfed in flames and becomes extremely hot, leaving no time for evacuation.
A backdraft can cause doors to blow off their hinges.

As shown in the video, a backdraft can cause a door to blow off. Never stand in front of a door.

Signs of an Impending Backdraft

Thick smoke forcefully escaping from gaps in windows or doors.
Sounds resembling breathing or whistling.
Windows rattling.
Windows or doors changing color, and glass breaking.

Door Control

To prevent a backdraft, controlling doors is essential. Avoid opening doors or breaking windows unnecessarily to minimize the influx of air.

Scientific Understanding of a Backdraft

A backdraft involves the rapid combustion or explosion of high-temperature pyrolysis products and flammable combustion products mixed with air. Several conditions must be met for a backdraft to occur within a compartment. The fire progresses to a ventilation-controlled state containing high concentrations of pyrolysis products and flammable combustion products, with low oxygen levels restricting combustion.

While flashovers and backdrafts are different phenomena, both involve rapid fire progression.

As illustrated, energy release from a backdraft is swift and short-lived. However, the fire can fully develop due to changes in ventilation caused by the overpressure and heat release from the backdraft.

Gas and Aerosol Laws

Gases and aerosols (tiny particles in the air) flow and take up the shape of their container, behaving as fluids. They expand to fill the entire volume of a compartment. Understanding fluid dynamics (the movement of gases and aerosols like smoke) is crucial to comprehending many aspects of fire behavior, especially backdrafts.

Gases expand proportionally to the absolute temperature (in Kelvin, where ( K = °C + 273 )). When the absolute temperature of a given amount of gas doubles, its volume also doubles.

Key Points to Understand About Backdrafts

Heating causes gases to expand.
Gas density decreases and rises when heated.
When confined and heated, gas pressure increases.
Increased pressure indicates higher temperatures.
When a fire occurs within a compartment, smoke rises to the ceiling and spreads horizontally. As the temperature increases, gas density decreases, causing less dense gases to rise. The difference in density between hot smoke and cool air creates two layers. The boundary between these layers is called the neutral plane. As the hot gas layer tries to expand, if it can’t, pressure rises. Fluid pressure is applied equally in all directions to reach equilibrium.

Gravity Currents and Air Tracks

The laws of gases help understand how smoke moves under fire conditions. Ventilation under fire conditions involves both smoke and fresh air. Generally, smoke is hot and fresh air is cool. This has specific effects on fire behavior. In fluid dynamics, “gravity current” refers to the primarily horizontal flow driven by density differences in a gravitational field. “Air track” describes the movement of smoke and fresh air in a compartment fire. Air tracks are influenced by the pressure created by heating gases and the density difference between hot smoke and cool air.

Smoke and air mix along the boundary of the gravity current. If the concentration of smoke (fuel) mixture reaches the flammable range and exceeds the ignition temperature, it will spontaneously ignite, causing a backdraft. The amount of smoke within the flammable range and the degree to which it is confined are major factors influencing this combustion reaction.

Firefighters and Backdrafts

Even experienced firefighters struggle to determine if the conditions for a backdraft are met, leading to numerous firefighter fatalities each year due to this phenomenon.

In the movie “Backdraft,” this phenomenon was ref

Reference page: “Key fire characteristics: combustion elements, smoke risks, toxic gases, and safe firefighting techniques.”

References　National Fire Protection Association (NFPA)

FIRE RESCUE CLUB

What is a Backdraft?