Defining the Difference between Backdraft and Smoke Explosions

Compartment fires are commonly discussed in the classical sense with the compartment fire growth history starting with the incipient phase leading into the growth phase that transition through flashover into the fully developed phase and ends in the decay phase. However, when a fire starts in a closed compartment where only ventilation is the leakage available, the fire can start to smolder and produce large quantities of CO and unburned hydrocarbons. These products of incomplete combustion accumulate in the compartment and create an extremely hazardous condition. If the conditions are right and a large vent becomes available, such as a fire fighter opening a door, a rapidly developing flame front spreads through the enclosure burning the available fuel that has mixed with the incoming air, culminating in a large fireball outside the opening. This catastrophic event is known to fire fighters as a backdraft and by definition requires a sudden change it the ventilation. Anecdotal evidence supports a different phenomenon in which a closed compartment suddenly erupts in flames breaking glass and even causing structural damage without a vent ever being open. This phenomenon is referred to here as a smoke explosion. Recent work has focused on improving our understanding of these poorly understood events and the conditions that precede them. A series of small scale experiments have been conducted burning a timber crib inside an enclosure with tightly controlled ventilation. Under certain fire conditions, the compartment will suddenly erupt, ejecting smoke and flames from the small openings in the compartment. This paper describes the experimental results from the smoke explosion research and compares the smoke explosion to the more familiar phenomena known as backdraft.