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'Black Fire', 'Auto-Ignition' or 'Hot Rich Flashover'?
Two Winnipeg firefighters, Captains Tom Nichols, 57, and Harold Lessard, 55, died and four other firefighters were injured in February 2007 when they were trapped on the second floor of a burning home in the St. Boniface area of Winnipeg Canada.

Above left is a similar fire in Ottawa demonstrating 'Black Fire' conditions

Before the fire department arrived, the fire had burned into the attic and the home's second-storey floor joists, city officials said.

The firefighters encountered a "black fire," described as "high-volume, turbulent velocity, super-dense black smoke accompanied by extreme head," officials said the report concludes. A 'black fire' suggests a flashover could happen, but one did not occur in this case, officials said  contrary to reports at the time of the fire. "A person says that all the bad things come together at once well, that's basically what happened to them," fire commissioner Doug Popowich stated. 

"The fire had seated itself well enough, it was well enough advanced that when the fire department entered that building, it was just at that very crucial time and over a very short few minutes of space, we went from what appeared to be a day-to-day type of response to a tragedy."

But just what is 'Black Fire' asks Paul Grimwood? It is certainly not a scientifically referenced term but one that has been used by 'old time' firefighters in the past.

It is reported on one website as 'Black Fire - This term has only recently been officially recognized. This describes a situation where HEAVY, dense, black smoke is being emitted by a fire. This smoke will be of high velocity, turbulent, high volume and extremely dense. It will also be hot. For all practical purposes this is a dense, superheated, cloud of fuel that is too rich to ignite. This smoke may be doing as much damage as fire. It can also be a sign of eminent' flashover.

But this is not entirely true for the term 'Black Fire' has been used for decades to describe boiling hot black mushrooms of smoke, issuing from a fire involved structure at some high velocity and under great pressure. It is not an event in itself but is representative of a fire that has reached an 'under-ventilated' state and is advancing rapidly towards some form of rapid fire phenomena.

What happened to those firefighters in Winnipeg? It is certain that the sequence of events developed rapidly. Just one week after the tragedy in Winnipeg, Manitoba, Canadians witnessed yet another shocking reminder of the dangerous nature of fire fighting when five Ottawa fire fighters were forced to jump to safety from the third floor windows of a blazing townhouse.

The picture taken from a fire in Ottawa on the left is not showing anything new. It demonstrates heavy black smoke issuing from two upper storey windows under great pressure. It also suggests, from the pattern of the smoke, that there is great heat in the smoke and that the fire is under-ventilated, creating a heavy rich mix of fire gases that have accumulated within. This is termed 'black fire' conditions. 

The potential for subsequent 'events' of rapid fire phenomena that might occur here are as follows -

  • Partial Auto-ignition of the fire gases on the exterior as they exit and mix with air (Fire Gas Ignition)
  • Complete Auto-ignition of fire gases inside the structure as air enters which in effect, is a Backdraft (auto-ignition is the trigger)
  • Partial Auto-ignition along the smoke interface, where incoming air meets the smoke
  • Thermal runaway - Leading to 'flashover'
  • 'Backdraft' - as air enters and causes a fireball to erupt within; the increase in burning at the fire source itself being the trigger
  • 'Smoke Explosion' - as superheated fire gases mix with air but receive ignition energy from a burning brand or other source within

Each of these events will cause the energy being released from the fire (the rate of heat release) to increase and the fire will burn uncontrollably and become hotter. Temperatures within the structure will therefore increase rapidly.

There is an event termed by some as 'Hot Rich Flashover', which has been used in some training texts to describe the event of 'Black Fire' (which is actually hot black smoke issuing under great pressure) mixing with air and igniting. This event is scientifically well established as 'auto-ignition' and is listed under the FIRE GAS IGNITION group of rapid fire phenomena. However, if the auto-ignition occurs inside the compartment, room or structure, the auto-ignition may actually serve as the trigger for a BACKDRAFT event, as it is air flowing in and diluting the gases that allows the 'rich-mix' gases to drop into their flammable range.

The term 'Black Fire' is not an event in itself but is a clear warning sign that an 'event' associated with rapid fire phenomena is about to occur. If these Winnipeg firefighters were 'caught' by some form of rapid fire phenomena it was not 'Black Fire', as stated, but rather a subsequent event, as described above.

For further information go HERE and HERE and HERE

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