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1. Backdraft is fueled by carbon monoxide (CO).
2. As soon as the envelope of the building is opened – such as breaking a window or kicking in a door – the backdraft occurs instantly.
“The first myth has been repeatedly dispelled by many of the researchers performing the engineering research,” Gorbett said. “They have found that the unburned fuel, known as pyrolyzates, are suspended in the smoke and serve as the fuel.
“This second myth is promulgated by every fire service manual I have seen through an illustration showing a firefighter standing by a window and breaking the window with a pike pole, followed by another illustration that shows the immediate backdraft occurring and blowing the firefighter back. The actual physics taking place is much more dangerous than the scenario depicted here. In a real backdraft, the gas layer (pyrolyzates) is not at its auto-ignition temperature, as assumed by this myth. If it were, the gas would burn right at that window and not result in an explosion, just a rollover.
“When the opening is made, firefighters make their way into the building having a false sense of security. They get themselves far into the building prior to the event occurring. The quarter-scale compartments that we have replicated backdrafts in are only two feet wide, three feet long and two feet high. We let the compartment flash over and then closed up the doorway. After we see the warning signs of a backdraft – pressurized smoke coming out every orifice of the compartment – we open the doorway, allowing oxygen into the compartment. On average, it takes 30 seconds for this event to come back out the opening. So imagine the delay this event would have when there are multiple hallways and separate compartments…the firefighters can make their way far into the building and be too far in to get out safely.”
Gorbett partnered with Pharr to co-author the book Fire Dynamics (Brady Fire). Pharr’s previous research coincided with Gorbett’s, but did not focus on backdrafts.
“When Greg joined us at EKU, we were using scale models to show students what was occurring, the fire effects and how they developed into patterns,” Pharr said. “We used scale-model burns in teaching investigations.” Students would build a compartment and furnish it with scale replicas of furniture and furnishings, then document where a fire started and study data they collected to see if it matched what they viewed, Pharr said.
“This evolved into using scale compartment burns in fire behavior and fire dynamics classes,” he said. “Students are able to see how fire progresses in a compartment – discuss the different conditions such as flaming combustion, the fire plume, the ceiling jets, rollover and flashover – to see those specific aspects of fire develop. And Greg was the one who said, ‘Let’s make a backdraft.’ We can pretty much predict the flashover but not necessarily the backdraft.”
The value of the research
In the investigation classes, the first fire students investigate is a quarter-scale model fire scene they have developed.
“We have burned everything from simulated classrooms to dorm rooms – just about everything you could think of,” Pharr said. “It’s pretty interesting to see what students come up with…Tactically, one of the things I’ve seen – especially in light of current research concerning ventilation – is that we need to better understand ventilation. For example, when you go into a burning house, as you open the door, the conditions inside don’t seem too bad because the airflow is rushing into the building. But it’s also fueling the fire and changing conditions that will rapidly deteriorate. Firefighters can see what happens and how quickly it happens. They can begin to grasp the fact that they don’t have the time to do all the things they like to do.”
One of the many successful former students and proponents of scale-model burn research is fire investigator Mark Campbell. He has found the relationship between the scale burns and real fires accurate.