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[Slide 1] 
[Shijin Kozhumal] Hello everyone, welcome back. We will continue our discussions on Enclosure fire dynamics and in this video we'll look into flame over and backdraft. 
[Slide 2]
Flame over which is also called as rollover it's a condition when an unburned fuel accumulates in the upper layer and ignites along a ceiling. It typically happens in in parking space, car parking garages—where there is a huge amount of ventilation available. So if there is some fuel material present and if there is a fire that vaporizes the fuel, some mixture of the unburned fuel vapors accumulated with some of the combustion products settle at the top side because the hot gases move up, and air is present always at the lower end, but there is very little fuel available at the lower end, so fuel is present at the higher end or the top end of the partially enclosed area. So the mixing of fuel and air happens somewhere in the middle. An interface like this, and fire spreads along that interface where the fuel and air can mix, and it cross. So here is a video where they tried to recreate a similar fire, but I think—I wouldn't call it a textbook definition of flame over, but you can see if there is a there is a fuel tank right it'll settle there will be someone burned we've settled there and that will for our along the ceiling how are this this description is a typical textbook example for a flame over. 
[Slide 2]
Next is backdraft. Backdraft happens when—if air, oxygen, is introduced into an oxygen depleted fire. Say for example, after a fire, or during the fire if there is enough fuel vapors produced, but if there is not enough sufficient oxygen. Typically we said even though there is 21% oxygen combustion requires around about 16 percent oxygen for flaming combustion, and at very low amounts of oxygen can one burn. But after some time, say if a door opens or if a window breaks, then there will be this this fuel rich combustion products are there. There is a lot of fuel vapor, not sufficient oxygen, but suddenly there is more oxygen coming to picture it will suddenly start flaming again. So that canceled in an immediate burst of almost like an explosion. Because of a lot of, plenty of, pre-mixed—partially pre-mixed mixture is already available there and cause a fireball. And this can be very dangerous, especially for firefighters when they try to attack a fire without realizing there is a partially burned mixture present, and that could cause a fireball this of course can depending on how much fuel was present, and how fast they ignite it can cause an explosion and a blast wave there is no escape from it because once it forms a blast wave at very high speeds, it spreads very fast. So hope you got a sequence of events happening in and during a backdraft. 
[Slide 3]
So here I have a video of such an incident firefighters were trying to extinguish a fire, and suddenly, because there were a lot of fuel vapors and partially burned mixture present and when some wind, or something broke, and suddenly there is a rush of air, and that all those partially pre-mixed mixture get sufficient oxygen. So if it was already around twelve thirteen percent oxygen that may not burn, but with a little more oxygen once it reaches around 176 percent oxygen here, suddenly that ignites and the fireball can even lead to an explosion and it happens so fast that firefighters may not even get enough time to escape. And so we had to be extra careful when attacking building on fire, even if there is no visible flame there if there is smoke coming out, or if there is some kind of whistling sound that suggests there is not enough sufficient ventilation. So there is always a chance of partially burned or unburned fuel mixture present there. 
[Slide 4]
This is similar experiment done of a collapse. Let us watch the video. So suddenly, if there is some opening, there will be an airflow, and due to some reason with the experiment with some hot surface, if there is an ignition source. We can see suddenly can lead to it flash of flame. So this happened so fast because normally if the fuel is releasing slowly, and if that mixes with air, there won't be enough flammable mixtures to cause an explosion or a rapid fire spread, but if there was previously a fire, or if there was previously some kind of pyrolysis happening at high temperature even without a flame. So there should be more fuel vapor present with very limited oxygen. It may not be able to burn completely, but suddenly when you have some door or window something opens, that additional air required flows in and mixes with the partially pre-mixed mixture already available there, and it can cause backdraft. 
[Slide 5]
I would recommend in addition to your normal textbook reading of the chapter, you can find some very useful information from NFPA 921, it's a much larger document, but at least the sections about the fire spread in a compartment and compartment fire development would be helpful for you to write your project support better. So thank you I hope this was useful, please complete your project work, I think you have a virtual lab this week, complete that. Complete the discussion board. Thank you for listening, and if you have any questions please post to the Virtual Office, and if you have anything more personally focused feel free to email either your instructor or Jetta. Thank you and have a good weekend.
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