Hey guys and gals got a question for you. I've been to a couple fires here in the last year where the fire was on the second floor with heavy smoke conditions but no smoke on the ceiling of the first floor. I've been looking over a lot of the NIST and UL flow path studies but can't seem to find anything about smoke flowing below its level of origin. The door has been left open on both fires but there was no change in either smoke behavior or movement. My question is: can smoke flow downward and if so, does leaving the front door open leave you in the flow path for fire advancement?
Smoke generally flows up because it is hot. No smoke on the floor below the fire usually means that not enough smoke has built up to fill the space enough to push it down to the second floor. This often would be the case because smoke rising will fill all the void spaces and seek an outlet above it. Once it fills the space it would begin to push down if it doesn't have a sufficient outlet to escape above. Now, the fire is generally doing the same thing and will burn upward, sucking air in the open front door and up to it. In my experience it's not usually going to rush back down the stairs at you unless it is somehow wind driven in that direction. For instance if a window breaks on the fire floor and wind is pushing the fire back into the house rapidly, it could be pushed back towards you and start burning faster in that direction. I have been to many fires in the second floor where there was no smoke on the first floor, but heavy smoke on the second. In most of those the smoke was finding ways out through the roof area. I personally have never seen that fire rush back down the stairs where it wasn't already burning even when the door was left open buy the entry crew.
Rob pretty much covered it. Generally fire and smoke want to move up. The only times I have seen smoke move down in a structure was from building up enough that it had no more room to go and down was the only direction (although that seems pretty rare and I would guess most structures would self ventilate by then), and also from bad techniques with applying water to the fire in the wrong direction and not having sufficient ventilation in place. There really isn't a scenario I can think of where you would see smoke going down in a structure for anything other than a long burning fire or very bad attack technique...