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I think I'm missing something here. How is the temperature differential between a fireplace and the top of that chimney different than the temperature different
by foota 9y ago
I think I'm missing something here. How is the temperature differential between a fireplace and the top of that chimney different than the temperature differential between the ground and the top of the super chimney?
- ChuckMcM 9y agoThe fireplace is dumping BTUs of energy into the air above it, the ground doesn't do the same.
- SubiculumCode 9y agoIsn't that just a difference of degree, not in process? I do not know if this works, but I can say that if heat is allowed to get higher in the atmosphere, then when it radiates freely, more will escape into space than if it radiates freely at ground level.
- ChuckMcM 9y agoNo. The thing you have to understand is that the air from sea level to the stratosphere, to a first approximation in stable conditions, has the same energy. There are local discontinuities where some air is slightly warmer than other air and so it goes up but by and large its a stable system. That said when you have a boundary layer between two areas with different thermal properties you get wind between them. If you like to go sailing as I do you will recognize this as 'lake effect' wind where the sun sets and the water dumps heat more slowly than the land, so you get an offshore breeze. Or in the morning when land heats up faster than the water and you get an onshore breeze. That occurs because there is a net energy differential between the two masses. There is no energy differential between higher altitude air and lower altitude air absent other weather effects (like water condensing from vapor to liquid or liquid to ice)
- pbhjpbhj 9y ago>first approximation in stable conditions, has the same energy < Nonsense, unless maybe you mean the same mass of air (are you including water vapour, particulates, hire are you accounting for differences in constituency [eg ozone levels]); even then it seems highly unlikely. Or by stable conditions do you mean ones that don't exist in reality (and in which case how is that useful as a model for the real atmosphere?).
- mikeash 9y agoThe air at the top of the chimney is cold because it has expanded due to lower atmospheric pressure. The air inside the chimney undergoes this exact same expansion, and loses the exact same temperature. A fireplace chimney is different because the air at the bottom starts out hotter than the ambient temperature. When it comes out the top, it has cooled but is still hotter than the ambient temperature at that altitude.
- foota 9y agoI see what you're saying wrt the difference. Thank you. The air once at the top wouldn't experience any expansion if it was the same density as the outside air, but what about the energy it picks up from expanding? Wouldn't it still have that? Of course it will have lost some from fighting gravity.
- mikeash 9y agoIn a stable atmosphere, all the various effects cancel out.
- pbhjpbhj 9y agoBut it's not a stable system, it's being driven with heat from the base. A simple hydrodynamic [physical] model should be easy to build in the lab.
- mikeash 9y agoThe chimney described here supports itself with the airflow it generates, which means it has to operate 24/7. That means it has to operate in stable air. It can't.