stupid physics questions

[gumboot]

No, a chiminea. Distinct from an open fire or brazier.

I mean, obviously Thompson and all that, but that's more about the difference between a crappy and a good chimney and it doesn't exactly explain how the constriction improves things.

Presumably something about laminar versus turbulent flow? Or an impedance-matching matter of getting the waste gas up to speed as efficiently (laminar-ly?) as possible to suck fresh air in without any risk of recirculation?

[zip accident]

No, a chiminea. Distinct from an open fire or brazier.

Oh, my misunderstanding, I thought the spelling was a joke, perhaps to do with Dick Van Dyke.

I mean, obviously Thompson and all that, but that's more about the difference between a crappy and a good chimney and it doesn't exactly explain how the constriction improves things.

Presumably something about laminar versus turbulent flow? Or an impedance-matching matter of getting the waste gas up to speed as efficiently (laminar-ly?) as possible to suck fresh air in without any risk of recirculation?

Hm, I think turbulent flow tends to be post-laminar (high speed), while the difference here is probably pre-laminar (low speed). OTOH, if you get a good flow going, that can make something quite energetic that is pretty much self sustaining.

I think it's partly to do with the pressure differentials between the top and the bottom, and how a fast flow can both exaggerate and promote that differential.

With the right/wrong combustible materials you can get a pretty good imitation of a ram jet going in a stove with a flue.

[pogo]

No, a chiminea. Distinct from an open fire or brazier.

Oh, my misunderstanding, I thought the spelling was a joke, perhaps to do with Dick Van Dyke.

My mistake too, I thought it just an odd spelling of chimney. Looks like it operates on the same principles: the narrow chimney on top will guide smoke away from the fire above head height, so it will spread out away from people and upwards. For situations where you cannot build a proper chimney.

[Jimaroid]

It's just the Venturi effect in action?

[gumboot]

I don't think venuri helps, because the chimney has to vent at some point, and the low pressure in the middle is only temporary.

Thing is, I know that there's a general upward flow of air over the top of a fire, because convection, and a consequent inrush of air from the surroundings. Any chimney might be expected to partition the outflow from the inflow so you get the most oxygen-rich air coming in and no eddy currents turning the old exhaust back down into the fire, but it's the huge efficiency boost of the constriction that I can't quite fathom.

The constriction obviously accelerates the gas, but only temporarily. According to the models that I understand the acceleration is only that required to keep the flow rate consistent with the wider part of the chimney, so the acceleration has no bearing on flow rate -- except that by adding any kind of manipulation you should anticipate drag which would make the flow a little slower.

But a proper chimney not only overcomes the drag it introduces but increases the flow rate beyond what is possible in an open space or a broader chimney.


(this is why I talk about chimineas, because you can't [shouldn't] replace the chimney in your home with an open space)

[zip accident]

I don't think venuri helps, because the chimney has to vent at some point, and the low pressure in the middle is only temporary.

How about Bernoulli?

Thing is, I know that there's a general upward flow of air over the top of a fire, because convection, and a consequent inrush of air from the surroundings. Any chimney might be expected to partition the outflow from the inflow so you get the most oxygen-rich air coming in and no eddy currents turning the old exhaust back down into the fire, but it's the huge efficiency boost of the constriction that I can't quite fathom.

The constriction obviously accelerates the gas, but only temporarily.

Temporarily for any particular molecule, but statically for the wave?

According to the models that I understand the acceleration is only that required to keep the flow rate consistent with the wider part of the chimney, so the acceleration has no bearing on flow rate -- except that by adding any kind of manipulation you should anticipate drag which would make the flow a little slower.

But a proper chimney not only overcomes the drag it introduces but increases the flow rate beyond what is possible in an open space or a broader chimney.

I think it's due to the expansion of heated gas, which makes hot gas lighter, so by stopping it from mixing with cooler air, you keep it rushing upward due to its lightness, and the draw brings in more oxygen, which makes the fire hotter, which makes the gas hotter, which brings in more oxygen. It's a positive feedback loop.

[pogo]

Not something I was familiar with before: this house and anywhere I’ve lived was built within the last 120 years or so, and modern chimneys were established long before then. But it‘s easy to see why the traditional design of a wide hearth with an opening above it would be worse.

You see them in old houses, though not often with a fire in nowadays. Big, wide fireplaces. They must have seemed very sensible then. A wide fire will radiate more heat. High enough to suspend pots for cooking or water heating, wide enough for two or three of them. And such a large permanent feature would have been prestigious, showing your wealth both in building it and fuelling it.

But it would be very bad for ventilation. The fire though wide would not burn very high and the smoke would then form clouds due to turbulence. It might then rise up the chimney, slowly. But it could easily be blown back down by a gust of wind, or blown out of the fireplace by a draught as a door opened.

Narrowing the flue would make the smoke rise much faster. There’s not just the fact that narrowing it increases the flow, but keeping it concentrated keeps the gasses hot to keep them rising all the way. This hot column of air is not going to be blown around by draughts and winds. The rapid exhaust gas movement will also ensure the fire keeps receiving a supply of oxygen so does not go out.

[gumboot]

The ideas of insulating the heated gas from the ambient temperature-gas, and of accelerating the gas so as to get it further away from the fire before it cools, are compelling.

The reinforcement of the updraft by providing more oxygen to the fire depends entirely on the chimenea actually having an effect in the first place, so I don't think I can work from there.

Googling around I learned that the taller the flue the more powerful the effect (makes sense; if we start at 0 we have the degenerate case of an open pit), and this troubled me for most of the day. In part because of this misleading image on Wikipedia, but I think I finally got it.

A chimney sustains a partial void in the atmosphere (being full of lighter-than-air gas) so that there a corresponding volume of fresh air needed to fill it -- the larger the chimney the greater the mass of replacement air and consequently the greater the pressure. Conversely, the void left by the upflow from a brazier can be in-filled (and, incidentally, mixed and cooled) by air immediately adjacent to it without that replacement air having to be sucked through the fire.

If the chimney gets too fat then the outflow will be slow and the gas won't be as hot, and eventually fresh air might start falling in from above.

The narrowness contributes to the speed (get it out before it cools) and the avoidance of backflow, while the length creates head.

Then you get fancy with the impedance matching horn business, which might give chimeneas their distinctive shape (if it's anything more than fashion) which emphasises the pinch more than the overall narrowness.


But there's always more to contemplate. Does the velocity, and corresponding drop in pressure, actually make the gas lighter still? Does a drop in temperature corresponding to the drop in pressure reduce the temperature differential between the flue gas and the chimney, thereby reducing the heat transfer and effective cooling of that gas?

[pogo]

I think that diagram and the section accompanying it over complicates things. It’s enough to think: hot air rises, and a narrow chimney stops it dispersing/mixing with cooler air and so keep it hot, so keeps it rising. The taller it is the greater the volume of hot air so the greater the effect. You can think of this as a sucking effect, or as the pressure of denser air at the base, whichever you prefer.

The constraint as it notes there is eventually the gas will cool down so stop trying to rise, and with drag will act as a brake on the rising gasses. So you cannot make it too tall. Probably not an issue in homes and small stoves like the chimnea. Probably the main design constraint on large factory and power station chimneys.

[gumboot]

The worst part of that diagram is that it indicates a low pressure at the top, when in reality the difference in ambient pressure between the bottom and top of any realistic chimney is going to be negligible.

It had me trying to imagine that a chimney might get so tall as to reach to a lower pressure atmosphere which would just suck all the gasses out of it or something, which was patently ridiculous both on scale and mechanism.

[5th Earth]

The worst part of that diagram is that it indicates a low pressure at the top, when in reality the difference in ambient pressure between the bottom and top of any realistic chimney is going to be negligible.

It had me trying to imagine that a chimney might get so tall as to reach to a lower pressure atmosphere which would just suck all the gasses out of it or something, which was patently ridiculous both on scale and mechanism.

Temperature differentials would work. Cool (read dense) intake air vs. hot (read low pressure) outlet air. On that subject...

Another factor is that, as the exhaust cools along the length of the chimney, the decreasing pressure of the exhaust will itself create a suction effect, as the air at the top of the chimney wants to be "smaller" than the air at the base of the chimney. Sort of a crude Stirling engine effect.

[gumboot]

The worst part of that diagram is that it indicates a low pressure at the top, when in reality the difference in ambient pressure between the bottom and top of any realistic chimney is going to be negligible.

Realised soon after posting that I cannot say this. The difference is going to be exactly the weight of the column of air over the CSA of the flue. If it were genuinely "negligible" then the weight of that column of air wouldn't be a consideration.

So I guess that technically the diagram showing lower pressure at the top than at the bottom is accurate.

[gumboot]

[Another factor is that, as the exhaust cools along the length of the chimney, the decreasing pressure of the exhaust will itself create a suction effect, as the air at the top of the chimney wants to be "smaller" than the air at the base of the chimney. Sort of a crude Stirling engine effect.

In that case, wouldn't it be easier to suck heavy air down from above the chimney rather than up from below it?

[pogo]

So I guess that technically the diagram showing lower pressure at the top than at the bottom is accurate.

But it does not matter. You heat air it expands. This makes it lighter than the surrounding cooler air, for a given volume. It does not affect the pressure, or at least pressure rapidly equalises through air movement.

As the hot air moves up a chimney the pressure of the surrounding air falls. But so does that of the rising hot air. So it’s advantage of being lighter persists, it is not diminished by the falling pressure as it rises. Even as the air thins it has, by weight, the same effect, providing it remains hot. Think how weather balloons keep rising even into space as pressure falls by expanding to match the pressure. They can keep rising (until they burst) as no matter how high they go helium is lighter than air.

This is why you see things like this:

At 419m tall air pressure would be measurably lower. But it does not make any difference. The main thing is the air must be warm all the way up, which will depend on the volume of air, the initial temperature, and how well the chimney is insulated.

Obviously you can’t keep building up forever. Eventually you run out of air. But long before that the air cools, and you pass the limits of how high we can build with current technologies.

[gumboot]

I'm not saying that it matters that they're is a pressure difference. Just that if it wasn't there then you would be in a situation where the chimney didn't work (so it does matter).

There's no inherent lift in hot gas. It's simply that there's heavier gas which wants to replace it. You can infer the mass of that replacement gas from the pressure difference between the top and bottom of the chimney. If the difference is zero then the mass must also be zero and it won't go.