Downdraft in the barrel?

[satamax]

Well, a little bit of theory i'd like to be checked. Exerpt of some stuff i wrote at permies.

Yep, there's a rapid cooling of the gases in contact with the barrel, but i'm not sure it creates enough advection movement to oppose to the convection.

It's all about temp differential and presure differential. But even if the cooled down gases have a highger density than the hot ones, they're still lighter than the ambient air at both ends of the system.

I have been wondering about that suposed downdraft pump effect in the barrel for a while.

I think the best answer is, when you put a barrel around a test rocket, always the draft suffers. Even tho the whole bottom is open. The gases are nowhere like nearing the dew point, nor cooling more than ambient, which would be necessary to create a draft.

All it might do is impede less the draft created by the heat riser than if the gases were hotter.

[Daryl]

Nov 12, 2014 13:30:35 GMT -8 satamax said:

It's all about temp differential and presure differential.

and thermal mass.    

[patamos]

One of the rules with the traditional double flue grundofens is that the length of the horizontal runs can be no greater than 75% of the vertical stack height.
Those heaters feeding into benches have about 3' of downward flow of gasses from the top of the firebox to mid bench height. Micro techniques such as widening the CSA in flue turns, and slightly reducing CSA along the length of flue runs act to stabilize the rate of gas flow. All of which helps to tweak the pressure differential as temperature is conducted into mass and/or radiated into space.

Rocket heat risers create a more powerful push than trad grundofen fireboxes. So the rules of flue run length are different.
I'm sure the torus at the top of the barrel is helping with reburn... but the 'power' of the downdraft is looking like a falsehood.

No disrespect to the progenitors of the RMH movement, but it is good to nut these things out as the evolution continues...

[DCish]

All it might do is impede less the draft created by the heat riser than if the gases were hotter.

At one point (sorry I can't search it out, on mobile right now with poor connection) Donkey put up a post with draft figures for various temp differentials and heights. His concluding hypothesis (as I recall it) was that a good chimney exhausting at above dew point temps is far more effective than the draft maintained by (as you suggest above) the difference between the riser draft and the comparatively lower draft of the barrel-cooled gasses.

[Donkey]

Nov 13, 2014 9:40:55 GMT -8 patamos said:

I'm sure the torus at the top of the barrel is helping with reburn... but the 'power' of the downdraft is looking like a falsehood.

No disrespect to the progenitors of the RMH movement, but it is good to nut these things out as the evolution continues...

I never bought into the re-burn in the barrel thing. That was an assumption based on temperature readings off of the outside of the barrel, I always had an alternative explanation for that. My friend Ernie published first, and speaks louder..

I'm not entirely surprised about the downdraft thing. I know that I helped to spread the idea, if it's truly bogus, I can help to stamp it out too.

[patamos]

Donkey, please clarify. Are you saying the temperature readings off the outside of the barrel are stemming entirely from the primary combustion in the lower heat riser?

I'm sure cooling gasses are gaining weight as they head down the barrel. But i imagine the push out of the heat riser is doing most if not all of the work... Curious as to how one would go about measuring those factors? Other than the observation that gasses move along fine in systems wit non-metal barrel downdraft chambers...

[Dan (Upstate NY, USA)]

Well if the vacuum downdraft theory is true (which I still think it is) then an unattached barrel hovered over the heat riser would NOT create a relative vacuum or low pressure area.

Why? Because any low pressure area it could create would be counter acted by sucking up the outside atmosphere as it cooled.

In an enclosed system with some kind of draft both the cooling effect in the barrel and the draft of the exit chimney create a low pressure area.

I base this off of my experience of using condensers in commercial power production. I could be wrong...

[satamax]

Wolf, condenser are another beast, they act with phase change, from steam, back to water. IIRC, it takes 4 times the energy to boil water, to produce steam. Then by condensing, that energy gets back in the atmosphere, minus losses.

In the barrel, what happens is that some gases cool down, but not all of theses. So they tend to stick upwards, and block the flow.

What i was saying earlier, when you put the barrel on without the chimney, it always stalls the system. So it's a radiator, but nothing else.

[Donkey]

Nov 16, 2014 15:04:52 GMT -8 patamos said:

Donkey, please clarify. Are you saying the temperature readings off the outside of the barrel are stemming entirely from the primary combustion in the lower heat riser?

Yep.. I don't think that there is a re-burn up in the barrel, not in a well tempered stove anyway. Seems to me that if your stove is working properly, there should NOT be anything left TO combust beyond the heat riser. The assumption was made that there is a secondary burn in the barrel because of a doughnut shaped hot spot that can be sometimes measured there. We've attempted to put an air feed in there to improve on the possibility to ZERO effect. Little holes have been poked into the barrel to attempt to see it in action, no love..
IMHO, this theory creates more questions than it answers which is my usual line of dismissal.

I'm sure cooling gasses are gaining weight as they head down the barrel. But i imagine the push out of the heat riser is doing most if not all of the work... Curious as to how one would go about measuring those factors? Other than the observation that gasses move along fine in systems wit non-metal barrel downdraft chambers...

Perhaps one or the other or both effects ARE happening in there.. It would be good to quantify these effects if possible.
All that said, we DO know that none of that is as effective as a properly running chimney.

[matthewwalker]

I've always been skeptical of the reburn in the barrel theory. The barrel gives the heat to the space quickly, perhaps helping draft. That's at odds with a final burn stage, in my opinion.

[Dan (Upstate NY, USA)]

What i was saying earlier, when you put the barrel on without the chimney, it always stalls the system

You can't just put a barrel over the top with closing in the system or your "condenser" has a huge air leak.


And you are right Max, condensing steam is multiple times more effective than cooling gases in creating a low pressure area.

But cooling gases does make a relative vacuum in a enclosed steady state system.

[shilo]

it's all about the dellta T