various experimental flues

[smarty]

Here is a pic of an assortment of cardboard formers that I'm going to cast up in ceramic shell.

The flattish forms are something I've wanted to try out in the heat riser to see if it has any effect on anything. I used to have an old tortoise stove with the motto"slow but sure" emblazoned on it. They were the complete opposite of a rocket of course but were one of the first stoves marketed for their efficiency.

Efficiency in those days being the ability to burn for as long as possible on one load of wood. they were an upright stove with a rather rockety appearance something like an L tube but with the wood standing upright in the insulated riser. At the top of this cylindrical firebox was a kind of flattened spherical shape with a little flap door on. I used to often watch the flames through this door following the curved contours and forming a kind of torus shaped eddy before flowing off out the flue at the back.

I reasoned that this held the heat in the top of the stove and increased the efficiency like a kind of afterburner. Since rocket stoves have the opposite aim from the tortoise stove I thought that I would try a torus shaped bulge to the riser at the bottom of the riser to maybe try to give one last mix to the gasses before they shoot up. I was also wondering whether it might hold the flames back in the top of the heat riser when the stove goes into flashover to keep the combustion in the insulated part of the stove.

I am shooting in the dark and just messing around really. I've no idea whether it will have a good or a bad effect. I'm hoping to pick up a thermometer soon to get a better idea of what's going on around the various parts of the stove.

[Deleted]

The most effective way to burn gas without pressurized air is the principle
employed in a bunsen burner, the venturi effect.

A narrowing speeds up the gas flow.
The faster flow drops the pressure to the wall,
which allows to suck air.

The bunsen burner expands then into a much larger volume,
to enforce better mixing with the air sucked into the epanding gas.

Note:
The gas is still pressurized, only the speed of the gas flow
causes an under presure to the wall and through holes to the air.
The expansion enforces better mixing with the sucked air.

Peter style rockets sucks air at an earlier point
and uses less expansion for mixing.
In principle expansion into a torus could be used
for better mixing.
May however result in more friction.

Dimples could be used to lower friction.
The cardboard formers could be wrapped by something
with an appropriate structure.

Despite the lowered friction by the vortices caused by dimples
the surface enlargement by dimples and the vortices may lead
to enhanced heat transfer.
Friction is reduced by eliminating the surface layer.

[peterberg]

Apr 2, 2013 3:18:07 GMT -8 @karl said:

Peter style rockets sucks air at an earlier point and uses less expansion for mixing.

What do you mean by "earlier point"? As far as I can tell, air is sucked through the p-channel right in the port where velocity is highest and expands behind it.
Please explain?

[Deleted]

Apr 2, 2013 4:12:41 GMT -8 peterberg said:

What do you mean by "earlier point"? As far as I can tell, the p-channel sucks air right in the port where velocity is highest and expands behind it.
Please explain?

A bunsen burner sucks air while the gas is already expanding.
Your stoves suck before expansion.

The reasons for the different approaches lie in
differences in the speed of flow and cross sectional areas
and the ability of the bunsen burner to adjust the amount of gas.

[peterberg]

Apr 2, 2013 4:28:51 GMT -8 @karl said:

A bunsen burner sucks air while the gas is already expanding. Your stoves suck before expansion.

Sorry Karl, you're losing me. You think you are explaining your point but it is rather the contrary.
The venturi effect, higher gas velocity coupled to a lower pressure, will suck in air. When the gas is expanding, velocity will drop and the pressure will go back to normal. I.e., there will be no suction anymore.
In your earlier post you stated this:

A narrowing speeds up the gas flow. The faster flow drops the pressure to the wall, which allows to suck air.
The bunsen burner expands then into a much larger volume, to enforce better mixing with the air sucked into the expanding gas.

In my opinion that's correct, your statement about my stove is not.
In my stove, the narrow port will speed up the flow. According to Venturi, this will create a slight underpressure. When the gas is already expanding (again), there won't be underpressure and no suction.

The Bunsen burner sucks in air and after that expands in a larger volume to enforce mixing, as you stated. I fail to see there's any difference compared to my stove, they are both relying on the same law.

[Deleted]

Apr 2, 2013 6:49:31 GMT -8 peterberg said:

The Bunsen burner sucks in air and after that expands in a larger volume to enforce mixing, as you stated. I fail to see there's any difference compared to my stove, they are both relying on the same law.

The burner sucks while gas is expanding, not after expansion.

www.seilnacht.com/Chemie/brenner2.html

Both are relying on the same law, but use it differently.
Your stoves could likely not suck as much air if they
would suck while expansion, at least not with only one small opening.

How can one fill a large bag faster with air ?
Bay wrapping it to a smal opening and blow directly with the mouth.
By blowing in the large opening at some distance.
Things are not always as it seems to be obvious.

[Deleted]

Sorry, but I could not find English sources with good pictures.
www.hamm-chemie.de/k7/k7ab/02_teclubrenner.htm
A Teclu burns hotter than a bunsen,
as it employs even more expansion and less resistance of sucked air.

Your stove implements the venturi principle much simpler,
but there is room for improvements.


Increasing the firing temperature by 200 ° C/392 °F would be theoretically possible.
As it would be significantly more complicated to make it might be only
for professional manufacturer of interest, or for melting of metal.

smarty you have shown some fun in experimenting.
Your ceramic shell technique makes experimeting relatively simple.
Maybe you will give it a try.
It would need a conical narrowing atop of the combustion chamber,
blowing into an even larger cone in which air will be sucked.

Maybe combining it with a Peter style to speed up flow before.

[Vortex]

English google translation of that page

[peterberg]

Apr 2, 2013 23:31:30 GMT -8 @karl said:

Your stove implements the venturi principle much simpler, but there is room for improvements.
Increasing the firing temperature by 200 ° C/392 °F would be theoretically possible.
As it would be significantly more complicated to make it might be only
for professional manufacturer of interest, or for melting of metal.

@karl,
Are you referring to a specific improvement and when that is the case, please elaborate? When you wrote this to mention you only think improvement would be possible on theoretical grounds, don't bother.

[Deleted]

Apr 4, 2013 1:12:09 GMT -8 peterberg said:

Are you referring to a specific improvement and when that is the case, please elaborate? When you wrote this to mention you only think improvement would be possible on theoretical grounds, don't bother.

For a burner its pretty simple:
Gas at a given pressure.
A narrowing to enhance speed.
An extension for expansion and mixing.
One or more openings for air.

The performance is determined by amount and speed of gas,
expansion and resistance to air.
The resistance to air is determined by size, form and amount of holes.
By a Teclu the air entrance is an adjustable conical ring.
A Bunsen has one or more restrictable holes.
The Teclu is more powerful due to less resistance to air
and more expansion for mixing.

In a rocket the gas is already mixed with air and burning,
when entering the narrowing.
The resistance to the air is determined by the cross sectional area
of the main entrance and the load ( the remaining load ).
The speed depends also on burning rate and temperature.

Your stoves have an additional entrance with limited cross sectional area,
due to the point of entrance.

Your charts show at times very little remaining oxygen,
which could be an indication that there may be a need for more air.
However due to the point of entrance a Peter channel cannot be larger.

for the vertical feed there are temperature drops related to the remaining oxygen.
As the burning gas is surrounded by hot mass the temperature
is somewhat leveled by it.

A channel can only be larger if it enters after the constriction.
Several points of entrance or a ring would be better.

Your charts show for the vertical feed that there is room for improvements
and the laws of physics determine how and where.

For the batchbox it is not clear from the charts,
thus rather theoretical.