Adventures with a tapered mold

[kirkerik]

very interesting - In my stove I used these salvaged soft bricks from a guy who had abunch left over from his kilns... anyway the bricks were "arch" bricks and all tapered on the 4" axis.

So - i'm thinking how do i use these to get the right tunnel/riser configuration? It dawned on me that I could change the shape of the j tube.

I ended up making a slight "belly" at the bottom of the heat riser w/ the shape of the bricks. almost an inch bigger in both directions - so from 49" sq. to ~ 62" sq. then back to 49"sq. at the elbow of heat riser. Have you tried anything like this? What results?

I'm in there on my stove right now replacing some cracked bricks on top of the burn tunnel so maybe i'll try the increasing volume in the burn tunnel as you have Donkey.

Sounds like you're getting some results from the expansion chamber idea even if they are not that discernable?

I read somewhere else that you have gone too large at the bottom of the heat riser?

kirk

[Donkey]

Aug 21, 2009 20:48:45 GMT -8 kirkerik said:

very interesting - In my stove I used these salvaged soft bricks from a guy who had abunch left over from his kilns... anyway the bricks were "arch" bricks and all tapered on the 4" axis.

So - i'm thinking how do i use these to get the right tunnel/riser configuration? It dawned on me that I could change the shape of the j tube.

I ended up making a slight "belly" at the bottom of the heat riser w/ the shape of the bricks. almost an inch bigger in both directions - so from 49" sq. to ~ 62" sq. then back to 49"sq. at the elbow of heat riser. Have you tried anything like this? What results?

I'm in there on my stove right now replacing some cracked bricks on top of the burn tunnel so maybe i'll try the increasing volume in the burn tunnel as you have Donkey.

Sounds like you're getting some results from the expansion chamber idea even if they are not that discernable?

I read somewhere else that you have gone too large at the bottom of the heat riser?

kirk

So.. How'd it go?? Have you noticed a difference?
I DID make an expansion chamber that was too large. It basically killed flow speed. The thing barely worked at all.

Increasing volume inside the burn tunnel by itself doesn't seem to do much. Increasing into the bottom of the tapered riser seemed to do a little.. Not totally sure.

Lately I've been wondering what would happen if I turned the tapered heat riser upside-down. I haven't done it yet and probably won't till October.. I'm going away on the 6th and won't be back for the month. Much to do between now and then.

[Donkey]

Ok.. So I've photographed and measured the tapered stove test bed.

Here's the thing all put together.
It's actually two tests in one. Firstly the one that is relevant to this thread, tapered heat riser. The other test is for bringing air in from the outside.. The horizontal stove pipe section is the air intake. It was intended for the sauna, but the client wanted to see the flame and didn't want the external air option.


Next pic, looking down with the heat riser removed. The top opening is where the riser sets. The middle opening is the feed box and the bottom is the external air intake.


And the final pic, from 3/4 view with INTERNAL measurements and an inset showing riser height.


A word on the air intake.. It works with a lid on top. Put a lid on and the air flows from the intake, to the top of the feed tube, then back down again. As a lid, I've been using that piece of Hardi-backer leaning against the bricks in the first image.

The dimension of the air gap with the lid on is 2".. All widths are the same, 5 1/8"

[Donkey]

Following the air from the intake through:

2" X 5 1/8" = 10.25 square inches. This is at the air intake with the lid shut. It's about 1/3 of the system size.

6 1/8" X 5 1/8" = 31.39 sq. in. Wood feed -- This is a little larger than six inch pipe target size, which is 28.26 sq. in.

6" X 5 1/8" = 30.75 sq. in. The bend going from the vertical to the horizontal.. If it were meant for a barrel to go over, this horizontal "burn tunnel" would be much longer. In this case it's one brick plus a little tile spacer..

9 1/8" x 5 1/8" = 46.77 sq. in. The longish slot upon which the heat riser sets.

8" x 5 1/8" = 41 sq. in. The junction between the square section and the round.. It's actually pretty difficult to accurately measure this bit, so I've treated it here as if it's a square section but it ain't. Its an 8 inch pipe section placed on top of the 9 1/8 x 5 1/8 rectangle brick section. This is a difficult-to-avoid narrowed area between the two parts..

8 inch pipe section = 50.24 sq. in. bottom of heat riser.. Tapering up to,
6 inch pipe section = 28.26 sq. in. top of heat riser.

[Donkey]

First thing that comes to mind when looking at the volumes:
SLOPPY!!
Just sloppy. Volumes jump all about, nothing is consistent.

It seems to me that this particular line of inquiry requires a certain amount of precision that this first test ISN'T showing.. It's probably why I'm coming up with nothing but question marks and "can't tells".. I'll have to do better next time.

[kirkerik]

donkey wrote:
"So.. How'd it go?? Have you noticed a difference?"

I have not noticed a difference. I do not have a side by side comparison nor have I built enough stoves to tell a difference.

kirk

[spacevan]

How long do you think that your vermiculite heat riser last for donkey? If it is protected from being impacted and moved or bumped wouldn't it last for a really long time maby 10+ years?

[Donkey]

Assuming no impacts, etc... Who knows?
Why not ten years or more? After using it, the clay is slightly fired.
I suppose that it might depend on the clay. Some may deteriorate over time in the heat, some might harden and last forever. ??
I'm sure the pearlite can handle just as much heat as these stoves can throw at it.
More of a question than an answer.

[russellinnc]

So it's been a few years now, with experience gained: Maybe some knowledge also. I am in need of building a 30kw/hr stove for intermittent use of 2-3 hrs, w/o much tending, preferably using an initial pack of wood dust/chips as the fuel.

From sailing & sailboat design I get the bluff body ref where a tapered shape in a current maximizes shear mixing at the minimum of drag because of the self extinguishing interaction of stacked and phase coupled vortexes having differing nominal diameters. Generally, things that are quickly extinguished don't changed the energy of things in thier environment so much (like putting out a small fire).

Example driven question: Why are the cooling towers of nuclear power plants shaped as they are? Can that shape be profitably applied to the rocket wood stove insulated stack?
Given the rotation imparted to compressible gases when making the j-tube transition this issue becomes more significant, rather like a blunderbus with a hard spiral in the bore. This issue is probably most significant at the check-valve-like directional static pressures created at the transitions to down-flow that minimizes back-flow seen as back pressure. Long story made short: a gentle spiraled transition one way and an abrupt transition the other (often seen in tidal estuaries).

The batch load wood dust fired heaters that I have seen and tried do not really seem to behave like a rocket wood heater should except at a small portion of their burn cycle. Seems like a conventional insulated stack placed atop the wood dust insulated burning stack might help. It will take me time and money to test that, and the essential doubling of device height has consequences for me that I would attempt to minimize if realistically possible.

As the charge of wood dust burns in its initially formed vertical stack it becomes wider at the top. The conventional insulated stack might be placed on a lid covering this vertical burn chamber/stack initially charged with 300,000BTUs of wood dust.