Rocket Stove Burn Tunnel Size

I've been trying to reign in my ADD enough to do a thorough body of research on rocket stoves lately, with the intent of building an experimental version in the next couple of weeks. Next month, I'll have the resources to buy some instructional material (book, DVDs, etc.), but I thought I'd take a shot at building one in the mean time to learn from my mistakes.

I've read several references to cross-sectional area of the burn tunnel versus that of the heat riser. Do I understand correctly that the burn tunnel cross section needs to be no larger than (and preferably smaller than) the cross section of the riser? This makes sense to me, given my understanding of actual rocket engine design (which is at best spotty).

If I'm using a 6" tube for the center of the heat riser, with a cross section of ~28 1/4", then the cross section of the burn tunnel should be less than that? Or no less than that? Exactly the same?

Thanks.

J.D.

It can be equal to (preferable) or greater than the burn tunnel CSA but NOT smaller.

The burn box can be quite a bit larger than the riser cross section. Check out my build thread.

Here's a drawing I've been working on for a cast tunnel. I expect to put an 18" long, 6" diameter core on top of it. I arrived at the 12" length between the riser tubes as half of the total of the 18" (core height) and 6" (tunnel height). Obviously I can vary a lot of the dimensions, however the plan is to use a 16" diameter barrel as a bell, so making the length between the risers less than about 9" means I'll be crowding the feed tube against the bell.

The plan is to cast the tunnel, then mortar it down to a cast slab, then pack cob around the whole thing, possibly adding a cleanout door at one end.

Any feedback is greatly appreciated.

J.D.

J.D. Ray wrote:I've been trying to reign in my ADD enough to do a thorough body of research on rocket stoves lately, with the intent of building an experimental version in the next couple of weeks. Next month, I'll have the resources to buy some instructional material (book, DVDs, etc.), but I thought I'd take a shot at building one in the mean time to learn from my mistakes.

I've read several references to cross-sectional area of the burn tunnel versus that of the heat riser. Do I understand correctly that the burn tunnel cross section needs to be no larger than (and preferably smaller than) the cross section of the riser? This makes sense to me, given my understanding of actual rocket engine design (which is at best spotty).

If I'm using a 6" tube for the center of the heat riser, with a cross section of ~28 1/4", then the cross section of the burn tunnel should be less than that? Or no less than that? Exactly the same?

Thanks.

J.D.

I think "rocket" refers to the sound, not the design. We're not going for thrust...

'The burn tunnel should be equal to or slightly smaller than the riser' is the general wisdom. I wouldn't sweat it too much.

This sounds like 3-Blind men trying to describe an Elephant ! Really !

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Actually, given that the system has burning fuel passing through a throat into a bell, it's pretty much a rocket engine; just not one designed for thrust. Thanks for confirming my understanding of it. With that, I can say that the gas expansion in the bell ensures draw because of the constriction in the tunnel. Without the constriction at the tunnel, the gas could expand both directions. One little air leak would cause back-burn in your feed tube.

I'll try to make one of these tunnels and report back. Wish me luck.

Cheers.

JD

A rocket engine not designed for thrust?? Well OK...

Constriction has little to do with it (not necessary), once you get the hot gasses rising the right way it draws.