Mass Rocket Stove Plans (Are my dimensions correct ?)

Greetings !

I just finished a set of blueprints for a mass rocket stove i might build this spring... (Everything is published here)

I was wondering if someone would validate the dimensions of my main cross-sections ?... (To get a proper draft)

I attached some of the main pictures to this post... the complete SolidWorks/STEP CAD files are available on my GrabCad.com account.

Thanks !

Here's some additional pictures to help out...

I can't say anything one way or the other about your dimensions, but wow, those are beautiful renderings. What did you use to model? And to render?

Cheers to you.

JD

Hi Ted,


Well, the first thing which comes to mind, your barrel to flue transition area is way too small. Even more regarding your barrel side gap which seems in the range of 11/16. Your flue transition hole has a perimeter of 18inches or thereabouts, X 11/16 that's a surface area of about 12 sqin. Too small compared to the 20sqin or thereabouts system size. Plus, the bottom of that transition area hole doesn't do much. And furthermore, you haven't accounted for laminar flow restrictions.

So, make your flue transition about 40 sqin, low and wide. Increase your barrel side gap to 1"1/2 or 2"

And please, don't use metal for the heat riser.



J.D. Ray, i think it's solidworks, as stated in the first post

J.D. Ray wrote:I can't say anything one way or the other about your dimensions, but wow, those are beautiful renderings. What did you use to model? And to render?

Cheers to you.

JD

This model and all detailled pdf plans are available to download on https://grabcad.com/ted.harris-1/projects

Everything was made in SolidWorks 2011. Renderings also in Solidworks.

Thanks for the comments

Thanks for replying...

In, "...flue transition area...",... are you referring to the 'C' or 'D' cross-section ?

The cross-section of the space outside the barrel which bring the gases to the 'D' opening is, as shown in the picture above, equal to 47sqin, which is more that twice the surface area of each of the other cross-sections... I cannot find any 12sqin cross-sections anywhere... can you tell me exactly where you can see that ?

About "laminar flow restriction"... you mean adding surfaces to generate turbulence ?

For your heat riser recommendation not to made it out of metal, i did hesitate between metal and a refractory cylinder... I will change the model to your recommendation. (Metal would get deformed ?)

Thanks and let me know where exactly you spotted 12sqin cross-section.

Satamax Antone wrote:Hi Ted,


Well, the first thing which comes to mind, your barrel to flue transition area is way too small. Even more regarding your barrel side gap which seems in the range of 11/16. Your flue transition hole has a perimeter of 18inches or thereabouts, X 11/16 that's a surface area of about 12 sqin. Too small compared to the 20sqin or thereabouts system size. Plus, the bottom of that transition area hole doesn't do much. And furthermore, you haven't accounted for laminar flow restrictions.

So, make your flue transition about 40 sqin, low and wide. Increase your barrel side gap to 1"1/2 or 2"

And please, don't use metal for the heat riser.



J.D. Ray, i think it's solidworks, as stated in the first post

One other thing !... Should my plans say "COB" instead of "Concrete" ?....
Would COB be less "craky" than concrete ?

Ted Harris wrote:Thanks for replying...

In, "...flue transition area...",... are you referring to the 'C' or 'D' cross-section ?

The cross-section of the space outside the barrel which bring the gases to the 'D' opening is, as shown in the picture above, equal to 47sqin, which is more that twice the surface area of each of the other cross-sections... I cannot find any 12sqin cross-sections anywhere... can you tell me exactly where you can see that ?

About "laminar flow restriction"... you mean adding surfaces to generate turbulence ?

For your heat riser recommendation not to made it out of metal, i did hesitate between metal and a refractory cylinder... I will change the model to your recommendation. (Metal would get deformed ?)

Thanks and let me know where exactly you spotted 12sqin cross-section.

Satamax Antone wrote:Hi Ted,


Well, the first thing which comes to mind, your barrel to flue transition area is way too small. Even more regarding your barrel side gap which seems in the range of 11/16. Your flue transition hole has a perimeter of 18inches or thereabouts, X 11/16 that's a surface area of about 12 sqin. Too small compared to the 20sqin or thereabouts system size. Plus, the bottom of that transition area hole doesn't do much. And furthermore, you haven't accounted for laminar flow restrictions.

So, make your flue transition about 40 sqin, low and wide. Increase your barrel side gap to 1"1/2 or 2"

And please, don't use metal for the heat riser.



J.D. Ray, i think it's solidworks, as stated in the first post

At the transition from barrel to flue (shown in C) the insulation around the riser blocks all but a circle at the edge of the pipe. 11/16 (the gap) times the circumference of the pipe is all the flow area available. If you make the transition wider it will help promote flow down the other side of the barrel.

So Ted,

the flue transition area is the transition from C to D to E. Despite what you think, your barrel side gap is too small, fluids in general when they touch materials, like walls of a pipe, get slowed down, down to stalling very close to the wall, the next molecules slide on the stalled ones, getting slowed by theses, and so on, so your flow is the fastest in the middle, and at standstill on the outer contact surfaces. his is the laminar flow effect. Thats why it is reccomended, not to go under 1 inch barrel gap. I would even say 3cm. Because the gas flow can't reach it's theoretical maximum speed in the system, due to the restriction caused by the laminar flow effect.

Then, back to the transition area, imagine the projection of the D square into the barrel, the gap between the barrel and the outer layer of the heat riser's insulation, has a cross sectional area. Your square has a perimeter of 18 inches normaly, if you say it has a 20 sqin surface. The gases enter D from it's sides, they can't reach the center of D, without pushing other gases first; they can't push the wall of the heat riser's insulation either. So your actual CSA between barrel gap and D, is the perimeter of D x the barrel gap. Aproximately 12sqin. I haven't bothered doing the exact calculation. I don't have solidworks, so i can't measure exactly.


Furthermore, since your gases come from the top, they have to take more bends to reach the bottom of D, so that creates more flow restrictions. And every change of direction in the flow creates turbulences, which in turn increase the drag onto the walls, so that reduces even more the speed of the flow.

http://en.wikipedia.org/wiki/Laminar_flow

About the metal, it can't resist spalling, temperatures inside a properly running rocket can reach 1200C°, near metal melting temps.



And about Cob, vs concrete. Well, for the mass, it doesn't realy matter. Concrete is easier to work with, and is easier to come by in bags, easier to mix too. But it's more expensive, had a little less heat storage capacity, and heat travels through it a smidge faster than in cob. It can't resist heat as much either. 400C° for concrete, absolute maximum. Good enough for the bench, but no good for the innards of a rocket.

There's something else i didn't see at first, your burn tunel is too long.

Satamax Antone wrote:There's something else i didn't see at first, your burn tunnel is too long.

Merci Satamax pour tes recommendations...

I don't want to get too technical but as you may have seen on my GrabCad.com page, i happen to be a specialist in CAE/FEA/CFD as i am the maker of ANSYS/Fluent and Icem series of applications.

Everything you said shows a good understanding of some fluid mechanic notions and that is not common among mere mortals !

The intention behind such a narrow space outside the barrel was exactly to cause a short "transition" phase between laminar and turbulent flow to maximize thermal exchange.

Again i don't want to get too technical here but one thing for sure, none of my fancy theories will ever prevail when compared to the experience of people like you. So i will modify the plans to increase the gap outside the barrel. (Thanks for that !)

While making the model, i was wondering the effect of this clearly longer horizontal burning chamber... (It was also intentional but not based on any factual information,.... just intuitive feeling.)

What is the rule of thumb regarding this ? Don't we want to expose the gas to the most heat possible ?

Ted, may be you could keep your gap, but you would have to do at least a 80 sq inches low rectangle transition area, i would think.
take the top lip of your rectangle plus the two sides, multiplied by the gap, and you should be ok.

The rule of thumb is 1/2/4 usualy.

Feed tube taken from the top to the mid height of the burn tunel being the 1

Burn tunel from the mid point of the feed tube to the mid point of heat riser being twice the 1st measurement

And mid height of the burn tunel to top of the heat riser being four times that 1st measurement.