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The physics behind a pocket rocket: what's wrong with this picture?  RSS feed

 
Sergio Santoro
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Location: Nicoya, Costa Rica
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Hi,

I've known about rocket stoves for a couple of years. I made them out of cob, bricks, I made biochar retorts, now I'm making the coffee can size rocket stove out of necessity to heat water for a bucket shower.
You know that video of geoff lawton's, where he shows the rocket water heater? That is my ultimate goal, but the management where I live don't quite want to spend the $400 worth of copper piping to have running hot water without proof that a rocket stove works.
I sometimes become angry for being asked to prove that fire heats water, but the fact of the matter is that all they saw was stoves that didn't quite perform as I said.

Now please look at these pictures. It's a big can of tomato paste, stuffed with some lime powder (first thing I had available) and a J tube of smaller cans of tomato paste. Yesterday I tried the L-tube model. Neither work properly. I heard there has to be a 1:3 ratio between the burning chamber and the chimney, or other say between the diameter of the opening where the kindler goes and the length of the chimney pipe.

To me a rocket stove is a special model that runs on very little fuel, makes no smoke and produces a lot of heat. I got the no smoke thing, but it never quite looked like a rocket reactor. Or at least, it did sometimes in the past, but not with this model.

I mean look at this youtube video!



He just puts two cans together, no special vents, no particular ratio, and that fire is raging. I had to extend my flu pipe, as you can see, way past the 3:1 ratio, and all I got was to have the flame not die off, but I had to constantly feed it with twigs, and the heat I was getting from the other end was not exactly what I need to boil water.

Ok, gonna post this, I hope I can insert photos and videos where I want. Otherwise, bear with me.

Thanks for any feedback!



Sergio
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Can stove
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John Elliott
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That video is not a rocket stove. What he's got going on there is a top-lit updraft. Watch how he keeps stuffing kindling in from the side to keep fuel under the burn zone. Another way you can tell it is not a rocket stove is the lazy flames coming out the top. If he had real turbulent flow, those flames would look quite different.

I know just enough about the physics of fluid mechanics to be dangerous though. If we have someone on this board who can calculate the Reynolds numbers for this combination of soup cans, I will defer to his expert opinion.
 
Dale Hodgins
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Location: Victoria British Columbia-Canada
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Those are both simple rocket stoves. They are not rocket mass heaters. The definition of the former is pretty loose. RMH have an insulated riser, usually have a metal barrel and they store heat in heavy materials.

Pocket rockets referred to on this forum are usually of the RMH variety. They are smaller than most and used to heat smaller spaces.

Both rocket stoves pictured above are suited to cooking, not to space heating.
 
Sergio Santoro
Posts: 256
Location: Nicoya, Costa Rica
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Thanks for the answers.... I guess.

Maybe I was too verbose and people only skimmed through my words. I'll rephrase it in bulletpoints.

+Here in Costa Rica it's the hot season, and no matter what season, I'll never need a rocket mass heater.

+I just need to be able to boil water without making smoke.

+By rocket stove I mean/meant some burning device based on an L or J shaped pipe that burns so hot and is so well oxygenated that the smoke combusts and the flame is raging

Can anybody help me achieve that, please?
 
Sergio Santoro
Posts: 256
Location: Nicoya, Costa Rica
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This is pretty much what I need.

I can't weld like him, though. I was just wondering what the ratio is between the length of the pipe, the openings, etc, so the fire roars. I remember playing with a wood stove as a kid, the fire would behave differently according to how much or little I would close the door.

 
John Elliott
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I am going to make a guess that your circular cross-section is not allowing for a turbulent enough flow to get a real rocket type burn. If it's not roaring like a rocket, the airflow is not turbulent enough.

Two possible fixes: (1) more air flow, even with the circular cross-section will up the Reynolds number of the flow into the turbulent region. If you get a bellows or a fan and actively blow into the horizontal of the L or J part and can get it to roar, then you know that was the problem.

(2) Use a rectangular can, like what olive oil or paint thinner comes in for your burn section.

When I make a rocket type stove out of stacked bricks, it has lots of 90 degree corners in the flow and it always draws enough air to roar.
 
Dale Hodgins
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That thing will piss a huge amount of heat from the metal sides. If it were cobbed over, much higher bottom of pot temperatures could be realized. The same thing built of firebrick or any clay brick would work better. If portability is desired, double walled units filled with perlite are a good option. Smoke will be reduced when a hotter burn is achieved. Go into google images and look for one you like. Many have plans when you go to the website. YouTube has many videos. Many completely failed projects are proudly displayed by builders who seem unaware of the need to insulate the burn tube. I don't think there is any magic length of burn tunnel that will solve all problems. Make sure that heat can't radiate out the side of the tube. This will allow for less fuel to be used and it will be safer for everyone.
 
Satamax Antone
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Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
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Dale Hodgins
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A sleeve to hold gasses against the pot could be placed on top of any simple rocket. The metal could be bent so that heat is diverted around handles. The Aprovecho design is pretty large for most of us. Notice the soot build up from incomplete combustion. The fire doesn't travel far before encountering the cool pot.
 
Sergio Santoro
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Location: Nicoya, Costa Rica
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Ok, thanks. I'll see how it goes with the changes.
 
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