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Posts: 46
Location: interior Alaska
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We know energy is proportional to the mass of a thing and its velocity. Not just mass * velocity but mass * velocity squared.

You could, for instance hunt for squirrels with a moose rifle but the fast heavy bullet has a lot of energy. Hunting moose with a squirrel rifle would be a really bad idea, the slow lighter weight bullet carries less energy.

Same with wheeled vehicles. Would you rather be hit by a tricycle going 0.2 miles per hour, or a pickup truck going 20 miles per hour?

Its the same with rockets. The thrust of a rocket is the mass of the fuel burned * the square of the velocity of the exhaust gasses. You got to include time with rockets. A rocket that burns five pounds of fuel in 15 seconds and exhausts its gasses at 200 miles per hour is making more thrust (for a shorter period of time) than a different rocket that burns five pounds of fuel in two hours and exhausts its gasses at 2 miles per hour.

With me so far?
 
pollinator
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The stew collected my ticket and I don't know where this is going soim both with you and lost- but as long as you know where your going ! BIG AL
 
Michael Scott
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Lets just do this in metric so everyone can play.

One easy (cheap) way to measure thrust would be to cut a piece of plywood about 20 x 20 cm that weighs 1 kilogram.

Get the stove running good. Set the plywood over the riser outlet. If the stove can lift the plywood off the riser mouth for even a fraction of an instant you know it was making 1kg of thrust. Pop that piece of wood out of the way, let your burn get reestablished and try the 2 kilogram piece.

Effective and cheap, but quite crude.

I suspect anyone making over about 5kg of thrust (roguhly) is gonna be burning through their drum top pretty quick.

 
Michael Scott
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In classical physics - the folks with the equations - a "rocket" is a thing that operates in space. It has no air intake. It carries both fuel and oxygen on board.

So in classic physics, with a rocket you weigh it at the starting line. If you know how fast the exhaust gasses were exiting the rocket nozzle and the change in the weight of the rocket (how much fuel and oxygen got expelled out the nozzle) - then you can solve for the change in velocity of the rocket.

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

Fortunately or not, the Tsiolkovsky equation does not work for a J stove, whether or not you call it a rocket stove. This isn't really rocket science.

A j stove is very much like the engine on a jet airplane. A jet engine has an air intake. Fuel is burned inside the engine. The exhaust gasses have a higher temperature and higher velocity than the intake air. The equation can be solved for foot pounds of thrust, or Joules, depending on how you feel about the number ten.
 
allen lumley
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when does they drink cart come round ! A. L.
 
Michael Scott
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allen lumley wrote:when does they drink cart come round ! A. L.



Now actually. I have been stewing on this for a couple days and only tonight recognized the rocket equations can't be made to work with what we are doing.

Sierra Nevada Porter at this end.

What I want to do is put a reasonably accurate number on the work my j stove is doing.

At +30dF with eight sticks of poplar it is doing a little bit of work. At -25dF with 20 sticks of spruce it is doing a lot more work --- both burns take 15-18 minutes. Every burn I have done in my prototype stove takes 15-18 minutes before I have to stoke it up again.

What I recognized tonight while I was making mesquite kabobs is the real variable is the mass (or weight if you must) of the wood in the feed tube. At +30dF I can get a clean 18 minute burn with perhaps one pound of wood in the feed tube. At -25df I can get a clean 18 minute burn with perhaps three pounds of wood in the feed tube - shouldn't matter if it is pine or ebony as long as I have the airflow to support it. The variable that Tsoilovsky can't account for is the intake air. I was trying to figure out how much nitrogen was passing through the stove and getting heated up per minute and realized it didn't matter.

I gotta go, what I want to do is not spend a thousand bucks on lab equipment.

I can weigh ($15) my wood of known moisture content ($30) as it goes in the stove.
I can time my burn. ($30)
I can measure the temperature of my exhaust gasses.($30)

What else do I need to measure to calculate thrust so we can compare apples to apples?
 
gardener
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IIRC, section, by gas velocity by gas density by time should give you a rough number. Speed can be measured by a pitot tube. Pression, if there's no venturi effect can be measured by a barometer. The rest is trivial.

And by the way, if you have exhaust gasses of a rocket stove leaving at 200 miles an hour, they wouldn't heat anything, They would be gone through the mass way too fast for heat exchange to happen
 
Michael Scott
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Looking at this article: http://en.wikipedia.org/wiki/Jet_engine_performance

Looking at the second set of equations, temperature at nozzle inlet versus temperature at nozzle outlet -- what if I measure the temp of my gasses at the outlet of the combustion chamber into the riser, the exit of the riser into the drum - and the exit of the drum into the mass?

Just thinking out loud.

I could build a nozzle out of single wall stove pipe, but i would have to ship it to you for us to compare apples to apples.
 
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Michael Scott,
Am I correct in thinking you want to quantify the thrust ( is that in Newtons?) of your rocket stove? And to predict its thrust under varying parameters of fuel load and fuel moisture content?
 
Satamax Antone
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Well Michael, i'm no good at equations usualy. I think the thrust of a rocket stove wouldn't move it if put horizontaly on V rails with V bearings. Even if polished.
 
Michael Scott
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Gavin Phillips wrote:Michael Scott,
Am I correct in thinking you want to quantify the thrust ( is that in Newtons?) of your rocket stove? And to predict its thrust under varying parameters of fuel load and fuel moisture content?



To be able to predict would be a big deal; I'll put it on my wish list, but I am not specifically working towards it.

I do want to compare.

The bare J stove I am prototyping in my back yard can burn a lot more wood with a small drop in ambient temperature.

Right now I have two data points. At +30dF it can burn 1.0 pound of wood in about 18 minutes. This is with a good hot burn, eye clean smoke, and the rocket noise telling me it has all the fuel it can handle.

Second data point, at +1dF the stove burnt 2.0 pounds of wood in 17:37 and was running like it wanted more fuel. I switched to a denser wood, loaded 2.5# without filling the feed tube and got back to the sound of the stove having all the fuel it could handle. Burn was 19:09.

Air density change from +30dF to +1dF was only about 6%.

I don't see how I can get good numbers without knowing how much air is getting sucked through the stove. Weighing and storing atmospheric air by mass is well beyond my budget, which has me looking at handheld anemometers. The other good thing about an anemometer is I could -probably- claim exemption from EPA (USA Environmental Protection Agency) regulation if I can prove my air to fuel ratio (by mass) is more than 50 parts air to 1 part fuel.

If anyone knows the specific heat of wood smoke I would be glad to know it- butI think I still need to know how much atmospheric air is in there with the wood smoke to get meaningful data out of velocity squared is equal twice the difference between h4 and h5.

I am also looking at data loggers, the kind that take industry standard temperature probes and O2 sensors and so forth. I found a two channel one yesterday for about US$400 with two temperature probes that will read up to +2500dF - but the operating range in the user manual is +32 to +122dF, so I couldn't count on it if I take it out in my yard at -25dF.

I will be watching for a 4 channel today if I have any free time. An O2 sensor to demonstrate free oxygen in the stove exhaust, coupled with a CO sensor to demonstrate the absence of CO in an eye clean burn will go a long way towards convincing someone at the university with a chromatograph to look for 2.5 micron particles in my exhaust stream.
 
Satamax Antone
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Michael, you're at the wrong place

http://donkey32.proboards.com/thread/355/small-scale-development

http://donkey32.proboards.com/thread/511/adventures-horizontal-feed

A daft idea which came to mind. Computer fans are voltage dependent for their speed. So the reverse is also true. Fit one snugly over your feed tube and measure the voltage. The good thing would be to find a way to do a speed chart for your fan. What with i don't know.

Anyways, you need to measure the air at the entrance. Since after the heat riser it has higher velocity, but less density. Can't you do a DIY pitot?
 
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Measuring at the riser comes with a number of problems. The biggest is variances in how the pipes are set up. Lets take 2 stoves with a 6" riser and a 10" outer shell on the riser. If I make one with the inner pipe slightly longer and the other with the outer pipe slightly longer and keep my insulating material below the level of the longer pipe lets see. For the first one 28 square inches and for the second I have 78 square inches for the thrust of the air to be spread over. So with identical thrust the larger one should lift nearly 3 times the weight. How well the board seals will also come into this. Next comparing across stoves pipe riser length will matter. What you are checking with your board over the riser technique is instantaneous momentum which is mass x velocity. Notice this is a linear relationship so more mass in motion has an equivalent amount of effect as more velocity. Finally it is the whole system that matters so checking with the system broken open in the middle really won't give you good information.
 
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