Michael Scott

Aldona, I encourage you to spend the US$15 on the .pdf booklet, link in my signature.

I have built now 15 prototype j stoves in my backyard, I have found over and over that getting away from the "known good", established dimensions just doesn't work.

If you stick to the dimensions in "the book" your stove will work the first time.

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.

Thanks for the ideas.

instrumented burn two discussion

There's something rotten in Denmark. No way can changing the air density alone from .0793 to .0843 lbs/ cubic foot ( a six percent change in air density) be the only factor that lets the stove handle more than double the fuel charge, from 1 pound to 2.5 pounds. Its just not reasonable. 19:09 is a little bit longer than 17:37 and 18:00, but not 2.5x.

It could be the heated exhaust gas is making better suction thus drawing in more volume of air per unit time at the colder temperature.

I went ahead with the burn tonight in spite of the wind because it is supposed to keep cooling off and I might not see ambient temps above zero again until late February.

Other hypothesis very welcome. How can this be?

Two instrumented burns tonight at +1 Farenheit.



Humidity is 65%, air density is .0843 pounds per cubic foot.

I have a bit of wind, a new variable for me. The flue exhaust was running at about 100dF average for both burns, trhough it did creep up almost to 200dF once during the spruce burn. I think I lose a lot more heat through the SWSP in light winds then I do in calm winds.

First I gathered 8 sticks of poplar, twice, and weighed them.





I am real comfortable desribing instrumented burn one above as the stove being charged with one pound of fuel and it running like that was all the fuel it could take at +30dF.

Tonight I got the stove lit and kindled with unweighed kindling - and then I was able to load all my poplar, 2 pounds, 0.2 ounces worth. The burn took 17 minutes and 37 seconds, and the stove was acting like it wanted more fuel.

I was at standing room only on the poplar, so I split some spruce while I was waiting for the poplar to finish and found the top load the stove could handle was 2 pounds, 8 ounces. What I did was weigh all the spruce I split, loaded the stove to all it would take (just shy of standing room only) and then I weighed the wood that wouldn't fit. Pics on request. Burn took 19 minutes and 9 seconds.

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.

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?

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.

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.

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?