Towards a wood fueled pulsejet

[vturbine]

I'm very new here, and my interest is in experimenting with wood as a fuel for a pulsejet. I've already talked about it a little here:
viewtopic.php?f=7&t=5378&p=64159#p64159

I've been trying to quickly read through the many posts on simple engines like metal jars, and pipewoods, but haven't yet even scratched the surface there, not to mention the more sophisticated types.

Out of curiosity today, and just to get my feet wet, I build a metal jar from 2-1/4" dia. (about 53 mm) metal tube, brazed to some 11 ga. sheet steel (about 3mm) on for ends. Length was 5" (125 mm). I started trying holes at 1/4", and gradually drilling larger until I got to 29/64" and it ran occasionally using denatured alcohol at that size. I'll probably try a little larger hole tomorrow.

I've done a lot of work on green wood chip (50% moisture content) combustion over the last 4 years. Heres a water cooled 24" green woodchip combustor I built -- the red glowing disk is the flameholder. This burner was capable of running 2 hours at a steady rate on 4 buckets of chips, while delivering delivered 250,000 btu of hot water to a heating system:

.

[Mark]

I don't know if you've tried methanol yet, but it is a little bit more perky than denatured alcohol and more forgiving in fuel/air ratios, especially in the smaller sized jars. You can buy HEET at Walmart, a nice dry wood alcohol. ha
http://www.brasslite.com/SiteImages/Misc/HEET.jpg

[PyroJoe]

This big draft engine is the most stable/robust thing I have constructed. Thrust is low. Heat is at the max.

When nothing else will run for whatever condition, I can always pull this one out and
fire it up. Pouring down rain or whatever.
Sounds about like a big v-8 on low idle, when she is pulsing slowly. Good throttle
range.

It becomes very hot, and is difficult to stand near. The IR radiation tends to cook
the lower legs. Becomes difficult to even look at. Have built special 2 foot long fuel stingers for this one.
If supplied a reasonable steady supply of warm fuel/gas, where I have marked the red
"x" she should start easily and run. The CC size will be dependent upon how much fuel
can be generated.

If the fuel supply is sparse, just reduce the CC volume to match what can be steadily
produced. Just remember it is a trade off, as the CC volume is reduced the less
tolerant she will be.

She gives enough vibration it should be easy to grate out the ashes and have a self feed mechanism.

There is something unusual about this engine, also including the draft engine I built from GRIMs design.
I call it the zombie effect. Many times the fuel can be completely shut off, count several seconds and then fuel slowly reapplied, and the engine will restart on its own. GRIMS engine can be shut off nearly 10 seconds and come back from the dead! ha

Will provide dimensions of this one if needed.

[PyroJoe]

CC is approx 10" Diameter (AC coolant bottle). Pipe is approx 2.3" I.D.(chainlink fence pipe from Home Depot)

How is your current burner/gassifier system setup?

Maybe the two systems can be combined or altered to match.
There will probably be some ratio of CC diameter to draft pipe height.

[vturbine]

Thanks pyrojoe and grim. I've got a slip roll that will bend anything greater than 3" so I can make cylinders and cones, or ovals, too, I guess by cranking in the rolls at spots. I happen to have that 2.3" fence post, too. In fact lots of tube and pipe metal collected over the years.

Heat isn't an advantage, however, in fact it's a waste of energy. Sound volume is important, and I'd like the cycle to be 50 Hz or faster. Actually 60 Hz would be interesting here in the US if you had a a good resonant system.

I would like to see those proportions PJ, if you have the time.

Notes on the jar I made. Drilling larger killed it -- I tried several sizes larger drills by 64ths of an inch. Nothing but a whoosh.

I then glued a 3/8" washer on top of the hole with muffler cement. And presto, it was running again. That washer is actually 0.45" in diameter, so it was agin just about 29/64" (or 11.5 mm)

It definitely runs better and longer in air than in a water cooling bath. Still using denatured alcohol.

[PyroJoe]

Have you considered multiple Rijke tubes tuned to 60 hertz?

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

Just use the heat from your current system to drive the sound. I can only imagine the volume that could be achieved with multiple tubes.

[vturbine]

Well I have thought about the Harwell thermomechanical generator (TMG) see here:

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

and here:

http://www.youtube.com/watch?v=p6qFMqszrIA

But as always the big problem with hot air engines is the thermal transfer rate through the walls, and the waste of the combustion gasses outside of the engine. That's where IC piston engines excel -- they use the combustion/exhaust gas energy directly, rather than trying to transfer heat into a secondary medium which does the work.

When I thought about it, it seemed that the IC analog to the Harwell TMG would be a pulsejet with a diaphragm or other method of moving a linear alternator. In that case the exhaust gas expansion is used, and the combustion is pulsing, not just a trapped hot air oscillation.

Rijke tubes don't seem like they develop much energy. Not at the rate that a pulsejet does. That's a guess, on my part. So I'm not very interested.

The big problems with running wood gas through a piston engine include tar formation on valves, or if extremely well filtered out, disposing of a very hazardous waste (wood tar/creosote). Also, cooling the gas stream to compress it better, and fly ash in the cylinders. I believe a pulsejet could handle all of these problems, and the heat of the combustion chamber would easily crack the tars into fuel gasses.

Yes I do think that my first wood pulsejet tries will be with a combined wood chip gassification/combustion chamber as the simplest beginning. That's why I started with a jar to get some experience.

[vturbine]

Pyrojoe, does the long end of your engine have to be recurved? Or did you just do that to get thrust out of both inlet and exhaust going in the same direction?

Since what I'm doing doesn't requre directional thrust -- could I just use a straight pipe?

[PyroJoe]

No, it doesn't have to be curved. Just curved it to try to maximize thrust, of which it was a poor producer.
The entire engine can be vertical if need be.

GRIM built a linear with tail pointed skyward a while back, straight up and down from intake to tail.

Will have to study the TMG to get a feel for what you have in mind.

[PyroJoe]

"L" draft engines have a straight tailpipe.

viewtopic.php?f=7&t=4682&st=0&sk=t&sd=a

Run like alcoholic banshees until out of fuel.

[vturbine]

pj, with the straight pipe mod, does it have to be a vertical, or can it be horizontal?

And with regard to the "L" draft type pipe engines, you also show a "J" engine -- does that run the same as the L's but just more safely re. spills?

On your straight pipe mod of the big engine, what is the pipe length and cone size?

[vturbine]

The TMG uses a diaphragm and springs and weight in a basic hot air engine design. The alternator is a linear type, basically just a moving coil in a big magnet. Output is AC.

Here's another type of hot air engine coupled linear alternator, only this time using a piston, but no crank/flywheel/etc. I'd call it a linear acoustic piston hot air engine. Though everybody else calls it a "lamina" engine.

http://www.youtube.com/watch?v=3r03h32YWzM

I've been putting all this stuff together in my mind with acoustic pulsejet chamber combustion instead of hot air. Do you see where I'm going with this? Wood (or other biomass) to electricity with one moving part.

Efficiency doesn't have to be very high because the fuel is cheap, replaceable compared to oil/gas/etc. Also doesn't require expensive conversions. Chips used raw and maybe even 50% moisture content. Chips can be made from brush and branches, doesn't require trees.

The hot air engines I've listed above cannot easily be scaled larger without BIG construction complexities (helium, high pressure crankcases, complex heat exchangers and regenerators). They work okay for a few watts when simple in design, but that's it.

The big problem in doing a pulsejet version is going to be extracting work without killing the cycle. My guess at this point. Well, other than getting one to work on wood gas. But I have a feeling that will probably be possible.

[PyroJoe]

Will need a day or so to measure the "as built" dims of the big draft engine, she should run horizontal, but is much easier to start vertical.

The "J" engine runs better than a "L" because it retains the alcohol at a specific distance from the intake. Safety is also a plus.

I see where your going with it, may be several ways to extract electricity from the process. Some involving no moving parts. I forget what it is called at the moment, a car company is using it to generate electricity from exhaust heat.

..Ha, found it, Thermoelectric effect:

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

http://www.gizmag.com/thermoelectric-ca ... mpg/10928/

from above link: "Volkswagen claims 600W output from the TEG under highway driving condition."

http://www.its.org/taxonomy/term/137

Joe

[vturbine]

Yup pj, I know about thermoelectrics. I have 3 custom made Peltier cells designed for thermogeneration.

The problem is the best of these devices is only 3% efficient. Photocells, by comparison are up to around 6% conversion efficiency. And thermoelectrics require a temperature differential, not just heat -- in other words a cold supply, plus they operate around 150 degrees, so the amount of energy they can transfer is small per unit size, and they can be destroyed by high heat where combustion is concerned. Surface temps are probably over 1000 F on a pulsejet, aren't they? These devices are also expensive.

Anyway, not a do-it-yourself item, making and stacking tiny pellets of bismuth telluride, but pulsejets? Seems like not only something do-able, but a whole lot of fun in the bargain!

re your pulsejet -- the vertical is probably better for wood gas anyway, if I were to combine a gas generator and a pulse combustor. I'd get natural draft for starting the gassifier section.

[vturbine]

Here's a schematic of a possible wood gas setup using your vertical and a conventional gasifier. The main problem I see here is that it requires a vacuum in the gas supply line to generate gas and create the gas gen inlet draft. I don't know what the net effect of the pulse jet will be since there is alternating positive and negative pressure -- but I assume that the net is negative at the inlet -- true?

We never want to pressurize the air inlet on the gassifier, since that can cause an explosion inside the generator. Generally the fuel lid is lightly spring loaded rather than latched, so it can act as a safety release. Also, negative pressure in the gas feed line helps prevent gas leaks and that is doubly important w/ carbon monoxide. (This schematic assumes outdoor operation, also.)

This isn't the only possibility, nor the simplest. I'll draw up some alternative versions.