pulsejet combustion in microgravity

[HellionFlier]

Hello, I'm new to this forum.

I am leading a team for NASA's Reduced Gravity Flight Program competition. It's open for all university and community college students. We are (tentatively) looking at pulsejets for our experiment. Pulsejets look attractive because they can be used in space (just put the oxidizer in its own tank, like a regular rocket engine). But we haven't found much information on microgravity combustion.

My understanding is that gravity affects convection currents. A candle has a teardrop shape under gravity because warm air rises to the top and cool air flows into the bottom, producing a convection current. In microgravity, there is no "preferred" direction, so the candle takes on a spherical shape and burns out quickly. My thinking is that the lack of gravity might change combustion dynamics inside the pulsejet in some way. It'd be hard to tell without a high speed camera because pulsejet combustion occurs in fractions of a second. Any thoughts on the direction I should take our research?

Much appreciated!

[metiz]

I don't think a pulse-jet will work in the vacuum of space. after the initial combustion, there's nothing for the pressure waves to "bounce against" to create a rarefaction wave, killing it after one half cycle

[Viv]

Hi, over the years I have read a few papers on combustion in drop chambers (tubes) to simulate microgravity combustion so the subject is investigated and published, I would start there with a simple search, your other assumptions are correct for how combustion takes place in microgravity and my own thoughts while reading the subject were that infrared radiation would play a large part, and also that the lack of convection currents and wall/boundary layer interactions would be very interesting

The key, and the advantage would be the ability of a pulse combustion type unit to overcome the problems of fuel mixing and expelling combustion products by utilization the momentum generated by pulse combustion and the acoustic interactions/pumping effects.

It seems at first glance that the limited boundary layer between fuel and oxidizer is the main problem to solve in microgravity combustion

Viv

[PyroJoe]

I somewhat echo metiz,
without ambient pressure there will be no re-compression wave for the next fuel air mixture charge.

It would be like building a diesel engine and not installing the pistons. I guess if you could pulse pressurize the combustion chamber with your fuel/oxygen injection system you may achieve a pulse type combustion but thats a little different than the average pulse jet.

A hybrid between a pulse jet and a Pulse Detonation Engine may be useful. I call these a Low Velocity Detonation Engine as they have a slower, less violent cycle which would allow more common/lighter materials for construction. Basically it would be like running a PDE in high speed deflag mode or running just under detonation velocities in a DDT engine.
Joe

[HellionFlier]

Just to clarify...

We are designing the experiment for NASA's Vomit Comet plane only, which does not carry a vacuum. We are merely investigating pulsejet combustion or something similar in microgravity. The plane creates up to 30 seconds of microgravity per parabolic arc, so that leaves plenty of time for many combustions to occur.

You're probably right about pulsejets not being able to work in vacuum, but I think our research will help everyone understand how well they would work in microgravity. That could be invaluable for designing high-performance pulsejet aircraft.

[PyroJoe]

Ah, so there is some ambient pressure. That brings it back to the realm of standard Pulse Jets, one difficulty maybe in how to handle a considerable volume of exhaust gas.

IMO about the only thing that would change, is the flame travel would be better centered in the combustion chamber. As strange as this may seem, I often have seen flame travel rise toward the top of combustion chambers that are positioned horizontal.
Joe

[Viv]

The most elegant and simple of experiments for the vomit comet is probably going to be a jam jar with a jell type fuel, should also be the easiest to safety proof

Viv

[PyroJoe]

It would need to be a specific type of jell, as most jars will burn off the surface layer of fuel from the jell then quit within a very few cycles. Also the jells often will insulate the fuel and create slow fuel vapor release rates.

[Mike Everman]

I think microgravity will make the pulse jet cycle much more orderly. I too have seen convection currents in pj's where one would think things were happening too fast for such things. We always imagine nice toroidal vortices in these ducts, but are rarely achieving them.

[Rocket Man]

If you have an oxidizer and a fuel then you have a rocket engine not a pulse jet.

[HellionFlier]

We are thinking of using a hydrogen-air mixture, because the exhaust would be mostly water - so no pollution in the Vomit Comet. The rules don't explicitly ban combustion, but there IS a maximum amount of propellant we can bring because of safety concerns.

Hydrogen would be very hard to work with, but I think it has the most potential because it's such a light gas. It also doesn't spontaneously combust in air, which is a safety plus. A enclosed box will be necessary to contain the exhaust gases. That means a small pulsejet.

Thank you for all your feedback so far! It's helped me a lot on the technical section of the proposal. If you have any technical or safety concerns, I'd love to hear your input. The proposal is due on Jan 25th, so there's plenty of time.

[Rocket Man]

Hydrogen gas is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. Hydrogen gas forms explosive mixtures with air in the concentration range 4–74% (volume per cent of hydrogen in air) and with chlorine in the range 5–95%. The mixtures spontaneously detonate by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).[13] Pure hydrogen-oxygen flames emit ultraviolet light and are nearly invisible to the naked eye. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerous acids.

I run my pulse jet engines on propane most of the time you can't ask for a better fuel it comes out under pressure so no fuel pump in required. As long as you have a constant fuel pressure you have constant fuel flow too so the engine runs fine all the time it is not affected by anything.

Any object that falls is fairly close to weightless = micro gravity. Any airplane that goes into a dive becomes weightless. How are jet engines, piston engines and a pulse jet engine on an RC model airplane affected my micro gravity.

[PyroJoe]

I typically steer folks away from tiny jets, but this may be a case where they could be utilized. If your going with hydrogen gas, a small valved Craft Jet may be a good fit.

-It is easy to create and replace the single valve.
-The exhaust generated would be of reasonable volume if only run a short time.
-The engine has a proven record and has been copied multiple times with success
-It would fit well within a small test cell. (maybe even multiple engines)
-If operated correctly, there would not be an overly large volume of hydrogen required at any given time

On the down side:

-Everything is in miniature scale, and probably not entirely representative of pulse jets

-It typically needs a starting air supply to bring it online,this would also setup the flow the jet requires, whether in zero gravity or here at ground level

-It would require at least two motivated technicians that could quickly understand/execute the starting procedure, and be able to maintain/diagnose/adapt to parameters needed for steady operation

-Heat, running hydrogen may overheat the engine in a very short time period. May require valve changes between tests (in flight) or just run it valveless:
viewtopic.php?f=2&t=995&st=0&sk=t&sd=a



A miniature Lockwood may also work, a few of those have been run with success.
Either way, it would be handy to batch produce several at a time after a proven design has been settled on. They are small so carrying a dozen could be done in a brief case.

http://www.youtube.com/user/LenapeFireT ... n4YVYdp8lE

[HellionFlier]

I'm beginning to think using hydrogen gas isn't such a good idea. The invisibility of a gas leak would be the biggest safety concern. It might cause NASA to reject our experiment, because they take safety very seriously on the Vomit Comet. Hydrogen can diffuse through solid steel, so preventing a leak would be very difficult.

[Viv]

Hi,

Yes Hydrogen could be tricky to handle but may I suggest you turn the problem on its head for a moment and attack it from another direction,

How much data are you going to collect? and how?

That may seem counter intuitive but it would give you an idea of how long your experiment had to run, this would then lead you to how big every thing had to be to complete the task and what it would look like.

Going back to the core of your post you said you wanted to observe microgravity pulsed combustion, that's an interesting topic but its not about building a jet engine and measuring thrust, its about pulse combustion in microgravity, so how will you observe and record this event for science? answer that question and we can start talking about what sort of combustor you need and fuel to run it.

Viv