Combustion Chamber Length.

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Post by Mike Everman » Sat May 15, 2004 3:19 pm

How did you generate it, Sam?
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Post by sam » Sat May 15, 2004 3:27 pm

Mike,
I've written a 1D fluid solver. I've added a combustion model and put some realistic boundary conditions in and it works! It wont be pefect but it models all the acoustics properly including temperature differences. I've had to force the turbulent mixing in the combustion chamber but that was always gonna be the case for a 1D simulation.
sam

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Post by Mike Everman » Sat May 15, 2004 3:52 pm

Nice. Not a typical skill set around here! Are you familiar with UFLOW? I'd like to chat off line a bit, if you'd contact me at everman(at)bell-everman(dot)com.
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Post by jmhdx » Sat May 15, 2004 4:23 pm

This is good stuff but I'm no nearer to knowing what a theoretically ideal pulse combustion chamber should look like.
With thanks.
Mike.

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Post by Mike Everman » Sat May 15, 2004 4:59 pm

Ideal is an interesting word! Hopefully we will figure it out one of these days. It's a tough question WRT any pulsejet dimension, and I think not enough ideas have been scientifically evaluated. the lengths discussed above are some good rules of thumb. since the combustion time is dependent in large part on the diameter, ideally it needs to match the frequency your fueling approach will support, and consequently the length of the duct. VERY difficult to determine without trial and error, if you have no instrumentation.

Stay tuned!

Pulse on!
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Post by jmhdx » Sun Jul 25, 2004 1:13 pm

Eric wrote:Well I tried to reply earlier but my computer crashed after writing up a nice big post... So here it is in the shortest possible way.

The diamter of the combustion chamber is not equal to its lenght. It is goverened by the dimensions of the valve system you make. Three recomended dimensions are given as the variable G. 1 each for the different types of valve systems (petal valve, high efficiency petal valve, and valve grid)

The most important part of the pulsejet is the lenght of the tailpipe, you can vary the lenghts of the C.C. and cone / exhaust pipe and it will still run as long as the overall lenght is right. The ratios that are built into the calculator are good for all around performance.

Good luck,
Eric
Well I Stayed Tuned and it's been a long time coming, thankyou Eric it's the only comment I've found on the subject. I have no idea whether a short fat or long slender chamber is best, certainly some transition between chamber and exhuast is necessary but I will not give up on straight pipe theory, it must work, it has to and the world will be amazed.
MIke.

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Post by Mike Everman » Sun Jul 25, 2004 3:19 pm

Boy, Mike, I hope that you weren't holding your breath! I mostly have conjecture and several tests planned. Bill Hinote goes on about the importance of the intake tube length; I'm inclined these days toward the importance of the exhaust throat location along the length (though my intakes and CC's are the same length, so we may be saying the same thing essentially). Everyone agrees that the inlet area is most important. Everyone also agrees that the CC is largely unused, the fresh charge being small in comparison. It's length is being driven acoustically, but really it's just a by-product of other dimensions.
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Post by mk » Sun Jul 25, 2004 6:31 pm

In my way of thinking CCs are mostly necessary for proper mixing of fuel and air, thereof "CC length to CC ID" ratios should become bigger at smaller engines.
Of course the acoustical fact plays a role, too. CCs are acting as Helmholtz resonators.
mk

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Post by jmhdx » Mon Jul 26, 2004 7:29 pm

That is a good way of thinking, and, I think you right that the id ratios lower as the tube gets larger, that would be down to a little extra dia adding up to a lot of extra surface area or volume.
I was concluding that a short wide cc would facilitate a larger number of 02 and fuel molecules and free radicals(heat energy) together at a particular instance in time in the linear direction resulting in a faster reaction for the given mass(a flat disc of reaction maybe better than waiting for the reaction wave to combust all products, the total reaction time of the same mass in a straight tube might be too slow) but Mike Everman pointed to the "sonic plugging" of the intake in a Lockwood type engine aiding mass outflow from the rear and Uflow backs this up.
Short and fat needs to be tried out at some stage I guess.

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Post by mk » Mon Jul 26, 2004 7:38 pm

I don't know how "fat" you're thinking of, in relation to the intake and exhaust openings, but I discovered that Lockwood-Hiller type engines are much easier to start with in relation bigger CC IDs. They are more forgiving to fuel flow - changes - and injectiors, too.
But you wouldn't get a very effective (considering fuel mass and thrust) engine...Milisavljevic could tell you a lot about.
mk

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