A question about homemade ramjets

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Anders Troberg
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re: A question about homemade ramjets

Post by Anders Troberg » Fri Jul 28, 2006 11:17 am

I place a fuel injector and a spark and somthing like a flamholder in the cycling duct, in order to re-accelerate the cycled air. will this do anything?
It will surely do one thing: give me a headache trying to figure out how the geometry should be...

Basically, you are building two ram jets, coupled in such a way that they each provide each other with ram pressure. What you are doing is trading fuel efficiency for low speed operation, which in some cases is a really nice trade to do.

You will most certainly need to provide some air flow to start the contraption, but that's also manageable.

What makes me somewhat cautious is that my next train of thoughts took me to a setup with two ram jets side by side, with crosswise tubes providing the feedback between them, but at that point I realize that the construction, although visually quite different from your original concept, was conceptually almost identical, except for the the engines feeding each other instead of themselves.

This clearly needs some more thinking. There may be some possibilities, but they require some work to find.

Najm
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re: A question about homemade ramjets

Post by Najm » Fri Jul 28, 2006 11:47 am

I place a fuel injector and a spark and somthing like a flamholder in the cycling duct, in order to re-accelerate the cycled air. will this do anything?
Heating the air will accelerate it also and if you gradually decrease the cross-sectiontional area of the duct then also the will get accelerated.
As I said before it works like an automatic assualt rifle and I will treat it like one in operation, so air is bled from the combustion chamber and is accelerated, the air due to its velocity entrains more air(for combustion) the
combustion process accelerates the air and gives it more energy and more heat energy could be given to the gases in the duct if wanted then the cycle repeats.I am assuming that less energy is needed to entrain more air into the
engine than is given out by the combustion process.
Any ideas?

Arman_awn
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re: A question about homemade ramjets

Post by Arman_awn » Sat Jul 29, 2006 4:26 pm

What material do you recommand to use? with how much thickness?
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Najm
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re: A question about homemade ramjets

Post by Najm » Sun Jul 30, 2006 8:02 am

For the prototype-if it can be called that- it can be made from mild steel because no expensive materials are to be used if the concept is verified by that model then it can be made from SS ,thickness of the material is not a thing to be concerned for.

Arman_awn
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re: A question about homemade ramjets

Post by Arman_awn » Tue Aug 01, 2006 11:33 am

What compression ratio is achievable by using a simple diffuser and a leaf blower?

Will using an element like a cone-shaped spike in front of the engine, in the diffuser section help?

I will be thankful if you answer both questions...
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Najm
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re: A question about homemade ramjets

Post by Najm » Tue Aug 01, 2006 12:06 pm

What compression ratio is achievable by using a simple diffuser and a leaf blower?

Will using an element like a cone-shaped spike in front of the engine, in the diffuser section help?

I will be thankful if you answer both questions...
I think compression can be calculated be finding the ratio of the inlet area to to the combustion chamber's area.The cone shaped diffuser also helps in increasing the pressure , its all about the difference in inlet area to the combustion chamber area.
Any ideas, opinions and answers by anybody else??

Arman_awn
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re: A question about homemade ramjets

Post by Arman_awn » Tue Aug 01, 2006 6:47 pm

Dear Najm, Thanks for your response.

Did you notice the leaf blower speed? I wanna use a home-vac for starting the engine...
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Najm
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re: A question about homemade ramjets

Post by Najm » Wed Aug 02, 2006 10:49 am

Leaf blower speed can be calculated by dividing the volume of air it takes in per minute by the cross sectional area of the exit nozzle.The volume of air taken in per minute is stated on the leafblower unfortunately since I have a second hand leafblower I couldn't calculate the air speed.
A home-vac can be used- anything that puts out air at velocity can be used for running a ramjet provided that enouugh air is supplied.
So when are you starting your project and do you have a refined diagram or idea of how your cyclical flow ramjet is going to be?

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re: A question about homemade ramjets

Post by Arman_awn » Fri Aug 04, 2006 2:36 pm

OK, What do you think about the flame holder? how it goes to be?
What size? What Shape? Remember the combustion chamber's diameter is 10 cm...
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larry cottrill
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re: A question about homemade ramjets

Post by larry cottrill » Fri Aug 04, 2006 3:53 pm

Arman -

For a practical flameholder in a jet that size, I would favor using an eight-legged "octopus" design just like the one in Steve's TLAR ramjet project. He borrowed that from the original Hiller helicopter tipjet design.

Of course, if you want to make more of a project out of it, you can make "textbook" conical flameholders. For an engine the size of yours, you could make these by slicing each one out of the "top" end of an ordinary 8.5 or 12 gram CO2 cylinder - these are high strength steel, beautiful stuff to weld. Since those are about 20mm OD, a couple of them would fit nicely in your engine.

The photo below is my paper model of a "textbook" ramjet with conical flameholders; however, the flameholders shown are a little small (or else more of them should be used, like maybe a cluster of four). Note their location, just beyond the rear end of the diffuser cone. The diffuser section on this model is a little short - my creativity was constrained by the paper cups used in our vending machines here at work ;-)

Good luck!

L Cottrill
Attachments
Model_ramjet_crop1_small.jpg
Crude model of classic "textbook" ramjet, with chamber cutaway showing twin flameholders at rear of diffuser section. Photo Copyright 2005 Larry Cottrill
Model_ramjet_crop1_small.jpg (94.4 KiB) Viewed 7118 times

Arman_awn
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re: A question about homemade ramjets

Post by Arman_awn » Fri Aug 04, 2006 5:23 pm

Dear Larry, Thanks for your response.

I myself prefer to use the eight-legged type of flame holder, because I think the fuel-air mixing qualities of this type is better. Is the cone-shaped Flame holder better really?
Another question: Can a cone-shaped spike in front of the diffuser(like those used on the SR-71's engines) increase the pressure ratio?

here is the picture of the 20 cm pipe I want to use as my combustion chamber.
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DSCI0002.jpg
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larry cottrill
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Re: re: A question about homemade ramjets

Post by larry cottrill » Fri Aug 04, 2006 6:51 pm

Arman_awn wrote:I myself prefer to use the eight-legged type of flame holder, because I think the fuel-air mixing qualities of this type is better. Is the cone-shaped Flame holder better really?
That should be fine. The conical flameholders are just the "classic" solution. The Hiller style star-shaped flameholder should work fine for low to moderate speeds.
Another question: Can a cone-shaped spike in front of the diffuser(like those used on the SR-71's engines) increase the pressure ratio?
I doubt that you could really make them work very well, unless you have a supersonic blower to use. Remember that the SR-71 is designed for high Mach number operation. Of course, I don't really know the right answer, as I am no engineer.

L Cottrill

Najm
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re: A question about homemade ramjets

Post by Najm » Sat Aug 05, 2006 7:34 am

The cone shape in the diffuser would work well if it is not designed for super sonic operation I made one ramjet with an intake like the SR-71's but it was of a different design, not the normal cone shape in the SR-71

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re: A question about homemade ramjets

Post by Arman_awn » Sat Aug 05, 2006 9:56 am

Ok, What was your design like? I mean, how can I desing a cone for subsonic speeds that works well?

Can provide me some info about a cone shaped diffuser that is suitable for my engine? I mean this cone would give the engine better perfomance.
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Najm
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re: A question about homemade ramjets

Post by Najm » Sat Aug 05, 2006 12:14 pm

Well my design was 2 cones joined at the point of their maximum diameter and that maximum diameter was at the intake of the ramjet .........I was trying not to make a coned intake so I inserted a cone into the intake, I dont think there will be much difference in thrust but the flame was more concentrated towards the centre of the exhuast,I don't know what that means though it might that it was producing more thrust or less thrust but I do not know about that as I didn't take any measurements.
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Really really crude picture of intake
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