A question about homemade ramjets

[Najm]

You could try something like the thing I have made in the picture becuase you do not want to disturb the boundary layer and I have not made anything like this till now this is only one of my ideas, this may work well ,also ask someone that the if the flame holder is before or after fuel injection.

[Arman_awn]

here is a pic of the Diffuser and CC made and coupled...
what's your opinion?

[Zippiot]

I have a question...

I wanted to try something I havent seen in ramjets yet, and usually you dont see things because they dont work but...

I will draw a pic soon, but lemme try to explain.

Has there been a ramjet where the expansion chamber is behind the flameholder. Picture this: a small aerosol can that is stuck inside of a bigger can. The smaller can is the intake, the part of it that lies inside of the bigger can is full of holes, acting as a flameholder. The fuel is sprayed into the smaller can so it mixes with the high speed turbulent air, then it is forced into the bigger can where it expands heats up and is burned.

I do see a problem with aspirating it this way though, as it seems more likely to backfire and burn another hole through my pot holders...

[larry cottrill]

here is a pic of the Diffuser and CC made and coupled...
what's your opinion?

Arman -

Looks quite good to me - nice work! Will the nozzle end be set up in a similar way? This is a very nice way to build it - you can try different geometries as long as you're willing to keep fabricating different pieces with the same flange dimensions. Nice idea, very logical approach.

L Cottrill

[Anders Troberg]

here is a pic of the Diffuser and CC made and coupled...
what's your opinion?

Nice! You can probably save some work by hunting for stainless bowls, there are plenty of them in that approximate shape, but with varying dimensions. It's a common way to make your own destillation apparatus here in Sweden...

[Arman_awn]

Dear Larry and Anders, Thank you. I used flanges to connect the diff. and cc together and I follow the same strategy as long as I'm working on this engine, and yes, the nozzle will be installed the same way :D.

I am looking for a way to secure the flame holder in the cc without disturbing the boundry layer...

[larry cottrill]

I am looking for a way to secure the flame holder in the cc without disturbing the boundry layer...

Arman -

It's not worth doing anything exotic. Your flameholder is positioned at the SLOWEST flowing part of the engine, and the widest diameter. Boundary disturbances will have very little effect at that point, certainly nothing to be worried about, in my opinion. Use conventional streamlined or round struts, and use your time to design/analyze the more crucial parts.

L Cottrill

[Arman_awn]

Hi, Dear Larry, thank you a lot...you are making ashamed with your helping...just thanks...

I wanna use propane as the main fuel for my ramjet, do you recommand this type of gasous fuel? how is the fuel supposed to be injected? Where should the fuel injectors be located? How many fuel injectors do I need?

[Zippiot]

It all depends if you are making an asipirated or injected ramjet. But seeing as how most subsonic rams are aspirated and supersonics are injected, 1 large center mounted injector will work. For best performance you want to make a ring of fire just behind the flameholder, this may require up to 8 small injectors but 3-5 will do just fine.

The turbulent air swirls and mixes air with fuel, making it quite hard to get a nice ring of fire...I imagine different flameholder designs will also efect this, try to calculate how the air will swirl inside the cc and figure out palcement by that.

Propane works great, it allows for easier static tests as the fuel will flow more stable and that leaves your hands free to position the air source.

[Arman_awn]

I've planned to make a ring using a tube with about 0.2 of dia and then to drill small holes (5 holes) on this tube. after that, I wanna connect the ending of the tube to my propane storage.

Will this work well for my ramjet?

[Arman_awn]

According to the calculations I've done, the diffuser geometry gives me a pressure rise of only 0.01!!!!
I have considered the leaf blower speed about 30 m/s, I don't know how much it is really...

Do you know at what speed does a home-vac run? I mean air speed..

on the other hand, does the flame holder blockage cause any pressure rise?

Is it possible to get compression ratios about 1:1.7~2?

[larry cottrill]

Arman -

You can probably do better with some experimentation, but as I said earlier, you're NOT going to get big pressure ratios with these kinds of air speeds. Low speed ramjets just aren't going to put out big thrust for their size, no matter what you do.

This is why I built the original Maggie so light (11 ounces or just under 3/4 of a pound, i.e. less than .37 kg) - I knew that high thrust was hopeless to expect. It's also why the areas are so large for a relatively short engine. Your only hope for a flight engine (at low speeds) is to make the area large while keeping the structure super light.

When I bought my leaf blower, it was one of the most powerful electric ones available, delivering about 120 MPH (around 200 km/hr, I think). Now, they have some electric ones that deliver air at 200 MPH (almost 300 km/hr!!!). That would do a lot better, but for flying a model, the question would be: How will we get our model up to that kind of speed so our little ramjet will even start contributing to thrust?

Low speed ramjets are extremely interesting, but it's hard to make them really practical for propelling anything. This is almost entirely because the pressure ratio will always be so limited - even if the design could be theoretically perfect! With the air sources we have access to, I doubt that we will ever achieve more than a 5 to 10 percent pressure rise - with everything working perfectly.

L Cottrill

[multispool]

Chamber obstructions will almost certainly cause a pressure drop.

You can make a flame holder out of sheet metal and mount it by sandwiching between the front flanges. I done this on a test chamber and it works fine!

[Najm]

You can increase the leaf blower speed by decreasing the diameter at the exit of the leaf blower's nozzle(of course this has to be done aerodynamically).The higher the air speed the better.
Leaf blower speed can be calculated by dividing the volume of air it puts out per minute(this information is usually printed on the leaf blower)by the cross sectional area of the nozzle.


BTW what is an aspirated ramjet? I mean where is the fuel added in the ramjet? I've always heard about injected designs.

[Zippiot]

Aspirated rams have the fuel added before the flameholder.

Rams not only need high speed air fed to them, they need high air flow also. Nozzling the blower does help a little...