A question about homemade ramjets

[Arman_awn]

Thank you all, my friends.

Dear Najm, I want to use your design of the cone and the design of larry's falmeholder(eight lagged type) together.

How can I join the cone, the flameholder and the fuel injection together and then join these thing to the body structure?
I'm going to use propane as the main fuel for the engine because it really needs nothing to be burnt...

[Najm]

Check out the "steve ramjet" topic(it was something like that) ,you will get a good idea on how to put a flameholder in a ramjet becuase I dont have much experience with them ,I used a conical flameholder and the intake cone is seperate from the flameholder and fuel injection , use an eight legged flame holder b/c I think they are better.

[Arman_awn]

1-You've said that the pressure ratio can be calculated by dividing the diffuser entrance area by the CC entrance area, for my case the ratio is 4, So can I say that the compression ration in my engine gets as high as 4:1?
Doesn't it depend on the leaf blower speed?

2- How should the fuel injectors be like? is propane the fuel of choice for my engine?

I'm wondering why no one other than Najm doesn't answer me and even read my thread....

[Johansson]

To get a pressure ratio of 4:1 the pressure after the diffuser must be four times higher than the atmospherical pressure, so it must move extremely fast. My guess is that a PR of 1.1:1 is more likely to be found inside any DIY ramjet unless it is used as a tipjet in a spinning test rig. The low PR is also the reason why ramjets are so fuel thirsty compared to a gas turbine...

//Anders

[Arman_awn]

OK, can a cone in the diffuser help an even slight increase in pressure ratio? how much?

[larry cottrill]

Arman and Najm -

The geometry you (Najm) show in your drawing will work, but I believe it will only work about as well as a simple conical diffuser. Here's why I think this:

Your "bullet" is really two cones, a short front-end cone and a much longer one (as it should be) at the rear, forming the diffuser. As the air flows rearward around the front cone, it will speed up before entering the diffuser section. You'd think this would be what you want, but it is actually self-defeating, because both the pressure and density of the air are reduced as it speeds up, just like the air flowing faster over the top of a wing! So, you DO achieve a higher pressure ratio in the diffuser itself, but you start out by robbing yourself of pressure before the air goes in !!!

So, what I'm saying is: There is nothing wrong with doing it that way, and hey, it looks pretty cool. But, it's easy to kid yourself into thinking that something like this will have some great advantage which it really doesn't have. (Of course, the action at supersonic speeds might be completely different ... but none of us are probably going to get there any time soon.) It just isn't physically "in the cards" for us to develop a high pressure ratio at low speed, no matter how cleverly we try.

Of course, I am no engineer - if anyone knows better, and can explain why what I've just said isn't reasonable, let him now speak!

L Cottrill

[Arman_awn]

I have a question about throttling: How is Throttling done in a ramjet engine?
I know we add more fuel to throttle the engine more and more, but in a ramjet, if we increase the fuel quantity being fed into the engine, what is responsible for the need to more air? do I have to run my leaf blower at a higher speed to throttle the Ramjet?

here is some pics of my diffuser I machined yesterday, and my combustion chamber or the body structure of the engine:

[Najm]

The geometry you (Najm) show in your drawing will work, but I believe it will only work about as well as a simple conical diffuser. Here's why I think this:

I know it will work like a simple conical diffuser, it was easier to make the intake this way that's why I made it this way ,I didn't say that there is a performance increase this way as the cross sectional area of the diffuser is increasing like a normal intake.
BTW it looks better than a normal intake even if it performs like a normal intake.
Also can anyone explain if it is beneficial to have the exhaust centered, I mean the flame wasn't taking up the whole cross sectional area of the exhaust.

[Najm]

Thrust changes by the fuel to air ratio , the leaner the mixture(fuel wise) the lesser the thrust and as it gets richer the thrust increases so throttling can be done by changing the amount of fuel flowing into the ramjet.
(If I have reversed the thrust's relation to how fuel lean or rich a mixture is then someone please correct me as I may have forgottten it.)

[Arman_awn]

I have a new idea about increasing the pressure ratio:

By using a conventional subsonic diffuser (like what I made) and two cones joined together at their maximum diameter in the rear section of this diffuser we can get higher pressure ratios.

The incoming air is slowed down and compressed in the conventional diffuser because of the increased volume and it is forced into the front cone and when it reaches the back cone once again it is comprsssed because of another increase in vloume...

What do you think about that?

[Arman_awn]

Hi,
Thanks for your response to my above question ):

I wanna know what's the bes way to secure the flameholder and the fuel injection system to the body structure without disturbing the boundry layer?

[Arman_awn]

Isn't there anyone here for help???

[Johansson]

WTF!

Asking questions that you probably can answer as well as anyone else is one thing, but yelling and screaming if no one answers fast enough is just rude.

//Anders

[multispool]

If I understand what your saying, then it wont work!
In the first diverging cone, the air pressure rises, density rises, velocity falls, exactly the reverse happens in the second stage, then in the third stage, rises back to what it was previously after the first stage, this density and pressure will of course be less than after the first stage due to frictional and other losses.

I'm not knowledgible with ramjets but would expect that if the flame holder is of the eight fingered style, it might be beneficial to bend the fingers back in a curve or cone shape in an effort to acheive stoiciometric mixing.

Ramjets are probably good to play around with but don't expect much (or any) measurable thrust from a low speed one. Compression through diffusion using a leaf blower is likely to be very low (as Anders has mentioned), and the air/fuel mixture will both need controlling for best running.

[Stuart]

I have a question about throttling: How is Throttling done in a ramjet engine?

You can throttle either by adding fuel or increasing airflow (if you start above the stoichiometric ration), that is the runaway nature of ramjets in operation. As long as you keep the flame lit through whatever combination, then you will increase thrust until you reach stoiciometric.