A question about homemade ramjets

[larry cottrill]

Najm -

In very small ramjets, I think it's MUCH better to mix fuel into the air somewhere out in front of the flameholders, because you have so little space (and time) for good mixing if you try to inject it behind. Of course, Steve didn't do that with his, and he got it to work all right (in his jet, the fuel is injected TOWARD the flameholder from behind, right in the middle). However, there is theoretically nothing wrong with pre-mixing the fuel and air forward of this point. On larger engines, where it's easier to get good mixing, injection after the flameholder should work fine.

The only real problem with forward mixing is that you could get combustion pretty far up in the diffuser zone if air speed is (or becomes) too slow. Of course, low intake air velocity will be a problem for any ramjet, no matter how the fueling is set up.

L Cottrill

[Zippiot]

Yeah I have been burned many times by the flame shooting out the front...
But once combustion started, it ran much better.

My fuel line shoots a stream right at the middle of my flameholder, which has a hole in it larger than the rest of the holes. This kinda mixes it between aspirated and injected, as fuel is mixed both before and after the flameholder.

[larry cottrill]

Yeah I have been burned many times by the flame shooting out the front...
But once combustion started, it ran much better.

Yes, with any ramjet there will always be some minimum speed, below which stability goes out the window.

My fuel line shoots a stream right at the middle of my flameholder, which has a hole in it larger than the rest of the holes. This kinda mixes it between aspirated and injected, as fuel is mixed both before and after the flameholder.

Unusual, but I really see nothing wrong with it, except that it would not work well of your fuel flow speed is way more than you need. As long as you control fuel flow carefully (which you have to, anyway), I don't see any problem with that. I guess its unusual layout might make it a little hard to comprehend what's really happening whenever it doesn't seem to be working quite right, but troubleshooting will probably be just a matter of gaining experience with it.

L Cottrill

[Najm]

Thanks for the reply, I've been using direct injection designs and they work pretty well as far as combustion is concerned(I lengthened the combustion chamber after the flame holder a bit).......I will try the aspirated design to see if it runs better.
BTW Arman how is the ramjet coming up? You have not posted for a week regarding your progress on it.

[Arman_awn]

Dear Najm, thank you for your worry about me :D

I have problem building the nozzle, it is a simple cone, but you know that these shapes are called "complex shapes" and they need to be precisely machined...If I face any problems I'll inform you as soon as possible..

thanks again for reply...

[Arman_awn]

1-Where the fuel injectors should be located? behind the flame holde (towards the diffuser) or in front of it?

2- How many fuel injector is needed? will 4 injectors suffice for my engine?

3- I've planned to use a "dotted" type flame holder. A circled metal sheet with many holes drilled on it, simple for construction...of course still the blockage remains only 30% of the entire area of the CC entrance area...

I need immediate help and answer to my quetions...

[larry cottrill]

1-Where the fuel injectors should be located? behind the flame holde (towards the diffuser) or in front of it?

I think it would work either way. Steve's jet was very successful injecting fuel into the flameholder from behind, so why not use that? Be careful to get you fuel spouts behind a dead area, NOT behind a hole where the air comes through.

2- How many fuel injector is needed? will 4 injectors suffice for my engine?

I don't think the number of injectors matters. It's getting them properly located that counts. Of course, multiple injectors gets the fuel more "distributed" in the front of the chamber - BUT, you need to make sure they are all flowing equally, so you have to build a multi-injector setup really carefully.

3- I've planned to use a "dotted" type flame holder. A circled metal sheet with many holes drilled on it, simple for construction...of course still the blockage remains only 30% of the entire area of the CC entrance area...

That sounds just fine to me - even lower blockage of the total area would be better, if you can get it reasonably.

Good luck! Be mindful of safety in your testing. Make sure you have a "quick cutoff" valve in your fuel line, in case of air supply failure!

L Cottrill

[Arman_awn]

Hello,
Dear Larry, I've machined built the flameholder now, and also secured it in the CC. Here is two pics of the flameholder (one with the diffuser removed and one without)...

one question: What kind of spark plug can I use in order to use it to start my engine?

I'll be glad to know your opinions about the work I've done.

[larry cottrill]

Your sparkplug can be almost anything you can find, located well to the rear of the flameholder. Theoretically (if your flameholder works well) it will only be needed for starting, anyway.

As to the flameholder, it looks fine to me, but I am NOT an aerospace engineer. You're at the level now where my opinions don't mean much - only testing and careful observation will really matter. My knowledge is all from an ancient textbook (early 1950s). Anything beyond that is pure speculation.

All I can say is test, test test! It's the only way you'll find out what you've done right and what needs modification. Your workmanship certainly seems first rate. Keep us posted on how it goes in finishing up and testing. It seems to me that providing an adequate air supply for static testing will be your biggest challenge.

L Cottrill

[Arman_awn]

Thanks Larry for the reply.

I have a question about the nozzle: What kind of material will be suitable for building the nozzle cone? is stainless steel sufficient?

How much thrust do you predict for my engine to preduce?

Is propane a suitable fuel for my engine?

[Irvine.J]

Hey mate, if you have seen the video's of my ramjet, which is completely wierd as the flame colour is dependant on heat of the engine... which is just plain wierd... it starts off as blue then becomes yellow ...wierd... (must have something to do with the impeder.) Heres the videos
http://www.pulse-jets.com/phpbb2/viewto ... 8&start=15

I used a Milo tin for the front and back nozzles. its very thin stainless steel, though I dont know the dimensions of your engine, mine is 42 cm long 9cm wide, and it seems to be handling it ok. (Though my max test time is 2 minutes at full brunt.) Milo tins are about .3mm thick, very thin indeed, if your using stainless of a reasonable thickness and good welds i'd say you'd be alright, dont quote me though, larry is the man lets see what he has to say. There is really no way to guess the thrust either as its partly dependant on how good your blower is, as the faster the air speed the greater the thrust, best bet is just to go ahead and give it a run... dont be shy now :D I wait with anticipation of your beast of an engine, make sure you upload a video!

[Najm]

Propane is suitable for running engines ...........I've been running my engines on methane gas and they run fine. Anything that burns can be used in a ramjet. Messerschmidt( I think the spelling is wrong) made a working coal fired ramjet.
Upload the dimensions of your ramjet only then can theoretical thrust be predicted, practically its always lower.
The nozzle can be made from SS but try not to overheat it as it could cause it to deform. Supply enough air to keep the engine cool.

[Arman_awn]

Hello, Thank you all my friends, you've provided me very good support and that's why I always have the energy to keep on completing my project, and in better words, our project. :)

I have a problem that got me to a stop for testing, What can I use as an injector? I don't prefer to use fuel pumps and somthing like that, I want to inject the fuel which is propane into the CC with the gas storage pressure assitance only...

[Arman_awn]

Some pics of the fuel injector, fuel pipe, the engine with the nozzle removed and so on....

[Najm]

No fuel pump is required when you are supplying gaseous fuels(propane) to the engine ....... the gas pressure is enough.
Nice looking engine.

Hey mate, if you have seen the video's of my ramjet, which is completely wierd as the flame colour is dependant on heat of the engine... which is just plain wierd... it starts off as blue then becomes yellow ...wierd...

Yes flame luminousity is dependant on its temperature.