Final Checks...the engine is ready to go!

[Arman_awn]

Hi,

I'm Arman, I haven't been here for a while and now I'm back here with you. If you remember me, you will probably know I have been on a ramjet project which I've talked about extensivly on the "A question about home-made ramjets..." topic.

By now, I have made almost all parts of my ramjet and as soon as the ignition system and the fuel system are ready I'll go for testing.
I've started this Topic in order to show you what I've done and how.

I'll be really glad to know your opinions about my work, and also to know the answers of the follwing questions (Pro's, It's your turn!):

1- The diffuser and the nozzle are both machined well and I think they are the nicest home-made diffuser and nozzle I've ever seen! Both of them will be welded to flanges in order for them to be connected to the flanges which are welded to the CC. What do you think about them? I mean the quality of constrauction and the probable performance.

2- In one of the pictures, the Flame holder is shown. A simple 'dotted' type flame holder that looks sufficient for my engine. How is the things with the flame holder from your point of view?

3- What's your opinion about the entire lenghth of the engine? doesn't it need any modifications?

4- the injectors are intended to delvier the fuel (butane) from behind the flame holder, how will that probably work?

5- Do you think the engine will run successfully? any suggestions form you pro's is appreciated...

I'm in a real hurry to test the engine, I'll be thankful if you participate in answering my questions sooner...

Thanks,

Arman

[larry cottrill]

Arman -

Basically, things look pretty good. The only two things I would comment on are that the chamber probably doesn't need quite that much length (but it shouldn't really hurt anything), and that you may find that you're trying to get too high a pressure gain out of your diffuser.

The diffuser appears to have an outlet/inlet diameter ratio of about 1.6 - this would be an area ratio of 2.56:1. That will only work out well if your blower delivers air at very high velocity. You will be able to tell the first time you get it running: If combustion crosses the flameholder into the diffuser, your airspeed is too slow for that ratio, and you'll need to cut the diffuser back. Please see my last night's post to thecheat in his thread for a fuller explanation.

You will learn a lot more the first time you fire it up! Good luck with it.

L Cottrill

[Arman_awn]

OK, for this diffuser area ratio, How fast does the air have to travel in order for the engine to be at "self-sustain" condition?

What's your idea about delivering a pre-comressed air to the engine? will the engine produce much more thrust?

Is the flame holder I showed a pic of suitable for my engine?

[Arman_awn]

Waiting for your repiles... :D

What's your idea about delivering precompressed air to the engine? is it possible? How does this affect the engine's total thrust?

[larry cottrill]

OK, for this diffuser area ratio, How fast does the air have to travel in order for the engine to be at "self-sustain" condition?

I don't have any exact way to calculate this, and besides, we don't have any data yet on how well this flameholder will work. If we make some "simplifying assumptions", we can get a ROUGH idea:

Let's assume that we need a minimum of 10 m/sec speed at the flameholder to keep the flame behind it - that surely can't be too far off. Let's assume that the density of the air doesn't change going through the diffuser (not quite true, but not a capital offense). The area ratio of 2.65 means that the inlet velocity would be 26.5 m/sec, a figure that should be easily achievable with a small electric blower, for example.

The problem is that the pressure gain available at that level will be very small. Remember, the chamber pressure can't be higher than the diffuser exit pressure. So, we are looking at an engine running "at idle". The CHANGE in air speed in the diffuser is 26.5 - 10 = 16.5 m/sec. To get the pressure in dynes/sq.cm, we use:
pdiff = V^2 / .2784 = 272.25 / .2784 = 978 dynes/sq.cm (or 0.978 millibars)
I think no matter how good your chamber and nozzle are, that's running pretty much as a "thrustless burner".

What do we have to do to get significant pressure? Let's say we want a diffuser pressure gain of 1/10 of an atmosphere, about 100 millibars. We would need a velocity drop of
V = 16.68 x sqrt(pdiff) = 16.68 x 10 = 166.8 m/sec !!!
And that's just the velocity difference - the inlet velocity would need to be significantly higher. Roughing it out this way, you begin to understand why everyone isn't flying around in low-speed ramjets!

What's your idea about delivering a pre-comressed air to the engine? will the engine produce much more thrust?

That's all right, if you want a "motorjet" rather than a pure ramjet. I personally like the motorjet idea, though purists find it distasteful. It is nothing but a ramjet with something bolted to the front that supplies lots of air, usually at enough speed that there is a slight pressurization. They are considered "inefficient", though it's always arguable exactly what that means. Of course, for an engine the size of yours, it would take a LOT of power to keep the front end elevated in pressure, even a little.

Is the flame holder I showed a pic of suitable for my engine?

I think it looks pretty good. If it seems too restrictive after experimenting with it a while, you can always enlarge all the holes a little for a less "draggy", more free-flowing design. Since your engine is so easily taken apart and re-asssembled, you could even make a set of "classic" conical flameholders that mount the same way, and compare how that works (keeping us all posted, of course ;-)

L Cottrill

[Irvine.J]

Hi larry, just want some help to understand the physics here...
"The CHANGE in air speed in the diffuser is 26.5 - 10 = 16.5 m/sec"
Is the "10" coming from needing 10mps to keep the flame behind the flameholder? And how do we know the ratio is 2.65?


Is 16.68 here a constant?
V = 16.68 x sqrt(pdiff) = 16.68 x 10 = 166.8 m/sec
Cheers.

[Arman_awn]

Dear Larry, Thanks a lot, You've clerified almost everything for me about the ramjets.

I have another idea about motor-jets. The idea is under Construction! as soon as I get to a logical result, I'll share it with you.

Again, Thanks for everything...

[larry cottrill]

Hi larry, just want some help to understand the physics here...
"The CHANGE in air speed in the diffuser is 26.5 - 10 = 16.5 m/sec"
Is the "10" coming from needing 10mps to keep the flame behind the flameholder?

Exactly - the value of 10 was just a guess, of course.

And how do we know the ratio is 2.65?

This is approximate, of course, based on measurements on screen of the photo Arman posted earlier (the one with the parts standing up). The area ratio is the square of the diameter ratio.

Is 16.68 here a constant?
V = 16.68 x sqrt(pdiff) = 16.68 x 10 = 166.8 m/sec
Cheers.

Yes. See this earlier thread:
http://www.pulse-jets.com/phpbb2/viewtopic.php?t=2207
Unfortunately, even though I say 'All constants now corrected' at the top, I still believe some must be off. I really need to go thru that whole thing again, and make sure it all makes sense. But, anyway, the idea is that a certain velocity of air creates a certain 'pressure' (actually called velocity pressure or velocity head). My idea in this thread is that, roughly speaking, this velocity pressure represents the momentum that is converted to static pressure as the velocity drop occurs. It cannot be highly accurate, but should give a rough "ballpark" idea of what we can get. There IS an accurate way of calculating this - I will try to find it and post it.

L Cottrill

[Arman_awn]

Hi, I'm now right back!

The nozzle flanges have been successfully constructed and now, the engine is complete and I've managed to test the engine by the next week.

I'll post some pics of the "NICEST HOME-MADE RAMJET EVER!" soon.

Thanks,

Arman

[Arman_awn]

Here is some pics of the "Monster" I've made:

[Arman_awn]

No one interested really???

any ideas are apriciated..

[Irvine.J]

Mate it looks AWESOME... the only idea I have is FIRE THIS MONSTER UP! Goodluck! It looks very very nice :D

[thecheat]

oh, you wanted feedback? heh heh... I was waiting for you to post a vid! looks GREAT! However, what're you going to use to provide the air? I don't think a blower will cut it...

[Arman_awn]

Thanks for your replies, the reason for not firing the engine up was that I haven't found a really powerful leaf blower yet.
as soon as I find a good and reliable leaf blower (supplying the airflow at it's max power) I'll fire that Dragon(!) up!

[larry cottrill]

Arman -

Yes, that looks like a beautiful build - I apologize for not congratulating you sooner! Get enough of an air mover in front of that, and I'm sure you'll be thrilled with how well it works! Just be prepared to spend some time experimenting with the right mixture of air vs fuel. Remember - Safety First !!!

L Cottrill