New ramjet experiments

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Irvine.J
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re: New ramjet experiments

Post by Irvine.J » Tue Dec 05, 2006 6:25 am

Anyone got an idea for an airspeed guage that wont melt??? Lol video of latest test run pending.

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re: New ramjet experiments-Bad day

Post by Irvine.J » Tue Dec 05, 2006 5:19 pm

Ok, so I cut back the exit diffuser slightly (1/2 an inch) ...
I also extended the gasline with another hose that was a different type to the one on the gas bottle. (I think this wasn't a good idea either)
And probably the most important thing, is when I started with the secondary baffle, it was flat, not bent back towards the exit diffuser.

In this run, a made a quick dodgey stand out of lego that really wasn't at all to my satisfaction, but I thought the hell with it I'll use it... I bent backwards the secondary baffle as to reduce the airstream interruption.... instantly I noticed I wasn't getting the flame behind the flame holder once again.... a few things to note in this video...

1- the test stand starts to slowly (though very little look closely) roll backwards. Then when a little more gas is applied it stops rolling.

2- no matter how much gas i apply, it does not move foward.

3- I move the nozzle closer to the inlet, and the stupid thing just sits there!

4- Notice the combustion again seems to be confined to the rear of the engine, it did not reach the PFH...

I'll bend the secondary baffle foward again with the slightly increased nozzle diameter... it wasn't a very successful day, the lego was not exactly a good idea, it didn't melt, but didn't have any weight, and was hard to secure it...the base didn't seem heavy enough so I had to weigh it down a little. I will make a small swingarm hopefully tonight to get a more fluid and less restrictive motion. Videos here... not very happy with myself right now...
Why leggo? I didn't have any spare wood lying around or wheels that I could use, i've since come up with a sweet idea!

http://mangyjet.freewebpage.org/rj.html

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re: New ramjet experiments

Post by larry cottrill » Tue Dec 05, 2006 6:09 pm

Irvine.J -

At the highest speed you're driving it, your flameholder isn't doing its job - on the video you can see the flame separating from the exhaust nozzle, just like you can get an acetylene flame to burn out away from the torch tip. This means all your combustion is being wasted behind your engine, under that condition.

I think your exhaust nozzle is too small, and is creating artificially high pressure within the engine. Essentially, the whole engine is acting as a mixer / nozzle, rather than developing thrust internally. In other words, there is simply a large pressure drop for the air going through, and a resultant velocity that exceeds the flame propagation speed of the air / fuel mixture.

Basically, I think you're pushing it too hard for its size, for the real ramjet action you want to achieve. Man, that must be a heck of a blower!

Don't be discouraged by this at all. What you're trying to do is simple - but NOT easy! A design like this will, realistically, probably only work well over a fairly narrow range of speeds. With this obviously very fast blower, what I would do is (a) open up the exhaust nozzle to a much smaller area ratio, maybe 2:1 or 3:1 at most, and (b) make the diffuser even longer with a smaller inlet - then see how it goes with various blower distances. You can always cut back the diffuser gradually to open it up if the front-end ratio seems excessive. I don't really think the basic design is bad at all - I really think the main problem is excessive choking at the rear end. It isn't right until you have stable combustion hanging right there in the chamber, close to the flameholder.

This might be a good time to review, once again, Steve's TLAR Ramjet video - that shows EXACTLY what you're after.

Best of luck!

L Cottrill

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re: New ramjet experiments

Post by Zippiot » Tue Dec 05, 2006 6:24 pm

So the flame should be behind but very close to the flameholder. My flameholder got red hot in runs lasting more than 10 seconds, that is why eventually the welds broke loose and the flameholder warped and tried to escape the jet. It didn't quite make it out but to my surpirze the jet continued to run until I cut the fuel.
I cant explain that one

You might be able to salvage this by making the nozzle convergent-divergent. Weld another cone that meets with the exhaust but has a lesser angle. I think of it as 2 paper cups, bottom to bottom. Just one is a slightly different angle, might be able to make that expansion behind the engine do a little work for you.


Can you post a picture of where your fuel is delivered and some close ups of how its attached and all. My maggie ram had a hole in a blocked pipe, facing backwards. It shot through a hole in the middle of the flameholder, but was placed infront of it. Fuel line placement is best done by guess and check...too bad its hard to patch a hole that size in thin steel...
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re: New ramjet experiments

Post by thecheat » Tue Dec 05, 2006 6:33 pm

Hey Irvine... ah blast it, I'll just call you 47... the new name just won't click in my mind...

47, you think you could post one of your videos? I still haven't seen one... I really want to see what "construct" you've made now...

PS: this may be a bad idea, but, if you've already gotten the pressure drop, would it be possible to rip out that rear flameholder, and attach a tube the size of the exit of the nozzle, then, place a flameholder in there? Dunno if it would work at all, but just an idea...
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re: New ramjet experiments

Post by Irvine.J » Wed Dec 06, 2006 7:29 am

Ok so heres a photo of my latest test rig. Its a swingarm connected to the strut of a trolley wheel "off something". It moves very freely, I can either lock it or unlock it depending on what I'm doing.

Firstly,
with the exhaust nozzle the same size,
and the secondary baffle is now returned to it vertical position...

I noticed the flame was now creeping infront of the flame holder, it was a deep blue inside the engine, however it just didn't look "Right".

I moved the air nozzle further away (about 6 inches) and toyed with the throttle on both the gas and airflow. It seemed a little harder to make this engine do anything really, it is starting to flame out if i give it too much gas.

I'm thinking seriously that the flame holder design is less then adequite, i feel its actually acting as a "nozzle" pushing gasses into the centre of the engine rather then slowing it down for proper combustion. I'm thinking this as when looking into the inlet watching the flame distribution pattern. What i'm going to do now... is before ripping out the secondary flameholder, trying to reduce the high pressure within the engine created by the airflow... hopefully the burning gasses will take over that role...

I will not post a video the above run as its kind of boring... however I cut 1 and 1/2 inches of the tailpipe so it now 5cm diameter so its area is 19.625sqcm... with some exciting results!
----------------------
Photos of the stand are below, notice it can swing both ways, no way, or just fowards if i want it to....
--------------------

==========================================
I have now cut back the engines exit diffuser, 1.5 inches as larry suggested (though it was a guestimate...still slightly smaller though nearly exactly the same as inlet... Instantly I noticed the flame is now sitting very tidy behind the primary flame holder... it was a tremendous blue, the exhaust flame is very blue, with only a small amount of yellow around the back... I'll wait till it gets a bit darker... i'm uploading the videos now from today, stand by for the night runs should be quite entertaining!

Now... also note that the baffle ratio is actually substantially higher then the primary flame holder ratio... this was intended as I had the idea the airflow would do the rest of the work keeping the flame in the CC and give me a slightly higher velocity difference... however I think now it really does need to be ground out a bit, perhaps as much as 1cm all around.

Day run is available now... or should be atleast shortly
http://mangyjet.freewebpage.org/rj.html

The cheat I'm going to upload those videos so you can simply download them shortly... gimme a bit you'll get them I promise.
Attachments
pic5.JPG
How the engine looks now
pic5.JPG (76.95 KiB) Viewed 7421 times
newjet 007.jpg
Complete swingarm setup...how the engine looked earlier...
newjet 007.jpg (210.56 KiB) Viewed 7428 times
newjet 008.jpg
Nut and bearing...trolley wheel.
newjet 008.jpg (203.07 KiB) Viewed 7428 times
newjet 012.jpg
Primary flameholder config.
newjet 012.jpg (163.61 KiB) Viewed 7428 times
newjet 016.jpg
Secondary baffle configuration.
newjet 016.jpg (168.91 KiB) Viewed 7427 times

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re: New ramjet experiments

Post by Irvine.J » Wed Dec 06, 2006 9:57 am

SON OF A ....
now this i really don't get, but maybe I can figure it out...

The night run really sucked. The flame had somehow crept its way foward of the flameholder again. I'm pretty much convinced its not getting proper mixing because of the flameholder, and I know how to fix it... I think my stinger's holes arent properly configured so I might knock the stinger in a little more or change it entirely.

This is actually quite shocking...
If you watch the video please watch it intently, notice i move the air nozzle foward and back, throttle up and down, gas up and down... did you see the heat pattern not at the exit nozzle...but right infront of the SFH or baffle?
When I stopped the engine...the baffle had been BENT BACKWARDS SIGNIFICANTLY... horrified... i bent it back into position, and went full bore on the blower with no gas... the blower could not bend at all... let alone as much as when the engine was running.

Toying with it by pushing it with my fingers, I realised it needed considerable force to bend them at all. I am thinking atm what this all means.
It suggests I suppose a large amount of pressure in the CC... I'm kind of pleased and annoyed... because it seems there is a large enough amount of pressure in there to bend my SFH but for some reason its still pushing through the PFH... GARR!

There is a tiny hole in the side of the engine you viewing, so you can see the flame is consistently within the cc, but the heat pattern suggest theres something not quite right with the PFH...

What to do next? cut out some of my SFH? I think it should be the next thing to be modified... its alot easier then installing a new PFH. I think I might try this before making the inlet smaller or removing more from the exit nozzle. Its interesting to not at equal gas volumes the blue flame now extends a much greater distance, but as larry pointed out its not really that much of tell tale to how well its functioning.

WHAT I DONT UNDERSTAND IS I DIDN"T MODIFY THIS ENGINE AT ALL AND FOR SOME REASON THE LAWS OF PHYSICS DON"T APPLY IN MY HOME!!! WHY ALL OF A SUDDEN IS THE FLAME INFRONT OF THE FLAMEHOLDER WHEN IT WASN"T BEFORE!!! STUPID UNIVERSE!!!!!!!!

The only other good thing I could say is when I increased the gas I got a monster blue flame, and the entire time there were presumably flames inside the cc...but I think most of its combustion potential is lost outside the tail.
Also be aware because this is made mostly from tins (as it cost me nothing) running it alot is basically causing it to decompose...it looked real purdy at the start...now it just looks ghastly, and some of the flameholder and baffle are begining to degrade substantially. The total runtime to this condition would be about 7/10 minutes.
Video will be ready in a few minutes...
http://mangyjet.freewebpage.org/rj.html
Last edited by Irvine.J on Wed Dec 06, 2006 1:08 pm, edited 4 times in total.

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re: New ramjet experiments

Post by Irvine.J » Wed Dec 06, 2006 9:58 am

Oh yeah I know what my flameholder problem is (atleast I think)...

One very important rule to remember in flameholder design may be stated as follows:
the airflow along the inside wall of the engine shell (this thin layer of air that clings to the wall is called the boundary layer) must not be interrupted by obstructions
because the boundary layer serves as an insulator between the high temperature flame and shell wall. Any interruption usually means hot spots or burnouts in the shell. A violation of this rule is illustrated in Figure #17.


I had a feeling this could be a major issue, but in the interests of this post and nutting it out I figured This (PFH) and the secondary baffle are now to be modified. My flameholer is not presenting enough flow around the outside for proper combustion I believe...5 minutes with the dremel will fix that!

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re: New ramjet experiments

Post by larry cottrill » Wed Dec 06, 2006 1:34 pm

James, you're definitely getting better at this! I'm convinced that you're probably getting a good run when the rear of the chamber is showing some heat.

A couple of possibilities for the forward-reaching flame problem: (a) Not quite perfect air supply centering on the intake; (b) Roughness in the diffuser, allowing separation of air from the diffuser wall in places; (c) Roughness at the intake rim, causing micro turbulence and separation; (d) Flameholder (your 'primary') too uneven to provide good flow around it, i.e. local spots of low speed turbulence, etc.

Try adding just a small but well-formed flare at the intake rim of the diffuser. True, this slightly reduces the diffuser area ratio, BUT it will help to make sure that you get laminar flow across the width of the diffuser, without pockets of separation extending down the sides. Remember, in flight, 100 PERCENT of the air going in the intake would be at EXACTLY the same speed, so perfectly laminar flow is guaranteed. It's a lot harder to do that with a narrow blower nozzle.

The yellow flame out the back is deceptive, I think - the flame that reaches forward into the front end is a nice blue, lean combustion flame that looks just right (except for its location!), so I think you're finding about the right mixture at the airspeed you're driving. The yellow tail flame is possibly just due to impurities picked up from your "junk" engine materials.

L Cottrill

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re: New ramjet experiments

Post by Irvine.J » Wed Dec 06, 2006 2:40 pm

Thanks Larry! I've notice that when lowering the quality of the video a few things don't show up the way I'd have hoped.

I had also thought the excess yellow was due to the impurities picked up due to a high rate of metal degradation, so thankyou for making a point of it ;-) (reminding me to ask if it could be contributing to the yellow)

I find it facinating how the addition of more gas actually makes the tailflame not only longer but far more dazzling blue. It looks white on the camera! Just a shame its innundated around the outside with all that yellow.

I'm conisdering before going ahead and cutting away with the secondary flame holder to install a new front one because its starting to look really shabby anyway. I'll run it, with a flared inlet nozzle first however, which will reduce the intake area a fraction like you mentioned too...should also tidy up that entry nozzle that looks more or less scrubby, but hey from my first post I said we are starting out that way :D

What do you think the cause of the SFH bending back like that, I'm inclined to believe it was a combination of heat and pressure... when you look at the SFH, theres not much room for it to get out really, could this explain the bending back? I think it would take quite a bit to get it to happen though...I look foward to following this phenomenon up some more.

Are we to believe there MIGHT be something positive out of calculating a precisely designed secondary plate that might improve the velocity difference? I know, initially it wasn't the plan to keep it after a run or two, but after seeing it bent back like that, the fact is its doing "SOMETHING" question is...what? Chances are, it will be removed very shortly though.
Till the next run tomorrow, all the best guys.
Last edited by Irvine.J on Wed Dec 06, 2006 3:20 pm, edited 1 time in total.

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Re: re: New ramjet experiments

Post by larry cottrill » Wed Dec 06, 2006 3:20 pm

Irvine.J wrote:What do you think the cause of the SFH bending back like that, I'm inclined to believe it was a combination of heat and pressure... when you look at the SFH, theres not much room for it to get out really, could this explain the bending back? I think it would take quite a bit to get it to happen though...I look foward to following this phenominon up some more.
Well, I think your SFH is proving that there's pressure developed in the chamber. I think its overall effect on engine operation as a whole is negative, though. In other words, sadly, I think it is turning out to be interesting, but not useful ...
Are we to believe there MIGHT be something positive out of calculating a precisely designed secondary plate that might improve the velocity difference? I know, initially it wasn't the plan to keep it after a run or two, but after seeing it bent back like that, the fact is its doing "SOMETHING" question is...what?
Again, sadly, I don't think so. I think what you're doing is setting up an artificial division of pressure and flow between your SFH and the exhaust nozzle. That would be cool if such a division could be shown useful - but I don't see that. A "jet" of any kind works because there is a nozzle that causes a "tradeoff" from pressure (random molecular motion) to jet velocity (highly directed motion). By doing part of that at your SFH, you're limiting the exhaust nozzle's chance to do it. And, creating more "back pressure" (a term which I despise) in the process.

It is just very difficult to imagine how creating an extra pressure drop in front of the one you want really helps anything. Of course, others may have a differing view, seeing some subtle advantage that I am not. It certainly has been interesting to see what happens with it there.

I think that at the rear of ramjets, the old truism is probably true: Simpler is better. It is usually considered important as part of good design that the flow path from the combustion zone all the way through the nozzle exit be as smooth as we can get it. This is for the sake of optimum gas momentum development from the available chamber pressure (which is fairly low in any engine we can build / test); also, to minimize internal drag losses.

L Cottrill

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re: New ramjet experiments

Post by Irvine.J » Wed Dec 06, 2006 3:35 pm

Nice, larry, that pretty much sums up that atleast in an experimental sence it was interesting to see the effects. I now can understand a little better the negative qualities of a second baffle after the velocity is already increasing speed, to slam into another breach, only to be slowed and jammed through another nozzle. I can speculate there maybe/possibilty )and its a big maybe) of refining it though, as to have two stages, one smaller then the next, then an extended or normal combustion chamber and exit diffuser but the calculations and machining of such parts would be beyond anyone other then nasa I suppose, and too expensive to warrant developing on a whim. For now, the SFH has gotta go, and I might change that flameholder over a bit later, keen so see what the flaring on the intake does though. I might just put 2 holes in each cross section of the front flame holder initially as well, that might help.
"back pressure" (a term which I despise) in the process.
We could call it... "Reverse deflection", "negative reflection" a combination of the two...or possibly "Turns around and lands in your face setting you on fire mollecular massflow return variable?" :D hehehe Bed time now though, chat to you all in the morning.

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re: New ramjet experiments

Post by Zippiot » Wed Dec 06, 2006 6:50 pm

Your jet is a different color in each section, did you use the same steel for each? Just looks cool

Give this a try: put a small piece of pvc pipe over your fuel injection, slide it back enough so that the fuel gets by. It slowly melts and makes the fuel air more rich as heat builds up. I tried it once and was very pleased, the jet seemed to gain more power than it had before, avoid the smoke.

One day I am going to melt a bunch of sugar and stick that inside, try to make a solid fueled ramjet.
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re: New ramjet experiments

Post by thecheat » Wed Dec 06, 2006 9:27 pm

Finally pictures!

I was reading through you post, saw the pictures, and first to mind was: wow... I didn't think you were supposed to break up the airflow along the walls!

I personally think that's a lot of your problem... Maybe you could make a doughnut shape like mine... or, go with one of those bland asterik type flameholders... :D
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re: New ramjet experiments

Post by Zippiot » Wed Dec 06, 2006 9:51 pm

Flow "should" be smoothe on the inside and outside. It does make a difference if you welds look like mine, but nice welds prolly wont affect it as much as you think.

Of course I cant see the inside of your jet so I can say for sure it isnt a factor.
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