Few Quick Questions...

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fterh
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Few Quick Questions...

Post by fterh » Tue Feb 03, 2009 7:42 am

I'm thinking of setting up a tiny engine but got some doubts.

It's not a pulsejet, but it works like one cuz there are no moving parts. I'll construct a chamber using maybe steel wire reinforced with several layers of aluminum foil (I'll worry about the melting points later). I'll maybe get some lighter fluid and then pump them into the chamber through those devices that convert a stream of liquid into a vapor mist and maybe attach a device that pumps air inside.

What I'm not sure of is:

- Will the mixture of lighter gas and air burn as expected, like a pulsejet? Or will the device (liquid to vapor) just have a orange flame from the end like a lighter?
- If it's like a lighter, how do I build it so it will actually burn rapidly producing huge amount of hot gases?

metiz
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Re: Few Quick Questions...

Post by metiz » Tue Feb 03, 2009 1:43 pm

Sounds like you want to build a ramjet and by the looks of your post I think it is save to asume you do not have a welder(?)

I don't recomend trying it with wire and foil. A ramjet needs a smooth surface to work properly and the temperatures will be so high that the aluminium will melt. strength migh also be an issue. If you realy wat to try your hands at a no-weld ramjet, try this one:

http://www.cottrillcyclodyne.com/Maggie ... aggie.html
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larry cottrill
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Re: Few Quick Questions...

Post by larry cottrill » Tue Feb 03, 2009 6:01 pm

metiz wrote:If you realy wat to try your hands at a no-weld ramjet, try this one:
http://www.cottrillcyclodyne.com/Maggie ... aggie.html
Thanks, metiz!

... But instead of the J-B Weld epoxy I worked with, use a furnace cement or fireplace cement -- OR the junk from those muffler repair kits. Something really designed from the ground up for high temps. Other than that one change, you can follow my construction scheme exactly. The front end (the diffuser section) would actually be better if it was more like a smooth cone; I just used what I could get my hands on at the time that looked good to me.

It also occurs to me, fterh, that what you're after is a kind of 'pressure jet' where you want the high speed fuel flow to drag air into the engine. That can also be done, but it takes pretty careful design to get anything useful out of the idea. All jet design work basically looks easier than it is, because "simple" doesn't generally equate to "easy", if you know what I mean.

L Cottrill

fterh
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Re: Few Quick Questions...

Post by fterh » Wed Feb 04, 2009 6:56 am

metiz wrote:Sounds like you want to build a ramjet and by the looks of your post I think it is save to asume you do not have a welder(?)

I don't recomend trying it with wire and foil. A ramjet needs a smooth surface to work properly and the temperatures will be so high that the aluminium will melt. strength migh also be an issue. If you realy wat to try your hands at a no-weld ramjet, try this one:

http://www.cottrillcyclodyne.com/Maggie ... aggie.html
Nice.. bookmarked :D

But what I'd really want to know is.. what factors determines if the combustion takes place in the chamber (like an explosion)or will it just burn at the end of the fuel injector.. like those bbq lighters Image

metiz
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Re: Few Quick Questions...

Post by metiz » Wed Feb 04, 2009 10:45 am

Thats what the flameholder is for (in a ramjet at least) It makes sure the flame does not blow out and also " forces" the fuel to be burned inside the combustion chamber rather then out by the exhaust.
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fterh
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Re: Few Quick Questions...

Post by fterh » Wed Feb 04, 2009 10:57 am

metiz wrote:Thats what the flameholder is for (in a ramjet at least) It makes sure the flame does not blow out and also " forces" the fuel to be burned inside the combustion chamber rather then out by the exhaust.
Blown out as in dying (no more flame)? And out by the exhaust refers to burning as it shoots out of the exhaust? :P

BUt I noticed that propane-fueled valveless pulsejets (from youtube) have a fuel injector without a flameholder.. hmm :?:

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Re: Few Quick Questions...

Post by phonedzero » Thu Feb 05, 2009 5:41 am

I think I am going to try and build one of these this weekend, and muffler cement is a fantastic idea! Stuff only gets harder the hotter it gets, just a little goopy to start with! Wonder if its is safe, chemically speaking to mix it with JB?

fterh
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Re: Few Quick Questions...

Post by fterh » Thu Feb 05, 2009 11:50 am

metiz wrote:Thats what the flameholder is for (in a ramjet at least) It makes sure the flame does not blow out and also " forces" the fuel to be burned inside the combustion chamber rather then out by the exhaust.
I did some research about the flame holder...

but I still don't understand:

1) How the flame holder prevents the flame from being blown out by the intake flow of air?
2) It's not answering my question :( on what determines if the fuel will explode and create thrust or burn at the end of the fuel injector like a bbq lighter

metiz
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Re: Few Quick Questions...

Post by metiz » Thu Feb 05, 2009 12:16 pm

fterh wrote:
metiz wrote:Thats what the flameholder is for (in a ramjet at least) It makes sure the flame does not blow out and also " forces" the fuel to be burned inside the combustion chamber rather then out by the exhaust.
I did some research about the flame holder...

but I still don't understand:

1) How the flame holder prevents the flame from being blown out by the intake flow of air?
2) It's not answering my question :( on what determines if the fuel will explode and create thrust or burn at the end of the fuel injector like a bbq lighter

1: As I believe it works: The flameholder creates a barrier for the gas and air so it has time to react with each other in the combustion process. Otherwise, the gas and air are blown away before they can react/ react in the wrong place It's just like blowing out a candle. If you put your hand directly behind the candle, you'll have a harder time blowing it out

2: Thrust is generated by the fuel that is burned. there is a certain volume of air in a ramjet (stationairy). If you add fuel and burn it, the air will get hot and expand. (boil's law) You now have a higher then normal air pressure in the cc of the ramjet that wants to go outside to stabilise. This coupled with the forward motion of the ramjet/ air source blowing in intake will force this high pressure zone out the back creating thrust
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fterh
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Re: Few Quick Questions...

Post by fterh » Thu Feb 05, 2009 12:22 pm

metiz wrote:
fterh wrote:
metiz wrote:Thats what the flameholder is for (in a ramjet at least) It makes sure the flame does not blow out and also " forces" the fuel to be burned inside the combustion chamber rather then out by the exhaust.
I did some research about the flame holder...

but I still don't understand:

1) How the flame holder prevents the flame from being blown out by the intake flow of air?
2) It's not answering my question :( on what determines if the fuel will explode and create thrust or burn at the end of the fuel injector like a bbq lighter

1: As I believe it works: The flameholder creates a barrier for the gas and air so it has time to react with each other in the combustion process. Otherwise, the gas and air are blown away before they can react/ react in the wrong place It's just like blowing out a candle. If you put your hand directly behind the candle, you'll have a harder time blowing it out

2: Thrust is generated by the fuel that is burned. there is a certain volume of air in a ramjet (stationairy). If you add fuel and burn it, the air will get hot and expand. (boil's law) You now have a higher then normal air pressure in the cc of the ramjet that wants to go outside to stabilise. This coupled with the forward motion of the ramjet/ air source blowing in intake will force this high pressure zone out the back creating thrust
Quick reply, thanks! What if the fuel burns at the end of the injector.. will there be thrust?

larry cottrill
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Re: Few Quick Questions...

Post by larry cottrill » Thu Feb 05, 2009 1:28 pm

In the classic "textbook" ramjet, the flameholders are fairly small, hollow cones with the tips cut off (in other words, conic frustra). Since they are placed at the rear of the diffuser section, the air flow is at elevated pressure but quite slow -- typically only a few metres/second. Air entering the front end of the flameholder slows down even further, practically to a crawl, and is also broken up into turbulence and vortexing. Basically, the flameholder is allowing fresh air/fuel mixture to get in, but really "puts the brakes" on it.

Any air/fuel mixture at a particular pressure has a certain flame speed at which the flame front propagates through the fluid. The flameholder slows the flow so much that it absolutely assures you that the flow velocity WILL NOT EXCEED that flame propagation speed. Thus, the flameholder always provides at least a single small region where the flame front WILL NOT be blown rearward. So, the flame front is stabilized, at least in the region of the chamber directly behind the flameholder(s).

If a ramjet is operated at a speed too high for its design, the flame front OUTSIDE the flameholders will be blown backward so the combustion chamber is not adequately filled with expansion gas. If the ramjet is operated at a speed too slow for its design range, the flame speed will exceed the flow speed at the rear of the diffuser and the flame front will move forward into the diffuser until it finds a flow speed that will balance it -- but this will be at a lower pressure than the engine is designed for. In either case, the efficiency of the engine is reduced. A simple ramjet will only operate efficiently in a fairly narrow range of flight speeds, where the chamber is properly filled with expanding combustion gas and there is no combustion in the diffuser section.

L Cottrill
Attachments
Model_ramjet_crop1.jpg
Model "textbook" ramjet. The two conical flamehiolders are clearly visible
at the front end of the combustion chamber / rear end of the diffuser section.
To be realistic, the diffuser should be somewhat longer than shown here.
Photo Copyright 2005 Larry Cottrill

metiz
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Re: Few Quick Questions...

Post by metiz » Thu Feb 05, 2009 1:34 pm

yeah what he said :P
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fterh
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Re: Few Quick Questions...

Post by fterh » Thu Feb 05, 2009 1:51 pm

metiz wrote:yeah what he said :P
Nice explaination, but too deep for me to understand.

Oh my country doesn't sell JB weld.. bummer.. :(

And I doubt a 15 year old can convince his parents to ship it in to build a ramjet.. :D

fterh
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Re: Few Quick Questions...

Post by fterh » Mon Feb 09, 2009 1:45 pm

Hey guys.. in my country we don't have autumn (sad) so nobody keeps a leaf blower..

Also if you have visited SG we live in those vertical blocks of buildings (HDB fyi :D) so setting off a ramjet will pretty much piss off the neighbors...

I'm planning to hit the parks to test it, I'm thinking of getting one of these monsters case fans (those PC casing fans, you know?) that roars like a hair dryer on max and has the same air flow, but that would suck too much battery power.. so I'm thinking of an aluminum 79CFM fan (http://www.xoxide.com/12alcasfan.html) but I'm not sure if it's high-powered enough though :(

anyone can advise?

fterh
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Re: Few Quick Questions...

Post by fterh » Tue Feb 10, 2009 7:37 am

Guys,

how do u ensure that the two stainless steel mugs, after cutting out the unnecessary portions will fit smoothly (like shown here: http://www.cottrillcyclodyne.com/Maggie ... aggie.html) How do u ensure that one of the mugs won't be like, much too big then the other?

:?: :?: :?:

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