## Few Quick Questions...

Moderator: Mike Everman

gameboyz
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### Re: Few Quick Questions...

I am such a procrastinator. I have had no progress since my last post till now I hate myself for that, but when I think of sawing it really dampens the mood

I will be going to a mall to buy another stainless steel mug tomorrow, and I think I'll use isopropyl alcohol for the fuel. Since it is volatile it should evaporate pretty quickly into vapour and mix with the air before combustion

p/s: sorry forgot to mention i'm fterh! ^^ I didn't create multiple accounts on purpose, but I forgot what exactly happened
careful there goes a finger; oh your leg too!

serverlan
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Location: Australia

### Re: Few Quick Questions...

Hi all,

I pose the quation about "why does the gas burn at the end of the tube" a while back also
[url]viewtopic.php?f=3&t=5240&p=62323#p62323[url]

I just thought I'd have a go at summarising what I have thnuked out since then.

A bunsen burn is a device that mixes fuel(gas) and air in a tube, then burns at the end.
They say it needs to be about eight times as long as it is wide, to give the air and gas time to mix.

It is possible to get them to burn at the wrong place, inside the tube, but usually they will burn at the end of the tube.

There is a fair bit of maths about, indicating that the flame will sit wherever there is a significant sharp increase in the diameter of the tube.
End-of-tube to open-air counts as such an increase.

My guess as to why this is so (apart from "cos the maths says so" ). is to do with flame and gas velocities.
If a tube leads from a high pressure zone, to a low pressure zone, then (given some boundary conditions are met)
the gas will accelerate to the speed of sound in the tube. The result is the hissing noise of escaping gas.
I'm guessing a bunsen burner does the same thing.
Once out of the tube, into open air, then the gas rapidly slows down.
Also note that the speed of a flame froint is a fair bit slower than the speed of sound.
So, the flame is unable to make its way upstream against the new gas flowing through.

The flame holders in ramjets act in a similar way. The gas enters the small hole, and is allowed to expand into the conical region. In doing so, it slows down. Thus the flame resides at the point where the gspeed of the gas flow coming in matches tthe flame speed moving back.
Cones are fairly forgiving
It doesn't muuch matter exactly where this balance occurs, so long as it occurs somewhere inside the cones.
And of course, the cone shields the flame from being blown out.

Does that help?

fterh
Posts: 39
Joined: Tue Feb 03, 2009 7:36 am

### Re: Few Quick Questions...

http://www.fabcode.org/2009/07/01/remar ... de-ramjet/

My progress so far, just wondering if the off-course cut is a severe mistake that will interfere with smooth operation of the ramjet.

And if butane is okay. I plan to use a hair dryer to blow air in and spray the butane in from the front along with the air

fterh
Posts: 39
Joined: Tue Feb 03, 2009 7:36 am

### Re: Few Quick Questions...

Any advice how to get the stainless steel strainer off the mold?

http://www.fabcode.org/2009/07/05/how-c ... so-flimsy/

Jutte
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Location: NZ

### Re: Few Quick Questions...

Have you tried heating the strainer part with hot water + putting cold water on
the mould part and gently twisting the lot?