small burner/emitter
small burner/emitter
(OP)
hello, while a mech e, my thermo and chem kinda suck. I'm more of a controls guy. So I am asking for some hlep on a device I am working on.
Basically, I want to make a small tube to glow as white as possible over as small a length of the tube as possible.
Basically I want to radiate 1000 watts of energy out of this small 'emitter' (it should be kept up at 1200-1500 degrees).
I plan on burning diesel fuel at the base of it. The flame will go up into the tube. I was thinking that I want to put in some way of taking the heat from the flame in the center of the tube and bring it out to the perimeter. Some internal fins or something.
The outside of the tube is in a vacuum so that heat only leaves the tube via radiation. Heat also leaves with the exhaust of course.
The scale I am talking about is perhaps a 1 cm dia tube. I want as much energy to leave this tube in as short of a distance as possible.
Are there any suggestion on how i design this. Then given a design, how do I model (calculate) what I am to expect in terms of how much energy leaves radiatively and how much leaves out the tailpipe (ignoring any conduction losses).
Basically, I want to make a small tube to glow as white as possible over as small a length of the tube as possible.
Basically I want to radiate 1000 watts of energy out of this small 'emitter' (it should be kept up at 1200-1500 degrees).
I plan on burning diesel fuel at the base of it. The flame will go up into the tube. I was thinking that I want to put in some way of taking the heat from the flame in the center of the tube and bring it out to the perimeter. Some internal fins or something.
The outside of the tube is in a vacuum so that heat only leaves the tube via radiation. Heat also leaves with the exhaust of course.
The scale I am talking about is perhaps a 1 cm dia tube. I want as much energy to leave this tube in as short of a distance as possible.
Are there any suggestion on how i design this. Then given a design, how do I model (calculate) what I am to expect in terms of how much energy leaves radiatively and how much leaves out the tailpipe (ignoring any conduction losses).





RE: small burner/emitter
I don't see that you could get anything better than about 2500 degF (1350 degC) in flue gas off diesel fuel so "white" may be "bright orange" at best.
How on earth are you going to atomise diesel fuel and create an expanding flame without getting fuel flow impingement on the tube. You may need to have a separate chamber below the tube and just use the flue gas to heat the tube.
You're looking for 1000 watts out so I bet you need at least 3000 watts in to stop your tail end temperature dropping off too much, = about 10,000 Btu/h (excuse my using Imperial units - it helps me to think) By my normal rules of thumb, that's a potential flame volume in the range of 0.05 cu.ft = 86 cu ins. Considering that you have to shape the flame and the air flow, it will need a lot of fiddling and trial runs.
The radiant surface you need could be around 12 - 15 sq ins. These values don't seem to go with a 1 cm dia tube.
To get the concentrated energy source you decribe, would you be better with a contained electrical coil?
RE: small burner/emitter
http://www.globar.com/ehe/globar.php.html
http://www.nernst.de/lamp/nernstlamp.htm
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: small burner/emitter
It cant be an electrical input to create the hot spot. Yes there may be some space before the actual burning chamber to get the gas and fuel to mix well. There are atomizer designs that should allow me to get a good burn.
Your value of 1350C jibes well with what I have come up with.
I dont quite get how your rule of thumb works. I dont understand why i need 3000W input to create my 1000W of radiant heat. That would imply, that ov and above any other efficiencies, I can only get about a 30% efficient burner.
RE: small burner/emitter
In order to achieve what you need, you'll need to supply a truckload of air into the fuel stream. Getting the proper mixing is always difficult. The expectation, therefore, is that much of your fuel will be burned inefficiently and not contribute much to desired effect. Consider that efficient diesel engines use fuel injectors and high compression ratios.
Why not use oxy-acetylene? The components are easier to mix and burn hotter.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: small burner/emitter
The lost energy (low efficiency) is due to the fact that after radiating some heat the leaving exhaust gases are still very hot.
RE: small burner/emitter
Ahhh... yes of course... I just didnt get that this would be 65% of the the total loss.
IR stuff,
diesel is a good fuel because it is readily available almost anywhere. kerosene (just another diesel grade) and Gasoline would be good. too. Methanol and alcohols dont burn hot enough.
I will have fans (or blowers) for air flow, so i am not worried about that.
RE: small burner/emitter
You're talking about a 1 cm diameter burn area. That'll require something like a 36:1 compression for air and fuel.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: small burner/emitter
RE: small burner/emitter
Getting back to the original post. What do you mean by radiate? A 100% emissive 1500 ºC blackbody in the shape of a 1-cm diameter only radiates about 187 W/cm of length. If you limit the length to 4 cm, you need more like 2500 ºC There aren't too many materials that can survive that.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: small burner/emitter
your insight there is exactly what I am looking for. I do not have a grasp of how you came to those numbers. Do you have a link or a textbook that I could look at to renew my knowledge of this type of thermodynamics. So, I'm not fixed onthe 1CM, It was a strawman.
I do subscribe to efunda.com and they have some very basic stuff there (which has helped me oh so often for back of the envelope calculations). But I think I need something better to get where you just got to.
RE: small burner/emitter
I think you need to describe in better detail why and what you actually need, and to not prejudice the discussion potential solutions.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: small burner/emitter