small burner/emitter

[Shandor]

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).

 

[Flareman]

Sorry to be late on your question and I might not be too useful anyway.

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?

David

 

[IRstuff]

Probably something like a Globar or Nernst glower is more appropriate:

http://www.globar.com/ehe/globar.php.html

http://www.nernst.de/lamp/nernstlamp.htm

TTFN

FAQ731-376

 

[Shandor]

Thanks.

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.

 

[IRstuff]

Seems to me that you've picked an awfully difficult fuel to work with.

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

 

[25362]

The lost energy (low efficiency) is due to the fact that after radiating some heat the leaving exhaust gases are still very hot.

 

[Shandor]

25362,

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.

 

[IRstuff]

A single burner on my stove dumps 15000 BTU. That requires VERY large burner to get efficient mixing of the NG with air. You're going to get an aerosolized fuel that wants to sink and condense. The more concentrated the fuel, the higher the required air flow into the concentrated volume.

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

 

[btrueblood]

Unleaded gasoline is also available everywhere, as you noted. It burns well in mantle lanterns (e.g. Coleman dual-fuel lantern), and the output is certainly white due to the rare earth oxides (I think its mainly thorium and cerium that does it - see wikipedia "gas mantle") in the mantle ash. A single lantern only puts out perhaps a hundred watts of visible light at most, though, and an awful lot of heat, so a 30% efficiency is probably a high limit, and 10% would be my WAG.

 

[IRstuff]

You have to be careful about that sort of generalization. A glowing white blackbody puts out a truckload of heat in radiated energy. That's why the sun feels hot on your skin.

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

 

[Shandor]

IR stuff,

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.

 

[IRstuff]

That's the result from the basic blackbody equation. There are a variety of blackbody calculators floating around on the web.

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

 

[btrueblood]

Generalization? This is just experience with gas mantle lanterns, IRstuff. The light output is about equal to a 60W-100W tungsten reading lamp, but I can boil a small pan of water on the (opaque, metal) rainshield cover. And, cerium oxide (and lanthanum oxide too) is a long way from being a black body, its true emissivity in the IR is comparatively low, whilst its emissivity in the visible region is comparatively high, thus if heated to flame temperatures (e.g. "red hot") it glows with a white light. There is a big hint for the OP there.