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Calculating Flame Velocity
4

Calculating Flame Velocity

Calculating Flame Velocity

(OP)
Can anyone give me guidance on calculating flame velocity.  We are using High velocity dual fuel burners(Natural Gas and Light Fuel Oil), pre heated air (450C), I can work out the volume of the burner outlet tile and I have got readings for gas pressure and volume or Oil pressure and volume dependant on fuel being used, combustion air pressure and volume.
So I think I've got enough numbers its just finding a formula.

 

RE: Calculating Flame Velocity

3
There may not be a simple formula of the type you're looking for unless you ask at the flame research level.

The flame velocity is just a way of describing the stable location of the flame reaction.  That is a heat transfer problem.  You essentially have to warm up the fuel to an unstable temperature and have enough air intimately mixed, locally, to permit a small exothermal reaction of a sufficient size that it cannot be quenched by unmixed reactants.  In your case you also have to evaporate some liquid droplets.
The heat transfer into the fuel usually occurs, primarily, by radiation from the reaction zone but there will also be radiation from the hot burner tile and that often swamps other heat transfer and allows the flame to stabilize on the block itself.

You are already starting with air at 450 degC which is getting pretty close to the autoignition temperature of the fuels you are using so you might even anticiptate an autoignition almost as soon as the air and fuel begin to mix.

Taking a really simplistic approach, if you consider that, as a stoichiometric mixture, with cold air and fuel, the flame speed of methane is about 1 fps (cold mixture), you can deduce that the heat transfer back into the cold mixture (from a normal flame) is effective at a rate of, roughly 21 Btu/sec/sq.ft and gets the mixture from 25 degC to 632 degC. From which one might conclude that the flame speed going from 450 degC could be 3.33 x faster (3.33 fps based on a cold mixture) or about 1.375 fps based on hot mixture.

The stabilizing effect of the burner tile is probably going to overpower this calculation if it is glowing (more than "red") hot.  Say the tile is at 1500 degC the radiation into a gas stream at 450 degC will be around 45 Btu/sec/sq.ft.

I'd be a little cautious of the light fuel oil, which I'm guessing, has an autoignition temperature around 260 degC.  The hot air will cause the droplets to vaporize and they will probably inflame immediately so you need to be sure that the atomisation is good otherwise you could get "cracking" within the droplet leading to soot formation, which may or may not burn out in the main flame.

If anyone out there does have a formula, please post it because I'd like it too.

Most burner people I know, faced with this complexity of problem, solve it empirically.
Good luck.

David

RE: Calculating Flame Velocity

Let me start by saying that I don't know of any formula to determine flame velocities and that there is little that can be added to what Flareman (David) has said.

When speaking of liquid H/C fuels, I think the duration of the combustion process -and speed and length of a flame- will depend almost exclusively on the physical processes of diffusion, convection and mixing of air (oxygen) and fuel, as well as on the different factors in heat transmisssion, and not on the chemical reaction per se, which would be almost instantaneous. Excess air is also added to balance out the irregularities in oxygen "consumed" in the entire flame mass.

In specially designed installations ignition is done in around 0.1 seconds, while the entire process of combustion takes place in less than 0.5 seconds. Heat releases per unit volume of the combustion chamber vary a lot: from oil refining process heaters at about 100,000 kcal/m3h thru steam boilers, locomotives, marine boilers, etc., and up to naphtha-burning I/C turbines with values higher than 100,000,000 kcal/m3h.

Atomization and mixing effects are enhanced when the "gaseous ambient" density increases. There are, of course, heat release (kcal/h) limits to the efficiency of atomization in one single burner since the kinetic energy of the fuel would not be enough to bring about an efficient mixing in a given time, and the flame would become longer and fuming; this is the case for splitting the heat duties among various burners.

I also believe that to get a perfect and complete combustion of any gas/liquid  H/C mixture all factors of the particular installation would have to be considered: atomizer, chamber volume, muffle, etc. to suit the characteristics of the "slower" burning fuel.

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