Hottest Part Of Oxy Fuel Flame (LP Gases)

[Ahmdo Molah]

Searching online for 'hottest part of a flame', it is easy to encounter info about oxyacetylene flames and that the hottest part is around the tip of the inner cone.

But for alternate fuels(LP gases) like propane, butane, etc., simple graphic info is very limited (so it seems). I did find some (below) indicating that for propane, the hottest part is farther away from the torch tip and the inner cone - somewhere around 2/2.5 times the length of the inner cone.

[ Wilhelmsen.com ]
https://www.wilhelmsen.com/ships-ser...ne-vs-propane/

"Propane releases only a small proportion of heat in the inner flame cone (less than 10%), so most of the heat in the flame is located in the outer cone.
Acetylene releases almost 40% of its heat in the inner flame cone."

Hottest Part Of Oxy-Fuel Flame (Wilhelmsen).jpg


[ Miller/Smith ]
* From the Smith Little Torch Manual (p.21) 1-3.Flame And Heat Placement BTU Output Per Cubic Ft Of Fuel Burned With Oxygen

The left column of the chart below indicates the approximate BTU at the tip of the inner cone and the longer tail of the flame.

Acetylene: 500 vs 970 ( 500 / 1470 = 34% at tip of cone)

Propane: 250 vs 2250 ( 250 / 2500 = 10% at tip of cone)

Flame & Heat Placement BTU Output... (Smith).jpg


[ DelphiGlass.com ]
https://www.delphiglass.com/blog/how...f-and-my-torch
from "Me, Myself, and My Torch" by Cere Ceddon

This is from a glass artisan's perspective.

Flame Zones (C.Seddon).jpg

"A - hottest part of the flame - used most of the time
B - used for metal fuming - gold or silver is vaporized at the tip of the inner cones
C - inner cones - composed of unburned gasses - cool and useless for heating
D - lower edge - used for precise, controlled heating - used for adding detail or stretching thin controlled tapers from rod
E - diffused heat area - used for pre-heating and for some tube work
F - outer reaches - used for preheating and for final heating of tubing just before blowing or stretching"


If anyone has more information on this matter, especially on flame temperature measurement methods used for determination, I'd be grateful.

[Mark Kelly]

If anyone has more information on this matter, especially on flame temperature measurement methods used for determination, I'd be grateful.

My first job out of Uni was in a research group and involved runnng an atomic absorbtion spectrophotometer which used an oxy acetylene atomiser*. That was a long time ago but from memory the flame temperature is determined by using a dual line absorbtion technique where the flame temperature is proportional to the ratio of the absorbance of the two spectral lines (I think it's a log relationship as it derives from the Boltzmann equation).

The reason for the flame temperature distribution is fairly simple: it depends on the relationship between gas flow rate and reaction rate. With acetylene, the reaction rate dominates so temperature reduces as you move out in the flame. With other gasses the reaction rate is slower so there is more combustion further out.


*if you think industrial grade acetylene is pricy, try high purity instrument grade.

[Ahmdo Molah]

Atomic Absorption Spectroscopy(AAS) is definitely interesting, but a bit more than I can chew at the moment . OTOH, I do get that LP fuels and their slower reaction rates result in the hottest zone being further out than the tip of an acetylene flame's inner cone.

What I'm wondering is how the sources I've mentioned above or their (unmentioned) 'sources' came to the conclusion that... "oh, the hottest point is... there!"

There was a time (not too long ago) when I thought a thermocouple, or an IR(infrared) camera would do the trick. I found out that thermocouples, pyrometers, and IR cameras can all be pretty inaccurate and/or inconsistent due to their inherent limitations.

[Mark Kelly]

The technique I mentioned allows measurement of the flame temperature in different zones.

See fig 3 in this paper for example (the two elements technique being discussed wasn't around when I was using an AA).

FWIW IR cameras are useful when the object is emitting in the infra red, below around 700 oC. Thermocouples can be used to directly measure flame temperature but you need special high temperature miniature ones and they disturb the flame.

[Ahmdo Molah]

I SEE!

After reading the entire research paper (like a cow...) I looked up "atomic absorption spectroscopy principle" and fortunately the very first site I encountered demystified the process bit by bit (without the fancy equations) in plain talk.
https://www.technologynetworks.com/a...cations-356829

So... if I'm not too off track... light from a light source with a filament of a specific material(s) (and therefore clearly defined spectral lines) is passed through a flame of a specified fuel (in an 'atomized state'), then passed through a filter (that blocks out unnecessary spectral bands), and a sensor(CCD, charge coupled device) detects the resulting spectral line intensity (after the partial absorption of source light by the flame).

It seems that in this method, the highest temperature location would be determined by the location where the absorption of source light is greatest (where the flame's reaction intensity and therefore the amount of relevant atoms are most concentrated).

Thank you!

[Mark Kelly]

Not quite.

The flame measurement is a bit subtler than just measuring absorbance. What you are looking for is the Boltzmann distribution, by measuring at two different wavelengths you can find the slope of that distribution which is proportional to temperature.

You cannot use a filament as source, if you did you would get a continuous spectrum rather than spectral lines (radiation from a solid is governed the Stefan Boltzmann law). To get spectral lines the material must be in gas form (eg each atom has six degrees of freedom), this is achieved using a hollow cathode tube which sputters atoms off the cathode material using inert gas ions accelerated from the anode.

Yes Boltzmann's distribution and Stefan Boltzmann Law are from the same guy: he invented stastitical mechanics which is why one of the seven foundational quantities of the universe is named after him (by Max Planck).