Flame Temperature 1

[49078]

This might seem like a simple question, but what is the difference between flame temperature, and adiabatic flame temperature?

cheers

 

[quark]

Yes, as simple as doing a google search. It is the difference between practical and theory. Adiabatic flame temperature is the flame temperature when there is no heat loss from the flame to surroundings or any other fluid and this can be calculated.

Generally flame temperature in fired furnaces is adiabatic near the burner tip.

 

[49078]

Thanks for you quick response quark, appreciate it.

Cheers

 

[25362]

First, and for the subject in hand, let's define combustion as the air-burning (oxidation) of a gas, liquid, or solid, evolving heat and often light.

Secondly, let's define what is a flame. In technical parlance, a flame is defined as the totality of the gaseous mass in a combustion reaction, disregarding its degree of luminosity.

Adiabatic flame temperature (AFT), as its name implies, is the theoretical highest (perfect and complete) combustion temperature that can be achieved in a space surrounded by heat-impermeable walls, ie adiabatic combustion, carried out with the minimum (stoichiometrical) amount of air. It is a "characteristic" of each and every fuel.

If air were supplied in excess, the adiabatic temperature would be reduced by the heat being absorbed by the xs air.

Some published values, in ^oC:

CO, 2,452
H₂, 2,210
Methane, 2,065
Ethane, 2,105
Propane, 2,115
Butane, 2,132
Ethylene, 2,343
Propylene, 2,255
Butylene, 2,221
Acetylene, 2,632

In actual practice, at temperatures [≥]1500^oC there are already appreciable reactions of endothermic dissociation of H₂O and CO₂ that bridle any further temperature rise. The decomposition products actually burn later on in lower-temperature zones.

Beside those effects, in industrial fired heaters, the combustion is never adiabatic, since a part of the liberated heat is transmitted in status nascendi by radiation to the cooler heat-absorbing sections of the heating chamber, and a small part is even lost by conduction through the chamber walls.

Besides, air is not always available to complete the combustion process at one go, and is frequently supplied in primary and secondary doses, a fact that adds to the reduction of the maximum attainable temperature.

Therefore flame temperatures, or combustion temperatures, would be the actual thermometric expression of the complex thermal balance in a particular combustion chamber.

Because of the varying spacial effects taking place in a real combustion chamber there is no exactly-defined flame temperature.

A more commonly-accepted definition would be the final key-temperature at which the combustion products actually leave the combustion space.

This extended post is incomplete, but is meant to give an idea on the subject in hand.

 

[hacksaw]

following on with 25362, it also generally assumes that combustion takes place with well defined chemical components driven to completion rather than through the various intermediate radical species.

 

[sailoday28]

25362 (Chemical)Excellent discussion.
"Some published values, in oC:" I assume that these values are based on standard conditions and that temperatures will vary dependent upon initial conditions of pressure and temperature.

Could we expand the discussion to include detonation and deflagration as related to adiabatic flame temperatures?

 

[49078]

25362

Thanx for the response, it was very informative. I have one question though, when you say

"The decomposition products actually burn later on in lower-temperature zones."

Are these zones just physical regions within a combustion chamber? or possibly after a time where a certain percentage of the fuel has burnt off? what exactly defines these zones?

cheers

 

[25362]

These are physically-djacent regions within the combustion gas mass in which these chemical groups find the conditions for complete combustion. No geometrical definiton.

The speed of their reactions is governed by molecular proximity, which in itself is enhanced by turbulence.

All the manifestations of combustion are somewhat related to the chemical composition of the fuel.

The chemical reactions involved are numerous and include the formation and disappearance of radicals and chemical groups.

Nevertheless it is interesting to note that sometimes completion of combustion is attained in fractions of a second.