Ask A Scientist© Chemistry Archive

name        Akemi R.
status      student
age         20s

Question -  Which results in a higher temperature, a luminous or a 
nonluminous flame? Why?

-------------------------
Akemi,

Assuming you might be referring to the flame of a common laboratory Bunsen
burner, the non-luminous flame is the hotter of the two. The luminous flame
contains particles of incandescent carbon that are the result of incomplete
combustion of the fuel gas. If the fuel is incompletely burned, the maximum
heat potential of the gas is not fully achieved.

When a weldor first lights his acetylene torch, the flame is brilliantly
luminous until he/she adjusts the flow of oxygen for efficient combustion of
the fuel gas. One could probably quickly pass one's hand through the
luminous acetylene flame without getting burned. It would be very foolish to
pass one's hand through the properly adjusted flame. No matter how fast you
were to move through the hot zone, you would very likely get a nasty burn.

Regards,
ProfHoff
=========================================================
The luminosity of a flame is not necessarily a measure of the flame
temperature. A flame is luminous because some chemical substance in the
flame is heated to a temperature that is sufficient to excite some electron
in that substance to a higher energy state. When the electron de-excites and
the energy difference between the excited electronic state and the ground
state is in the visible region of the electromagnetic spectrum, the flame is
luminous.
A good example of this is to put a small amount of table salt on a piece of
wire [hold it with a pliers so you don't burn your fingers] you will see the
brilliant yellow color characteristic of sodium.

In contrast, the flame that occurs from the combustion of hydrogen and
oxygen is very very hot but it is virtually colorless because there are no
substances [predominantly H2, O2, and H2O] that have electronic states that
emit radiation in the visible part of the electromagnetic spectrum.

Vince Calder
=========================================================
Assuming that the flames release energy at the same rate the non-luminouse
flame will result in a hotter flame temperature.

Because in a luminous flame some of the energy is put into the light and is
therefore not available to heat the gases in the flame.

Greg Bradburn
=========================================================