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.