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Michael Porter on the importance of the Primary flame and Parallel burner nozzle

Primary Flame
A neutral flame isn't the only goal to be looked for in a burner. After establishing that your flame is neither oxidizing or reducing, you want to try for a single wave front, which indicates total combustion in the primary flame. Why would you want this? Because it's the opposite of a forest fire; instead of being spread out, the heat is produced in the most compact flame you can make. There are many advantages to such flame control. In the first place, the smaller the area of a flame, the higher the flame temperature that can be achieved from the production of all those expensive BTUs. Also, complete primary flame combustion allows sealing of the burner entrance, since no secondary air is needed for complete fuel combustion; this will do as much to increase internal forge temperatures as do the higher flame temperatures. Together, these two advantages amount to a huge boost in heating ability.

Parallel (step) burner nozzle vs. tapered nozzle
There are two burner nozzle designs that are popular on forge and furnace burners at present: The first design is an internally tapered nozzle, which  increases nozzle area as exiting gas moves forward, thus  providing a low pressure point that helps "glue" the flame to the nozzle exit; the same low pressure area also helps prevent "burn-back" into the burner's mixing tube, because the tube has a higher pressure than the nozzle area.

 Pressure reduction in tapered nozzles placed over a pipe or tube is also gained from area increase due to the nozzles larger inside diameter at its narrow end; the thickness of the mixing tube wall itself creates a small increase in nozzle area. 

I decided to experiment with a thicker shoulder (step) by inserting a spacer ring between the mixing tube and nozzle, allowing the same total increase in nozzle area without need for adding shape (taper) to the nozzle. I found that the greater benefit comes from combustion taking place deeper within the nozzle, which then super-heats, creating a larger ignition area than is found with a tapered nozzle. More ignition area produces better combustion; especially as flame speed increases in response to greater fuel/air input.