Types of Flame

Laminar and Turbulent Flames 

 Just like regular fluid flows, flame propagation can be grouped into laminar and turbulent categories.  The character of a flame can even be quantified using Reynolds numbers.  The exact science of assigning a flame a Reynolds number is not really within the scope of this report, so the discussion here will center on qualitatively defining the character of the flames.  Laminar flames are steady and tend not to move much. Candles tend to have laminar flames. A Bunsen burner flame is another common laminar flame.

Unpredictability is one of the main characteristics of turbulent fluid flow. This trait carries over to turbulent flame propagation as well. Turbulent flames are unsteady and random. A common example of a turbulent flame is a campfire.

 

    
Laminar Turbulent

    

 

Premixed

When the oxidizer and fuel are already mixed before combustion, the resultant flame is premixed. These flames are basically constant pressure and move at a velocity a little less than a few meters per second. The speed a premixed flame propagates at is dependent on the curvature of the flame and the local flow condition. (http://epswww.epfl.ch/aps/meet/DFD99/baps/abs/S470.html) 

Whenever an combustion needs to occur within a small confined area, premixed flames are the flame used. For this reason, Gas fired furnaces & stoves, as well spark ignition car engines utilize premixed flames are used. Premixed flames can be both laminar and turbulent

 


Premixed

 

Diffusion Flames

Diffusion flames occur when the fuel and air streams enter the area where combustion occurs separately and mix during combustion. The outer boundary of a diffusion flame is defined by the interface between the fuel and oxidizer.  This boundary area is called the reaction zone. The temperature of the flame is at its maximum within the reaction zone and the flame’s color is a light shade of blue. The inner area of the diffusion flame is the place where there is fuel but no oxidizer. No actual production of heat occurs in this inner area, but this area is warmed up as heat from the reaction zone transfers into it. This region is a yellowish color. This color is a result of the blackbody radiation emitted by soot particles. Soot is produced when the heat from the reaction zone transfers to the inner area of the flame, and heats up the fuel.  (http://www.tfd.chalmers.se/~svan/fmt_gk01/lectures/lecture_9.htm ) The combustion in diesel automobile engines happens because of diffusion flames. Like premixed flames, diffusion flames can be laminar and turbulent. 


Diffusion