1. Effect of aerodynamic confinement – external recirculation

When an [GLOSS]axial turbulent diffusion flame[/GLOSS] is confined within a combustion chamber or furnace, the length of the flame will increase if there is recirculation of combustion products around the flame jet. The dimensionless [GLOSS]Craya-Curtet number[/GLOSS] may be used to establish the existence of an external recirculation zone (Combustion File 16) and to estimate the increase in flame length that it causes (Combustion File 15).

The relationship is:

(L_fc/ L₀) = 0,48 + 0,32(m)^0.5     (1)
where:
– L_fc is the confined flame length (m) operating at 5% excess air
– L₀ is the stoichiometric mixing length of an unconfined flame from the same burner (m) (Combustion File 4)
– m is the dimensionless [GLOSS]Craya-Curtet number[/GLOSS] (Combustion File 16)

2. Additional effect of thermal confinement – combustion intensity

Experimental studies of natural gas axial diffusion flames undertaken at industrial scale by the GEFGN have shown that the effects of external recirculation on confined flame length or not fully independent of flow rate, ie there is also an influence of thermal confinement. Where applicable, this may be expressed by the direct incorporation of the fuel flow rate into equation (1) to give:

(L_fc /L₀) = 0,0256Q^0,373m^0.5 + 1      (2)where Q is the natural gas input (Nm³/h).

This equation has been verified on the GEFGN furnace for the following range of operating parameters:

100 Q 300 N m³/h
7 m I_t 30
3,5 I_t 10 N/MW

with I_t the specific thrust on a thermal basis (Combustion File 18)

3. Graphical method

The combined effects of aerodynamic and thermal confinement are shown graphically in Fig 1.

  Fig 1: Influence of confinement on flame length
Acknowledgement
This Combustion File has been extracted from the “Recueil Des Fiches Techniques” published in the Revue Generale de Thermique.
We are unaware of the original author’s name. However we would like to thank the author and Gaz de France for the use of the information.