The structure of a premixed turbulent flame

Abstract

An experimental study of the structure of a premixed turbulent flame propagating in a duct-confined, stoichiometric propane-air mixture has been carried out. Care was taken to reduce any effects of axial pressure gradients. By suitable changes in grid geometry, the turbulence intensity and scale of the approach flow were varied independently over a wide range. The results of these experiments show a strong link between the mechanisms of turbulent flame propagation and the flame-generated turbulence. Thus, three distinct regions, each having different structural characteristics in regards to the effects of turbulence scale on flame-generated turbulence, may he identified. The physical processes for each region, namely a ‘wrinkled-pulsating’ behaviour of region 1, the ‘jet-like’ characteristics of region 2 and the ‘eddy entrainment - combustion in depth’ mechanism of region 3 are described. A comparison between the mean and the turbulence properties of a flame and of a coaxial jet of lighter fluid (hydrogen) spreading into a turbulent, co-flowing air stream has been made. Based upon this, the contribution of shear-generated turbulence to total flame turbulence is calculated. A definition of flame-generated turbulence is then proposed. It is shown that in some cases the flame damps the turbulence whereas in most other instances it generates additional turbulence. The total magnitude of the relative flame-generated turbulence intensity does not exceed about 12 %. Finally, it is shown that the flame damps the unburnt stream fluctuating vorticity in region 1, has no effect in region 2 and augments the vorticity in region 3.