A Numerical Study of the Transition of Jet Diffusion Flames
Abstract
A numerical study on the transition from laminar to turbulent of twodimensional fuel jet flames developed in a coflowing air stream was made by adopting the flame surface model of infinite chemical reaction rate and unit Lewis number. The time dependent compressible NavierStokes equation was solved numerically with the equation for coupling function by using a finite difference method. The temperaturedependence of viscosity and diffusion coefficient were taken into account so as to study effects of increases of these coefficients on the transition. The numerical calculation was done for the case when methane is injected into a coflowing air stream with variable injection Reynolds number up to 2500. When the Reynolds number was smaller than 1000 the flame, as well as the flow, remained laminar in the calculated domain. As the Reynolds number was increased above this value, a transition point appeared along the flame, downstream of which the flame and flow began to fluctuate. Two kinds of fluctuations were observed, a small scale fluctuation near the jet axis and a large scale fluctuation outside the flame surface, both of the same origin, due to the KelvinHelmholtz instability. The radial distributions of density and transport coefficients were found to play dominant roles in this instability, and hence in the transition mechanism. The decreased density in the flame accelerated the instability, while the increase in viscosity had a stabilizing effect. However, the most important effect was the increase in diffusion coefficient. The increase shifted the flame surface, where the large density decrease occurs, outside the shear layer of the jet and produced a thick viscous layer surrounding the jet which effectively suppressed the instability.
 Publication:

Proceedings of the Royal Society of London Series A
 Pub Date:
 November 1990
 DOI:
 10.1098/rspa.1990.0132
 Bibcode:
 1990RSPSA.431..301Y