Three-dimensional simulations of isobaric premixed flames freely propagating in narrow circular channels: breaking of symmetry

Abstract

In this work we perform three-dimensional simulations to study the breaking of sym-

metry of lean premixed isobaric flames propagating in narrow channels of circular

cross-section subject to an inlet Poiseuille flow. In particular we consider the effects

of the flow rate, the channel diameter, wall heat-losses for flames with the Lewis

number (Le) equal or less than unity.

Numerical analysis shows that for Le = 1 and under adiabatic conditions, a

supercritical bifurcation occurs at flow rates exceeding the critical value and the

axisymmetric flame becomes unstable, giving rise to a nonaxisymmetric flame. For

flames with Le < 1, a subcritical bifurcation occurs and we report multiplicity of

stable solutions: the flame can be axisymmetric and non-axisymmetric for the same

values of parameters. It is important to underline that both types of solutions are

stable, and thus can be observed experimentally. In the presence of wall heat-losses,

the critical flow rate corresponding to the bifurcation point increases. This indicates

a promoting effect of wall heat-losses on flame shape symmetry. We also find that

an extinction gap can appear when increasing the flow rate. For the smaller Lewis

number (Le = 0.5) we observe a complex flame dynamics, showing the coexistence

of steady and oscillatory combustion regimes.