A lean mixture of propane/air flows over a backward facing step and ignited. The mixture reacts with the flame front being stabilized by the
presence of the backward facing step. The geometry reproduces that of an experimental setup of Pitz and Daly.
Physics and modelling
The fluid flow and turbulence effects are represented by large-eddy simulation since a comprehensive set of Reynolds-averaged simulations
has failed to reproduce the flame instability demonstrated in experiments. Combustion is modelled using the LES formulation of the Weller 3-equation model,
solving the transport equations for the regress variable, flame wrinkling and laminar flame speed.
Animations
The four images presented below show the iso-surface of the regress variable , effectively representing the flame front. The iso-surface is coloured
by the local laminar flame speed, which varies according to the local strain rate. The solution reproduces the flame instability that occurs in experiment caused
by the combination of Kelvin-Helmholtz and Rayleigh-Taylor instabilities. An important factor in the unstable behaviour is the flame response of the local strain
rate, which is high enough to locally extinguish the flame. Close to the step, the flame is two-dimensional and the high strain rate considerably reduces the
laminar flame speed. Further downstream, the strain rate is lower and three-dimensional effects become more visible.
, effectively representing the flame front. The iso-surface is coloured
by the local laminar flame speed, which varies according to the local strain rate. The solution reproduces the flame instability that occurs in experiment caused
by the combination of Kelvin-Helmholtz and Rayleigh-Taylor instabilities. An important factor in the unstable behaviour is the flame response of the local strain
rate, which is high enough to locally extinguish the flame. Close to the step, the flame is two-dimensional and the high strain rate considerably reduces the
laminar flame speed. Further downstream, the strain rate is lower and three-dimensional effects become more visible.
