No abstract

No abstract

Direct Numerical Simulations (DNS) have been conducted to study the response of initially laminar spherical premixed methane-air flame kernels to successively higher turbulence intensities at five different equivalence ratios. The numerical experiments include a 16-species/25-step skeletal mechanism for methane oxidation and a multicomponent molecular transport model. Highly turbulent conditions (with integral Reynolds numbers up to 4 513) have been accessed. The effect of turbulence on the physical properties of the flame, in particular its consumption speed S c , which is an interesting measure of the turbulent flame speed S T has been investigated. Local quenching events are increasingly observed for highly turbulent conditions, particularly for lean mixtures. The obtained results qualitatively confirm the expected trend regarding correlations between u ′ /S L and the consumption speed: S c first increases, roughly linearly, with u ′ /S L (low turbulence zone), then levels off (bending zone) before decreasing again (quenching limit) for too intense turbulence. For a fixed value of u ′ /S L , S c /S L varies with the mixture equivalence ratio, showing that additional parameters should probably enter phenomenological expressions relating these two quantities.

Direct Numerical Simulations (DNS) of turbulent premixed flames burning hydrogen, synthetic gas and methane have been performed, relying on detailed chemical and transport models and taking into account volume viscosity. In this manner, it becomes possible to quantify the impact of this last contribution. It is shown that laminar flames are not modified by volume viscosity, while the local structure of turbulent flames may differ considerably when taking it into account. A noticeable impact is even observed on global flame properties. The modifications induced by the volume viscosity transport term are extremely small at first, but are sufficient to lead to completely different realizations at a later time due to the chaotic nature of turbulence. Noticeable modifications induced by volume viscosity are found for syngas as well as for hydrogen flames. For such hydrogen-containing fuels, the differences appear to remain unchanged when increasing the turbulent Reynolds number. On the other hand, turbulent flames burning methane show no significant impact due to volume viscosity. Since an accurate computation of the volume viscosity transport term is possible on available supercomputers, it is thus recommended to take it into account for detailed studies of turbulent flames burning hydrogen-containing fuels, in particular for DNS, while this term can be safely neglected for higher hydrocarbon flames.

No abstract