A direct numerical simulation study of turbulence intensity effects on Nox formation in freely propagating premixed flames

摘要

Direct numerical simulation (DNS) is utilized to study the effectof turbulence intensity on NO formation in two-dimensional (2D)freely propagating CH4/air turbulent premixed flames indecaying isotropic turbulence. Chemical reactions are describedby a reduced chemical kinetics mechanism for methanecombustion based on GRI-Mech3.0, involving 28 species and268 elementary reactions. Six DNS cases of different turbulentvelocities were performed, with a constant turbulent length scale.NO formation pathways for the low and high turbulence intensitycases are compared. In the low intensity case, all the pathwaysfollow the trends of freely propagating laminar flame results. Inthe high intensity case, however, the pathway distributions arewider. Moreover, in highly turbulent flames, the net contributionof the prompt pathway, the main contributor for NO productionin laminar flames and low turbulence intensity case, becomesnegative, consuming NO. The reactions involved in the promptpathway are analysed and compared with their laminarcounterparts. It is concluded that in the high intensity case smallturbulent eddies are able to transport some radicals to the highertemperature regions. Specifically, hydrocarbon radicals such asCH, CH2 and CH3, are transported to these regions where NOreacts with them.