Investigation of the Effect of DMMP Addition on the Methane–Air Premixed Flame Thickness

Flame thickness is an important parameter in both laminar and turbulent flame studies. To provide some basic understanding of the effect of the fire inhibitor on the laminar flame thickness, numerical calculations of methane–air premixed flames doped by dimethyl methyl phosphonate (DMMP) were conducted. The results show that the flame speed depends highly on the reactions: \( {\text{HOPO}}_{2} + {\text{H}}_{2} = {\text{PO}}_{2} + {\text{H}}_{2} {\text{O}} \); \( {\text{PO}}_{2} + {\text{H}} + {\text{M}} = {\text{HOPO}} + {\text{M}} \); \( {\text{HOPO}} + {\text{OH}} = {\text{PO}}_{ 2} + {\text{H2O}} \); and \( {\text{HOPO}} + {\text{OH}} = {\text{PO}}_{ 2} + {\text{H}}_{ 2} {\text{O}} \) The laminar flame thickness increases with the increase of the DMMP addition. The preheat sub-zone in the flame front is more vulnerable to the inhibition effect of DMMP. Based on the opposed-flow flame calculations with different outlet velocities, the results indicate that the preheat sub-zone is more dependent on the local stretch rate than the reaction sub-zone. To figure out the reason why the flames become thicker after DMMP addition, the flames’ chemical structures are extracted and discussed. It is found that the chemical reactions in the flame zone are retarded and the upstream gas flow velocity is artificially reduced to make the flame surface stay in a certain area in the calculation. Accordingly, the residence time of the reactant mixture increases, and the CH₂O and OH diffuse and distribute in a wide area. Therefore, the radical-based flame thickness increases with DMMP addition.