Experimental study of the influence of dimethyl methylphosphonate on methane/air coflow diffusion flames using OH-PLIF

Experiments were conducted to evaluate the inhibition efficiency of dimethyl methylphosphonate (DMMP) on methane/air coflow diffusion flames. An optical diagnostic system (hydroxyl planar laser-induced fluorescence) was used to measure the OH radical distribution in the flames. The minimum extinguishing concentration (MEC) of CO2, as a function of DMMP addition, was measured to characterize the inhibition efficiency of DMMP. Results show that, when the concentration of DMMP is higher than 0.6%, DMMP's marginal inhibition efficiency reduces to almost zero. It is found that the normalized peak OH concentrations keep unchanged with a value of 0.9 for flames at conditions near extinction limit. This is firstly observed based on experimental measurements. Furthermore, using DMMP alone cannot extinguish the flame even with 4.59% DMMP addition. When the inhibition efficiency of DMMP gets saturated, a large amount of phosphorus-containing molecules agglomerate around the flame. These particles act as sinks for the active phosphorus-containing species with a high inertial force, which are hard to diffuse horizontally into the flame. Therefore, it is concluded that the generation of the particles is a reason for the saturation phenomenon of DMMP in extinguishing the coflow diffusion flames.