Study On The Inhibiting Mechanism Of Hydrocarbon Flames By DMMP

The occurrence of fires has serious consequences for people’s property,life and lives.The application of fire extinguishing agents can reduce the incidence of fires and reduce the damage caused by fires to a certain extent.Due to the protection of the ecological environment,the efficient halon fire extinguishing agents will be phased out,and it is extremely urgent to find new alternative fire extinguishing agents.Phosphorus containing compounds are of high efficiency on fire inhibition.In this paper,dimethyl methylphosphonate(DMMP)was used as a representative of halogen-free phosphorus-containing compounds(PCCs).Systematic experiments and simulation studies were carried out to study the fire extinguishing performance and mechanism of DMMP on methane flames.The fire-extinguishing experiments were first carried out based on a small-scale co-flow diffusion flame.Since using DMMP alone cannot extinguish the flame when the partial pressure of DMMP is lower than its saturated vapor pressure,the minimum CO2 volume concentration required to extinguish the flame on base of specific DMMP volume concentration,that is,the minimum extinguishing concentration(MEC),is used to characterize the inhibition efficiency of DMMP.The flame stability was observed with DMMP and CO2 addition to the air.It was found that when the concentration of DMMP was less than 0.2%,MECCO2 decreased rapidly with the increase of DMMP concentration.However,when the concentration of DMMP was more than 0.6%,further addition of DMMP brought about negligible decrease of MECCO2.It is defined that when the addition of DMMP is more than 0.6%,the fire extinguishing efficiency of DMMP is saturated.There are two mechanisms to explain the saturation of DMMP extinguishing efficiency:the effective phosphorus species in the flame are concentrated into fine particles and the peak concentration of active radicals(H,O and OH,etc.)in the "reaction kernel" is reduced to chemical equilibrium level.In order to study the intrinsic mechanism of the saturation of DMMP’s flame inhibition efficiency,OH-PLIF measurement technique was used to measure the OH concentration distribution in flames.It was found that when the amount of DMMP was fixed,as the concentration of CO2 increased,the intensity and scale of the flame"reaction kernel"gradually decreased.When DMMP was added alone,with the increase of DMMP concentration,the intensity of the "reaction kernel" decreased rapidly,and then decreased slowly.Especially when the inhibition efficiency was saturated,the intensity of the "reaction kernel" remained basically unchanged.According to the distributions of OH concentration in the flames near the extinction limit,when the concentration of DMMP was more than 0.09%(the inhibition efficiency was not saturated),the intensity of the "reaction kernel" remained basically unchanged.Thanks to the reflection/fluorescence of the phosphorous particles on the pump laser,the OH-PLIF system can capture the formation of phosphorus-containing particles in the flame.The results showed that when the inhibition efficiency of DMMP was saturated,a large amount of phosphorus-containing particles were formed around the flames,and the amount of particles increased significantly with the increasing of DMMP addition.The formation of particles results in the concentration of the effective PCCs participating in the reactions in the flame to be saturated as the addition of DMMP keeps increasing,thereby causing the inhibition efficiency to be saturated.In order to investigate the flame inhibition efficiency of DMMP without considering the formation of phosphorus-containing particles,this paper experimentally measured the laminar burning velocities of DMMP inhibited methane/air premixed flames based on Bunsen method.The decrease of the laminar burning velocity is used as an indicator of inhibition efficiency.It was found that as the addition of DMMP increased,the flame inhibition efficiency also had a saturation effect.Four sets of phosphorus-containing flame suppression mechanisms were established to predict the experimental results,and then detailed numerical simulation studies were carried out based on the best matching mechanism.The flame temperature,flame thickness and the sensitivity of flame propagation speed to the reaction rate of phosphorus-containing elements were analyzed with the addition of DMMP.It was found that the elemental reaction PO2+H+M=HOPO+M(R812)had the greatest influence on the flame propagation speed.The heat release and H consumption of R812 and the whole phosphorus-containing reactions were statistically analyzed,respectively.It was found that with the increase of DMMP addition,the contributions to the heat release of both objects increased gradually,and the contributions to H consumption of both objects decreased gradually.On the one hand,heat release promotes combustion,on the one hand,H consumption suppresses combustion;and the combined action of both effects leads to saturation effect of DMMP flame suppression efficiency.Finally,numerical studies on the thermal and chemical effects decoupling for the elementary reaction were carried out.Different from the previous physical and chemical effects decoupling by changing the reaction mechanism,this paper realized the real-time thermal effect and chemical effect decoupling by modifying the energy equations and the composition equations in the open source program Chemkin-II.It is found that the chemical effects of R812 and DMMP on laminar burning velocity are greater than their thermal effects.