Hydrocarbon Flame Inhibition By Halon Replacements

The study of fire suppressants that have high efficiency and low environmental impact is a fundamental and important topic of the fire research.Production and application of Halon 1301 have been banned by the Montreal Protocol because of its ozone-depleting potential.Although the Halon 1301 is legally allowed to be used in the aircraft cargo bay fire protection,the research of halon replacements continues to be imperative.In the present study,experimental and numerical study of the influence of a certain number of halon replacements,Novec-1230(C₆F₁₂O),HFC-125（C₂HF₅）,2-BTP（C₃H₂F₃Br）,HCFC-123（C₂HF₃Cl₂）and fine water mist（H₂O）on the laminar hydrocarbon flames were performed.Laminar flame speed and extinction stretch rate are both global flame phenomena,as it provides the basic information regarding the chemical reaction rate,thermal diffusivity and heat release of the flame under certain conditions of temperature,pressure and equivalence ratios.Experimental data of the laminar flame speed and extinction limites are considered to be a useful tool for characterizing the effectiveness of fire suppressants,and the accurate knowledge of the two parameters has served to validate and optimize chemical kinetic mechanisms.In order to determine the laminar flame speeds and the extinction stretch rate accurately,in this work,a twin-flame opposed jet counterflow flame configuration coupled with particle imaging velocimetry（PIV）technique were developed,and it was used for the investigation of the flame propagation,flame appearance,velocity vector field and extinction limits of the inhibited hrdrocarbon flames under a wide range of equivalence ratios and agent loadings.In the laminar premixed flames,the PIV was used to determine the 2D flow field of the counterflow flames,and the velocity profile along the stagnation streamline and the characteristics of velocity vector field was studied.Two parameters Su,ref and K were obtained to determine the unstretched laminar flame speeds by the computationally-assistant nonlinear extrapolation method.For the nonpremixed laminar flames,the extinction stretch rate was determined by approximation method.In addition,we compared and analyzed the inhibition performance of the replacement agents in premixed and nonpremixed flames.In the numerical simulations,a comprehensive kinetic reaction mechanism was assembled to model hydrocarbon/air flames with four added replacement agents.The newly-developed reaction mechanism was validated by the present experimental results.To gain an insight into the difference of the inhibition effectiveness of the replacement agents in lean and rich flames,thermodynamic equilibrium calculations were performed to study the peak and equilibrium chain-carrier radical concentrations which were assumed to be correlated with the flame speeds.A sensitivity analysis and reaction pathways for hydrocarbon flames with added replacements were conducted to understand the inhibition mechanism.The flame propagation of the syngas/air premixed flames with vater vapor addition was studied.The non-monotonic behavior of the flame speed as the H₂O concentration increases was captured.In the study of the chemical and thermal effects of the H₂O addition,it was found that the chemical effect of the H₂O increased the flame speed of the low H₂/CO syngas flames.The flame inhibition effectiveness of the replacement agents was strongly dependent on the equivalence ratios and agent loadings.For initially lean flames,enhanced combustion in the presence of the sub-inerting agents of the Novec 1230,HFC-125 and 2-BTP were observed.The critical equivalence ratio for the inhibition and enhancement of Novec 1230 addition was in 0.63<Φ<0.68.For methane or propane flames,HFC-125 can increase the laminar flame speeds by 20%at 0=0.6 and Xa=0.02.For 2-BTP,the laminar flame speed shows the saddle-shaped behavior as agent loading increases.It first decreased for low agent loadings,and then increased as additional agent was added,and again decreased as agent loadings further increased.At 0=1.0,the effectiveness of the replacement agents decrease in the order of 2-BTP>HCFC-123>HFC-125.In addition,the extinction data for non-premixed flames revealed that the resistance of the agents to extinction decreases in the order of C₂HF₅,C2HF₃C₁₂,and C₃H₂F₃Br,which is consistent with the suppression effect on flame speed in premixed flame.The inhibition mechanism was analyzed in the numerical study.For the Novec 1230 and HFC-125 inhibiting flames,the radical concentrations are reduced by the radical trapping mechanism,while for 2-BTP and HCFC-123,the catalytic radical recombination cycle drive the radical concentrations to low levels.The increase of the adiabatic flame temperature demonstrates the fuel-like properties of the replacement agents,which can be lead to the increase of the laminar flame speed.The innovation and contribution of this work are summarized as follows:A advanced research platform is developed for the evaluation of the inhition effectiveness of the halon replacements.A new counterflow flame configuration with PIV technique was developed for the measurements of the laminar flame speeds and the extinction limits of hydrocarbon flames with added replacement agents,and new experimental data were obtained for the validation of the reaction model.