Suppression Of Hydrogen-air Explosions By Hydrofluorocarbons

With the accelerating depletion of globalized fossil energy reserves and the increasing aggravation of environmental pollution problems,hydrogen energy as a new type of secondary energy has received widespread attention from countries around the world.However,since hydrogen having a small ignition energy,a wide range of explosion limit,and high calorific value,and therefore it can easily lead to accidents during production,storage and transportation.In order to explore the inhibition effect and mechanism of HFCs on hydrogen explosion,a premixed open space flammable gas cloud explosion experimental system was built,and two HFCs including trifluoromethane and pentafluoroethane were selected as suppressants.A soap bubble method is adopted to study hydrogen-air explosion suppression.The flame propagation behavior was obtained by high-speed camera and schlieren system at various hydrogen-air equivalence ratios and suppressant concentrations.The effect of suppressant concentration on the flame instability at various equivalence ratios was analyzed according to thermal expansion and flame thickness.Meanwhile,the explosion pressure behavior was observed through the pressure transducer,with the aim to study the influence of suppressant concentration on the maximum explosion pressure,the maximum rate of pressure rise,the positive pressure impulse and negative pressure impulse.The chemical kinetics of the explosion suppression process was analyzed using CHEMKIN,and the effects of suppressant concentration on the laminar burning velocity,laminar flame structure,active radical distribution and reaction path of premixed gas combustion were studied.Experimental research shows that both trifluoromethane and pentafluoroethane suppressants can effectively suppress flame instability,reduce flame burning velocity and explosion pressure,and thereby reduce the risk of hydrogen explosion.Of the two suppressants,pentafluoroethane is more effective than trifluoromethane in explosion suppression.In stoichiometric and rich flames,the addition of both suppressants can reduce the thermal expansion ratio and increase the flame thickness,thereby weakening the effect of hydrodynamic instability on flame instability.The effect of hydrodynamic instability is relatively weaker in lean hydrogen-air mixtures.When the content of pentafluoroethane is high,the burning velocity of the spherical expansion flame is extremely low,resulting in increased buoyancy instability,while the buoyancy instability is not shown in the spherical flame added with trifluoromethane.The pressure behaviors of the premixed gas,including the maximum explosion pressure,the maximum rate of pressure rise,the positive pressure impulse and the absolute value of negative pressure impulse,all decrease with the increase of suppressant concentration,while they first increase and then decrease with the increase of equivalence ratio.According to the analysis of reaction kinetics,trifluoromethane and pentafluoroethane destroy the chain reactions mainly through the combination of fluorine-containing radicals with H,OH or O active radicals,thereby reducing the combustion reactivity.Pentafluoroethane has a better suppression effect than trifluoromethane.With the increase of suppressant concentration,the peak and equilibrium concentrations of H,OH and O radicals gradually decrease,with the reducing effect most obvious for H,the main fluorine-containing elementary reactions that inhibit combustion by consuming H,OH,and O are R861?CF₃+H<=>CF₂+HF?,R948?CF+OH=CO+HF?,R925?CF₂+O<=>CF:O+F?.After the addition of trifluoromethane,the consumption of H,OH and O is mainly achieved by the following fluorine-containing element reactions:R861?CF₃+H<=>CF₂+HF?,R875?CHF₃+OH<=>CF₃+H₂O?,R925?CF₂+O<=>CF:O+F?.After the addition of pentafluoroethane,the main fluorine-containing elemental reactions that consume H and O are the same as trifluoromethane,namely R861?CF₃+H<=>CF₂+HF?,R925?CF₂+O<=>CF:O+F?,and the consumption of OH is mainly achieved through the reaction of fluorine-containing element R948?CF+OH<=>CO+HF?.This study provides theoretical reference for the application of trifluoromethane and pentafluoroethane in hydrogen explosion suppression,as well as the basic data of further study of hydrofluorocarbon suppression mechanism.