Study On The Performance And Mechanisms Of A Fire-extinguishing Foam With Nanoparticles

Oil fire is one of the main dangers in the field of oil storage and chemical industries.The aqueous film-forming foam(AFFF)plays an important role in pool fire or flowing fire extinguishing.However,the shortages of traditional foam in performances limit its efficiency in fire-extinguishing,especially in large fires.Therefore,it is significant in industry application to find an adequate fire-extinguishing foam with superior performance.It has been found that small particles can be used as stabilizers and sunscreens of foam,by forming a three-phase foam.However,the effects of particle addition on the performance of AFFF are still unclear.Therefore,in this paper,a new three-phase fire-extinguishing AFFF had been prepared based on the AFFF foam liquid and nanoparticles.The changing rules and influencing mechanisms of the three-phase foam performance,including the expansion,stability,rheology,heat insulation and burnback performance,etc,were studied by different experiments and simulation.The influence of foam composition and operation factors on the foam performance was analyzed.In order to obtain the complete distribution characteristics of temperature and heat flux in the foam layer during the thermal insulation process,a relatively perfect heat transfer model of the foam layer exposed to thermal radiation was constructed based on the microstructure and heat transfer characteristics of the three-phase foam.The expansion and heat insulation characteristics of the foam layer were simulated and analyzed.The main contents and conclusions in this paper are summarized as follows.The effects of factors of foam composition and operation conditions on the expansion and stability of the foam were studied by the tests of foaming and stability.It is found that the silica nanoparticles could significantly increase the foam stability,being better than other particle materials.In the foam with silica nanoparticles,it is better to use particles with the same surface wettability.The foaming ability and stability of the foam with hydrophilic particles are better than that of the foam with hydrophobic particles.Furthermore,the effect of the specific surface area of the hydrophilic particles on the foam stability is also less than that of the hydrophobic particles.As the particle concentration increases,the foam drainage time increases significantly,especially when the particle concentration exceeds 1.0 wt%.When the foaming expansion ratio(FER)is not more than 17,the stability of the three-phase foam increases with the increase of FER.High ambient temperature benefits the foaming but reduces the foam stability.The process of change of the foam stability decreasing with the temperature rise could be divided into three stages with the descending speed being first fast and then slow.To evaluate the performance of the three-phase fire-extinguishing foam more scientifically,quantitative models of FER,foam stability,and rheological characteristics of the foam were proposed based on the experimental data.The experiments also imply that the stability of the foam containing hydrophilic particles increases continuously with the increase of surfactant concentration,while the stability of the foam containing hydrophobic particles increases first and then decreases.Essentially,the stability of foam is determined by the synergistic effect or resistant effect between particles and surfactants in the foam.To substantiate the above mechanism,the motion and adsorption of the amphoteric fluorocarbon and hydrocarbon surfactants on the surface of silica particles in the solution were studied by the molecular dynamic(MD)simulation.It was found that the surfactant in the solution has no obvious effect on the wettability of hydrophilic particles.As a consequence,the synergistic effect always exists in the foam with hydrophilic particles.However,while the surfactant molecules are adsorbed on the surface of hydrophobic particles,the hydrophilic groups are oriented outwards,resulting in increased hydrophilicity of the particles.So in the foam with high surfactant concentration,the wettability of hydrophobic particles has been changed.Moreover,the competitive adsorption between the particles and surfactant molecules occurs due to the limited adsorption space on the gas-liquid interface in the foam.Both factors affect the stability of the structures of the hydrophobic particles on the gas-liquid interface,leading to the resistant effect.The experimental results show that the suitable concentration of AFFF agent in the three-phase foam is 3.0%.Besides,the foam has good adaptability to the salinity of the solution that the sea water could be used to prepare the foam liquid.The heat insulation and burnback performance are also the important performance of the fire-extinguishing foam.The insulation characteristics of the three-phase foam layer exposed to radiant heating from the high-temperature fire were experimentally studied.It is found that the temperature and heat flux gradually decreases from top to bottom in the foam layer exposed to the radiant heating environment due to the obvious endothermic effect of the foam layer.In the heating process,the state of the foam layer presents three successive stages,i.e.the expansion stage,the balanced stage,and the collapse stage.In the expansion stage,the inside gas is heated and expanded,and water vapor is produced by liquid evaporation in the foam,causing the thickness of the foam layer increases gradually.In the balanced stage,the layer thickness reaches the maximum value and is maintained around the maximum due to the collapse of bubbles.At this stage,the evaporated vapor diffuses upward and overflows,keeping the inside pressure of the foam layer at a stable value.Meanwhile,the temperature of the upper layer is kept close to the boiling point of the liquid.In the collapse stage,the evaporation and drainage of liquid lead to the decrease of the liquid phase holdup in the foam layer,resulting in the continuous fracture of bubbles and the collapse of the foam layer.Besides,the effects of foam composition and operation conditions on the insulation and the burnback performance of the foam layer were also studied.It is found that the insulation and the burnback performance of the foam layer can be enhanced with higher particle concentration or lower foam expansion ratio(FER).Moreover,the larger intensity of the external heat radiation,the faster heat transfer in the foam layer,and the shorter life of the foam layer.According to the knowledge of the microstructure and heat transfer characteristics of the three-phase foam layer,a relatively perfect heat transfer model for the foam layer exposed to thermal radiation was built at the end of the thesis.Four distinct features related to the heat transfer in the foam layer were considered in the model,including the gas-liquid-particle coupling heat conduction,the interaction force among the three phases,the absorption and scattering effect of the liquid and particles on the thermal radiation,the evaporation and endothermic effect of liquid.By the new model,the expansion process of the foam layer can be simulated,and the heat flux distribution and insulation mechanism in the layer are analyzed and improved.Together with the experimental results,it is found that the hydrophilic particles in the foam mainly enhance the thermal stability of the foam layer with little contribution to the radiation resistance of the foam layer.While the hydrophobic silica nanoparticles can simultaneously enhance the thermal stability and radiation resistance of the foam layer,that the hydrophobic nanoparticles are more conducive to enhancing the insulation performance and to prolonging the life of foam layer.The results of this study provide theoretical guidance for the application of the three-phase foam in fire-extinguishing and can be a useful reference for the optimization of foam composition and operation conditions.