| Water spray was widely used in firefighting field due to its advantages such as non-pollution,high efficiency and scour smoke particles.Domestic and foreign scholars have carried out a lot of studies on water spray fire fighting.However,most of the published studies were performed on the macroscopic level,which could not reveal the essential law of the interaction between water spray and pool fire.There are a lot of basic research problems during the process of water droplet impacting on liquid fuel,such as the attenuation effect of the fire plume on the droplet velocity,the interaction process between the droplet and burning fuel,the heat and mass transfer between droplet and burning fuel,and the fire expansion mechanism for water spray suppressing the high temperature pool fire.So it is necessary to explore the interaction process between water droplet and liquid fuel surface from microscopic perspective.Therefore,in order to deepen the knowledge of the basic research problems involved in the fire fighting process by using water spray,the microscopic process of single droplet interaction with burning fuel surface is focused in this paper.A scientific understanding of the interaction process between single droplet and liquid surface can provide a better basis for understanding the efficiency and mechanism of water spray suppression.Three typical phenomena were found in this study and the influences of pool temprature and droplet Weber number on the phenomena were analyzd.Systematic experiments were conducted to explore the interaction process for single droplet impacting onto heated fuel surface(the fuel temperature is ranging from 30 to 78?)by using high speed digital camera.Three typical phenomena including Penetration,Crater-jet and Surface bubble were observed and plotted on a We-?map.The effects of the changes of the liquid physical and chemical properties,which were caused by changing fuel temperature,on impact morphology were discussed qualitatively.The effects of pool temperature and droplet Weber number on the characteristic parameters,which includes the parameters of crown structure and the energy conservation and transfer during crater and jet formation process,were discussed quantitatively.The distribution law of typical impact phenomenon was revealed,and the prediction model of water droplet impacting velocity,maximum crater depth and maximum jet scale were established.Systematic experiments were conducted to explore the interaction process for single droplet impacting onto burning fuel surface(Fuel boiling temperature does not exceed 120 ?).The key nondimensional parameters determined by the dimensional analysis were obtained and the impact outcomes were plotted on a(?dvt2D0/?d)0.375(?dvtD0/?d)0.25-lg We map.From the perspective of energy conservation and transformation,the transformation processes of the outcomes were analyzed qualitatively.The influences of the fuel physical and chemical properties on the phenomena were analyzed.The effects of the droplet Weber number on the characteristic parameters of the collision outcomes including the maximum crater depth,the jet length scale,the maximum crown height were discussed in detail.Prediction model of maximum crater depth and jet length scale were deduced and built up.The impact velocity after droplet passing through the flame was predicted and the data showed that the difference between the theoretical impact velocity and the actual velocity is ranging from 5.5%to 15.8%,which quantifies the attenuation effect of the fire plume buoyancy on the kinetic energy of the droplets.The development law of steam explosion phenomenon was revealed,and the influences of pool temperature,droplet liquid type and Weber number on vapor explosion were analyzed.The model of maximum absorption heat of single water droplet was established.A series of experiments were performed to explore the dynamic characteristic for single droplet interacting with heated fuel surface(the fuel temperature is ranging from 160-340?).Four types of liquid droplet including water droplet,droplet with 5%NaCl additive,droplet with 3%AFFF additive and HFE-7100 droplet were used in the experiments.The results showed that vapor explosion can be observed when the fuel temperature exceeds 210 ?.The heat and mass transfer process of the droplet were analyzed and the state of the droplet mainly consists of three stages:?:the water droplet temperature rises from room temperature to 100 ?;?:the phase of the 100 ? water changes to 100? water vapor;?:100? water vapor continues to absorb heat and the vapor temperature rises from 100 ? to final temperature Tfinal(Tfinal ? fuel temperature).In other words,after the water droplet immersed in the fuel pool,water droplets continuously absorbed heat from the oil and then evaporated into steam.Finally,the water vapor was released above the fuel surface.According to the state of the water droplet,the model of maximum absorption heat was built up.The effects of the fuel pool temperature,the droplet diammter and the droplet Weber number on the vapor explosion time were analyzed.It is foud that the higher the fuel temperature is,the earlier the vapor explosion occurs and the vapor explosion time increases with the droplet diameter and droplet Weber number.The differences of fire vapor explsoion intensity after the impact of different kinds of droplet on the high temperature oil pool were analyzed,and the order is as follows:water droplet>droplet with 5%NaCl additive>droplet with 3%AFFF additive>HFE-7100 droplet.The instantaneous expansion model of the flame after the droplet collision on burnig fuel surface was established.The dynamic characteristic for single droplet impacting on burning fuel surface(the fuel temperature is larger than 340 ?)was experimentally investigated.Three types of liquid droplet including water droplet,droplet with 5%NaCl additive and droplet with 3%AFFF additive were used in the experiments.The effect of the vapor explosion on the flame expansion was qualitatively analyzed.The mechanism of the instantaneous expansion of the flame after the droplet enters the high temperature oil pool was revealed.The development model of the instantaneous expansion of the flame was established.The effect of vapor explosion on flame expansion was quantified by tracking the flame area and volume.It can be found that vapor explosion is one of main causes of the flame expansion. |
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