Studies On Combustion Characteristics Of Typical Liquid Pool Fire And Its Smoke Control Method Using Water Mist Curtain

With the acceleration of urbanization in China,many kinds of tunnels emerge rapidly.In the event of a fire in the tunnel,high temperature,heat radiation,smoke particles and toxic and harmful gases will be generated,which will seriously endanger the lives and property of personnel.Due to the environmental hazard of traditional halogenated alkane and other fire extinguishing agents,water mist is gradually applied and popularized in tunnel transportation and other fields as a clean and efficient fire extinguishing technology.The development of fire smoke control technology based on water mist is expected to effectively block the spread of fire smoke,while cooling,extinguishing,and attenuating flame heat from radiation.However,the research on fire smoke control technology based on water mist is limited.Previous researches were carried out on scale reduction experiments and full scale numerical simulation experiments.Moreover,the researchers do not agree on the effectiveness of the water mist curtain to fire smoke control in the tunnel.Therefore,this thesis proves the effectiveness of the water mist curtain to fire smoke control,by the means of mesoscale experiments,full-scale experiments and full-scale numerical simulation experiments.At the same time,the mechanism of fire smoke control using water mist curtain is also analyzed.In order to understand and analyze the method and mechanism of fire smoke control in tunnel,this thesis carried out an experimental study on the combustion characteristics of diesel pool fire with different diameters and liquid level depths in the tunnel.The diesel burning rate,CO yield and temperature distribution are measured and analyzed.The results show that when there is no longitudinal wind,and the burning rate of the pool fire does not change significantly with the depth of the liquid level.However,the CO yield significantly decrease with the increase of liquid level depth.When the diameter of the pool is large enough,the burning rate of the pool fire is reduced as the liquid level depth decreases.However,the CO yield does not significantly decrease with the increase of liquid level depth.When the longitudinal wind is applied,the burning rate of the pool fire will increase firstly and then decrease,and the CO yield has the same change trend,both the burning rate and CO yield get the maximum when the velocity is about 0.3 m/s.When there is no longitudinal ventilation,the rate of smoke sedimentation is determined by the burning rate and the depth of the liquid level.When the longitudinal wind is applied,the lower wind speed promotes the burning of pool fire,and the temperature of the tunnel ceiling increases.When the wind speed increases to 0.8 m/s,the maximum temperature position inside the tunnel begins to move down.Considering the purpose of fire smoke control method in the tunnel,a diesel pool fire was considered which yields large amount of smoke.The combustion behavior of a pool fire gives rise to its temperature field,which controls fire chemistry including soot formation,smoke yield and the way of smoke spreading.Therefore,it is helpful to understand the time-varying dynamic structure of a pool fire soot particles within a fire confound the measurement of its temperature field using traditional fire temperature measurement methods such as thermocouples.Soot accumulation on a thermocouple changes the effective instrument time response and leads to measurement bias associated with radiative exchange including both gain and loss-depending on the specific thermocouple environment.Therefore,we use the methanol pool fire to replace the diesel pool fire and study its temperature field at NIST experimental platform.One of the advantages of using methanol is that soot particles will not be generated.Through the measurement of the temperature of the fire field in the mesoscale methanol pool fire using an in-house probability fitting routine,the average temperature field of the flame center surface and the transient temperature field in one pulse period are obtained,which improves our understanding of the combustion behavior and dynamic characteristics of the pool fire.The results show that the flame temperature measured by the thermocouple changes periodically with time,and the proportion of different temperatures in the same temperature range accords with the Gaussian distribution.For the first time,the temperature of the high temperature point of the mid-scale pool fire was measured by the fiber measurement method.The average temperature in the flame was calculated by the cumulative distribution function(CDF)method,and the validity of the CDF calculation method was proved by the Weckman's experimental measurement data.And calculate the corresponding temperature field.And using the Gollner method,the average temperature field distribution map is calculated.Fitting the relationship between the average temperature values obtained by Gollner method and CDF method,we calculate the change of transient temperature field in one cycle of turbulent pool fire.The results help to understand the dynamics of turbulent pool fire,and it can be used to verify the fire model considering the transient temperature field.Improve the understanding of the causes of smoke during combustion and the process of smoke entrainment.Based on the study of typical pool fire combustion characteristics,we build a mesoscale multi-force synergy experimental platform to study smoke control in the tunnel using water mist curtain.In-depth study on the effect and mechanism of fire smoke control using water mist curtain in a complex environment tunnel.The results show that,in a complex environment,the water mist curtain can effectively control the spread of tunnel fire smoke,improve the visibility of the protected side,and reduce the concentration of CO.Through experiments and numerical simulations,the mechanism of fire smoke control by water mist curtain is analyzed.It is found that during the application of water mist,large eddies are formed on both sides of the fog curtain,which helps to settle nearby smoke particles.At the same time,it is found that different nozzles produce fine water mist curtains have different effects on tunnel fire smoke control.The two nozzles used in the experiment have the most ideal pressure in controlling fire smoke.For the windless conditions,the calculation formula for calculating the smoke falling distance which induced by the water mist curtain was developed,and comparing with the experimental results to verify the validity of the calculation formula.On the base of the study about the smoke control effectiveness with the use of water mist in a mesoscale tunnel experimental platform,full-scale experiment and simulation experiment were carried out to study the smoke control effectiveness.The results show that the water mist curtain nozzle independently developed by our research group,in the full-scale high-space simulation experiment,the fire separation can be effectively carried out,and the flame radiation intensity of the protected side can be reduced,fire smoke spread can be delayed.In the full-scale numerical simulation experiment,the water mist curtain can control the fire smoke spread,and it is found that the increase of the number of nozzles or rows of water mist curtains,the smoke vent which setting on the ground,and use of the combination of water mist curtain and smoke blocking,can aid to increase the effectiveness of smoke control.These results provide guidance for the engineering application of the water mist curtain in controlling the tunnel fire smoke.