Study On The Effect Of Chamber Pressure On Burning Characteristics Of Pool Fires

Compartment fire is one of the most classical issues in the fire research history,and only a few studies have focused on the chamber pressure.With the development of economy,there are more and more pressure chamber.Fires can also occur in pressure chamber,where the internal pressure is not equal to a standard atmospheric pressure,such as aircraft cargo compartments and nuclear containment shells during a pressurized period.The understanding of the influence of chamber pressure on fire behavior can provide theoretical guidance for fire safety design and promote fire prevention technology.In the initial stage of the fire in the pressure chamber,the heat release rate is small,while during the full development stage,the enviromental parameters(pressure,temperature,and gas atmosphere)would influence the development of fire.In this paper,the influence of chamber pressure(environmental pressure,initial pressure)on the burning characteristics of pool fire was studied through experimental investigations and theoretical analyzes.Main fundings include:(1)The effect of enviromental pressure on burning characteristics of small scale pool firesFirstly,the influence of enviromental pressure on the burning characteristics of small scale pool fires was investigated.To achieve the stable burning of pool fire,the experimental platform of pressure chamber was designed.In the chamber,ethanol pool fires in 2.0-cm-and 1.5-cm-diameter small scale vessels were conducted under a wide pressure range of 1-5 atm,where burning rate,flame shape and radiation fraction were measured.The effect of pressure on the heat transfer from the flame to the fuel pool controls the change of burning rate with pressure.As pressure increases,burning rate decreased slightly because the effect of pressure on conductive heat transfer was dominant.Then,burning rate increased significantly with pressure,because the effect of pressure on convective heat transfer and radiant heat transfer gradually became important.The prediction model for the flame geometric parameters of laminar diffusioin flame was proposed based on the dimensionless analysis,where the influencing factors of the flame shape parameters were summarized.To verify the proposed model,the dimensionless oscillation length was correlated with dimensionless flame geometric parameters.Base on the flow model above,a soot model similarity was developed,where the soot concentration was expressed in terms of the laminar smoke point height and the flow conditions,and a global relationship for non-dimensional radiation fraction was introduced and verified.(2)The effect of inital pressure on burning characteristics of pool fires in the closed chamberSecondly,the effect of initial pressure on the burning behavior of ethanol pool fire was studied.A series of ethanol pool fires with diameters from 4 cm to 10 cm was carried out in a pressure chamber with interior dimensions of 0.6 m(height)× 0.4 m(diameter),and the different initial pressures range from 0.6 atm to 2.5 atm.The fuel mass,flame video,and oxygen concentration were measured in the experiment,furthermore the change in flame appearance,evolution of mass burning flux,and self-extinction time were analyzed and discussed.The mass burning flux increased significantly with initial pressure and the mean mass burning flux can be correlated with the average pressure using exponential function,where the value of exponent increased with dimater.The results showed that the pressure has little influence on the remaining oxygen concentration at extinction.The oxygen concentration follows Gaussian distribution.The mean value is about 12.9%,and the variance value is about 0.37%.Based on the momentum conservation equation for the gas movement driven by buoyant force,the non-dimensional parameter was derived to illustrate the change of flame structure.The flame structure of pool fire was firstly divided:laminar flame,tip flickering flame,sinuous meandering flame and turbulent flame.The oscillation intensity,which was redefined using the probability contour of the flame,could be related with the proposed non-dimensional parameter.(3)The development of fire models for fire pool fire in the pressure chamberFire models for fire pool fire in the pressure chamber,including the heat transfer model,the prediction correlation of self-extinction time and the model for the oscillation of pool fire were poposed.Considering the efect of pressure,a theoretical formulation was proposed to estimate the radiation flux received by the fuel surface from the chamber wall and hot gases,with which the relationship between the modifed mass burning fux of pool fire and pressure was obtained.By considering the effect of initial pressure,an empirical form for predicting the self-extinction time was developed,which showed a good agreement with the experimental value in our study and previous test.Measurements of the time-varying temperature field in a l m pool fire burning in a quiescent environment using a thin filament pyrometry(TFP)method were presented.The change of temperature field in one cycle was obtained,and the analysis provided quantitative insight into the complex and dynamic structure of the oscillation.The influence of ambient pressure and gravity on oscillation frequency depended on the flameoscillation mechanisms,the tip flickering flame or the non-tip flickering flame.Based on the one-dimensional movement of vortex along the axis,a theoretical model was proposed to explain the effect of ambient pressure and gravity on frequency,and then the obtained relationship between Strouhal and Grashof was verified by the measured value.