An Study On The Combustion Behaviors Of Two Asymmetrical Square Fires In Open Space

With the development and progress of the economy,the frequency of multi-source fires has increased significantly in recent years.Multi-fire source fire,also known as group fires,refers to a fire in which two or more adjacent fire sources in a discontinuous distribution occur at the same time.It is a discrete and non-continuous combustible combustion phenomenon.The Group fires occur in areas with adjacent and discrete combustible materials such as forests,ancient wooden buildings,and underground spaces.The burning intensity of multi-fire sources fires is much higher than that of single fire burning,which greatly increases the difficulty of fire rescue and seriously threatens people’s lives and property safety.Therefore,studying the burning law of multi-fire source fire is different from single-fire combustion,and has important guiding significance for improving fire monitoring and evacuation rescue in scenes that are prone to multiple fire sources such as cities and forests.In this paper,propane gas was used as fuel,square burners with square sides D=10cm,D=15cm were used,the spacing was set at 0～60cm,and the heat release rate range was between 10.8kW and 64.8kW.A total of 168 sets of dual fire source experiments and 18 sets of single fire source were carried out to explore the flame interactions at different intervals and heat release rates.According to the flame shape of the stable combustion stage at different intervals,the interaction between the asymmetric dual fire sources are divided into three stages:complete merged,incomplete merged,and no merged.It qualitatively describes the interaction behavior between the fire sources at each stage.By introducing the flame fusion probability Pm,each stage is quantitatively divided.The article studies the interaction of asymmetric gas fire sources in an open space,qualitatively describes the merging probability and flame height change with distance and heat release rate,and analyzes the reasons for the changes from the air entrainment mechanism.Dimensional analysis method is used to fit the flame merging probability of working conditions 0.05<Pm<0.95 many times,and a flame merging probability prediction model suitable for working conditions except for complete merging(Pm≤0.05)and no merging(Pm≥0.95)is proposed.It is concluded that the probability of flame merging is inversely proportional to the 1.01 power of the dimensionless distance(S/(D1)),0.19 power of the dimensionless HRR QD1*,and 0.54 power of the dimensionless HRR QD2*.By comparing the index,it is concluded that the distance has the greatest influence on the merging probability,followed by Q2,and Q1 the least.Taking the air entrainment mechanism of the asymmetric fire source as the theoretical basis,the flame height prediction models at different stages are established respectively.The flame height expressions at different stages are proposed:(1)Complete merging height.It can be regarded as a square burner whose combustion area is the sum of the area of two burners.The side length of an equivalent single fire source is used as the characteristic length to dimensionlessly the flame height and HRR.The dimensionless flame height(Zf,FM)/(DFM)is proportional to the 0.4 power of the dimensionless HRR QDFM*.(2)Incomplete merging height.Considering factors such as HRR,burner sides,and spacing,an asymmetric dual fire source air entrainment model is proposed.The entire area is divided into flame area,additional area,and environment area.The Bernoulli equation,the law of conservation of mass and the Burke-Shumann theory are used to obtain the expression of the dimensionless flame height(Zf,IM)/(Zf,S=0).The parameter γ which characterizes the air entrainment rate ratio on the free entrainment side of the flame zone at different intervals is introduced.When the incomplete merging is found through calculation,this parameter can be regarded as a constant,the value is 0.96.The relationship between the flame height and the side length of the burner,the distance between the burners and the HRR of the fire source is further obtained.(3)No merging height.Comparing and fitting the fire sources of D=15cm and D=10cm respectively,it is found that the height of the flame is close to the height of a single fire at this time,and it can be directly calculated using the Heskestad empirical formula.