Experimental And Theoretical Research On Multiple Fires Burning With Gas Fuel

Multiple fires burning is a special fire phenomenon when multiple flames from separate fuel beds burn simultaneously in a certain space range.It is commonly observed in wildland,urban and industrial fires.In contrast to single fire burning,the flames of multiple fires have interaction with each other.What's more,multiple fires burning may result in flame merging,spot fire and fire whirl and these make the fire more hazardous and complex.In the researches of single fire,based on the shape of fire source,the fire could be classified into axisymmetric fire and line fire.For the flame merging of multiple fires,there may exist different merging cases,for example,axisymmetric fires merging into axisymmetric fire,line fires merging into line fire and axisymmetric fires merging into line fire.In this paper,we use fire array,two rectangular fires and two square fires to stand for these merging cases and to study multiple fires burning systematically.This paper presents experimental investigation and theoretical analysis on flame merging behavior and combustion characteristics of multiple fires.New methods to identify flame merging are proposed.The flame merging process is described quantificationally and the change of physical mechanism in the flame merging process is discussed.For different flame merging states,the variation trends of flame height are studied and several scaling laws,empirical models and theoretical models are developed.The similarities and difference between multiple fires and single fire are examined from flow field structure and heat field structure.The detailed work and results of this thesis are summarized as follows.(1)Focusing on the flame merging criterion and flame height of merged flames,the experiments of fire array with even number of flames are conducted.By changing the number of flames,the spacing between burners and the heat release rate,flames with different merging states are obtained.By analyzing the variation trends of centerline temperature,it is found that a physical transition occurs with the decrease of fire spacing.The lower region in the centerline of fire array changes from the heated air region to the flame region and then to the fuel-rich region.When the flames change from unmerged state to completely merged state,the maximum centerline temperature first increases and then decreases.Thus the temperature of completely merged flames could be used as a reference flame merging temperature.Based on these,A new method change rule of flame height,the experiments of two square fires are conducted.By changing the size of burner,the spacing between burners and the heat release rate,flames with different merging states are obtained.It is found that nearly no flame tilt is observed when the flames get merged and the flames may first get touched with each other at the base of flames.So the flame interaction between two square fires is weak and it is hard for flames to get merged.Dimensionless model is also developed to describe the flame merging probability.The variation trends of the ratio between flame height of two flames and single flame height are discussed.It is found that the flame height ratio increases from 0.85 to 1.15 with the increase of dimensionless heat release rate when the burner spacing is zero and this could be explained by the change of air entrainment model.