Interaction Behaviors Of Two Symmetrical Square Fires Under Different Confined Conditions

Two or more nearby fires burning simultaneously is termed as multiple fires,which is commonly occurred in forest and urban spaces where the discrete combustibles are distributed.The burning of multiple fires can easily develop into devastating mass fires.When the fires are located sufficiently close to each other,the air entrainment around the flames is restricted and the resulting pressure drop between fires would make the flames tilt to each other and even merge together.Flame merging will increase the flame height and burning rate when compared to a single fire,and it might even cause the fire whirl and jump fire under specific conditions,increasing the possibility of the fire ignition and spread.Under the superimposition effect of heat feedback,the multiple fires in confined spaces is much more destructive and uncontrollable.The deep understanding of the development of multiple fires has a great significance in improving the fire prediction,alarming and controlling in forest and urban fires.Therefore,this paper aims to study the burning behaviors of two identical square fires under different confined spaces.The method combining the theoretical analysis and experimental study is adopted to systematically study the fire plume behaviors induced by two fires burning in open space,below an unconfined ceiling and in a long confined space.The main work contains:A set of experiments was conducted in open space to study the flame shape,burning rate and flame outward radiation of two identical square fires.First,the propane was used as fuel.The flame merging probability based on the statistical concept was adoped to quantify the merging extent.Based on the derivation of the relationship between air entrainment velocities in the confined and free areas,the piecewise correlation for predicting the flame height was developed.Then the two cuboid flames model by taking the flame tilt angle into account was proposed to predict the flame outward radiations received by targets facing upward.Second,using the heptane as fuel,the burning rate increament model was established under the superimposition effect of heat feedback.The weighted multi-point source model based on the flame volume ratio was proposed to calculate the flame outward radiations received by targets facing the flame,and the accuracy of the model was validated by comparing with the conventional flame models and experimental results.A set of experiments was conducted for two identical square propane burners burning under an unconfined ceiling.The effects of the ceiling height,heat release rate and burner edge spacing on the ceiling flame length and longitudinal ceiling gas temperature were investigated.It was found that as the spacing increases,the longitudinal ceiling flame length increases while the transverse ceiling flame length decreases.Based on the law of energy conservation and by introducing the additional heat release rate exhausted by the ceiling flames,the correlations for predicting the longitudinal and transverse ceiling flame lengths were established respectively.It was revealed that the maximum ceiling gas temperature is weakly affected by the spacing,then the model for predicting the maximum temperature was developed based on the single fire plume theory.It was validated that the classical correlation for predicting the ceiling gas temperature distribution proposed by Heskestad and Hamada is no longer viable for the two fire plumes below the ceiling.The free flame height was introduced to modify the plume radius,the new model for predicting the ceiling gas temperature distribution was therefore established for two impinging flames.A set of experiments was conducted in a long confined space to study the ceiling gas temperature,flame shape and outward radiation induced by two identical square fires which are located in the longitudinal centerline of the space.First,the propane was used as fuel.The correlations for predicting the longitudinal ceiling gas temperature distributions were established for the weak plume and strong plume,respectively.And the relationship between the total flame length including the vertical and longitudinal ceiling flames and the normalized heat release rate was established by introducing the perimeter of air entrainment.Second,using the heptane as fuel,it was found that the fuel mass burning rate increases first and then decreases with increasing the spacing.And the maximum total heat release rate induced by two pools located along the longitudinal centerline was more than 4 times of the single pool fire.Based on dimensional analysis and introducing a correction factor,the correlation for predicting the longitudinal ceiling flame length was proposed.Finally,a method of simplifying the two merging flames into a single large flame was proposed,then the flame model of the sum of a vertical cylinder and a ceiling thin disk was assumed to calculate the flame outward radiation and its proportion.