Studies On Burning Characteristics And Associated Flame Behaviors Of Elevated Pool Fire Under Airflow

Fossil fuel tank fires in chemical plants and refineries are dangerous and difficult to extinguish,and a tank fire might cause a large area of tanks to be involved in a fire.Meanwhile,the effect of ambient airflow in the wild outdoors would make the fire hazard more complex and increase the fire risk.From a scientific point of view,the storage tank fire that occurs outdoors under the influence of ambient wind can be abstracted into the problem of pool fire combustion issue under ambient airflow.Pool fire is one of the classic scientific problems in the field of fire and combustion,which has both basic scientific significance and practical application value,and mastering and understanding its combustion characteristics and flame behavior are conducive to guiding the revision of the safety design specification of tank farms,facilitating the negative impact of disasters,promoting the fire early warning,prevention and control capabilities of urban industrial parks.A great deal of previous studies have been widely carried out on the fire dynamics characteristics of an isolated pool fire under wind,whose fuel outlet is flush with the ground level resembling oil spill fire accidents.However,there is a lack of research on tank fires at a certain height above the ground whose air entrainment behavior is not restricted by the ground,especially the multiple tank fires under the ambient airflow.Therefore,in this paper,based on the fire accident with multiple storage tanks ignited at the same time in a tank farm,the burning characteristics and flame characteristics of an isolated elevated pool fire and double elevated pool diffusion flames under the action of ambient airflow are carried out.A series of bench-scale experiments were performed,the flow and combustion structures were analyzed in combination with the fire numerical simulation software,and the morphological characteristics,burning rate,near-field radiation characteristics and interaction between elevated pool fires under the influence of cross airflow were systematically investigated.Based on the classical dynamic theory of pool fire,the basic knowledge of flow and heat transfer,similar principles and dimensional analysis,the horizontal flame length of an isolated pool fire and the evolution of near-field radiative heat flow with airflow speed were revealed.And the dynamic characteristics of the flow between flames,heat feedback interaction,mass loss rate and the evolution of the flame macroscopic three-dimensional structures of the double tandem pool fires with various spacings and ambient airflow speeds were researched.The specific research contents are as follows:(1)The experiments about pool fires elevated above the ground with certain height and pool fires flush with the ground level were conducted under airflow.The morphological characteristic parameters of elevated pool fires and ground pool fires with different pan sizes and HRRs under different airflow speeds were studied,including the momentum-dominated control horizontal flame length,buoyancy-dominated horizontal flame length and total horizontal flame length.Using FireFOAM fire dynamics simulation software,the similarities and differences of flow and heat transfer characteristic structures in these two scenarios were discussed and analyzed.Based on the air entrainment volumetric flow and momentum-buoyancy interaction relationship,a dimensionless physical model of total flame horizontal length under two scenarios was constructed.Using propane,methane and ethylene as fuels,the inclined axis temperature of the isolated gaseous pool fire and the near-field radiative heat fluxes were measured,and the variations of the local width of the flame along the vertical height direction under airflow were quantified.On the basis of the solid flame model,the change of flame geometry and temperature along the flame axis was taken into account in the hierarchical cylindrical radiation model,finding that the modified model predicts better for the near-field radiative heat flux values.(2)The combustion experiments of wind-blown double elevated pool fires employing n-heptane and ethanol as fuels were conducted.The effects of fuel types,separation distances and ambient airflow speed on the interaction between dual pool fires,the tilting behavior of the flame and the mass loss rates were studied.The merging behavior of double pool fires were qualitatively analyzed and discussed.The inclination angles of merging and non-merging flames were quantified.Based on the upstream and downstream pool fire tilting behavior when they are non-merging,the force status of two flames were systematically partitioned and explained.By comparing the tilt angle prediction formulae of downstream pool fires and single elevated pool fires,a semi-empirical prediction expression of the’effective wind speed’ considering the blockage effect caused by the first pool fire was presented.Secondly,according to the heat feedback interaction mechanism,the differences of the upstream and downstream pool fires burning rates relative to that of a single elevated pool fire at the same airflow speed were theoretically analyzed.Finally,based on the stagnation film theory,the burning rates of double pool fires and isolated pool fire that were in the realm of convection heat transfer were quantified.(3)The combustion experiments of a pair of elevated gaseous pool fires with symmetrical or asymmetric heat release rates under airflow effect were examined.The effects of fire source power symmetry,separation distances and ambient airflow speeds on the three-dimensional macroscopic morphological structures of the two fires were studied.The three-dimensional diffusion combustion behaviors of double pool fires were recorded,the horizontal length of the flame and the flame vertical height along the flow direction,and the characteristic length scales of flame flapping behavior normal to the flow direction were quantified respectively.Based on the competition mechanism between buoyancy flux and airflow momentum flux etc,a normalized physical characterization model of horizontal length and vertical height of merging and non-merging flames at the airflow momentum-controlled regime was established.According to the similarity analysis,the characteristic flow structures between the double pool fires were studied,the key feature parameters were extracted to normalize the time-averaged maximum flapping amplitude of the diffusion flame.