Experimental Study On Combustion Characteristics Of N-heptane Pool Fire With Different Aspect Ratios

Generally speaking,fire source shapes in real life can be assumed to three types:axisymmetric,rectangular and line.For example,tank combustion and vehicle fire can be regarded as an axisymmetric and rectangular fire source,respectively.Transmission lines,pipelines and flame front of a spreading forest fire can be abstracted as line sources.Fire source shape need to be simplified and assumed in fire research.A lot of research on axisymmetric fire source have been conducted.Based on the regular shape,the fire combustion behavior and the corresponding characteristic parameters are relatively easily assumed.And the burning rate,flame height,the centerline temperature distribution and flame radiation were analyzed.However,the research for the rectangular and line fire source was relatively scarce.The shape and size of the fire source will influence the air entrainment and the combustion behavior of flame.And the evolution law of relevant characteristic parameters will be different,which may eventually cause inestimable consequences to the development process of fire and endanger human life and property.The gas fuel(such as methane,propane)was mainly used in previous studies on rectangular and line fire.And the flame height and the centerline temperature rise distribution were analyzed.However,the combustion behavior of the pool fire is more complex and there are few relevant studies.So it is necessary to conduct the pool fire experiment with different fire source shape and analyze the main characteristic parameters such as the burning rate,flame shape,temperature distribution and flame radiation of the rectangular and line fire.It can provide the certainly theoretical basis for preventing fire and protect human safety effectively.In this paper,the combustion behaviors of rectangular and line pool fires are analyzed by experiment research,including the following two aspects:Firstly,a series of experiments of rectangular n-heptane pool fires have been conducted in open space.A set of fuel pans with the same width of 8 cm but different aspect ratios of length to width(n=l/w:1,2,3,4,6,8,10,12 and 14)have been used.The mass loss rate(MLR),flame shape,temperature distribution and radiant fluxes received by external vertical targets were investigated.The mass loss rate per unit length was used to characterize the transition from square,rectangular to line fire source.It was found that the mass loss rate per unit length and flame height increase in three stages with the increase of n.The boundary of rectangular fire and line fire is n=10.Based on traditional processing method of the flame height,a new method of two-dimensional description was introduced to represent the flame shape of rectangular fire.By dividing the pool into several parts along the pool length(longitudinal),a piecewise function between the flame height and the longitudinal distance was proposed.Spatial temperature distribution was obtained by the measuring method of thermocouple array.Based on the two-dimensional flame shape,the radiant fluxes to external targets were calculated by applying a modified weighted multi-point source(WMP)model.The proposed model was validated by comparing with the single point and cuboid flame models and using the experimental data.Further,the n-heptane line fires burning in the pools with different aspect ratios were conducted in open space.A set of fuel pans with larger aspect ratios(n=8,10,12,14 and 20)but different width(w=2cm,5cm,8cm)have been used.The mass loss rate(MLR),radiation distribution(along the flame plane)and radiant fraction were investigated.When the pool width is constant,the mass loss rate increases with the increasing n and the flame shape presents similar characteristics.An obvious bifurcation phenomenon occurs when w=5cm.The relationship between the radiation and flame height was established by using slit radiometers.The effect of the measuring position and aspect ratio on radiation distribution were discussed.Combined with the WMP model,radiant fraction of line fires was derived based on the radiation distribution and the relationship between the radiant fraction and aspect ratio was established.Compared with data calculated by the WMP model,a new multi-cylinder flame model was proposed for radiation characteristic from the case of flame bifurcation.