Study On Horizontal Fire Propagation Characteristics Of Single And Two-parallel PMMA Slabs

Flame spread over solid combustibles is an important stage in building fires,involving complex physicochemical processes such as gas-phase combustion,solid-phase pyrolysis and complicated process of heat and mass transfer.In real fire scenarios,the factors affecting the flame spread include material properties(sample width,sample thickness,etc.)and external environmental parameters(ambient pressure,oxygen concentration,etc.).Moreover,in the case of multi-fire spread situations,external radiation factors are also involved,such as the melting and dripping caused by burning of polymer materials,and the ignition of surrounding combustibles by high-temperature heat radiation.Study on flame spread over solid combustibles is helpful to understand scientifically the mechanism of flame propagation and develop effective fire extinguishing techniques.Horizontal flame spread over solid combustibles is a typical form of countercurrent fire spread.In addition,a steady-state stage flame spread process is easily achieved,a large number of experiments and theoretical studies on this form flame spread have been conducted to explain the evolution of flame spread and explore the control mechanism.Two-dimensional simplified flame spread models based on the assumptions,such as infinite width,have been proposed.However,the three-dimensional fire propagation over uninhibited solid is common in the actual fire scene.Compared with the two-dimensional fire propagation,the flame spread rate in three-dimensional case is most likely faster and thus more harmful.Therefore,research on the horizontal fire propagation over solid combustibles in three-dimensional case has important practical guiding significance for mastering the basic fire propagation evolution and heat transfer control mechanism.The typical polymer material PMMA was taken as the research object,and research on the flame spread over both single and two-parallel PMMA slabs vertically arranged horizontally were carried out in this paper.Based on the self-made horizontal flame spread experimental apparatus,the effect of slab sizes(thickness,width)and the vertical separation distance between two-parallel slabs on the flame spread characteristics parameters,such as flame spread rate,flame height,flame length,was studied experimentally and theoretically.The measured radiant heat flux in the preheating zone at the flame front was compared with the theoretical value,to reasonably explain the heat transfer control mechanism of the horizontal flame spread over multiple-fire spread over horizontal plates.The experimental study found that the pyrolysis front is U-shaped,during the horizontal flame spread of PMMA slabs.In addition,the flame spread rate,flame height and flame length decreases first and then increases with the increase of the width of the sample in both single and two-parallel cases,which mainly results from changes in radiation heat transfer.In the condition of single PMMA slab,as the width of the sample increases,the radiant heat flux in the preheating zone of the flame front shows a tendency to increase.The deviation between the theoretical and calculated values of the radiant heat flux are within 12%.Under the condition of two-parallel PMMA slabs,the flames are always merging in the range of the separation distance between two separated slabs tested in this research.The flame spread rate,flame height and flame length are obviously larger than that for single PMMA slab.All of them increase first and then decrease with the increment of the separation distance between two separated slabs,the separation distance corresponding to peak value changes with sample thickness.The variation of the radiant heat flux on the upper and lower of the PMMA surface with the separation distance between two separated slabs has a synchronous tendency with the variation of the flame spread rate,and the deviation between the measured value and the theoretical value of the radiant heat flux in the preheating zone is within 18%,indicating that the radiation effect plays a dominant role in the horizontal flame spread of the two-parallel PMMA slabs.