Studies On Three-dimensional Downward Flame Spread Over Uninhibited PMMA Slabs

Flame spread over solid combustibles is an important problem in building fires.Thus,a large number of experimental and theoretical studies were conducted on the flame spread over solids.Flame spread over solid is a very complicated problem,involving heat and mass transfer and chemical reactions for both gas and solid phases,therefore,in-depth understanding of the combustion of solid combustibles and the flame spread mechanism is essential in fire research and is of great significance in fire assessment and rescue.Although flame spread process is affected by the solid properties and other external factors,the combustion of solid and the fire spread rate prove to be determined by the total heat transfer rate to the solid.Part of the heat generated by the combustion of solid combustibles is transmitted to the solid surface by radiation and convection,and the other is emitted to the environment through radiation.Under the heating of flame,solid is pyrolyzed and flammable gases are released.As a result,a stable diffusion flame is formed at the solid surface.For solid at the unburned region,the temperature of solid rises when heated.The solid is ignited when the pyrolysis temperature is reached,and the flame spreads as a consequence.Countercurrent flame spread is an important form of flame spread over solid combustibles.Because the countercurrent flame spread is relatively stable,the research of flame spread mechanisms over solid were conducted mainly on countercurrent flame spread.In order to simplify the flame spread process,most of the investigation for countercurrent flame spread were conducted under the assumption of infinite width of sample.However,solids are unlimited and the boundaries are open in the real fire scenarios.Therefore,it is essential and of great significance to study the flame spread under open boundary conditions,which is defined as three-dimensional(3-D).A series of experiments for 3-D downward flame spread Polymethyl methacrylate(PMMA)plates with different sizes were carried out under different ambient pressures to investigate the effects of size and ambient pressure on 3-D downward flame spread.Through the analysis of the overall energy balances of the whole sample and preheating region at the steady-state stage,3-D downward flame spread models were proposed.Based on the theory of 3-D downward flame spread over a single slab,three-dimensional downward flame spread over two-parallel slabs with different separation distances under different ambient pressures was studied.The dependences of 3-D downward flame spread rate,the angle of pyrolysis front and the averaged flame height on separation distance under the two-parallel conditions were analyzed.Some detailed results are summarized as follows:The experiments of 3-D downward flame spread over uninhibited PMMA slabs with different widths and thicknesses were conducted under normal ambient pressure condition.The effects of sample width and thickness on 3-D downward flame spread were studied.It was found that 3-D downward flame spread rate increases with the increasing width,and finally tends to be stable for larger sample width.The variation of flame spread rate with sample thickness is consistent with the classical two-dimensional countercurrent flame spread results.According to the characteristics of PMMA residues at the steady-state stage,a simplified model for 3-D downward flame spread was proposed,which mainly includes three characteristic angles.Based on the analysis of the overall energy balance for the solid at the steady-state stage,the flame spread rate and mass loss rate at the steady-state stage were predicted and agree well with the experimental results.In addition,a comparative analysis of the relationship between the averaged flame height and the mass loss rate was performed.To investigate the influence of ambient pressure on 3-D downward flame spread,the identical experiment was conducted at five cities with different altitudes,Hefei,Xining,Geermu,Lhasa and Yangbajing.It was found that the effects of ambient pressure on the angle of the pyrolysis front varies with sample thickness.For thermally-thin PMMA(2 mm),the angle of the pyrolysis front does not change with the ambient pressure.However,the angle of the pyrolysis front becomes sensitive with ambient pressure for larger sample thickness,Based on the analysis of ambient pressure on the heat flux at the pyrolysis surface and preheat region,the model of 3-D downward flame spread under different ambient pressures was proposed.Through the prediction formula of flame spread rate,the relationship between the normalized pyrolysis front angle and the normalized ambient pressure was proposed and verified with the experimental results.The mechanism of 3-D downward flame spread over a single PMMA slab and two-parallel PMMA slabs were studied.The experiments of 3-D downward flame spread were conducted over both a single and two-parallel slabs at three cities with different altitudes.The study found that the 3-D flame spread rate is controlled by the ignition rate at the side-edge.Based on the control volume at the side-edge and the energy balance relationship,a theoretical model at the preheating region of 3-D downward flame spread was proposed.Different from the results for surface downward flame spread,3-D downward flame spread rate decreases with the reduced ambient pressure.The main reason accounting for the outcome is that the reduced pressure increases the thermal diffusion scale and the radiative heat loss.In order to further verify the accuracy of the 3-D model,the model was applied to the two-parallel case coupled with a radiation model.The prediction formula of the flame spread rate for parallel plates was proposed and verified with experimental results.In addition,the relationship between the flame height and flame spread rate was obtained according to the diffusion flame theory.The influence of separation distance on the flame height under two-parallel case was analyzed and discussed.