Research On Effect Of Radiation Attenuation And Flow Characteristic Of Pyrolysis Volatiles On Radiant Pyrolysis And Ignition Of Solid Combustibles

Radiant pyrolysis and ignition of solid combustibles is a complex process with heat and mass transfer in solid and gas phase, chemical reaction and fluid flow. Radiant pyrolysis and ignition of solid combustibles is the early stage in fire and is closely connected with other followed important fire processes, so it is always an interesting subject in fire research. Subjected to external radiant heating, the heated area of solid combustible starts to decompose when a certain temperature attained, the pyrolysis volatile emanates from the solid combustible and mixes with fresh air. Once the concentration and the temperature of combustible mixed gas are sufficient, the radiant ignition is triggered. In radiant ignition process, the incident radiant heat flux is absorbed and attenuated by the pyrolysis volatiles. The attenuation will retard the thermal decomposition and delay the time to ignition. It is still not known whether the radiation attenuation effect is significant in real fire situations. The radiant ignition simply is treated as a heat and mass transfer process with thermal decomposition in solid phase by some people before. However, by the difference of radiant auto ignition at different incident distances subjected to a same heat flux, it is found the consideration with single solid phase for radiant ignition is limited. Furthermore, the most of radiant ignition studies before are all carried out in bench scale condition, the correlative assumption and conclusion should be verified in the larger fire conditions.The radiant pyrolysis and ignition of solid combustibles has been studied experimentally and theoretically in this paper. The main content includes:the radiation attenuation, dilution and accumulation effects of the pyrolysis volatiles, and effect of the heat transfer in gas phase on radiant ignition.The radiant heating in fire is simulated by a resistant radiant heater. The degree of radiation attenuation of incident heat flux of pine and PMMA before ignition is obtained (6-14%). Based on Beer's law, a more reasonable parameter for determining the radiation absorptivity of pyrolysis volatiles of different fuels is presented, and the insufficiency of Kashiwagi's correlative conclusion is indicated. Then, the radiation attenuation effect has been successfully introduced in an improved PDE radiant ignition model based on Kung and our model before. It is found that the main calculated results have all been influenced obviously by the radiation attenuation effect, which illustrates that the consideration of radiation attenuation by pyrolysis volatiles in radiant ignition models is necessary.For the insufficiency of the consideration with a single solid phase for radiant ignition, the quantitative effect of the radiant distance on radiant ignition of solid combustibles has been investigated experimentally. It is found the time to auto ignition and the critical incident heat flux to auto ignition both increase with the increase of radiant distance which doesn't propose by other researchers before. Based on the quantitative concentration dilution characteristic of combustible pyrolysis volatiles and the non-monotonic distribution of gas phase temperature between radiant heater and sample surface obtained, the radiant auto ignition phenomenon above is well explained. The importance of the heat and mass transfer in gas phase for radiant auto ignition is pointed out.The effect of sample size on the radiant auto and piloted ignition of solid combustibles is studied experimentally. It is found the time to ignition decreases greatly with the increase of sample size, especially in low heat flux range (q"in<40kW/m2). For heating under a same heat flux, the mass flux in ignition decreases with the increase of sample size. As the increase of sample size, the flow mode of pyrolysis volatiles plume changes from the diluting to around with small size sample to accumulating to center with large one. It is found the measured concentration of pyrolysis volatiles in the sample center increases with the increase of sample size, which validates the corresponding hypothesize by other people before and emphasizes on the limited acceptance of the one dimension flow assumption of pyrolysis volatiles in the gas phase part of radiant ignition model.