Study On Pyrolysis And Ignition Process Of PMMA Under Periodic On-off Radiation

Radiation plays a leading role in many indoor fire scenarios,which determines the spread and development of fire.And it is the main heat transfer form in ignition and transmission of real fire.In actual fire scenario,radiation cannot be constant.Therefore,compared with the constant heat flux,it is more realistic and meaningful to study transient heat flux.Study of the ignition time,pyrolysis and ignition mechanism of the heat flux of solid combustible can help us better predict the occurrence of fire and reduce the loss.Thermal thick PMMA(polymethyl methacrylate)was chosen as material of experimental research in this article.An oscillation heat flux experimental platform was designed and built.By changing the oscillation period,a series of ignition experiments were carried out.At the same time,the corresponding physical model was established.The results of the experiments were verified and complemented by using the method of numerical simulation.Combining with the knowledge of heat conduction,we carried out deep theoretical analysis to explain the experiment phenomenon and finally established the prediction model of ignition time under periodic radiation.The surface and the in-depth temperatures oscillate with the periodic radiation.These oscillations are more obvious near the surface,but are hard to be observed at the bottom of the sample.The temperature attenuation is negatively correlated with in-depth distance and is positively correlated with the thermal diffusion coefficient of the material.At the time of ignition,the temperature of the different cycles eventually reaches the same value,so the ignition temperature can be regarded as a property of the material-specific.There is a time lag between the external heat flux and the surface temperature and the time lag between the depth temperature and the surface temperature.The time delay is related to the cycle length,in-depth distance and thermal diffusivity.The mass loss rate was almost zero in the initial 100 s,and began to increase steeply after about 100 s.It oscillated with the oscillation of the heat flux,and the critical mass flux at ignition was in the range of 1-1.4 g/m s.The theoretical ignition time data are in good agreement with experimental data.Compared with constant heat flux,the periodic heat flux delayed the occurrence of ignition,but with the increase of the cycle length,ignition time will decrease.That is to say,the ignition time decreases with the increase of cycle length,which is because that the average energy density increases as the cycle length increases.