Experimental And Theoretical Study On Pyrolysis And Opposed-flow Flame Spread Of Typical Non-charring Polymers

Pyrolysis and flame spread of solid combustible are important subprocedures in initial and development stage in fire which determine the subsequent process, and thus have been the focus of numerous researchers all over the world in the last decades. Pyrolysis is also the one of sub processes of flame spread, and mutual relation exists between both of them. More and more attention is paid by the investigators to the special spread behaviors in fire of non-charring polymers used extensively as structure and decorative materials in modern buildings. When sample is heated by external heat flux, thermal degradation reaction occurs in the non-charring material. Volatiles generated by the polymer are transferred from interior to the surface of samples by bubbles and then diffuse to the ambient environment. Combustible premixed gas fuel mixed by the volatiles and air is ignited at the surface of polymer on which combustion reaction take place and sustained flame is established over the material. Some of the energy produced by the combustion is used to heat the combustion products and some other is released to the environment by radiation. The rest, feedback heat flux, is used to preherat the virgin material by convection, radiation and conduction. Combustible fuel is released to maintain the gas phase combustion when the virgin polymer is heat. This continuous cyclic process leads to the sustained flame spread process over the polymer. The flame spread rate of non-charring polymer is much larger than that of charring polymer due to the absence of the char layer which emerges during the pyrolysis of traditional charring polymer, such as paper and wood, and block the external incident heat flux. Futhermore, the melt and flow characteristics of non-charring polymer will also enhance the spread rate.Experimental study and numerical simulation about pyrolysis of three typical non-charring polymers, Poly(methyl methacrylate)(clear PMMA), Poly(acrylonitrile butadiene styrene)(ABS) and High Impact Polystyrene (HIPS), under three different heat flux in cone calorimeter and nitrogen atmosphere are conducted in this thesis. Based on the study of pyrolysis, experimental and theoretical study on opposed flow flame spread over non-charring polymers under natural convection condition with finite dimension are performed, namely, the effect of thickness, width, ambient pressure and finite dimension on the flame spread process. Specific content is summarized as follows:For the prolysis, experimental investigation about one dimensional pyrolysis is conducted under low, medium and high heat flux in cone calorimeter and nitrogen atmosphere for PMMA, ABS and HIPS. The validity of the non-contact temperature measurement method is verified by the comparison between the bottom surface temperature measured by thermocouples and infrared thermal camera. The piecewise linear relationship between thermal conductivity of material and temperature is obtained through the measured bottom surface temperature and mass loss rate in experiments coupled with the ThermoKin model of Stanislav I. Stoliarov. Furthermore, an one dimension pyrolysis model of non-charring polymers, which considers the absorption methods of incident heat flux at the surface of sample and thermal degradation reaction in interior of solid, is developed based on the experiments. Investigation and comparison about surface and in-depth absorption hypotheses, commonly used in literatures, are conducted in the model, and the result indicates that different absorption methods have significant influence on top surface temperature and the temperature distribution in heat penetration layer, which can be reflected in the distinct discrepancy of ignition time in air atmosphere. Both simulation results with different absorption assumption are acceptable when the details of the micro pyrolysis in the polymers are not the focus. Also, the correctness of the established model is validated by the comparison between experimental and simulation results about mass loss rate and bottom surface temperature.For flame spread, experimental and theoretical analysis of opposed flow flame spread are performed under natural convection condition in different environmental pressure. The results show that the opposed flow flame spread rate increases with the increasing thickness when the width of sample is small, which is distinctly different with the conclusions of that two dimensional infinite width condition. The explanation is that the lateral combustion at the two sides accelerates the spread process for finite dimension material. With fixed thickness, the flame spread rate inversely proportional to the width of sample. In different environmental pressure, Hefei, Xining and Lhasa, the mass loss rate, flame height and spread rate increase with increasing ambient pressure. In low atmospheric pressure, chemical reaction kinetics and flame feedback heat flux in gas is the controlling mechanism for flame spread rate. When the Damkohle, which characterizes the gas phase chemical reactions, decreases below a critical value, no sustained flame can be maintained over the surface of polymer. A simplified model is developed in this thesis based on the theoretical analysis from the perspective of heat and mass transfer, in which the spread rate is a function of dimension of sample, leading edge angle and thermal and physical parameters of material. Meanwhile, the model provides the feedback heat flux measurement method for pyrolysis and preheated zone by measuring the flame spread rate and mass loss rate. Also, the good agreement between the experimental and analytical results verifies the validity of the simplified model.