Experimental And Theoretical Study On Effect Of Radiation And Convection Of Ceiling On Pyrolysis, Ignition And Flame Spread Of PMMA

The process of pyrolysis, ignition and flame spread of combustible solid is a common phenomenon in forest fires, building fires and industrial fires. Pyrolysis and combustion of solid fuel will release large amounts of toxic and harmful gases, which will seriously affect people’s safe. Thus the pyrolysis, ignition and flame spread process of combustible solid is a basic issue in fire safety. Understanding of the pyrolysis、ignition characteristics and flame spread mechanism of solid fuel fire is an important part of fire science research.In modern architectures, there are two main types of combustible solids used as construction material and decorating materials:charring and non-charring materials. Carbon is the main component of solid residues of pyrolysis production for the charring combustible solids. But content of char in pyrolysis of non-charring is less than 5%. The typical materials of charring solids and non-charring solids are wood and PMMA (Poly(methyl methacrylate)). The pyrolysis and combustion of non-charring solids will produce large amounts of toxic gases and there will be dripping and flowing phenomenon, therefore the fire hazardous of non-charring materials is greater than charring materials. So studying the pyrolysis, ignition and fire spread of non-charring solid materials has important theoretical and practical significance.The roof is a fundamental part of the buildings, for a roofed compartment, the development of fire out of control can be divided into four stages:ignition stage, fire growth stage, fully developed stage and extinguish stage. In an actual building fire, after a period of development, if there lack of effective control and fighting, the fire will grow very rapidly, when it reaches fully developed stage, flashover may occur. Flashover has a great threat to people’s life and property.Most fire disaster of the year 2015-2016 occurred in roofed structures, and the ignition and fire spread stage in a fire has great influence on the upcoming growth stage and flashover stage. So the study of ceiling effect on ignition and fire spread of combustible solid fuels is of great importance.At first, the present article analyzed the pyrolysis and ignition of non-charring material (PMMA) under radiation in different ambient pressure condition and under the different radiation orientation experimentally and theoretically; then based on the study of pyrolysis and ignition, the effect of ceiling on fire spread of vertical placed PMMA is studied experimentally and theoretically. The main components are as follows:1、Experiments are carried out at different altitude conditions in Lhasa (high altitude, low pressure) and Hefei (low altitude, atmospheric pressure) to study the pyrolysis and ignition characteristics of solid fuels. It is found the under same heat radiation conditions fire is easy to occur in Lhasa. Based on the experimental results, we built a solid pyrolysis model and a gas phase model in which counter-flow assumption is adopted. Combining the solid and gas phase models, a uniform models is built. The uniform models can be used to explain the effect of altitude, ignitor energy on ignition of combustible solids;2、The effect of seven different sample orientations from 0° to 90° on pilot and non-pilot ignition of PMMA exposed to radiation has been studied with experimental and numerical methods. Some new and significant conclusions are drawn from the study, including a U-shape curve of ignition time and critical mass flux as sample angle increases for pilot ignition conditions. However, in auto-ignition, the ignition time and critical mass flux increases with sample angle a. Furthermore, a computational fluid dynamic model have been built based on the Fire Dynamics Simulator code to investigate the mechanisms controlling the dependence on sample orientation of the ignition of PMMA under external radiant heating. The results of theoretical analysis and modeling results indicate the decrease of total incident heat flux at sample surface plays the dominant role during the ignition processes of auto-ignition, but the volatiles gas flow has greater influence for piloted ignition conditions;3、Experimental and theoretical analysis of studied the ceiling effect on vertical placed PMMA. Found that in different widths, in the case of samples of different lengths, due to ceiling radiation under fire conditions spread rate ceiling and weight loss rate is higher than the case where there is no ceiling. There is no ceiling on the roof and two series of experiments, each of the heat release rate per unit area of variation and size remained the same, but in the stage of full fuel, under the roof there are significantly higher than the thermal release no ceiling situation. The experimental and theoretical results show that the ceiling radiant heat feedback to the wall under the ceiling results in the case of samples with a significant difference between the incident surface heat flow, which is the cause and without a ceiling in both cases the main reason for the spread of the fire-related parameter changes;4、In the laterally fire spread of solid fuel under ceiling, the lateral fire spread rate presents a "U" variation with the increasing of ceiling height; and we can find that m∝Wn, n changes from -0.28 to 0.38; while xp-xb ∝w"’. m increases from 0.28 to 0.53.