OPPOSED-FLOW FLAME DEVELOPMENT OVER MELTING POLYMERS - MODELING AND GLOBAL ENERGY BALANCE ANALYSIS

摘要

A numerical model for flame spread over polymeric surfaces is investigated. The physical mechanisms considered in the condensed material include phase change (melting), in-depth pyrolysis reaction and heat conduction. The physical mechanisms considered in the gas phase, using the Oseen approximation, include heat transfer, fuel and oxidizer transfer, and finite-rate combustion kinetics. The mechanisms at the interface include heat conduction of both phases, radiant emission from the surface, and diffusion of pyrolysis products. The dependence of the flame spread rate on phase change and thermal properties is investigated by varying three non-dimensional parameters, St, (k{sub}l){top}-,and (C{sub}(pl)){top}-.Quantitative comparisons of flame spread rate indicate that the numerical model matches very well with an analytical formula. An energy balance analysis is performed for both ignition and flame spread processes in order to establish the importance of various heat transfer mechanisms and develop a simple theory. The linear relationship between ignition time and 1/St (or (C{sub}(pl)){top}-) from the numerical results is predicted by a simple theory. Good quantitative agreement between this theory and the numerical results is found for variable (C{sub}(pl)){top}-, and fair agreement for variable St. For variable (k{sub}l){top}-, only qualitative deductions can be obtained from this theory. The mechanisms of flame spread at the steady state are also interpreted by performing the energy balance analysis on various control volumes selected.