Combustion Processes Laboratories

In this work, an enthalpy-temperature hybrid method is proposed for the numerical solution of generalized phase change problems, and applied to the prediction of polymer pyrolysis and ignition. The basic idea of this method is to treat both enthalpy and temperature as independent variables, and to solve the conservation equations and the constitutive equations (enthalpy-temperature relations) simultaneously. The formula of the enthalpy-temperature relations are not necessary the same for different phases, but can be chosen independently according to the characteristics of physical problems and the convenience of numerical analysis for each respective phase. Therefore this method applies to the problems regardless of the form of the constitutive equations. It overcomes the difficulty or even impossibility encountered in the traditional enthalpy-temperature method, of which either enthalpy or temperature must be consistently and explicitly expressed as a function of the other over all the phases. The method is first applied to a one-dimensional classical freezing problem for method demonstration and verification. It is found that the numerical results of temperature history and the position of phase change interface agree well with the analytic solution existing in the literature. The method is then applied to the numerical simulation of the pyrolysis and ignition of a composite material with a polymer as the matrix and fiberglass as the filling material. Three models of oxygen distribution in the molten layer are considered to explore the melting and oxygen effects on the polymer pyrolysis. Numerical calculation shows that high oxygen concentrations in the molten layer enhance the pyrolysis reaction, resulting in a larger amount of pyrolysate, but in lower surface temperatures of the sample. It also shows that distribution of oxygen in the molten layer has a strong effect on pyrolysate rate, and therefore on ignition and combustion of polymers. Comparison with available experimental data indicates that a model of oxygen distribution in the molten layer that is limited to a thin layer near the surface describes best the ignition process for a homogeneously blended polypropylene/fiberglass composite.