Mathematical/numerical modeling on moisture evaporation, pyrolysis and smoldering of porous media

Abstract

“I was able to see further because I was standing on the shoulders of giants” Sir Isaac Newton A full procedure of numerical modeling on Moisture Evaporation, Pyrolysis and Smoldering (MEPS) of porous material has been extensively discussed. From the starting point on fundamental theories development on MEPS modeling, implementation on numerical techniques to the computational code realization, an in-depth discussion was illustrated in this thesis. The effects of material physical and chemical structure, anisotropic characteristics, heat and mass transfer in porous media which affecting the behavior during evaporation, pyrolysis and smoldering are extensively reviewed. A new mathematical model for evaporation, pyrolysis and smoldering of porous media which included sophisticated chemical and physical processes involved has been developed. The model comprises detailed considerations on fundamental multi-phase heat transfer (conductive, convective and radiative), momentum and mass transfer (evaporation, pyrolysis and smoldering). It was found that the proposed model cannot be solved by standard Finite Volume method alone due to the complexity of proposed mathematical model. Some specific numerical modeling problems such as thermal non-equilibrium between phases, pressure coupling inside low permeability material, un-saturated moisture evaporation and high order chemical reactions need specific treatment. Several innovative methods on tackling these problems were introduced. These treatments were found robust in solving modeling problems and without introduced much additional computational demand. Details of these methods were illustrated in thesis. With the merits of those innovative techniques, the proposed model was solved successfully and implemented in a computational code which written in FORTRAN 90. The general procedures and the remark points utilized in the MEPS computational code are illustrated. The computational code was applied to predict the moisture evaporation and pyrolysis of wet wood. The results show that current model is superior in the prediction of mass loss rate and the surface temperature in different heating conditions. It demonstrated that the current model can be used to describe the sophisticated chemical and physical process. Meanwhile, a further modeling validation was undertaken on smoldering of polyurethane foam. The predicted temperature history during smoldering was found excellent agreement with the measured data. The phenomenon found in the experimental, included preheating, settling, front marching and quenching, are also captured by the present model. A preliminary extension of the model for the simulation on autoignition, pilot ignition and flame spread is also discussion in the late section. These simulations involved the comprehensive solid-gas combustion. It demonstrated that the proposed model is capable on modeling solid-gas combustion which only some minor modifications are needed to be made on both equation set and numerical implementation. This possibility greatly reduces the computation effort on performing solid-gas combustion.

Date of Award	2 Oct 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	W. Z. LU (Supervisor)