Relevant model complexity for non-charring polymer pyrolysis

Abstract

The choice of the heat, mass and chemical mechanisms included in a pyrolysis model is often subjective, and detailed justifications of the inclusion or exclusion of the different mechanisms are infrequent. The implicit assumption that models with a higher number of mechanisms reproduce more accurately the reality has led to the recent growth of complexity in pyrolysis modelling seen in the literature. However, as we show in this work, the comparison of several conceptual models predicting the same experimental results does not support this assumption, but reveals the presence of unnecessary complexity and multiple sources of uncertainty. Using a novel approach corresponding to a mechanism sensitivity, the influence of the heat, mass and chemical mechanisms on the transient predictions of surface temperature and mass loss rate (non-flaming conditions) for PolyMethylMethAcrylate (PMMA) samples is investigated. While a small change in the chemical degradation mechanism has a large effect on the predictions of the mass loss rate, the surface temperature is not affected. The heat transfer mechanisms appear to have however a significant effect on both quantities of interest. This study demonstrates that the use of complex chemical mechanisms (e.g. multi-step reaction scheme or oxidative reaction) is not justified if the mechanisms of the heat transfer are kept simple. It is therefore recommended to use consistent levels of crudeness dictated by the heat transfer.