On the level of complexity of the energy equation for solid heat transfer in flame spread

Modelling flame spread requires an adequate representation of multiple interacting processes to achieve satisfactory predictions. In fire safety engineering, the selection of conceptual models to represent these phenomena is typically subjective and often lacks detailed justification. This study investigates the influence of varying the level of complexity in the condensed phase on the predictions of time to ignition, and in-depth temperature profiles under flaming conditions. The condensed-phase energy equation was assessed and numerically decoupled from the gas phase using an experimental dataset for a 1 × 30 × 100 cm 3 PolyMethyl MethAcrylate (PMMA) sample. Results showed that accurate predictions of time to ignition can be achieved without describing the front surface heat losses; however, the variation of gas-phase temperature must be considered if these losses are included. A detailed description of boundary conditions was found to have a limited impact on in-depth temperatures compared to the inclusion of heat of pyrolysis, in-depth radiation, and vertical conductive heat transfer within the solid. While model error generally followed the expected complexity–accuracy curve, local minima and maxima have emerged in a few locations, highlighting the potential for counterintuitive behaviour in intermediate models. • Complexity–error predictions curve shows non-monotonic trends • Taxonomies revealed compensation between solid and gas-phase processes • Assuming ambient temperature caused significant errors in ignition time predictions.