Understanding the role of water evaporation and condensation in applied smoldering systems

Abstract

Accurately resolving the way energy is stored and transferred is critical to applied smoldering systems in porous media. Water evaporation and condensation are important mechanisms in the energy conservation of smoldering. This is particularly important when significant water presence can affect the potential for self-sustained smoldering as it occurs in a broad range of practical applications. This paper describes a one-dimensional smoldering model considering water phase change. The model was calibrated to a series of wet smoldering experiments of GAC varying initial water saturation from 5 to 20%. The results show that introducing simple calibration constants on the thermal properties of a wet porous medium can accurately predict the transient and spatial resolution of the progression of evaporation, condensation, and smoldering reactions. A sensitivity analysis suggests that wet smoldering could be improved by increasing fuel concentration and injected airflow, which provides practical insights into wet smoldering applications. In addition, the model was applied to establish five characteristic zones for the wet smoldering front: evaporation, condensation, pre-heating, smoldering, and cooling. The evaporation and condensation zones challenge smoldering propagation due to the high water saturation. The pre-heating zone represents the region separating the smoldering front and evaporation zone, which serves as an important dry buffer and requires a minimum thickness (4 mm) to avoid extinction.