Experimental and numerical investigation of weak, self-sustained conditions in engineered smouldering combustion

Engineered smouldering applications require self-sustaining conditions where the fuel is consumed without any addition of energy beyond ignition. In practical applications, it is preferred to create scenarios that are robust (i.e., far from extinction). In this work, smouldering column experiments were conducted, spanning conditions from robust self-sustaining through weak self-sustaining and to extinction by systematically lowering the applied air flux and the fuel concentration. A previously developed one-dimensional numerical model, which was validated under robust smouldering conditions, was used to simulate the experiments. Experiments showed front deformation in the weak and extinction cases due to multi-dimensional effects. This deformation resulted in a redistribution of the air flow, altering the global energy balance at the centre-line. The model was found to be unable to accurately reproduce these weak and extinction cases with the original validation parameters. A sensitivity analysis revealed that treating the heat of combustion as a fitting parameter, and letting it vary beyond its realistic maximum, enabled the model to create a condition of sufficient energy at the centre-line and was the best way to quantitatively simulate the weak and extinction cases for this specific scenario. This reveals that the model assumptions, particularly the assumption of one-dimensionality, are sufficient for robust smouldering but are insufficient for weakly self-sustaining scenarios. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.