Sustained flaming ignition of Quercus fabri leaf-litter fuel beds by a smouldering cigarette: Effects of moisture, packing ratio and airflow

Ignition of porous lignocellulosic fuel beds by weak smouldering sources is governed by moisture, bed-structure and airflow conditions, yet the transition to sustained flaming remains poorly quantified. We examined Quercus fabri leaf-litter fuel beds under a full-factorial combination of five fuel moisture contents (FMC) levels on a dry-mass basis, five packing ratios and five nominal airflow velocities, with 30 replicates per combination (3750 trials). Sustained flaming ignition occurred in 55 trials (1.47%), all at the FMC levels of 0—15% and nominal airflow velocities of 3—5 m/s; treatment-specific probabilities ranged from 0 to 16.7%. Conditional time-to-ignition ranged from 67 to 195 s. Ignition probability generally decreased with FMC and increased with packing ratio and airflow, whereas conditional time-to-ignition increased with FMC and decreased with packing ratio. Classification models achieved ROC-AUC values of 0.8630—0.8721 and PR-AUC values of 0.0700—0.0791 under grouped cross-validation; XGBoost performed best for conditional time-to-ignition ( R 2 = 0.905; RMSE = 9.976 s). SHAP analysis identified packing ratio and airflow as leading contributors to ignition occurrence and FMC as the leading contributor to time-to-ignition. These results characterise a low-probability ignition response in a porous lignocellulosic fuel bed and identify the observed moisture and airflow transition intervals within tested experimental conditions.