Combustion characteristics and dual-constraint predictive models of continuous spill fires on sand substrates

Liquid fuel spill fires pose significant hazards to petrochemical facilities and human safety. This study investigates the thermal and mass coupling mechanisms governing gasoline spill fire behavior on sand substrates via a continuous spill fire platform and integrated experimental-theoretical analysis. Sand particle size (0.125-0.65 mm) and bed thickness (5-25 cm) were systematically varied at a constant fuel supply rate (40 rpm). Sand substrates fundamentally alter classical pool fire self-limiting behavior through a dual-constraint mechanism coupling energy supply and Darcy-flow mass transport. Coarse sand (0.38-0.65 mm) exhibits single-peak HRR decay, with maximum HRR reaching 18.7 kW under thin-bed conditions (5 cm). Fine sand (0.125-0.212 mm) displays distinctive two-stage attenuation after fuel supply termination: an initial high-HRR plateau sustained by capillary-trapped fuel, followed by a low-HRR plateau governed by mass-transport limitation as the evaporation front migrates deeper. Under the present experimental conditions, a critical retention thickness of 15 cm is identified, where fine sand achieves complete fuel retention (0% leakage) and medium sand (0.212-0.38 mm) achieves an optimal balance between retention capacity and infiltration resistance, effectively converting high-intensity uncontrolled combustion into low-intensity manageable states. A seepage-depth modulation factor is proposed to quantify nonlinear suppression of combustion intensity, and a dual-constrained two-zone combustion model (gas-phase combustion zone coupled with liquid-phase seepage zone) is developed based on mass conservation. These findings provide a mechanistic basis for passive fire-protection design in fuel storage and transportation infrastructure.