A cost-effective CFD model for large-scale liquid fuel spill fires

This study presents a novel CFD model for ignitable liquid fires in fuel spill scenarios. The liquid fuel simulation is based on a 2-D thin-film ( similar to 1 mm) model previously developed in FireFOAM to treat water transport and fire suppression. The model is extended to simulate thick ( similar to 1 cm) liquid transport by adding temperature and velocity profiles at the sub-grid level. A sub-grid heat transfer model is implemented for in-depth heat transfer. Thermocapillary convection is modeled using a parabolic velocity profile driven by the local surface temperature (or surface tension) gradient. The current approach captures the key physics associated with the liquid spills, liquid heat transfer and fire spread with low computational costs, avoiding the challenge and expense incurred by multiphase flow ( e.g , volume of fluid) methods, which must track the moving liquid-air interface. Model validation is conducted using static methanol pool fires (0.3 and 1.0 m diameter), and flame spread over a rectangular pool (0.3 m x 0.9 m) containing n-decane and mineral seal oil, representing low and high flash point fuels, respectively. Grid convergence studies reveal that a 10-mm gas-phase grid and a 0.5mm sub-grid in the liquid-phase in-depth direction are sufficient for modeling liquid fuel fires. Predicted heat release rates (HRR) and fire spread rates are in good agreement with the measurements. Simulations show that thermocapillary convection plays an important role in transporting internal energy away from heated liquid in the burning region and promoting flame spread over the liquid pool, especially for higher flash point liquids. This work establishes a novel CFD modeling framework for computationally efficient simulations of liquid fire spread that is amenable to engineering-scale configurations and provides insights into the relative contributions of various heat transfer mechanisms present in such fire scenarios.