An investigation of pyrolysis and ignition of moist leaf-like fuel subject to convective heating

The burning of a thin rectangular-shape moist fuel element, representing a living leaf subject to convective heating, was investigated computationally. The setup resembled a previous bench-scale experimental setup (Pickett et al., Int. J. Wildland Fire19, 2010, 153-162), where a freshly harvested horizontally oriented manzanita (Arctostaphylos glandulosa) leaf was held over a flat flame burner and burned by its convective heating. Computations were performed by FDS coupled with an improved version of Gpyro3D. This improvement was concerned with the calculation of the mean porosities in the computational cells to account for the net volume reduction that the condense phase experiences within the computational cells during moisture evaporation and pyrolysis. The dry mass was assumed to consist of cellulose, hemicellulose and lignin undergoing the pyrolysis reactions proposed by Miller and Bellan (Combust. Sci. Technol.126, 1997, 97-137) for biomass. The reaction scheme was initially validated against published experimental and computational TGA results. Then, the burning of leaf-like fuels with three initial fuel moisture contents (40%, 76%, 120%), selected as per the range of experimentally measured values, was modeled. The time evolutions of the normalized mass were good for the modeled fuels with 76% and 120% FMCs and fair for the one with a 40% FMC, as compared to the experimental burning results of four manzanita leaves with unspecified FMCs. The computed ignition time was also in good agreement with the measurement. The computed burnout time was somewhat shorter than the measurement. Modeling revealed the formation of unsteady flow structures, including vortices and regions with high strain rates, near the fuel that acted as a bluff body against the stream of the burner exit. These structures played a significant role in the spatial distribution of gas phase temperature and species around the fuel, which in turn, had an impact on the ignition location. Fuel moisture content primarily affected the temperature response of the fuel and solid phase decomposition.