Pyrolysis and Combustion Characteristics of Leaf-like Fuel under Convection and Radiation Heating

The heating of a fuel element representing a vertically oriented living manzanita leaf was computationally investigated. The computational conditions resembled a previous experimental setup where manzanita leaves were exposed to an upward stream of hot gases supplying the convective heating and a radiant panel providing the radiative heating. The simulations confirmed the experimental observations as to what the impact of the heating mode is on ignition. The convection-only and combined convection and radiation modes resulted in ignition and flaming whereas the radiation-only mode did not. The leaf mass loss versus time, the heating pattern of the leaf, the ignition time, and the flame spread pattern in simulations were in good agreement with the experimental findings. Furthermore, the simulations revealed the impact of the heating mode on thermochemical evolution of the leaves, formation of boundary layers on the leaf faces, ignition, and formation and propagation of the flame. An analysis based on the postprocessing of the numerical data showed that except for the first second, the fuel may be considered thermally thin. Ignition occurred near the lower corners of the leaf, which were exposed to the highest heat fluxes compared to the rest of the leaf. In the combined heating mode, the fuel pyrolyzed faster and ignition occurred earlier because the overall heat flux was larger. The computed heat release rates per unit volume showed that the flame is formed in the lower part of the leaf, and then moves and propagates upward. The computations revealed that the overall heat release by combustion of pyrolysis gas was not sensitive to the mode of heating.