Concurrent flame spread over discrete thin fuels

An unsteady two-dimensional numerical model was used to simulate concurrent flame spread over paper-like thin solid fuels of discrete configurations in microgravity (0 g with 20 cm/s) and in normal gravity (1 g). An array of ten 1 cm-long fuel segments was uniformly distributed in the flow direction (0 g) or in the vertical direction (1 g). A hot spot ignition source was applied at the upstream leading edge of the first fuel segment. The separation distance between the fuel segments was a parameter in this study, ranging from 0 (corresponding to a continuous fuel) to 3 cm. Using this setup, the burning characteristics, spread rate of the flame base, and the solid burning rate were examined. The flame base spread rates in both 1 g and 0 g cases increase with the separation distance. This is due to the flame jumping across the gaps. For the solid burning rate, the dependency on the separation distance is different in 1 g and 0 g cases. At a flow velocity of 20 cm/s in 0 g, the flame reaches a limiting length and the flame length is approximately the same for all fuel configurations. As the separation distance increases, the heating length (the fuel area exposed to the flame) decreases, resulting in a decreasing total heat input and a decreasing solid burning rate. In 1 g, the flame is long and extends to the last fuel segment before the first fuel segment burns out. This suggests that the heating length is approximately the same in all simulated cases (∼total fuel length). However, the flame standoff distance decreases when the separation distance increases. This results in an increasing total heat input and an increasing solid burning rate. Terrestrial experiments were conducted to validate the 1 g model. The experimental results agreed reasonably with the model predictions of burning characteristics, burn durations, and flame spread rates.