Effect of convective losses on the spread of flame over pine needle beds

An FDS-based numerical model was developed considering the appropriate kinetic parameters of moisture release and pyrolysis reactions using the thermal decomposition model. Fire propagation experiments were conducted over pine needles for three fuel loadings (0.8, 1.2 and 1.6 kg/m²) and three different wind velocities (0, 1 and 1.8 m/s). Fire spread rate, mass loss, flame height, depth and angle, and bed temperature were used to validate the numerical model. The Byram convective number (N_c) was calculated for numerical and experimental data and the fire spread regimes were identified for different wind velocities and fuel loads. A dimensionless correlation for bed convective heat loss in terms of bed structure and wind velocities was developed. For a given bed structure, the convective cooling increases as the wind velocity increases. This is due to the movement of the air towards the fire front from the downstream side of the bed. A separate correlation in terms of bed optical thickness and Froude number was also developed for calculating the critical velocity at which the convective heat loss starts to decrease for a given bed structure. N_c was calculated for the critical velocity cases and a regime for rapidly propagating fires was identified. The extent of validity of the developed correlations is also discussed based on the critical velocity values. The findings are analysed from the trends and behaviour of particle convective heat loss data and air velocity contours for different bed structures and wind velocities.