Numerical study on opposed-flow flame spread over discrete fuels – The influence of gap size and opposed-flow velocity

Abstract

Flame spread over solid fuel is a common phenomenon in fire scenes. This work looks into the relationship between the flame spread and the gap length between fuel beds at different opposed-flow velocities. An unsteady numerical model is developed to study the opposed-flow flame spread over discrete fuels. The influence of the gap size between the discrete fuels and the opposed-flow velocity on the opposed-flow flame spread is investigated, and the results are validated with an analytical model. The characteristics of solid burning, gas fuel and flame spread rates are examined. The results of temperature and gas fuel distribution show that the heat transfer from the flame to the discrete fuels decreases as the gap size increase. The pyrolysis front lags behind the leading position of gas thermal structure because the discrete fuels behind the flame front can provide fuel gas to the flame front by diffusion for combustion. The mass diffusion plays a significant role in the flame spread across the gaps. The average flame spread rate increase first and then decreases with the increase of gap size, because of the competition mechanism between the flame jump effect and the heat transfer. Besides, the flame cannot spread across to the next discrete fuel if the gap size increases to a critical value. The analytical and numerical results showed that the critical gap size is proportional to the reciprocal of opposed-flow. This work should be significant to predict the fire growth of discrete fuels and its potential hazards.