Firebrand burning under wind: an experimental study

Abstract

Background

Spot fires play a significant role in the rapid spread of wildland and wildland–urban interface fires.

Aims

This paper presents an experimental and modelling study on the flaming and smouldering burning of wood firebrands under forced convection.

Methods

The firebrand burning experiments were conducted with different wind speeds and firebrand sizes.

Key results

The burning rate of firebrands under forced convection is quantified by wood pyrolysis rate, char oxidation rate and a convective term. The firebrand projected area is correlated with firebrand diameter, char density, wind speed, and flaming or smouldering burning. A surface temperature model is derived in terms of condensed-phase energy conservation. We finally establish a simplified firebrand transport model based on the burning rate, projected area and surface temperature of firebrands.

Conclusion

The mass loss due to wood pyrolysis is much greater than that due to char oxidation in self-sustaining burning. The burning rate is proportional to U^1/2, where U is wind speed. The projected area for flaming firebrands decreases more rapidly than that for smouldering ones. The firebrand surface temperature is mainly determined by radiation.

Implications

Knowledge about firebrand burning characteristics is essential for predicting the flight distance and trajectory in firebrand transport.