Interactions of fires of neighbouring shrubs in two- and three-shrub arrangements

A physics-based computational model was utilised to better understand the interactions of fires generated by burning of neighbouring shrubs. The model included large-eddy simulation for flow field turbulence and a two-phase approach for the coupling of solid fuel and gas phases. Two different arrangements consisting of two and three identical shrubs placed adjacent to each other were considered. All shrubs were simultaneously ignited from their base with the aid of separate ground fuels. Both crown and ground fuels were modelled as porous media with thermophysical properties of chamise and excelsior respectively. Modelling results indicated that the peak mass-loss rate and the vertical fire spread rate within a shrub decrease when the shrub separation distance increases. At zero separation, heat release rate normalised by the number of shrubs is enhanced by 5 and 15% for the two-shrub and the three-shrub arrangements, respectively. Generation of strong vorticity by higher gravitational torque appeared to be the cause for enhanced burning in the three-shrub arrangement. This effect was seen to be much weaker for the two-shrub arrangement. Interactions between the individual fires cease for a centre-to-centre distance of 1.5 and 2 times the shrub diameter for the two-shrub and the three-shrub arrangement respectively.