Background: A deeper physical understanding of flame behaviour is necessary to make more reliable predictions about forest fire dynamics.

Aims: To study the container size effect on the combustion characteristics of herbaceous fuels.

Methods: Dead samples were put in cylindrical containers of different sizes, and were ignited at the lowest circumference of the basket in the absence of wind.

Key results: In the growth phase, there is an anomalously fast relaxation of the fuel mass accompanied by a super-diffusion of the emitted gas species, whereas in the decay phase, there is a stretched exponential relaxation and the gas species sub-diffuse through the porous fuel. The crossover between these two anomalous processes occurs when the flame is fully developed. Thomas’s correlation between flame height and fuel bed size, and the two-third power law dependence of the normalised flame height on the Froude number, fit the experimental data well. The flame height variation with burning rate exhibits a hysteresis cycle, implying the existence of memory effects on flame formation.

Conclusions: The observed relaxation regimes and hysteresis cycle seem to drive fire dynamics and behaviour.

Implications: Further investigation of the influence of the fuel geometry and porosity on these properties is necessary.