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.

The fuel flammability, combustibility and fire spread properties depend on several parameters. Among these parameters the heat flux, moisture content, wind and the fuel load. Flammable ignition occurs when the emitted organic volatile components flow mixed with air reaches to a minimum rate corresponding to the lower flammability limit. The gas flow of these components depends on the fuel quantity (load) and its temperature. Therefore, ignition time depends on the fuel load. In this work the effect of the load on ignition time of live Pinus Halepensis needles is investigated using a cone calorimeter (providing three incident heat flux intensities). The average fuel moisture content is around 50% in wet basis. Ignition time exhibits an exponential trend for all the incident heat flux intensities considered with a correlation coefficient R^2>0.96. The exponent corresponds to a characteristic load m ̅_0 around 1 kg/m² slightly dependent on the incident flux within statistical errors. For very small loads compared to m ̅_0 ignition time is nearly independent of load, because the time required for water evaporation is neglected compared to that required for volatiles mixture with air to reach the lower flammability limit. For larger loads (in the range 0.38 kg/m² to 1.27 kg/m²), the time required for water evaporation introduces a shift in ignition time which increases linearly with load. Finally, for large loads compared to m ̅_0, the fuel bed thickness becomes larger than the optical length, and only its top layer is heated, transmitting thus heat to the internal layers by conduction. Therefore, the internal layers evaporate water and cool the upper one, leading to exponential of ignition time with the load. For dry fuels, ignition time appears independent of load.