Experimental study of the burning characteristics of dead forest fuels

Abstract

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.

A comparative study of the combustion dynamics and flame properties of dead Mediterranean plants

“…The results presented in Table 2 suggest that, on one hand, the dimensionless heat release rate is in the range Q 1 < < 100 fmax for shrubs (in agreement with the results of Pinto et al 2017), Pinus Pinaster (in agreement with the results of Dupuy et al 2003) and Eucalyptus. On the other hand, 0.1 < 1 fmax for straw, which is consistent with the results found by Sahila et al (2023) where l d ~( ) max c fmax 2/3 . This suggests that, for the same load and bulk density, the fuel type affects the scaling behaviour of the buoyancy-driven turbulent diffusion flames.…”

“…Forest fire is a very complex phenomenon involving physical processes that occur at different spatial scales ranging from the microscopic (smallest) scale, where the three phases of the fuel can be distinguished (solid, liquid and gas), to the gigascopic (largest) scale, where a fractal analysis of its pattern becomes feasible (Séro-Guillaume and Margerit 2002;Sahila et al 2021). At intermediate scales, many works have already been devoted to examining the behaviour of these parameters in the case of pool fires (Zabetakis and Burgess 1961;Tarifa 1967;Kung and Stavrianidis 1982;Babrauskas 1983;Koseki and Yumoto 1988;Koseki 1989;Klassen and Gore 1994;Chatris et al 2001), fire whirls (Martin et al 1976;Lei et al 2011;Pinto et al 2017) and natural fires (Thomas 1963;Dupuy et al 2003;Sun et al 2006;Sahila et al 2023). In the present work, an experimental study of the burning characteristics of several Mediterranean forest fuels was carried out where the turbulent diffusion flame was subjected only to buoyancy forces.…”

“…Anomalous diffusion and relaxation processes have been observed in various systems (Richardson 1926;Scher and Montroll 1975;Koch and Brady 1988;Bunde and Havlin 1996;Küntz and Lavallée 2001;Bennett et al 2003;Dobrovolskis et al 2007;Matthäus et al 2009;Llievski et al 2018). Sahila et al (2023) found recently that these anomalous processes drive the burning dynamics of dead straw, and they introduced a Kohlrausch-Williams-Watts (KWW) equation to accurately describe the time evolution of the source's heat flux.…”

“…It does not take into consideration other mechanisms such as the decomposition and formation of molecules(David 1975). In earlier work,Sahila et al (2023) studied the combustion characteristics of dead straw for different container sizes, and demonstrated that Eqn 4 can also describe a relaxation process where M f is replaced by a relaxation function ϕ (t) = (M f (t) − M fmin /(M 0 (t) − M fmin )) obeying Eqn 2 with K = 1/τ, τ being a relaxation time that measures the rate of decrease of the relaxation function (see Eqn 3), i.e. the rate of consumption of the fuel during a characteristic period (growth or decay).…”