Ignition/non ignition phase transition

Abstract

The flammable/non flammable transition is investigated both experimentally and numerically within the framework of statistical mechanics and percolation-type phase transitions. The heat flux threshold for ignition is usually estimated deterministically by fire research community. It is defined actually by ASTM standards as the average between the minimum flux allowing ignition and the maximum one which does not allow it. The probabilistic aspect of the ignition transition is demonstrated here, and a new method for the estimation of the ignition critical heat flux is proposed. Dead wheat straw and live pinus halepensis needles are investigated experimentally using a cone calorimeter and numerically using a simple physical model. The experimental data on ignition time are different from those obtained numerically. This difference is due to the fact that the numerical model assumes auto-ignition whereas experimental data are obtained by using a pilot flame. It is found that the variation of ignition time with the incident heat flux obeys a universal power-law behaviour near the heat flux threshold. The critical heat flux for ignition estimated from this law is also obtained using the ignition probability method. This critical heat flux is as low as it cannot be obtained using the ASTM standards. The power-law exponent is found compatible for both dead straw and pinus halepensis suggesting a universal phase transition. Further discussions on the comparison between flammability transition and spread/non spread transition are provided.

Is there a critical fuel moisture content for flammability?

“…Chapter 3 -Fire Management / ² and its magnitude is fixed to 17 kW/m². This magnitude of the heat flux is far from the critical heat flux for pinus halepensis needles, which is about 10 / 2 (Sabi et al 2018). The ignition is controlled by a pilot flame located 1 cm above the sample surface (Torero 2016), and the ignition time is recorded.…”

“…The power exponent in Fig. 3 seems to be around -0.8 independently of the moisture content and of the fuel nature (see Sabi et al 2018), indicating a universal phase transition (Stanley 1971). Therefore, for any moisture content, there exists a sufficiently large heat flux allowing the fuel ignition.…”

“…The heat flux received by the fuel from the epiradiator used by Trabaud might be much smaller than 17KW/m² (provided by our cone calorimeter). The heat flux provided by the epiradiator could be smaller than the critical flux for ignition, which is around 5.5 / 2 for pinus halepensis needles (see Sabi et al 2018). In order to answer this question, we assume the critical moisture as an exothermic/endothermic phase transition.…”

“…For each of these moisture contents, only one ignition test was successful, whereas far from the moisture threshold, in all tests, the ignition process occured. Therefore, a probabilistic behaviour occurs near the moisture threshold, which should correspond to the critical heat flux (Sabi et al 2018). In order to check whether the ignition threshold is due to the heat flux or the moisture content, a set of tests with fresh and dry samples were realized for different values of the incident heat flux ".…”

“…In order to check whether the ignition threshold is due to the heat flux or the moisture content, a set of tests with fresh and dry samples were realized for different values of the incident heat flux ". The critical heat flux for ignition defines the endothermic /exothermic transition (Sabi et al 2018). The ignition time behaves near this transition as: tign α (q"-q"c) -…”