Bruno Coudour

The present work provides a detailed experimental and numerical study on suppression of a heptane medium-scale pool fire by applying water mist. The customarily described well defined structure of the flame does not exist any longer by application of water mist. It is observed that the size and shape of the flame vary randomly and continuously due to sudden and short mist vaporization periods. An injection of the finely divided water mist (50μm) into a buoyancy-induced fire has a combustion-supporting effect of a heptane pool fire. In no mist case, contribution of the radiation heat flux to the pyrolysis rate of liquid fuel seems predominant. However, with addition of water mist, an increasing trend in burning rate is related mainly to the significance of convective heat flux due to moving of the flame region towards the liquid surface. This behavior suggests that extinguishment is achieved by total clearance of the liquid surface rather than from reduction in flame-to-liquid surface heat feedback. The numerical work provides the detailed temperature and species fields in addition to radiation/convection heat fluxes over the liquid surface during the addition of a water mist by using various drag force models.

The objective of this work is to assess the accuracy and limitations of two different semi-empirical soot models: the Laminar Smoke Point (LSP) and soot-yield approach. A global soot formation model based on the LSP concept is embedded within FDS6.7. Quantitative comparisons were made from turbulent buoyant pool fires between several computational results and well-instrumented experimental databases on the soot volume fraction, mass loss rate, heat release rate and gas temperature. The LSP model in combination with soot oxidation and surface growth is validated for most of the methane, ethylene and heptane turbulent buoyant pool fires, covering a wide range of fuel likely to form soot. This paper aims to broaden the scope of the validation of the available semi-empirical soot modelling. For the porous methane and ethylene burner, the LSP model was found to provide a better description of the soot volume fraction. The overall visual soot distribution is also numerically reproduced with the soot-yield approach, but as expected, there are some differences between the prediction and the measurement regarding the magnitude of soot volume fraction. The computed radiant heat flux was compared with experimental data for heptane flame, showing that predictions using both the LSP and soot-yield models were found to be twice the value of experimental data, although the measured HRR (Heat Release Rate) is reliably reproduced in the numerical simulation. For the heptane buoyant pool fires, a sufficient accuracy of the numerical model is confirmed only in some of the locations as compared to the experimental results. It is demonstrated that neither the temperature nor the soot volume fraction can be reliably calculated in the necking flame flapping region when the pyrolysis rate of condensed fuel (heptane) is coupled with radiation/convection heat feedback. This implies that an accuracy of prediction on the turbulent buoyant pool fires depends on the studied fire scenario regardless of the semi-empirical soot models.

Background: Wildfires are a growing threat, especially in Mediterranean climate areas during periods of drought. The wildfire research community continues to investigate propagation mechanisms considering thermal transfer and fluid mechanics and sometimes a simplified chemistry taking into account the main combustion products such as CO, CO2, H2O, H2 and CH4. However, volatile organic compounds (VOCs) cannot be neglected and have a major role in vegetation ignition. Despite this, knowledge on the influence of wildfire on plant emissions remains lacking. Aims: This work addresses the effect of combined hydric and thermal stresses on the physiology and emissions of Rosmarinus officinalis and Cistus albidus, which are widely consumed in Mediterranean wildfires. Methods: Plants were submitted to hydric stress inside a greenhouse, then to thermal stress inside a heated hermetic enclosure while collecting emissions with adsorbent elements for analysis. Simultaneously, heating experiments were carried out with leaves sampled to analyse their emissions. Key results: The heating tests showed significant variations in monoterpene, fatty acid and alkane emissions between stressed and unstressed plants with different results between Rosmarinus officinalis and Cistus albidus. Conclusion/implication: Further experiments with other plant species and quantification of emissions may help to better know why Mediterranean wildfires are a particular threat and how to improve firefighting methods.

Flashover phenomena in wildfires have been reported by firefighters and have caused a number of fatalities among their ranks

Sometimes forest fires have unpredictable and dangerous behaviors for human lives. One of these behaviors leads to accelerating forest fires, characterized by an augmentation of the rate of spread and of the energy released by the fire. The phenomenon mechanisms are not well understood but a thermochemical approach supports that volatile organic compounds coming from heated vegetation and smoke may create a flammable atmosphere near the fire front; according to the literature, α-pinene (C₁₀H₁₆) and benzene (C₆H₆) were found to be the main compounds. Therefore, we studied α-pinene/benzene/air mixtures by varying the initial temperature from 75°C to 180°C, equivalence ratio from 0.7 to 1.4, and fuel proportion. We focused on two combustion characteristics: laminar burning speeds and Markstein lengths. We observed a maximal laminar burning speed shifted from 1.1 equivalence ratio, when mixtures are rich in α-pinene, to 1.2, when mixtures are rich in benzene. With the increase of benzene percentage, laminar burning speeds also increase and the flame becomes more stable.