Spontaneous ignition of vertically positioned wood samples under time-dependent heat flux

Abstract

Dynamic heating regimes are observed during structural and wildland fires. Most experiments to date use static/constant heating regimes, whereas ignition characteristics under time-dependent heat flux has been poorly studied. Existing apparatuses have limitations to study this effect, such as heating conditions and sample size/position. In this study, we conduct experiments on a custom-made apparatus to investigate the spontaneous ignition and convection cooling effect on this ignition of different vertically positioned wood species subjected to both static and dynamic heat fluxes. All experiments were conducted on cypress wood. Temperature, mass and ignition time were recorded during the experiment. Samples were exposed to 30 kW/m2 static heat flux and increasing heat flux. Preliminary results showed that for static heat flux two out of four samples were ignited at 100 sec and 78 sec with mass consumption 9.5% and 8% respectively. Two other samples were smouldered intensively during the heating period without flaming ignition observed. Their consumed mass was 28 and 31 % after 5 minutes. For increasing heat flux, ignition occurred at 743±7 sec with mass consumption 20.7±0.8 % for all four samples. Infrared video showed that flaming ignition for increasing heat flux started after the phase when the wood sample surface cracked into rectangular areas. This led to the heating of a deeper section of the sample, thus generating more pyrolysis gases, which was indirectly confirmed by the two times greater temperature at 3 mm depth for increasing heat flux. The next step of the proposed research will be to investigate the convective cooling on spontaneous ignition using various types of wood species. Different static and dynamic heating regimes will be used and their influence on the ignition process will be evaluated.