Ignition of wood under time-varying radiant exposures

•The non-piloted ignition temperature of spruce wood is highly variable.•Ignition temperature is more variable for time-varying exposures than for constant.•A numerical heat transfer model yields good predictions of ignition time.•Predictions of ignition time are more accurate when the heating rate is greater.

Many studies have utilized a small-scale experimental apparatus such as the cone calorimeter to investigate the piloted ignition of wood exposed to constant levels of incident heat flux; however, there is a deficiency of similar studies related to the non-piloted ignition of wood exposed to time-varying heat fluxes which might represent more realistic fire exposures. In this study, a method was established for producing well-controlled, time-varying exposures using the conical radiant heater of a cone calorimeter. Experiments were conducted in which the incident flux, time to non-piloted ignition, and back-surface temperature of spruce wood were measured. Measured data were used in combination with a numerical heat transfer model to compute the time-dependent temperature distribution through each specimen, and thereby deduce the surface temperature at ignition. From the 30 specimens tested, the average surface temperature for non-piloted ignition of wood was determined to be 521±10 °C. From this surface temperature range, the heat transfer model was used to predict the range of time over which non-piloted ignition was likely to occur for a given time-varying exposure. This procedure was found to produce excellent predictions of ignition time for the time-varying exposures considered in this study. In addition, several existing ignition models were considered, and their suitability for predicting the non-piloted ignition of wood was assessed.