Impact of linearly increasing heat flux on the ignition of wildland fuels

An experimental and theoretical study was conducted on the ignition delay time of a forest fuel layer under controlled varying radiative conditions. Experimental data were collected using the Idealized-Firebrand Ignition Test (I-FIT) apparatus, which uses a cylindrical heater to emit a linearly increasing incident heat flux. Data acquisition ensured precise temporal resolution throughout the entire test. The experimental ignition time was then compared to predictions from a theoretical model based on the energy balance of the fuel layer. A simplified analytical solution was obtained for temperature evolution, providing an estimated ignition delay time at different incident heat fluxes. This methodology provides an estimation of the critical heat flux without requiring additional experiments. Special attention was given to the ignition temperature criteria by comparing estimated values with measured quantities. To understand how assumptions affected the simplified analytical solutions, a numerical solution was also compared. The narrow scatter observed confirms the reproducibility of the thoroughly characterized I-FIT apparatus. As the heating rate slope increases, the ignition delay and total energy required decrease. While the model provides useful first-order predictions, it relies on simplifying assumptions whose implications are discussed transparently.