Radiation effects on horizontal flame spread under non-steady airflows

Thermal radiation from the flame to the unburned fuel plays a key role in horizontal concurrent flame spread by heating the fuel surface and influencing the spread of the flame. This work investigates thermal radiation in horizontal concurrent flame spread under non-steady forced airflow conditions. Experiments are conducted using black cast polymethyl methacrylate (PMMA) sheets with a thickness of 1 mm as fuel. A non-steady airflow with a sinusoidal profile was applied in each experiment, with a baseline of 300 mm/s, amplitudes of 100 and 200 mm/s, and frequencies of 1/32, 1/16, and 1/8 Hz. Radiation emitted by the flame is partially captured through heat flux gauge measurements and interpreted using geometrical data extracted from video footage. View factors between the flame and target surfaces (heat flux gauge and unburned sample) are calculated using a contour integral technique, with contours derived from parametric representations of the flame geometry based on flame height, burnout front, pyrolysis front, and flame tip position. This allows estimation of radiative heat transfer to the heated zone. Results show that the magnitude of flame radiation to the sample does not change significantly under non-steady airflow; however, the size of the heated zone exhibits a strong transient response. This response is attributed to the transient variation in flame extension length over the unburned surface. Additionally, two-dimensional spatial distributions of incident radiative heat flux to the heated zone are calculated and analyzed. The average flame spread rate is estimated by integrating the incident radiative heat flux, neglecting the convective contribution, which resulted in an underestimation of 18%–36% compared with the experimental flame spread rate.