Abstract

This study investigates the influence of convective and radiative heat transfer on horizontal concurrent flame spread under non-steady airflow over black-cast polymethyl methacrylate (PMMA) sheets. Bench-scale experiments are conducted in a flow duct under sinusoidal airflow, with controlled baseline airflow, flow amplitude, and frequency. Convection and radiation are separated from the total heat flux measurement to a total heat flux gauge in the heated zone. For the lowest amplitude and steady-flow conditions, radiation is the primary contributor when the separation between the pyrolysis front and the heated-zone target is large; at smaller distances, convection becomes the primary contributor. Conversely, for the highest transient flow amplitude, the dominance alternated frequently, exhibiting a pronounced intermittent behavior as the flame approached the target. For the highest amplitude, the flame spread rate measurements suggest that the flow imposes a strong influence on the solid-phase, whereas no such effect is observed at the lowest amplitude. For the highest amplitude cases, the instantaneous flame spread rate exhibited a temporal trend opposite to that of the incident radiative heat flux and the imposed oscillatory airflow. The observed flame behavior in this study corresponds to regimes of mixed and natural convection only. These results reveal that, even without reaching forced convection, oscillatory airflow can strongly modulate heat transfer and flame spread dynamics through its interaction with buoyancy-forced flows. Novelty and significance statement This work provides the first experimental investigation of heat-transfer mechanisms during horizontal concurrent flame spread under oscillating airflow conditions. By resolving radiative and convective heat fluxes ahead of the flame, the study reveals how transient forced flow modifies heat feedback to the unburned fuel during flame propagation. Unlike steady airflow conditions, oscillatory flow introduces time-dependent variations in both the magnitude and time evolution of heat transfer, leading to intermittent and phase-dependent coupling between flame motion and heat feedback. The results show that the interaction between oscillatory forcing and buoyancy can produce alternating regimes in which radiation or convection temporarily dominates the heating ahead of the pyrolysis front, even when the overall flow remains in the mixed or natural convection regime. These findings provide new experimental insight into transient flame-flow interactions and support modeling of flame spread under time-dependent flow conditions.