Experimental and theoretical analysis of the smoldering-to-flaming transition in moist pine needle beds under unsteady wind flow

Background Smoldering in forest litters is prone to transition to flaming under dry and windy conditions. Most studies focus on smoldering-to-flaming (StF) transition under steady wind conditions, however, a knowledge gap persists regarding periodic wind oscillations in wildfires. Aims This study aims to elucidate the effects of unsteady wind on the StF transition in moist pine needle beds (PNBs), focusing on moisture content (MC), maximum wind speed, wind frequency and direction. Methods Laboratory-scale experiments combined with Fast Fourier Transform (FFT) technique and theoretical analysis were conducted. Key results Higher wind frequencies reduced the peak smoldering temperatures and lateral spread rate, delaying the StF transition. Theoretical analysis derived that the pressure penetration depth is inversely proportional to the square root of the frequency (i.e. δp∝f−1/2), indicating shallower gas penetration at higher frequencies. This limits oxygen transport to deeper char layers, suppressing deep-seated char oxidation . Conversely, the predicted gas penetration velocity increased with wind frequency (i.e. uy(t)=−φπfκP∞μ(1−1e)ηρ∞U0Asin(2πft+ϕ)), enhancing surface convective cooling. Conclusions The dual mechanism of suppressed deep-seated char oxidation and intensified surface cooling impeded the necessary heat accumulation for the StF transition, explaining the observed StF transition delay under high-frequency wind conditions. Implications This work advances the fundamental understanding of the StF transition behavior in unsteady wind fields closely linked to realistic wildfire scenarios.