Experimental study of downward flame spread over a thin cellulose-based sample in reduced and enhanced pressure

Downward flame spread, a canonical burning configuration, provides an effective platform for investigating material flammability and fire dynamics owing to its steady characteristics. Previous studies primarily employed reduced-pressure environments to simulate flame behavior in micro- and partial-gravity using pressure-modelling approaches. Although reduced-pressure effects have been extensively examined and several correlations relating flame spread behavior to ambient pressure have been proposed, their validity under elevated pressures remains largely unexplored. This experimental study investigates flame spread behavior over a thin filter paper under both reduced and enhanced pressures relative to atmospheric conditions and examines extinction phenomena at low pressure. Experiments were conducted in a combustion chamber over a wide pressure range (30–250 kPa). At high pressures, the flame exhibits a flat front across the sample width and spreads steadily downward. As pressure decreases from 250 to 60 kPa, flame height remains approximately constant while the sample heating length increases, attributed to enhanced thermal diffusion at reduced pressure. The flame spread rate was also determined. At high pressures, the measured spread rate agrees with the predicted thermal-regime value ( V f , th ) , whereas at lower pressures it decreases approximately linearly with inverse pressure, following V f / V f , th ≈ 1 − C / P . Below 50 kPa, the flame becomes dimmer and develops a curved front, and partial flame spread is observed. Periodic local extinction and regrowth along the flame width produce oscillations and lateral swinging until complete extinction occurs. Analysis indicates that reduced flame spread rates and extinction at low pressure result from increased radiative losses and diminished chemical kinetics.