Near-limit downward/opposed flame spread, flame front oscillation and limiting oxygen concentration of B-LDPE tube in normal- and hypergravity

Near-limit downward flame spread over cylindrical thermoplastic materials under various gravities, oxygen concentrations and opposed flow velocities were investigated experimentally. Black low-density polyethylene tubes with stainless-steel (SS) and copper (Cu) cores were tested. Key parameters, including flame spread rate (FSR) and limiting oxygen concentration (LOC), were measured, and flame oscillations were analyzed using continuous wavelet transforms. Results show that, for SS-core samples, gravity-induced stretch effects reduce the flame temperature, weakening the dripping effect. Dripping seldom occurs near LOC. The trends of FSR and LOC are consistent with blow-off theory. The dominant frequency for oscillatory flame front increases with gravity, while the amplitude decreases owing to shorter dripping lengths. For Cu-core samples, far from extinction, the metal core acts as a heat source, enhancing dripping and FSR, especially in hypergravity. Near the LOC, the Cu-core shifts to a heat sink, thus the flame-flow interaction begins to dominate the FSR. LOC for Cu-core sample is higher than that for SS-core sample, and increases more mildly with gravity and opposed flow. Flame oscillations are more intense in hypergravity due to a stronger dripping flow, but it becomes less important near LOC. This study provides insights into the flammability of thermoplastic material, contributing to fire safety in spacecraft.