Microgravity combustion of polyethylene droplet in drop tower

Microgravity experiments of polyethylene (PE) droplet combustion were conducted by a 3.6-s drop tower with the gravity level of 10⁻³~10⁻⁴ g to investigate the burning behaviors and fire hazards of molten thermoplastics in the spacecraft. Pre-ignited droplets with a diameter of about 3 mm were continually generated and detached from burning PE tubes. Once the drop capsule started free-fall, droplets entered the microgravity environment with an initial velocity of 10–35 cm/s (Stage I). A comet-shape flame with an intense bubbling and ejecting process of the moving droplet was observed, and the burning-rate constant (K) was found around 2.6 ± 0.3 mm²/s. After the droplet landed on the floor, it could rebound with a near-zero velocity, showing as a spherical flame (Stage II). The combustion of PE droplet followed the classical d-square law with K = 1.3 ± 0.1 mm²/s. The measured large burning-rate constant (or the volume shrinkage rate) of the moving droplet was caused by the robust bubbling process, which reduced the bulk density of molten PE and ejected unburnt fuel (about 25% of total mass loss). However, the actual mass burning rate of the PE droplet should be smaller than most hydrocarbon liquids because of a smaller mass-transfer number (B ≈ 2). The flame burning rate of PE droplet is 4 ± 1 g/m²-s per unit flame-sheet area that may be used to estimate the fuel mass-loss rate and fire heat release rate in microgravity. This novel microgravity combustion experiment on the thermoplastic droplet could expand the physical understanding of fire risk and hazard of plastic material in the spacecraft environment.