Far-field signature of fire in low gravity: Influence of ambient oxygen content and pressure on size distribution of smoke particles

Abstract

Fire detection in space habitats is hindered by the unique characteristics of smoke particles released by weakly buoyant flames. For the first time, non-invasive measurements of particle concentration, size, and light absorption properties are performed in the downstream flow of a flame spreading in microgravity. The studied flame spreads over Low Density PolyEthylene (LDPE) coated Nickel/Chrome (NiCr) wires in an opposed oxidizer flow. The far-field signature can then be contrasted with previous measurements performed over the same configurations and related to the morphology of soot particles sampled in and behind the flame.Following measurements regarding the impact of gravity, the influence of ambient pressure and oxygen content on the characteristics of the particles is reported in microgravity. Increased pressure boosts the soot production rate, leading to higher concentration levels of particles detected for any class of particle size. Higher pressure also causes larger aggregates that exhibit a higher level of polydispersity among the constituting primary particles. This tends to enhance light absorption, more specifically at larger particle size. Less intuitively, increased oxygen content accelerates the pyrolysis rate, therefore hastening soot particles’ maturity, and ultimately increasing the detected soot concentration. In addition, enhanced oxygen content reduces organic carbon within particles, boosting light absorption regardless of the particle size. The far-field signature and its sensitivity to the ambient conditions are in decent agreement with observations of soot particles collected at the trailing edge of the flame. As a result, combining higher oxygen content and lower pressure in future space habitats as projected by the main space agencies can cancel out the effect on the detection threshold of a given flame if this threshold only includes the concentration level.