Microgravity droplet combustion: An experimental investigation on the influence of sooting and radiation on droplet burning

Single isolated droplet combustion experiments performed in microgravity provide an ideal environment to advance the understanding of non-premixed flames. The absence of buoyancy results in a spherically-symmetric non-premixed flame and measurements can be used to validate theories based on one-dimensional analysis. In many of the early droplet studies, the effect of sooting was not considered due to the complexity involved in incorporating soot formation mechanisms and belief that sooting for fuels such as heptane was not important (based on degree of sooting under normal gravity conditions). An experimental study is presented on the influence of sooting and radiation in microgravity droplet combustion. The microgravity experiments were performed in the NASA 2.2s drop tower, NASA-GRC 5.18s Zero-G facility and the Japan Microgravity Center (JAMIC) 10s facility. Soot volume fraction was measured using the method of light-extinction/tomographic inversion for n-heptane droplets with initial diameter ranging from 0.84--2.9 mm. Measurements helped to clarify the influence of initial droplet size on sooting behavior.; Droplet combustion studies performed in microgravity greatly simplify studies of soot formation, growth, agglomeration and oxidation since all processes are radially directed. Characterization of soot morphological properties including the primary particle size, radius of gyration, fractal dimension and mass fractal prefactor term are necessary for the analysis of these soot processes. Soot was sampled using the method of thermophoretic sampling from microgravity droplet flames. These represent the first such measurements performed for spherically symmetric droplet flames. The morphological properties of soot produced in microgravity droplet flames are significantly different than those measured in normal gravity experiments.; Understanding how soot is formed in microgravity droplet combustion requires understanding of sootshell formation. The first experimental observation of the mechanism of sootshell formation, from initiation near the flame and transport of the soot toward the sootshell is discussed. These experiments demonstrated that thermophoresis transports the soot particles away from the high-temperature region toward the droplet to ultimately reside at the sootshell location (due to the balance with Stefan-flux). The experimental measurements were compared to models of sootshell formation.; One method to control soot emission from combustion systems that employ non-premixed flames involves the use of alternative fuels and the blending of fuels of varying sooting propensity. Sooting and burning behaviors of various alkane fuels (hexane, heptane, nonane and decane) were measured. Insights gained from these measurements are discussed.