Soot-field structure in laminar soot-emitting microgravity nonpremixed flames

Abstract

Due to the intrinsic interest in laminar flame regimes that cannot be attained in normal gravity (1 g), the microgravity environment (0 g) offers an attractive and promising perspective to enhance our understanding of soot formation mechanisms in diffusion flames. An experimental investigation conducted at the 2.2-s drop tower of the NASA Lewis Research Center is presented to define the soot-field structure within 0-g laminar jet nonpremixed flames which operate above their smoke point. Parallel work on earthgravity flames is also presented to facilitate comparisons and define the effect of gravity on the soot fields. The experiment considers jet diffusion flames of nitrogen-diluted acetylene burning in quiescent air at atmospheric pressure. A full-field laser extinction technique is utilized to determine the transient soot spatial distributions in axisymmetric flames with fixed flow conditions corresponding to Rc=O(100). Results are presented for a nonsmoking normal-gravity flame and its nonbuoyant counterpart which releases soot from its blunt tip. Quantitative measurements on the transient postignition character of the soot field in 0 g are presented for the first time and indicate that millimeter-diameter burners result in brief soot-field transients in microgravity. At a fuel flow rate which is nearly twice that at the smoke point in 0 g, the soot field after the initial transient period sustains its annular structure throughout the luminous nonbuoyant-flamezone. The maximum soot volume fraction measured at 0 g is nearly double that at 1 g for the same flow rate, thus attesting to the higher sooting tendency of nonbuoyant flames. The results indicate that the prolonged residence times under microgravity promote soot growth more than oxidation, thus producing more soot in 0 g. Side-by-side comparisons of the soot distributions in 0-g and 1-g flames demonstrate the increased spatial resolution afforded in microgravity flames.