3mm droplet of heptane burning in microgravity as part of the FLEX experiment conducted on the ISS.  Atmosphere is 33.4% oxygen (balance nitrogen) at a pressure of 0.713 atm.  This back-lit view shows the droplet as it burns as well as small particles of soot which coalesce.  The flame itself is generally not visible.
Paul Ferkul / NASA Glenn Research Center
2009-10-24
NASA Glenn Research Center