The optics of small diffusion flames in microgravity

Light refraction by small diffusion flames in microgravity is analyzed, following previous observations, of the tendency of their shadows to disappear as zero gravity is approached. The theory of density profiles for spherically symmetrical flames is investigated both for the final steady state and as a function of time after the onset of zero gravity. The effects of the spreading of gradients are considered in terms of their consequences for schlieren and shadow recording. Using the drag on flame ions by electric fields to simulate microgravity by opposing natural convection, a highly magnifying schlieren-, shadow-, and deflection-mapping system is used to compare these theoretical results with optical records of small natural gas flames. There is good agreement between the two. We conclude that, while schlieren and shadow marking may not always vanish completely in zero gravity, depending on the sensitivity of the optical system, they deteriorate so much that classical deflection methods based on steep refractive index gradients are not very; suitable for the study of small diffusion flames in microgravity. As regards other optical methods, however, the cancellation of natural convection by acting on flame ions by electric fields does appear to offer the opportunity of carrying out, for example, planar laser-induced fluorescence (PLIF) studies on the bench top instead of having to use parabolic flights or drop towers.