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Simulating microgravity in small diffusion flames by using electric fields to counterbalance natural convection

A principal motivation for combustion research in microgravity is the absence of buoyancy–driven flows. To accomplish an equivalent microbuoyant condition in the laboratory, we use flame–ion generated winds driven by electric fields to balance local convection. The object of such bench–top microgravity simulation is to facilitate a wide range of optical and laser diagnostics that can probe the structure of small diffusion flames without the considerable experimental obstacles and costs involved in microgravity drop tower or flight tests. In preliminary experiments, a temporary (of the order of seconds) microbuoyant point of balance was achieved for the entire flame, which was, however, suitable only for instantaneous recording, being subject to oscillations and disturbance by random convection currents in the laboratory as well as gradual changes in flame structure. This paper describes a localized steady–state balance condition, permitting iterative measurements over extended time periods, which we have achieved by appropriate modifications of the electrode configuration. To demonstrate this methodology, the temperature field of a small non–premixed methane–air capillary flame under electric field control is probed with holographic interferometry and coherent anti–Stokes Raman spectroscopy.To help understand the steady state of balance, rates of ion formation were also measured for comparison with the normal buoyancy case. With the appropriate electric field, the temperature measurements of the capillary flame show a reduction in the temperature gradient closely approximating to a purely diffusive thermal profile, such as would be expected to result from inhibition of the local buoyancy–driven convection.