The effect of support fibers on micro-convection in droplet combustion experiments

This study reports experimental evidence of gas phase micro-convection induced by support fibers used in droplet combustion experimentation. Soot aggregates formed during combustion of n-octane and n-decane droplets (initial diameters ranging from 0.5 mm to 5 mm) provide natural seeds to reveal the thermal and flow asymmetries involved. The experiments are carried out in an environment that reduces the influence of forced and buoyant convection for both free-floating (unsupported) and fiber-supported droplets. Under these conditions, the soot trapping patterns (due to a balance of thermophoretic and flow-induced drag) would be spherical. However, this situation is only observed for unsupported droplets, or for fiber-supported droplets when the fiber is small relative to the droplet diameter. For D-o < 1 mm a ground based drop tower employed two 14 mu m diameter SiC fibers to fix the droplet's position during burning; unsupported droplets were also examined. For D-o > 1 mm the International Space Station provided capabilities for anchoring test droplets onto a single 80 mu m SiC fiber, and for deploying unsupported droplets. Results clearly indicate that a non-symmetric gas flow field exists in some cases (i.e., for 1 mm < D-o < 3 mm, with an 80 mu m fiber) near to where the fiber enters the droplet. This gas motion originates from the presence of the fiber that introduces asymmetries in the temperature and flow fields resulting in localized force imbalances on the soot particles, which cause vortical flow patterns near the fiber. This may in part be explained by flow asymmetries induced by droplet shape distortions coupled with heat exchanges between the fiber and surrounding gas and conduction into the droplet, resulting in a Marangoni flow near the droplet surface. For very small fibers (or for unsupported droplets) spherical soot shells are found suggesting that no thermal and flow asymmetries exist. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.