Droplet combustion of blended fuels with alcohol and biodiesel/diesel in microgravity condition

Abstract

Based on a drop tower built in the lab space, an environment of reduced gravity was employed to study the droplet combustion of multi-component fuels, specifically of diesel mixed with alcohol (methanol, ethanol, and 2-propanol) and biodiesel. The falling duration was about 0.68s and was sufficient for complete burning of a droplet with a diameter as small as 0.5mm that was suspended at the crossing point of two ultra-thin fibers with a diameter of 2.5μm. The fibers were made of ceramics with low heat conductivity and imposed negligible interference on the droplet that was less subject to deformation as compared to large droplets that were usually tested. In the study, biodiesel was not added merely as an alternative fuel to diesel, but also to provide an interface for the mixing of alcohol and diesel. In the lack of asymmetric convection that was generically rendered by buoyancy or external flow, the burning characteristics were investigated with alleviated complexity while a soot shell could be formed under the flame surface with spherical symmetry. The work can hence be adopted for qualitative understanding of soot formation at the first place, for which quantitative measurement may not be directly performed on the spot, for different fuel mixtures. It was found that the addition of alcohol in a diesel–biodiesel mixture could enhance the burning rate, reduce the preheating delay, and mitigate pollution of soot particles as well as the tendency to form rigid layers. Furthermore, the introduction of microexplosion through the mixing with alcohol of much higher volatility may lead to substantial disintegration and faster combustion of fuel droplets.