Distinctive combustion stages of single heavy oil droplet under microgravity

• Published in: Fuel (Guildford) Vol. 82; no. 3; pp. 293 - 304
• Main Authors: Ikegami, M., Xu, G., Ikeda, K., Honma, S., Nagaishi, H., Dietrich, D.L., Takeshita, Y.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.02.2003; Elsevier
• Subjects: Applied sciences; Combustion of liquid fuels; Combustion. Flame; Droplet combustion; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Heavy oil; Microgravity; Multicomponent fuel; Temperature history; Theoretical studies. Data and constants. Metering; Vaporization mechanism; Vaporization mechanism; Temperature history; Droplet combustion; Microgravity; Heavy oil; Multicomponent fuel; Diesel fuel; Combustion; Droplet; Vaporization; Temperature measurement; Petroleum residue; Heavy fuel oil
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/S0016-2361(02)00257-0

This report presents an investigation on the combustion of single droplets comprised of heavy oil and oil mixtures blending diesel light oil (LO) and a heavy oil residue (HOR). The tests were conducted in a microgravity facility that offered 10 s of free-fall time. Fine wire thermocouples supported the droplets, resulting in a measurement of droplet temperature history. Additional data were the droplet and flame size history. The results identified four distinctive burning stages between ignition and extinction for heavy oil (C class) and HOR–LO blends. They are, in succession, the start-up, inner evaporation, thermal decomposition (pyrolysis) and polymerization stages. The start-up stage denoted an initial transient period, where the LO components burned from the droplet surface and the droplet temperature increased rapidly. The latter three stages featured pronounced droplet swellings and contractions caused by fuel evaporation and decomposition inside the droplet. An evaporation temperature demarcated the start-up stage from the inner evaporation stage, and this temperature corresponded to a plateau in the temperature history of the droplet. Two additional temperatures, termed the decomposition and polymerization temperatures, indicated the ends of the evaporation and decomposition stages. These temperatures were similarly identified by plateaus or inflection points in the time–temperature diagram. The evaporation temperature gradually decreased with increasing the initial LO mass fraction in the droplet, whereas the other two temperatures were almost independent of the oil composition. All three temperatures increased with decreasing initial droplet diameter, but the dependence was very slight. Based on the results, the combustion of heavy oil droplets appears to be dominated by a distillation-like vaporization mechanism, because of the rapid mass transport within the droplets caused by the disruptive burning.