COMBUSTION DYNAMICS OF BIO-DERIVED, SURROGATE, AND TRANSPORTATION FUEL SYSTEMS

Abstract

Experimental results are reported that address fundamental multiphase burning characteristics of transportation fuels and their blends with selected biofuels, as well as the combustion of transportation fuel surrogates. Included are n-butanol, hydrotreated renewable diesel and jet fuels derived from microalgae (AHRD and AHRJ, respectively), and surrogate fuels including iso-octane and mixtures of n-heptane and iso-octane. The transportation fuels examined are ethanol-free gasoline, #2 diesel, and Jet-A. The configuration employed is an isolated droplet burning in an environment that promotes spherical symmetry of the combustion process with room temperature air at atmospheric pressure. The main parameters of the experiments are the initial droplet diameter (Do) and blending fraction. Estimates of the heat transfer regimes show that as Do increases, burning can transition from where radiative process on the droplet burning is negligible to where it is important. The estimated range is from about 0.5 mm or less to well over 1 mm. To access such a large dynamic range, we carry out experiments in a ground-based drop tower at Cornell that provides about 1.2 s of experimental run time for Do < 0.8 mm, and the International Space Station (ISS) that allows for a virtually unlimited experimental time, with the experiments reported here having Do up to about 5 mm and corresponding burning times of nominally 15 s. Soot formation is a prominent feature of the experimental observations, and we report the development of an algorithm that allows quantifying the amount of soot from digital video images of the droplet burning process. The results show that n-butanol droplets have burning rates that are almost identical to gasoline even though their sooting propensities are markedly different. Moreover, the combustion rates of AHRD droplets are close to those of #2 diesel droplets in spite of their significant chemical and sooting differences. AHRJ droplets burn slightly faster and produce less soot than Jet-A. These observations indicate that bio-derived fuels examined here are attractive additives to transportation fuels to reduce the consumption of petroleum-based fuels and reduce particulate emissions through blending. The results for surrogate fuels show that the droplet burning rate decreases throughout the range of droplet size investigated for iso-octane and the n-heptane/iso-octane mixture because of increasing radiative losses from the droplet flames when Do increases.