Experimental study of butanol droplet combustion in a turbulent, elevated-pressure environment

Abstract

To assist in the search for clean liquid alternative fuels for future combustion engines, the combustion characteristics of butanol droplets were experimentally investigated. The experiments were conducted under varying flow turbulence intensity, $q^{0.5}$, (up to $0.5$ m/s), pressure (up to 11 bar), and ambient compositions, including different levels of $O_2$, $C{O}_2$, and $N_2$, at room temperature. A fan-stirred spherical chamber, equipped with four pairs of axial fans, was used to generate a controlled turbulent flow field with negligible mean velocity. A 500 µm butanol droplet was suspended on a single micro-fiber at the center of the chamber and ignited using a coil resistance wire. The combustion process, including the droplet time history and luminous flame structure, was recorded using two synchronized cameras. The findings revealed that increasing ambient pressure enhances the burning rate, $K_b$, while the effect of turbulence varies with pressure. At low pressure, turbulence has minimal impact, whereas at higher pressure, $K_b$ increases with $q^{0.5}$, before declining due to temporary luminous extinction (TLE). Butanol exhibits a lower $K_b$ than heptane; however, under high-pressure, high-turbulence conditions, their $K_b$ values converge. Additionally, butanol droplet combustion is influenced by $O_2$ and $C{O}_2$ concentrations. Oxygen improves $K_b$ by delaying TLE, while $C{O}_2$ reduces $K_b$ by lowering flame temperature and thermal conductivity of the ambient mixture.