Ignition and Combustion Characteristics of Heptane-Based Nanofluid Fuel Droplets

Nanofluid fuels are promising fuels in the fields of spaceflight and aviation. Their stability is a critical constraint for potential applications. Oleic acid can enhance the stability of nanofluid fuels, but it influences the ignition and combustion performance of nanofluid fuels. In this study, the effects of various concentrations of oleic acid and nanoparticles (NPs) on the ignition and combustion performance of the heptane-based nanofluid droplets were studied using a high-temperature tubular resistance furnace system at 600–700 °C. The ignition delay times, ignition temperatures, and combustion processes of all samples were measured. The results show that with the increase of temperature, the ignition delay times and ignition temperatures of the pure heptane, heptane with addition of oleic acid, and heptane-based nanofluid droplets decrease. The addition of oleic acid into the heptane, its ignition delay time and ignition temperature significantly increase with the increase in oleic acid concentration, which are greatly increased by 60 and 40%, respectively, for the heptane droplet with 2% oleic acid compared with the heptane droplet. Oleic acid makes the heptane droplet to undergo microexplosion, surfactant combustion, and secondary extinguishment. With the increase in temperature and concentration of oleic acid, the microexplosion and the number of generated bubbles are more obvious. Al NPs have little influence on the ignition delay time of the heptane droplet with oleic acid at 600 and 650 °C, while the enhancement of nanoparticles is significant at 700 °C. In addition, Al NPs can decrease the ignition temperature of heptane-based nanofluid droplets and the decreased percent reaches 17%, but the promoting effect is weaker than the inhibition effect of oleic acid. With the increase in concentration of Al NPs, microexplosion of the heptane-based nanofluid droplet will be enhanced.