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.