Distinct evaporation and combustion behaviors of suspended and unsuspended nanodiesel droplets

This work reports the main evaporation and combustion characteristics of diesel droplets doped with different concentrations of alumina and ceria nanoparticles (NPs) for a range of conditions scarcely explored and relevant for combustion applications: high-temperature and reducing/oxidizing atmospheres (0/10 % O₂). Due to the potential influence of the particular experimental conditions, all tests are performed using two different setups: a free-falling droplet (FFD) rig and a suspended droplet (SD) facility, following a systematic study that is considered especially pertinent for particle-laden fuels. The reported results demonstrate, for the first time, a great influence of the test method on some of the observed behaviors, which can perfectly justify some contradictions and even inconsistencies observed in previous works. Tests on unsuspended nanodiesel droplets provide smooth evaporation curves until the onset of a single and violent microexplosion that shatters the droplets, whereas the testing of suspended droplets yields a fluctuating evaporation process, with a wide range of sequential disruptive phenomena of different intensities (swelling, puffing, weak microexplosions). These clear differences point to the impact of the suspension filaments on disruptive behaviors for the range of conditions explored, even when very thin ceramic fibers are employed. In spite of these differences, some common features have also been identified. Namely, the addition of NPs does not drive significant changes in the droplet evaporation rate, probably due to the small impact of thermal radiation for the tested conditions. However, the onset of disruptive phenomena shortens the liquid conversion times as compared to neat diesel, with an earlier occurrence as the NP concentration increases, especially for FFD tests. Among the two tested nanoparticles, ceria shows significantly stronger disruptive events and also a progressive reduction in evaporation rate for unsuspended droplets, which is consistent with the formation of a less permeable shell for this kind of NP.