Nanofuel Droplet Evaporation Processes

The concern about the level of toxic emissions from the use of fossil fuels in internal combustion engines is widely held. Several alternatives have been suggested to mitigate this concern including the use of biofuels in the engines, hybrid internal combustion–electric power systems and electric propulsion systems. In the last decade there has been progress with adding nano-sized particle additives to hydrocarbon fuels with the aim of improving the thermo-physical properties. The nano-sized metallic particles increase the surface-to-volume ratio of the resultant nanofuel suspensions. Reductions in the emissions levels from the combustion of these nanofuels have been reported; these improvements derive from the reductions in ignition delay, and therefore, higher burning rates arising from increases in the evaporation rates of the fuel droplets. Thus, droplet evaporation mechanisms influence the ignition time of the droplets, and consequently the ignition delay time. Optimizing these parameters can help to reduce the emissions from the internal combustion engines. The study presented here examines the up-to-date results of work carried out by various researchers on the droplet evaporation mechanisms of nanofuel droplets. The predominant processes presented as being responsible for the enhancement of the droplet evaporation rate are that the nanoparticle additives increase the droplet fuel temperature by radiative absorption, and that at high temperature values the agglomerates of the nanoparticles heat up residuals of the liquid fuel causing fuel droplet disruptions and micro-explosions. The various parameters that affect these and other nanofuel droplet evaporation mechanisms are presented. A case is made for further studies in this area.