A comprehensive review of liquid fuel droplet evaporation and combustion behavior with carbon-based nanoparticles

Nanofuels (NFs) are an innovative fuel category where nano-scale metal or carbon-based particles are suspended within liquid fuel (LF) to enhance performance, combustion efficiency, and emission characteristics of internal combustion devices while preserving the base fuel properties. Carbon-nanoparticle-based nanofuels (CNFs) have recently attracted attention for their potential to significantly enhance combustion performance and reduce emissions. CNFs offer advantages such as lower toxicity, a reduced environmental footprint, and cost-effectiveness compared to metal-based alternatives. Carbon nanoparticles exhibit potential in enhancing liquid fuel combustion characteristics, particularly when used at low particle concentrations (≤0.30 % w/w), which is likely to be optimal for improving the burning rate. This enhancement can be attributed to their superior heat absorption and transfer properties, improved atomization mechanisms, and impact on combustion kinetics. This review investigates the potential of CNFs and examines the mechanisms by which they alter combustion and evaporation characteristics. Empirical evidence indicates that the increased evaporation and burning rates of CNFs are primarily due to improved radiation capture and heat transfer. The behavior of ignition is closely related to the aggregation and distribution of nanoparticles within CNF droplets, which affects fuel evaporation dynamics. Additionally, increased micro-explosion intensity and generally reduced micro-explosion frequency are observed during CNF droplet combustion. Factors such as particle size, concentration, morphology, and thermo-physical properties play crucial roles in influencing changes in evaporation rate, burning rate, ignition delay, burning period, and micro-explosion characteristics. Studies conducted at droplet, spray, and engine scales consistently support the positive effects of CNFs observed at the droplet scale. These improvements lead to enhanced combustion parameters, better engine performance and a significant reduction in harmful emissions. However, concerns remain about the potential presence of nanoparticles in exhaust emissions and their implications for the environment and human health. This review offers a comprehensive analysis of CNFs, providing insights into their potential applications and identifying areas that require further research.