Droplet-Scale Combustion Analysis of Third-Generation Biodiesel–Diesel Blends

Biodiesel derived from waste cooking oil (WCO) and animal fats is a promising alternative to fossil fuels, offering environmental benefits and renewable energy potential. However, a detailed understanding of its combustion characteristics at the droplet scale is essential for optimizing its practical application. This study investigates the combustion behavior of biodiesel–diesel blends (B5, B10, B15, B20, B25, B50, B75) and neat fuels (B0 and B100) by analyzing combustion rates, pre-ignition time, burning time, droplet morphology, and puffing characteristics. The results demonstrate that biodiesel concentration strongly influences combustion dynamics. Higher blends (B50, B75) exhibit enhanced steady combustion rates due to increased oxygen availability, while lower blends (B5–B25) experience stronger puffing events, leading to greater secondary droplet formation. The global combustion rate follows a non-linear trend, peaking at B10, decreasing at B25, and rising again at B50 and B75. Pre-ignition time increases with biodiesel content, while burning time exhibits an inverse relationship with combustion rate. Four distinct puffing mechanisms were identified, with lower blends producing finer secondary droplets and higher blends forming larger droplets. Puffing characteristics were evaluated based on puffing occurrences, intensity, and effectiveness, revealing that puffing peaks at B25 in occurrence and at B10 in intensity, while higher blends (B50, B75) exhibit notable puffing effectiveness. This study addresses a critical research gap in droplet-scale combustion of WCO and animal fat-derived biodiesel across a wide range of blend ratios (B5–B75). The findings provide key insights for optimizing biodiesel formulations to improve fuel spray atomization, ignition stability, and combustion efficiency in spray-based combustion systems, such as diesel engines, gas turbines, and industrial burners, bridging fundamental research with real-world applications.