Study on three droplet sequential burning characteristics of coal direct liquefied diesel

Coal direct liquefied diesel (DDCL) is a new alternative fuel with high cleanliness and quality. Its hydrocarbon composition and physicochemical properties are quite different from those of petrochemical diesel. In order to study the difference in evaporative burning characteristics between DDCL and diesel during the sequential burning of multiple droplets, three fuel droplets are suspended on fibers in order by a micro-injector. The first droplet was ignited by an electrode igniter, and then the sequential burning process of three droplets was captured by a high-speed camera. An automatic processing program is written based on Matlab software to calculate the equivalent diameter of each fuel droplet. The result shows that the burning process of the DDCL droplet can be divided into three stages: initial thermal expansion, steady burning, and late micro-explosion burning. DDCL is much easier to form the burning mixture and has better flammability than diesel. The droplet spacing and fuel type have significant effects on ignition delay. The square of the normalized diameter of droplets decreases linearly with time during the steady burning period, which follows the D2 law. The reduction rates of normalized squared diameter for DDCL droplets are higher than that for diesel. The pyrolysis of macromolecular components at high temperature produces many small molecules and low boiling point components, which induce the micro-explosions in the late burning stage. The micro-explosion intensity of DDCL droplets is obviously lower than that of diesel.