Experimental Research on the Disruptive Evaporation and the Motion Characteristics of Secondary Droplets for Emulsified Biodiesel with a Suspended Droplet Configuration

[Image: see text] The secondary atomization of droplets is one of the means to improve the efficiency of diesel fuel injection atomization. As a promising biomass fuel, emulsified biodiesel showed a good prospect in improving the atomization effect of diesel engines. In this study, a high-temperature and pressure-resistant evaporator was designed to simulate diesel-like conditions, and the evaporation and combustion experiments of emulsified biodiesel droplets were carried out. The morphological changes in the droplets were dynamically captured using a high-speed camera. According to the collected images, the evaporation characteristic parameters, the dynamic parameters of droplet motion, and the correlation between the original and secondary droplets were quantitatively analyzed. The gain effect of secondary atomization for droplets on the diesel engine spray was evaluated. The results showed that the emulsified biodiesel underwent nucleation, agglomeration, puffing, and explosion during the evaporation process, while the classic d(2) law only meets a few cases of this fuel. The effect of high temperature was reflected in reducing the normalization time of droplet agglomeration and explosion, while the higher pressure inhibited the expansion of the droplets, thus slowing down the expansion rate and restricting the droplet volume. Driven by the water content, the time of droplet explosion was closer to the droplet lifetime. The evaporation process of the secondary droplet was similar to that of the original droplet over a reduced scale of 1–2 orders of magnitude. (D(d)/D(d)(0))(2) ≈ 1 was a necessary condition for the secondary droplets to be produced in large quantities. The average equivalent diameter of the droplets was distributed in the range of 80–140 μm, and the secondary atomization caused expansion of the spray range by 20–40%. Expansion of the range of secondary droplets was beneficial to shortening the ignition delay and increasing the combustion rate.