Experimental study on puffing characteristics of biodiesel-butanol droplet

In order to research puffing characteristics of biodiesel-butanol blends, single droplet experiments of BUT25 and BUT75 were conducted under the condition of atmospheric pressure and 800 °C. Experimental results showed that BUT25 only demonstrated core mode, while BUT75 demonstrated both surface and core mode simultaneously. The core mode was a periodic process including bubble nucleation, bubble expansion and bubble rupture inside the droplet. The surface mode was featured by the appearance of many micron sized child droplets. Two new parameters were proposed to analysis puffing characteristics, namely the similarity degree and the deformation degree. The puffing process usually contained the transient heating phase, fluctuation evaporation phase and equilibrium evaporation phase. Both the transient heating phase and equilibrium evaporation phase were stable evaporation processes. Analysis indicates that the three phases of the puffing process are more appropriately distinguished through the profile of similarity degree than that of the normalized squared diameter. Furthermore, the similarity degree of BUT25 displayed a wavy structure and BUT75 displayed a trapezoid structure in the fluctuation evaporation phase. On the other hand, the deformation degree remained unchanged in stable evaporation processes but exhibited a sharp rise at the instant of bubble expansion or rupture.