# Secondary Atomization of a Biodiesel Micro-Emulsion Fuel Droplet Colliding with a Heated Wall
## 3 Results and Discussion
### Effect of Water Concentration in Fuel Composition  
It is also important to estimate the effect of changes in the water proportion in fuel micro-emulsions on the characteristics under study. For that we studied the interaction of a composition with a D+FAME/W ratio of 80/20 (16 vol.% water concentration) and 90/10 (8 vol.% water concentration) with a substrate heated to 300-500 \({}^{\circ}\)C (Figure 11). For D+FAME/W = 80/20, the mean diameter variation of the secondary fragment can exceed 50%, but with a lower proportion of water (D+FAME/W = 90/10), the variation becomes comparable to the measurement error (15-20%). Given that the two studied  
Figure 10: Effect of emulsifier concentration in WiBDME (C-3 and C-5) on hydrodynamic disintegration characteristics of corresponding fuel droplets (mean child droplet diameter (**a**) and evaporation surface area (**b**) at various surface temperatures). WiBDME parameters are close (Table 3), the result can be explained by the factor of temperature. It means that a considerable amount of water in a WiBDME droplet will promote the film-boiling-induced disintegration [16] with abundant heat supply to a spreading drop lamella. This will be accompanied by rapid phase transformation and consecutive breakup (Figures 5a and 6a). This process is rather inconsistent in general and the droplets generated as a result significantly vary in size (Figure 11a). With D+FAME/W = 90/10 (i.e., C-4 composition is close to a diesel/biodiesel blend), the drops break up into almost identical fragments for any studied temperature \(T_{0}\) (Figure 11a). This has a negative impact on the integral characteristic (Figure 11b): it hardly varies with the growing \(T_{0}\) and remains moderate (about 5.5-7.5) with an increase in _We_ unlike \(S_{1}\)/\(S_{0}\) of C-1 droplets whose surface area increased more than 10 times after disintegration (Figure 11b). Therefore, the higher the water proportion in the micro-emulsion, the greater the variation in the size of resulting fragments (Figure 11a), and, correspondingly, integral quantitative characteristics (Figure 11b). Figure 11: Effect of water concentration in WiBDME (C-1 and C-4) on hydrodynamic disintegration characteristics of corresponding fuel droplets (mean diameter of child droplets (**a**) and fuel evaporation surface area (**b**) at various surface temperatures). The experimental curve trends in Figures 7, 8, 10 and 11 are similar in terms of the mean diameter of child droplets and variation of the liquid evaporation surface area. The effect of water fraction on child droplet generation is determinative for some of the secondary atomization techniques, in particular, for droplets colliding with each other or due to thermal micro-explosive breakup. It is shown in [36] that micro-explosive breakup response time is minimal for water proportions of 3 vol.% or 97 vol.%, i.e., for very low or very high values. This is due to high heat capacity and high vaporization heat of water. These two parameters are several-fold higher than those of any liquid fuel. As a result, an increase in the water proportion also increases the overall heat capacity and heat consumption in evaporation. This also prolongs droplet heating to the breakup point. The lower the proportion of water, the faster its local overheating is achieved, which is sufficient for disintegration. The surface tension of the liquid combustible component is much lower than that of water. So, even when water was overheated locally, the parent droplet rapidly broke up. Under heating, the surface tension, viscosity, and density of the combustible and non-combustible components went down. Therefore, a fine aerosol formed due to micro-explosion was not unexpected. However, our experiments [36] showed that large concentrations of water (up to 97 vol.%) in heterogeneous droplets also provide a micro-explosive breakup. Most likely, this happens because such a small volume of the liquid combustible component heats up very fast.