# Secondary Atomization of a Biodiesel Micro-Emulsion Fuel Droplet Colliding with a Heated Wall
## 3 Results and Discussion
### Comparing the Results of Disintegration Characteristics Analysis by 2D and 3D Recording  
One of the objectives of this research was to establish the difference in the integral characteristics of hydrodynamic disintegration when using 3D recording and 2D recording of the outcome. Similar to the tests, the results of which are presented in Sections 3.1-3.5, we conducted experiments with three-dimensional recording of quantitative parameters and compared several \(d_{\text{s}}\)/\(D_{0}\) and \(S_{1}\)/\(S_{0}\) for the two measurement techniques (Figure 12).  
Figure 12: Comparison of integral characteristics of hydrodynamic disintegration for 2D and 3D recording: (**a**) C-1, \(T_{0}\) = 300 \({}^{\circ}\)C; (**b**) C-1, \(T_{0}\) = 500 \({}^{\circ}\)C; (**c**) C-4, \(T_{0}\) = 500 \({}^{\circ}\)C.  
For the three-dimensional recording, the parameter \(d_{s}\)/\(D_{0}\) was on average 24.5% lower at all times (Figure 12); the parameter \(S_{1}\)/\(S_{0}\) was on average 28.5% higher. The difference is largely explained by the difficulty in the evaluation of smaller-diameter secondary fragments during the 2D recording, because their images were overlapped by larger droplets in the measurement area. This resulted in overestimated \(d_{s}\)/\(D_{0}\) and correspondingly decreased \(S_{1}\)/\(S_{0}\). In general, the hydrodynamic disintegration characteristics can be evaluated using simpler and less laborious 2D recording technique, but only with the use of an adjustment coefficient. The overall trends of \(d_{s}\)/\(D_{0}\) decrease and \(S_{1}\)/\(S_{0}\) growth with the increasing _We_ remained unchanged for both 3D and 2D recording techniques (Figure 12).  
The experiments in [31] showed that the preliminary heating of fuel droplets may increase the effect of two factors. The first one is a decrease in the surface tension and viscosity, increasing the amount of fine child droplets after primary and secondary atomization. The second one is the increased evaporation rate of a droplet and a vapor buffer layer forming around it, acting as thermal insulation by deterring the heat flux to the droplet surface. This vapor buffer layer also decreases the relative impact velocity when heated droplets interact with each other. This factor promotes the interaction of droplets in the rebound regime, i.e., preserving their initial size. This way, the increased droplet vaporization can hinder its disintegration, despite the decrease in the surface tension and viscosity. The two factors have an opposing effect on the characteristics of the child droplets. It was shown in [31] that for initial droplet temperatures of 20 \({}^{\circ}\)C and 80 \({}^{\circ}\)C, these characteristics may vary from 15% to 50%. Thus, we can formulate a hypothesis that the experimental results of the current research are the upper estimate of \(S_{1}\)/\(S_{0}\) achievable in a combustion chamber, because fuel droplets will be heated up to a high temperature prior to their collision with a wall. As a result, the formation of a buffer vapor layer around the droplets is of high probability.  
For further research, it is proposed that we should study the kinematic stability and rheology of the biofuel microemulsions under a wide range of ambient temperatures, as well as to explore the effects of various bio-additives (e.g., fatty acid methyl ester of sunflower oil) on the drop impact processes, and to proceed with the tests on the primary and secondary atomization of the fuel microemulsion flows. The latter is difficult to implement without the results of the tests with impinging single drops.