## 3 Results and Discussion
### Analysis of Fuel Ignition and Combustion
As noted above, the fuel droplets used in the experiment have an elongated shape when spreading over steel surfaces with a texture in the form of parallel microchannels (AB, A2B, and 2 AB). Therefore, in the case of the AB, A2B, 2 AB surfaces, it is incorrect to consider the dependences \(\tau_{d}=f(d_{s})\). The experimentally obtained contact diameters of the spreading fuel droplets were used to calculate the projection areas of the fuel droplet/heating surface interface (\(S\)). The fuel droplets on the P surface have a spherical segment form and their base area is close to a circle. Therefore, \(S=\pi\cdot d_{s}{}^{2}/4\), where \(d_{s}\) is the contact diameter of the fuel droplet. On the AB, A2B, and 2 AB surfaces, the droplet base is close to an ellipse. Therefore, the projection area was determined by the formula \(S=\pi\cdot(d\updownarrow)/2\cdot(d\leftrightarrow)/2\), where \((d\updownarrow)\) and \((d\leftrightarrow)\) are the contact diameters of droplet fuel measured in parallel and perpendicular directions with respect to the microchannels. It can be seen from dependences \(\tau_{d}=f(S)\) (Figure 7) that the larger the projection area of the fuel droplet/heating surface interface, the shorter the ignition delay time of fuel droplets (oil in the usual liquid state, emulsion, and slurry) is. The larger \(S\) provides the larger contact areas of the droplet with the heating surface. Consequently, the intensity of heat transfer from the heating surface to the droplet is higher. Therefore, the droplet warms up faster and the evaporation intensity of the combustible components that make up the fuel is higher. The required concentration of combustible substances near the surface of the fuel droplet is achieved in less time to implement gas-phase ignition. It was found that the ignition delay times of fuel droplets located on surfaces textured by laser radiation are shorter than on a polished surface for all types of fuels used in experiments (oil, emulsion, and slurry) for all other conditions being equal (Figure 7). For example, the difference in the ignition delay times of fuel droplets located on surfaces 2 AB (the smallest values of \(\tau_{d}\) were recorded) and P (the largest values of \(\tau_{d}\) were recorded) in the case of oil is 25%, emulsion is 28%, and slurry is 55%. It is to be noted that among the formed textures AB, A2B, and 2 AB, the best result in intensifying the ignition was achieved on surfaces with the 2 AB texture configuration. For example, the difference in the values of \(\tau_{d}\) recorded on the 2 AB and AB surfaces is 18% for the ignition of oil droplets, 17% for emulsions, and 36% for slurry. As noted above, the decrease in ignition delay times is due to the larger contact area between the fuel droplet and the heating surface. As is seen in Figure 7, the contact area between the fuel droplet and the heating surface increases in the sequence P-AB-2 BA-2 AB, other conditions being equal. This is due to the fact that in the same sequence (P-AB-2 BA-2 AB), fuel droplets spread better over surfaces. Figure 7: The ignition delay time of fuel droplets versus the projection area of the _fuel droplet/heating surface_ interface under a mixed heat supply. It is obvious that the best spreading on the AB, A2B, and 2 AB surfaces was facilitated by the fact that these surfaces demonstrated superhydrophilic and superliophilic properties after laser processing. The wettability inversion of metal surfaces (the transition from hydrophilic/hydrophobic to superhydrophilic) after their processing with laser radiation has been studied quite well [34]. It should be noted that the 2 AB texture configuration contributed to a greater spreading compared to the AB and A2B texture configurations. This is connected to the fact that on the 2 AB surface, most of the texture consists of micro- and nano-sized elements formed due to ablation and further crystallization of the material.