## 3 Results and Discussion
### Analysis of Fuel Ignition and Combustion
It is noteworthy that the fuel droplet/heating surface interface, through which heat is transferred from the heating surface to the droplet, depends not only on the spreading area of the fuel droplet but also on the surface roughness. Obviously, with increasing roughness, the heat transfer surface area increases. As noted in Section 3.1, the increase in surface area due to roughness can be estimated using the 3D roughness parameter Sdr. The Sdr values (Table 2) show that the increase in surface area due to roughness increases in the sequence of the P-AB-2 BA-2 AB surfaces. In other words, it is in the same sequence in which the fuel droplets spread better. It is possible to conclude that among the texture configurations used in the experiments in the form of parallel microchannels, the 2 AB texture configuration has the greatest positive effect on reducing the ignition delay time (Figure 7), not only due to better spreading of fuel droplets over it but also due to the greatest increase in surface area as a result of developed roughness. It is known that fuel droplets can ignite and burn out in one of the following conventionally distinguished regimes: evaporation, boiling, puffing, and microexplosion [35; 36]. For oil in the usual state, there is no secondary atomization when heat is supplied to the droplet; it evaporates and boils. The puffing and microexplosion regimes are typical only for multicomponent fuels. Figure 8a shows the number of puffing initiations of emulsion and slurry droplets under a mixed heat supply. The intensity of puffing, as well as the size of the burnout region, where a combustible vapor-gas mixture is formed during physical and chemical processes, largely affects the intensity of combustion of multicomponent fuels. The greater the number of puffing initiations (Figure 8a) and the larger the burnout region (Figure 8b), the faster the droplets of multi-component fuels will burn out (Figure 8c). In order of increasing the burnout time of liquid fuel droplets, the steel surfaces are arranged in the following sequence: 2 AB-AB-A2B-P. Figure 8 shows that the smallest values of the number of puffing initiations and the maximum size of the burnout region were recorded during the ignition and combustion of liquid fuel droplets on the polished surface. Under such conditions, the burnout time will be longer than on textured heating surfaces. The highest values of \(N\) and \(D_{\text{max}}\) and the lowest values of \(\tau_{\text{burn}}\) were recorded on the 2 AB surface for the fuels used in the experiments, other conditions being equal. For example, for a slurry droplet, the burnout time on the 2 AB texture is two times less than that on the polished steel surface. Thus, it can be concluded that physicochemical processes occurring under the conditions of fuel ignition and combustion are realized much more intensively on the 2 AB texture compared to the use of polished, AB, and A2B textures as heating surfaces. It should be noted that the values of the combustion characteristics (\(N\), \(D_{\text{max}}\), and \(\tau_{\text{burn}}\)) presented in Figure 8 for a mixed heat supply to the AB and A2B surfaces differ slightly from each other and lie in the range of the confidence interval. It is also necessary to clarify that the comparison of the ignition and combustion characteristics of the three types of fuels is incorrect due to the different compositions and, consequently, different physical and chemical properties. In addition, the same volumes of droplets of oil in the usual state, emulsion and slurry, were used in the experiments. However, due to the different component compositions, their masses differed from each other. Therefore, on the same surface, for example, 2 AB, the times of complete burnout of an emulsion droplet are shorter than the \(\tau_{\text{burn}}\) of a slurry droplet, despite its more intense puffing.