## 2 Experimental Equipment and Methods
### Experimental Setup  
An experimental setup presented in Figure 1 was used when conducting an experimental study of the ignition and combustion processes of composite liquid fuel droplets. The experimental setup makes it possible to simulate the heating conditions of a fuel droplet, identical to the heating conditions in the boiler furnace when the fuel droplet moves at the speed of the heated air flow (i.e., droplet speed relative to the air flow is zero). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Indictors** & **Method** & **Value** \\ \hline Cetane number & ISO 5165:1998 & 51 \\ \hline Cetane index & ASTM D 4737—10 & 46 \\ \hline Flash point in an closed crucible, \({}^{\circ}\)C & ASTM D 93—13 & 55 \\ \hline Ash content, \% & ASTM D 482—13 & 0.01 \\ \hline Mass fraction of sulfur, mg/kg & ISO 20884 & 10 \\ \hline Mass fraction of polycyclic aromatic hydrocarbons, \% & IP 391/1995 & 8 \\ \hline Density, at 15 \({}^{\circ}\)C, kg/m\({}^{3}\) & ASTM D 4052—18 & 820–845 \\ \hline Mass fraction of water, mg/kg & ISO 12937:2000 & 200 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Typical physicochemical characteristics of diesel fuel. Figure 1: Scheme of the experimental setup. A motionless high-temperature air medium was created in the cavity of a Loplif50/500-1200 tubular muffle furnace (Laboratory Equipment and Instruments, Russia). The inner diameter of the ceramic furnace tube is 50 mm; its length is 450 mm. The temperature varied in the range of 20-1200 \({}^{\circ}\)C and was controlled by a built-in type K thermocouple. Due to the fact that the ends of the tubular muffle furnace were open during the experiments, the gaseous products of the combustion of the fuels' single droplets diffused into the environment. Composite fuel particle size (diameter of about 2 mm) is relatively small compared to the size of the internal volume of the muffle furnace ceramic tube (diameter 50 mm, length 450 mm). Therefore, in this study, oxygen concentration did not change significantly during the burnout. Fuel particles/droplets (depending on the aggregate state with varying their initial temperatures over a wide range) of different compositions were dosed with the same mass of 5 mg and a diameter of about 2 mm. The mass of suspensions was determined using an AF-225 DRCE analytical balance (ViBRA, Tokyo, Japan): the minimum recorded mass was 1 mg, the maximum recorded mass was 92 g, the discretization was 0.01 mg, and the accuracy class was Special (I) according to OIML R 76-1: 2006, IDT. Before the experiments, all fuels were thermostated at a constant temperature for 15 min. An Arctiko ULUF 15 freezer (ARCTIKO A/S, Esbjerg, Denmark) was used to cool fuels in the temperature range from minus 60 \({}^{\circ}\)C to minus 40 \({}^{\circ}\)C, and a thermostatic cell of a LOIP FT-205-25 liquid cryothermostat (Laboratory Equipment and Instruments, Saint Petersburg, Russia) was used in the temperature range from minus 20 \({}^{\circ}\)C to plus 60 \({}^{\circ}\)C.  
In each series of the 5-8 experiments (Figure 1) conducted in identical initial conditions; the muffle furnace was heated to a given temperature (\(T_{g}\)). Single fuel particles/droplets located on a ceramic holder were introduced into the furnace cavity with a speed of no more than 0.1 m/s along the symmetry axis of the ceramic tube up to its center (Figure 1). The SPSH20-23017/2000Z manipulator (ZAO Mechatronic Product Factory, Moscow, Russia) simultaneously moved the ceramic holder and the high-speed video camera (Figure 1). This made it possible to record and analyze the processes during the entire induction period from fuel heating to its burnout. It should be noted that the experiments for different fuel compositions were conducted in identical conditions. The end openings of the muffle furnace (Figure 1) were open during ignition and combustion of the fuels. The oxygen concentration in the chamber was about 20.5% and did not change significantly during the burnout of a relatively small fuel particle/droplet.