## 2 Materials and Methods
### Study of Liquid Hydrocarbon Characteristics
#### 2.5.3 Ignition and Combustion  
Droplet ignition and subsequent combustion of liquid hydrocarbons were studied using the experimental setup with a schematic diagram shown in Figure 5.  
The main component of the experimental setup (Figure 5) is a temperature-controlled furnace (Rusuniversal, Russia) used as combustion chamber, supplied with a digital temperature controller (measurement error \(\pm\) 1-3 \({}^{\circ}\)C) 0.012 m\({}^{3}\) in volume. The setup also includes Fastcam SA4 high-speed cam-coder (Photron, USA) with a 1024 \(\times\) 1024-pixel (20 \(\upmu\)m pixel size) image format, 12-bit color depth, and 500 frames per second shooting frequency, and the platform of the coordinate mechanism was designed to feed liquid fuel droplets into the furnace with less than 1 mm movement error. Analysis of the gas-phase products was performed by once-through gas analyzer Test-1 (BONER, Russia). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value, mm** \\ \hline Nozzle outer diameter d\({}_{1}\) & 20 \\ Fuel inlet channel diameter d\({}_{2}\) & 8 \\ Air inlet channel diameter d\({}_{3}\) & 4 \\ Height of the annular gap d\({}_{4}\) & 2 \\ Nozzle outlet diameter d\({}_{5}\) & 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Values of geometric characteristics. Figure 4: Scheme of the pneumatic injector. The study of the ignition and combustion of single liquid hydrocarbon droplets included several stages. Firstly, the required heating temperature was set in the temperature-controlled furnace (in the range of 450-700 \({}^{\circ}\)C, with an intermediate step of 50 \({}^{\circ}\)C) recorded by a chromel-alumel thermocouple. These temperatures correspond to the conditions of sprayed liquid fuels supply to the combustion chamber of modern boilers [29]. Next, a Lepipet NOVUS 10-100 \(\upmu\)m mechanical dispenser was used to apply a liquid hydrocarbon droplet to the metal holder (made of steel wire). Then, the coordinates were set by the PC and the coordinate mechanism fed the sample to the center of the combustion chamber. Droplet diameter was measured using Tema Automotive software and accounted 1.5 mm (\(\pm\)0.1 mm). Video recording started when the movement of the rod with a fuel droplet towards the combustion chamber began. The resulting gas-phase combustion products (CO, CO\({}_{2}\), NOx, and SO\({}_{2}\)) were recorded using a once-through gas analyzer. Gas was extracted through a hollow rod (6 mm inner diameter) with the mechanical holder fixed on it. The resulting gas-phase products entered directly into its hollow part and then flew into the gas analyzer by means of a smoke exhauster due to the close rod proximity. The flow rate of gases was 3 L/min. The time period for scanning attained 3 s.  
Several parameters were analyzed for comparative assessment of the droplet ignition and combustion characteristics at different air temperatures. They are ignition delay time, flame combustion time, and total combustion time recorded by a high-speed camcorder. The ignition delay time was considered the time from the moment the holder with a liquid hydrocarbon droplet entered the chamber focus until the start of the visible surface glowing, which corresponded to the start of combustion. The flame combustion time was recorded starting from the moment of visible glowing until flame failure. The total combustion time was determined by the interval from the moment of visible glowing to complete attenuation of the resulting carbon residue. All experiments were conducted under the same conditions at room temperature of 20 \(\pm\) 1 \({}^{\circ}\)C and relative humidity of 65 \(\pm\) 3%. The systematic measurement error was less than 4% when measuring the characteristic droplet size; less than 0.3% when measuring the ignition delay time and flame combustion time; and \(\pm\) 3 \({}^{\circ}\)C when measuring the air  
Figure 5: Experimental setup for studying droplet combustion of liquid hydrocarbons. temperature.