## 3 Experimental Process and Results  
Prior to conducting the droplet ignition experiments, the internal temperature of the heating vessel was set to 1073 K. The temperatures at five points on three cross-sectional planes within the heating vessel were measured using thermocouples through the small orifice; the measurement points are shown in Figure 6, with the measurement results presented in Table 1. The maximum temperature difference was less than 10% of the set temperature, and the uncertainty at a 95% confidence level is \(\pm\)24.7 K, indicating that the temperature field within the heating vessel could be considered uniform with weak air convection, thereby satisfying an experimental condition for high-temperature static air. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Measurement Point** & **Plane A** & **Plane B** & **Plane C** \\ \hline
1 & 1092 K & 1047 K & 1086 K \\
2 & 1177 K & 1117 K & 1168 K \\
3 & 1061 K & 1043 K & 1056 K \\
4 & 1179 K & 1140 K & 1178 K \\
5 & 1095 K & 1076 K & 1091 K \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measurement results of temperature at each point. Figure 5: The relative position of the thermocouple and the drop. The temperature inside the heating vessel was set at the start of the experiment. After the temperature had stabilized, the fuel droplet was suspended on the fiber and subsequently introduced into the heating vessel at a constant speed via the sliding guide. Simultaneously, the high-speed camera began to record the combustion evolution of the droplet throughout the reaction process at a 5000 frame rate. If droplet ignition occurred, the fuel loading volume in subsequent experiments was reduced until ignition could not be achieved. Observed results of the droplet ignition process are shown in Figure 7.  
To minimize the influence of the fiber's insertion on the internal flow field of the vessel and to ensure that premature ignition did not occur before the droplet reached the camera's observation position, following repeated tuning and experimentation, we set the movement speed of the support fiber to a constant 20 mm/s. The high-speed camera recordings indicated that, before reaching the observation position and undergoing ignition, the droplet has sufficient time to resume stable evaporation, and the droplet exhibits minimal oscillation and deformation, which can be considered negligible. Therefore, it is  
Figure 6: Temperature measurement point inside the heating vessel. (**a**) Three cross-sectional planes; (**b**) five points on the planes. Figure 7: The ignition process of an n-heptane single droplet. considered that both the droplet and the surrounding flow field are in a relatively stable state at the moment of ignition. Under the combined influence of gravity and surface tension, the droplet assumes an ellipsoidal shape in the normal-gravity environment. However, in the microgravity environment, the droplet assumes a standard spherical shape. Therefore, to facilitate the modeling of droplet ignition in microgravity, the standard spherical diameter of the droplets was used as the measured parameter in this study. The spherical diameter is determined by calculating the volume equivalence between the actual ellipsoidal droplet and its standard spherical shape. Given the major axis diameter \(d_{a}\) and minor axis diameter \(d_{b}\) of the ellipsoid, the equivalent diameter \(d_{0}\) of the standard spherical droplet is derived as follows:  
\[d_{0}=d_{a}^{\frac{1}{4}}\ d_{b}^{\frac{2}{2}} \tag{1}\]  
The major and minor axis diameters of the actual droplets can be calculated using the images before ignition. By measuring the actual diameter of the fiber and its imaged width, a scale can be established, as shown in Figure 8. Subsequently, through image analysis, the transverse and longitudinal dimensions of the droplet can be measured, enabling the calculation of the actual diameter of the major and minor axes. To ensure measurement accuracy, three clear images before ignition were selected for each experimental group.