## 5 Conclusions  
In this study, we first designed a heating vessel to simulate a high-temperature static-air environment. Subsequently, hanging-droplet experiments were conducted to measure and investigate the relationship between ignition temperature and initial diameter for single droplets of ethanol, n-heptane, and n-dodecane fuels. The analysis of the experimental results revealed that, within the temperature range studied, the minimum initial diameter required for the ignition of single droplets of ethanol, n-heptane, and n-dodecane decreases as the ambient temperature increases; conversely, it increases as the ambient temperature decreases. Furthermore, at the same ambient temperature, the minimum initial diameter required for ignition is the largest for n-dodecane droplets, followed by n-heptane, with ethanol droplets requiring the smallest minimum initial diameter. In the analysis of the experimental data, this study, building upon the classical Frank-Kamenetskii method, derived a predictive model for single-droplet ignition in a high-temperature static-air environment in normal gravity. By incorporating the empirical-formula-corrected Nusselt number using the Spalding number, a predictive model for single-droplet ignition in the microgravity environment was proposed. Comparisons between the experimental results and existing observations revealed that this semi-theoretical, semi-empirical ignition model effectively characterizes the ignition conditions for single droplets, thereby laying a foundation for future related research. According to the future research plan for the CSS, once the microgravity liquid combustion module is successfully developed and deployed, microgravity combustion experiments of fuel droplets will be conducted to further investigate the combustion behavior and characteristics of droplets in a microgravity environment.  
Figure 20: Comparisons between the ignition models in different gravitational environments and experimental data for n-dodecane (\(A=5.12171\times 10^{-17}\); \(n=7.22623\)).  
**Author Contributions:** Conceptualization, H.Z.; methodology, X.L. and H.Z.; validation, X.L. and Y.F.; formal analysis, X.L.; investigation, X.L., Y.F. and X.P.; data curation X.L.; writing--original draft preparation, X.L.; writing--review and editing, H.Z., X.L., X.Z., X.Y. and Y.F.; supervision, H.Z. and X.L.; project administration, H.Z.; funding acquisition, H.Z. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.  
**Acknowledgments:** The authors sincerely appreciate the support provided by the China Space Station Combustion Science Experimental System Project.  
**Conflicts of Interest:** The authors declare no conflicts of interest.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Serial Number** & **Ignition Temperature \(T_{\infty}\)/K** & **Equivalent Diameter \(d_{0}\)/mm** \\ \hline  
\hline \hline \end{tabular}
\end{table}
Table 1: _Cont._  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Serial Number** & **Ignition Temperature \(T_{\infty}\)/K** & **Equivalent Diameter \(d_{0}\)/mm** \\ \hline  
\hline \hline \end{tabular}
\end{table}
Table 3: Experimental conditions for n-dodecane.