## 4 Closing  
As the follow-on work of our previous study, this paper has shown an overall relationship of pressure effects on upward flame spreading behavior and burning characteristics of an inclined FPU foam board. Considering the particular fuel type, we have given a careful investigation on the three-dimensional upward spreading process of FPU, also with special burning phenomenon observed. Based on fire dynamics analysis, comprehensive understandings of individual and coupling effects (of pressure and orientation) on FSR, burning rate, flame length and each heat transfer component are proposed or formulated with semi-quantitative correlations theoretically, which are consistent with the test results. The work is believed to be helpful for fire hazardous evaluation or supplementation of fire scientific theory under sub-atmospheric pressures. Future work is planned to be carried out with more combustibles and much lower pressures.  
Figure 4: Variation tendencies of flame tilt (**a**), pressure indexes (**b**), heat transfer intensity (**c**) and burning characteristics (**d**) by influences of inclination or pressure.  
**Author Contributions:** Initial conceptualization, R.T. and X.M.; data curation, X.M. and X.Z.; formal analysis, R.T., X.M. and Y.Z.; funding acquisition, R.T., X.M. and X.Z.; methodology, R.T. and Y.Z.; project administration, R.T.; writing--original draft, R.T. and X.M.; writing--review and editing, X.Z. and Q.Z. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the National Natural Science Foundation of China (No. 52076084) and Huaqiao University Scientific Research Foundation (No. 16 BS801). [PERSON], [PERSON] and [PERSON] are also supported by the Open Project Program of State Key Laboratory of Fire Science (HZ2020-KF07, HZ2019-KF13 and HZ2018-KF09).  
**Acknowledgments:** We thank [PERSON] for assistance with the experiment's design.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclature**  
\(c_{p}\)Specific heat of fuel (J g\({}^{-1}\) K\({}^{-1}\))  
\(g\)Gravity (m s\({}^{-2}\))  
\(L_{f}\)Flame length (m)  
\(in\)Burning rate (g s\({}^{-1}\))  
\(p\)Air pressure (kPa)  
\(\dot{q}^{\prime\prime}_{f,conv}\), \(\dot{q}^{\prime\prime}_{f,rad}\)Convection and radiation preheating flux from fire (W m\({}^{-2}\))  
\(\dot{q}^{\prime}_{conv}\), \(\dot{q}^{\prime\prime}_{rad}\)Convection and radiation heat feedback from fire (W m\({}^{-2}\))  
\(\dot{q}^{\prime\prime}_{TPU,\,cont}\)Conduction preheating flux from FPU (W m\({}^{-2}\))  
\(\dot{q}^{\prime\prime}_{p}\)Preheating heat flux to pyrolysis zone (W m\({}^{-2}\))  
\(S\)Burning area (m\({}^{2}\))  
\(T_{ig}\)Ignition temperature (K)  
\(T_{\infty}\)Ambient temperature (K)  
\(U_{w}\)Velocity of air entrainment (m s\({}^{-1}\))  
\(V_{p}\)Flame spread rate (m s\({}^{-1}\))  
wWidth of fuel board (m)  
\(x_{p}\)Distance of pyrolysis zone (m)  
\(\alpha\)Pressure index of preheating flux  
\(\beta\)Pressure index of flame length  
\(\delta_{p}\)Length of preheat zone (cm)  
\(\theta\)Angle of inclination (\({}^{\circ}\))  
\(\rho\)Density (kg m\({}^{-3}\))  
**Subscripts**  
0 Normal pressure  
dDecreased pressure  
dwDownward  
\(f\)Fire or flame  
uwUpward  
\(\infty\)Ambient