## 3 Results and Analysis  
### Upward Flame Spreading Behavior with Different Orientations  
Typical configuration of upward flame spread over the FPU board is illustrated in Figure 1 based on experimental observation. Figure 1a shows the different zones and heat transfer mechanisms, where a significant large inverse \"V\" shape pyrolysis zone could be found in tests with inclination angle \(\theta\) exceeding 30\({}^{\circ}\). This kind of \"V\" shape was also reported in downward flame spread, e.g., [17; 18], but with a much smaller scale and blunter included angle. The large inverse \"V\" pyrolysis zone is formed under both effects of flame tilt and ambient air entrainment by increasing \(\theta\). In addition, it makes the issue more complicated than downward spread, since spreading became a genuine two-dimensional (in surface or 2-plus-1 dimension considering thickness) process as,  
\[\delta_{p}=\delta_{p}(y,t),\ x_{p}=x_{p}(y,t) \tag{1}\]  
To facilitate discussion, parameters along the central axis of the FPU board (\(y=w/2\)) are used for analysis below. Overall comparisons of characteristics between upward and downward flame spread are conducted in Section 3.4 for better understanding.  
It is noticeable that two particular phenomena occurred during spreading (burnout zone in Figure 1a):  
1. Melting flow combustion by high temperature molten fuel, owing to the thermoplastic-like FPU material selected, could bring heat and mass from upper to lower position through a rapid path, and also might make fire development worse as the secondary ignition source.
2. Flash burning in partial surfaces appeared intermittently, which was induced by the continuous lean decomposed gas (and some fuel vapor with small molecules) generated from the deep side of the \"burnout\" zone. As the customized FPU material is modified with clay bulk and special foaming agent, char-residue was formed along the surface after combustion, which would prevent the fuel board from complete burnout. Lean combustible gas or vapor would be released by smoldering, although pyrolysis front had already passed. Moreover, the porous structure of FPU will be helpful in gas release from depth, which has potential to be ignited by flame, hot plume or even melting flow combustion to turn into flash burning.  
Figure 1: Configuration of upward flame spread over flexible polyurethane (FPU) foam (**a**) and variation of heat transfer intensities with increasing inclination (**b**).  
Burning completeness, mainly presented as burnout layer height, is also related to inclination by visual judgement. Figure 1b shows the comparison of heating intensities and burnout layer shape with \(\theta\). It is clear that with the increasing inclination, heat feedback to the pyrolysis zone was weakening, yet preheating to the preheated zone was enhanced due to the slanting and stretched flame, leading to a faster spread rate (or velocity) but less thermal penetration in the burning area. As a consequence, the burnout layer height and/or remaining fuel became thicker in general. Further, it becomes difficult to sustain self-propagating flame spreads for large \(\theta\) conditions since critical heat feedback flux is required, which is also proved in [PERSON]'s work [6] that fire extinguished at \(\theta\to 90^{\circ}\) for RPU boards.