Prediction of the critical condition for flame acceleration over wood surface with different sample orientations

To understand the inclination effects on flame spread over wood surface, a set of flame spread experiments were carried out for different sample orientation angles from −50° to 20° in the Hefei Plain (at the altitude of 50 m) and in the Tibetan plateau (at the altitude of 3658 m). At both altitudes, a transition zone was found at 10–20° orientation for flame spread rate, the preheated length and flame angle from the horizontal. The transition zone was an external manifestation of the change of flame spread from steady state to acceleration. A new relationship of Πc = 1 was established to predict the occurrence of acceleration based on theoretical analysis. Experimental data at the two altitudes suggested that the critical value of Πc is about 1.1–1.2, which has a good agreement with the theoretical value.