Investigation on the double-sided concurrent flame spread behavior of log planks

Abstract

This paper presents an experimental and theoretical research on the double-sided concurrent flame spread behavior of inclined planks. Pine log planks with a size of 100 cm long, 30 cm wide and 1 cm thick were used with six angles of inclination (θ). Results showed that the flame and the pyrolysis front on the upper surface present an inverted "V" shape at θ =15° to 45°, and the lower surface flame is always clinging to the plank surface, which is significantly different from the combustion of small-sized wooden samples. The flame spread rate is positively correlated with θ. Besides, it is found that the pyrolysis length (Lpy), preheating length (Lpr) of upper surface and the flame length of lower surface (Lf,lowersurface) have a relation of Lf,lowersurface≈Lpy+Lpr, indicating the significant influence of the lower surface flame on the double-sided flame spread. Heat received by the upper surface is mainly the radiative heat flux from the flame on the surface, and a physical model considering flame sizes and shapes is established, whose results agree well with the experimental data. Theoretical analysis was conducted on the heat transfer from lower surface, and found the conductive heat flux from the lower surface flame is of the same order of magnitude with the radiative heat flux from the upper surface flame. It indicates the heat from the lower surface flame has large contribution to the flame spread behavior. In addition, the influence of tung oil, which is widely used in timber buildings, on the flame spread behavior was also researched. It shows that brushing tung oil on the plank surface could increase the flame size, spread rate and heat release rate.