Experimental investigation of the burning behavior of small logs with various external radiative heat fluxes

Wood is widely used in building materials, furniture and handicrafts due to its excellent thermal insulation properties, mechanical strength and ornamental value. However, wood is a combustible material that undergoes pyrolysis, ignition, charring and cracking. Most current studies on the burning characteristics of wood have used relatively homogeneous artificial boards or logs without defects. However, in practical applications, wood structures vary among different species, the wood surface grain is not uniform, and there are many structural defects in the logs, such as knots. These factors cause the wood burning characteristics to differ significantly from that of a homogeneous board. This study experimentally investigated the burning of small logs made of white pine, radiata pine and Chinese fir. The burning behaviors were compared with different external radiative fluxes with measurements of the burning phenomenon, heat release rate, and ignition characteristics. The results show that the wood species, the grains and the defects (knots) all affect the wood burning characteristics. Increasing the external heat flux reduces the ignition time and increases the peak heat release rate. In the range of operating conditions in this paper, the difference of the log ignition time at the external radiative heat flux of 15 kW/m² in the repeated tests is about 35%, which is higher than that of the homogeneous artificial wood. This trend indicates the effect of structural heterogeneity on the combustion characteristics of logs, but this difference decreases with the increase of the external radiative heat flux. For the same tree species, the combustion characteristics of sparse and dense grain samples are different. The highest ignition time difference is more than 500 s, and the charring rate is higher in the direction parallel to the grain than in the direction perpendicular to the grain. The Chinese fir contains a large number of knots, which can reduce the ignition time and increase the peak heat release rate at the low external radiative heat flux.