Objective  The combustion of underground fire is a slow, flameless, long duration smoldering, which does great harm to forest. Daxing’anling region is a frequent area of forest underground fire in northeastern China. This paper aims to study the effects of different heating times and humus particle sizes on the vertical combustion of underground fires, aiming to provide a reference for the prevention, monitoring and suppression of underground fires in the area.

  Method  Taking the typical forest type in the Daxing’an Mountains, i.e. Larix gmelinii forest as the research object, according to the indoor control simulation of scorching experiment data, SPSS was used to perform two-factor analysis of variance, and the Origin software was used to draw. The effects of three heating times and five humus particle sizes on the spreading speed and the highest temperature at different depths during the vertical combustion of underground fire were studied.

  Result  During the vertical burning process of the forest underground fire, the temperature of the highest burning at a depth of 3 cm was only affected by the heating time and the difference was significant (P < 0.05), but the effects of humus particle size on the difference were not significant (P > 0.05). The highest temperature of combustion at a depth of 6 cm was affected by the heating time (P < 0.05) and the particle size of humus (P < 0.05). When the depth was greater than 6 cm, the maximum combustion temperature was only affected by the humus particle size and the difference was significant (P < 0.05). The spread rate during the vertical combustion of forest underground fires was only affected by the humus particle size. There were significant differences between the spreading speeds (P < 0.05).

  Conclusion  The highest temperature of underground forest fires burning at 3 cm and 6 cm depth rises with the increase of heating time, and the burning temperature is the highest when heated for 2 h. When the vertical combustion depth is 9−21 cm, the humus particle size is 40 mesh, the burning temperature is the highest. Among the effects of humus particle size on the spread speed of forest underground fires, the spread speed of humus particle size 20 mesh is the fastest; when the humus particle size is 60 mesh, it is the criticality in the process of underground fire combustion. The highest combustion temperature and spread speed are lower.