Abstract: To investigate the flame merging probability of dual fires in a naturally ventilated tunnel, a series of experiments were conducted using a 1∶10 scale model of the tunnel. The study focused on how the position and spacing of the fires affected the merging probability. The results showed that the merging of fires in different positions can be categorized into three stages: continuous merging, intermittent merging, and non-merging. In the continuous merging stage, the probability of flame merging is always 1. In the intermittent merging phase, the probability of flame merging decreases rapidly as the distance between the fires increases. In the non-merging phase, the probability of flame merging is always 0, and a greater distance between the fires is needed for the probability of flame merging at the wall-adhering position to reach 0. In the range of dimensionless fire source spacing between 0.27 and 0.80, when the dimensionless fire source lateral position is less than or equal to 0.06, the probability ratio of flame merging decreases linearly with the increase of non-dimensional lateral position of the fire source. When the non-dimensional lateral position of the fire source is greater than 0.06, the ratio of flame merging probability no longer changes significantly with the increase of non?dimensional lateral position of the fire source.

Key words: naturally ventilated tunnel, dual fires, flame merging probability, fires spacing, transverse position of the tunnel