Abstract: In order to investigate the influence of the ground effect on the flame merging characteristics of dual ignition sources when a double fire occurs in a naturally ventilated tunnel, a series of experiments were carried out using a 1:10 small-size tunnel model. The focus was to investigate the effect of fire source spacing on the flame merging probability and the height of the flame merging point when the surface of the dual fire sources is flush with the tunnel floor. The results show that: fire smoke flows out of the openings at both ends of the tunnel, and the wind speed generated by the fresh air supplemented from both ends of the tunnel causes the flames to still tilt towards each other when the fire sources are more widely spaced, and the flame tilting angle is about 10° when the spacing is sufficiently large. A dimensionless quantity is introduced to represent the law that the flame merging probability varies with the change of fire source spacing and heat release rate. In addition, the experimental data in this paper are also compared with the previous work, and it is found that the ground effect contributes greatly to the flame merging behavior of dual fire sources in a longitudinal tunnel arrangement. The relationship equation between the ratio of the height of the flame merging point to the tunnel height and the modified dimensionless heat release rate is also given.

Key words: ground effect, road tunnel fire, double sources fire, flame merging probability, height of the flame merging point