Due to the particularity of double fires attached to the tunnel wall, its flame characteristics are quite different from those in open space or no wall attached fires. A series of experiments were carried out in the naturally ventilated rectangular reduced size tunnel model. It is shown that with the increasing of fires spacing s, the flame height Hf first increases slightly, then decreases continuously, and remains stable finally. Under the same fire source size, double fires attached to the wall is 1.2 ~1.4 times the flame height of single fire attached to the wall. Under the same conditions, due to the limited entrainment of flame plume when attached to the wall, the flame height of double fires is 15% ~ 17% higher than that at tunnel central axis. The maximum flame height of fires attached to the wall is higher

than that at tunnel central axis or open space. Based on air entrainment theory, the empirical formula for calculating the merged flame height H_f is established, which shows that H_f gets the minimum value when the dimensionless spacing is 1.8D.

double fires, rectangular tunnel, fire attached to the wall, air entrainment theory, merged flame height