A series of experiments was carried out using a 1:10 small-scale model tunnel to investigate the effect of different fire source elevations on the flame merging characteristics of dual fire sources under natural ventilation. CFD numerical simulation software was used to analyze the impact of fire source elevation on flame merging characteristics and air entrainment from the perspectives of lateral velocity, longitudinal velocity, vertical velocity and air entrainment rates. The effects of fire source spacing and heat release rate on the probability of flame merging and the height of the flame merging point under different fire source elevations were investigated. It is found that as the fire source spacing S increases, the flame tilt angle theta first increases and then stabilizes for the first time. With a further increase in S, theta increases again and stabilizes for the second time. The first stabilization is mainly controlled by the interaction between the fire sources, while the second stabilization is primarily influenced by natural ventilation from both ends of the tunnel. As the fire source elevation increases, the flame merging probability decreases with the addition of air entrainment between the fire sources. What is more, when the fire source elevation exceeds 8 cm, the effect of the fire source elevation on the flame merging probability is no longer obvious. The prediction model of flame merging probability under different fire source elevations is proposed. Meanwhile, the dimensionless fire source elevation h/D and the modified dimensionless heat release rate Q(DS)& lowast; are used to express the dimensionless height of flame merging point Z(m)/H-ef. The piecewise function of Z(m)/H-ef and Q(DS)& lowast; is obtained. WhenQ(DS)& lowast;<= 88,h/D <= 8, the relationship between Z(m)/H-ef and Q(DS)& lowast; could be described with exponential function and whenhD>0.8, the fire source elevation has little impact on Z(m)/H-ef. WhenQ(DS)& lowast;>8, Z(m)/H-ef is approximately constant and increases with fire source elevation.
