Multisource fire accidents frequently occur in tunnels, and their combustion characteristics are complex, influenced by both the parameters of the source itself and air entrainment and thermal feedback of adjacent fire sources. The combustion characteristics of multisource fires are related to the size, fuel type, and spacing of neighboring sources. In a dual-source fire in a tunnel, adjacent dual sources may compete for air, leading to flame merging. During flame merging, special flame phenomena occur, such as flying fire and fire whirlwinds, increasing the probability of fire spread and the degree of danger. Thus, building a reduced-size tunnel model to systematically analyze the evolution mechanism of multisource fire behavior is necessary.

Herein, we built a 1∶6-scaled tunnel model and conducted scaled experiments to investigate the dynamic evolution characteristics of restricted symmetric dual-source fire in natural ventilation tunnels under different fire source spacings. We studied the combustion rate, flame morphology, flame fusion, and ceiling temperature distribution. In particular, the ceiling temperature was further divided into near- and far-fire-source regions. We aimed to reveal the evolution of an asymmetric entrainment mechanism involving flame tilting and the fusion of a restricted symmetric dual-source fire in a tunnel. In addition, we constructed a quantitative model for the highest temperature rise of symmetrical dual-source fires under different fire source spacings and lateral offset distances. Thus, the dynamic characteristics of the tilting or even fusion of a restricted symmetric dual-source fire were elucidated.

This study found that the mass loss rate, heat release rate, and flame height of the fuel were negatively correlated with the lateral offset distance and fire source spacing. When the dimensionless distance between the fire sources (S/D) was less than or equal to 1, flame fusion occurred, and a peak temperature occurred below the ceiling. As the distance between fire sources increased, the maximum temperature distribution transitioned from a "single peak" to a "double peak". When the S/D≥4, the two fire sources essentially burned independently, and the peak number of the maximum temperature curve changed to 2. The dimensionless maximum temperature rise in the near-fire-source region exhibited an exponential decay relationship with the dimensionless lateral offset distance. A prediction model for the maximum temperature rise in the near-fire-source region was established and verified through comparison with experimental data from previous study and this study and numerical simulation results.

This study provides basic data to further elucidate the combustion mechanism of dual-source fire in tunnels under different lateral offset distances. Moreover, this study offers a theoretical basis for smoke control and emergency rescue during multisource fires in tunnels.