Numerical simulation study on the fire plume behavior and smoke spread characteristics of transverse double fire sources in ultra-wide tunnels

Numerical simulations were performed to examine the fire plume behavior and smoke spread characteristics of transverse double fire sources in ultra-wide tunnels. The effects of tunnel width, total heat release rate, and the dimensionless parameter S / W (where S is the distance from the fire source to the sidewall and W is the tunnel width) were analyzed to clarify the fundamental mechanisms governing plume interaction and smoke transport. The results show that variations in S / W lead to three characteristic plume development modes: wall-attached plume, approximately independent plume, and inward-tilting plume merging beneath the ceiling, which strongly influence the ceiling temperature field. A pronounced double-peak ceiling temperature distribution appears when the S / W is small, and it gradually transitions to a single-peak profile as the plumes merge. The plume merging distance exhibits a distinct non-monotonic dependence on S / W , increasing initially and then decreasing, and reduces to zero when the two plumes merge before reaching the ceiling. Moreover, the maximum ceiling temperature rise follows a similar three-stage evolution with S / W , and a piecewise empirical model is established to describe its relationship with fire power and S / W . Additionally, the longitudinal ceiling temperature decay remains well represented by an exponential form, with the decay coefficient increasing at both low and high S / W . The smoke layer height becomes larger at both small and large S / W values due to restricted entrainment on the wall-adjacent side and competitive suppression of inner-side entrainment, respectively. These findings providing essential guidance for evaluating thermal hazards and smoke movement in ultra-wide tunnels.