Influence of the closed end on the smoke propagation and temperature profile in urban utility tunnel fires

Urban utility tunnels have relatively small cross-sections and isolated environment, which is different from the common transportation tunnels. Thus, the fire situation in the utility tunnel is more severe, and the spread of smoke has special characteristics due to the isolated environment. Considering the detection, early warning, and prevention of utility tunnel fire, it is essential to understand the characteristics of smoke movement and temperature variation during the occurrence of fires in the restricted environments of utility tunnels. This study employs numerical simulations to investigate the influence of closed end on the smoke transport and temperature variation of the ceiling in utility tunnel. The range of the heat release rate is 3 to 15 MW, and the longitudinal distance (d) d ) between the closed end and the fire source varies from 3 to 30 m. The results indicate that due to the presence of the closed end in utility tunnel fires, the side of fire source near the closed end is prone to accumulating more smoke and forming a thicker smoke layer. Moreover, the backflow of smoke after colliding with the closed end will cause disturbance to the stability of the smoke layer on the other side. As the fire source progressively travels away from the closed end, the disturbance caused by the accumulated smoke progressively weakens, and the concentrated smoke is more likely to reach a relatively stable state. Finally, a correlation is established, utilizing the starting location of the one-dimensional spread stage as a reference point, to anticipate ceiling smoke temperature variation over the longitudinal direction of the tunnel. To validate the applicability and reliability of the proposed correlation, a comparison is made with the data from previous studies on both closed-end and open-end tunnels.