The smoke stratification and the smoke descent along a tunnel are of the utmost importance for personnel evacuation. The paper investigates the smoke descent along a tunnel during a naturally ventilated tunnel fire. A theoretical model is developed to predict the smoke depth below the ceiling along the tunnel. A series of numerical simulations of full-scale tunnel fires are conducted to compare with the developed model, and some coefficients such as the entrainment coefficient are determined from the simulation results. The concepts of critical moment and critical distance are proposed to characterize the smoke descent along the tunnel. The results show that as the smoke spreads longitudinally, the smoke depth below the tunnel ceiling continuously increases. The temperature decay along the tunnel due to heat losses and air entrainment at the smoke layer interface is considered as the main parameter for the smoke descent. After the vitiated air returns back to the fire source, the smoke stratification in the entire tunnel will be significantly reduced. The smoke layer depth along the tunnel based on the temperature distribution is relatively stable in the process of smoke development, which is not sensitive to the HRR, but influenced by the tunnel width, and this method could only be used before the critical moment. The outcomes of this study could provide references for a better understanding of smoke movement in naturally ventilated tunnels and provide technical guidelines for fire safety designers.
The authors would like to acknowledge the Tunnel and Underground Safety Center (TUSC) for the financial support. Thanks to Qinghua Guo for the valuable discussions and technical assistance during his stay at RISE. Shengzhong Zhao and Haoran Yang were also financially supported by the National Natural Science Foundation of China (No. 52208115 ), the Natural Science Foundation of Shandong Province (No. ZR2020QE279 ) and Plan of Guidance and Cultivation for Young Innovative Talents of Shandong Province.