Numerical and experimental study on the critical velocity and smoke maximum temperature in the connected area of branch tunnel

The objective of this study is to investigate critical velocity and smoke maximum temperature beneath the ceiling in the connected area of branch tunnel with varying fire locations. The fire sources were located in the divergent connected area of the branch tunnel, to imitate traffic accidents near the branch point. A 1/20 scale model branch tunnel was built including main line before branch, main line after branch, and ramp. Experimental tests and numerical simulations were performed to explore smoke movement characteristics with longitudinal ventilation. The results showed that the enlarged cross-sectional area in branch tunnel caused the shortening of the back-layering length, and a modified model of back-layering length was proposed. The higher tunnel height in this work affected the critical condition of large fire; it caused a larger transition point of dimensionless critical velocity. A revised model was proposed for the maximum temperature rise of tunnel fires in the connected area of branch tunnel. The critical velocity kept unchanged when the branch angle increased from 0° to 20° because there is little change in the longitudinal smoke temperature. As the local tunnel width of fire source was increased, the required critical velocity was increased while the local effective velocity kept nearly the same.