Numerical studies on the synergistic effects of smoke extraction and control performance by mechanical ventilation shafts during tunnel fires

High smoke extraction efficiency and a relatively stable smoke layer stratification are both expected in tunnel ventilation systems. The purpose of this paper is to explore the overall performance of mechanical board-coupled shaft under different ventilation strategies. A total of 57 simulations were conducted, and the effects of the distance between the shaft and board ($h_D$) and ventilation velocity on the overall performance were investigated. The results indicate that the performance of smoke extraction and control will be improved by the application of mechanical ventilation and board. Smoke movement patterns under different working conditions were different, for cases of $h_D\le 0.40m$ the smoke could propagate through the whole tunnel without backflow, while for cases of $h_D>0.40m$, the backflow exists and the smoke movement can be separated into three periods (propagation, stagnation, and retraction). The critical criterion of backflow was investigated and a simple model was deduced to estimate the maximum propagation length. Moreover, the dimensionless time for the smoke flow to reach its maximum propagation length was established. Finally, a comprehensive index $\phi$ was proposed to evaluate the synergistic effects of smoke extraction and control performance. These studies may provide positive significance for the ventilation design.