Effect of unpowered ventilation caps and shaft parameters on fire smoke spread in the natural ventilation tunnel with shafts

Shaft-type natural ventilation offer a sustainable and cost-effective alternative to mechanical systems for the mitigation of smoke hazards. This study investigated the behavior of smoke with the application of unpowered ventilation caps in 1/15 scale shallow-buried urban road tunnels with shafts, and analyzed parameters such as the shaft axis wheelbase, shaft width, shaft width-to-height ratio, and heat release rate (HRR). The results show that the dimensionless HRR as an independent relationship with the dimensionless ceiling temperature rise in both the presence and absence of the unpowered ventilation cap. According to the experimental results, dimensionless length of smoke backlayering increases significantly with the increase of the HRR. In addition, the smoke backlayering length is positively correlated with the shaft width, and the backlayering length decreases and then increases with increases of the axis wheelbase. Moreover, the chimney effect is facilitated with the shaft height increase, which enhances exhaust effect, and the backlayering length decreases with the increase in the shaft height when the aspect ratio is greater than 0.5. Once the critical shaft width-to-height ratio is reached, the change in the shaft height no longer has an effect on the backlayering length. Finally, a dimensionless backlayering length calculation model based on the shaft parameters with different shaft exhaust effects is developed by combining the discriminant factors. The findings have crucial implications for fire prevention and safety management, and provide a valuable reference for future studies and engineering applications.