Theoretical prediction on supply air performance and CO dilution effect of the pressed-in ventilation system during the construction period

•Theoretical model to assess the effectiveness of pressed-in ventilation is proposed.•ΔLv,max and fan’s operating points are analyzed for different fan 2# location.•The spatial–temporal CO concentration inside tunnel after blasting is predicted. The effectiveness of pressed-in ventilation system significantly affects the environmental condition inside the tunnel after blasting. Previous studies rarely focus on the ventilation scenario with two fans installed in series at a certain interval on the leaky ventilation ducting, which is very common for super long tunnel. The concept of the combined fan is introduced to consider the equivalent pressure rise created by the fan 1#, air leaky ducting 1 and the fan 2#. A theoretical model of long-distance air supply calculation suitable for this scenario is proposed to predict the flow rate to the working face and the fans’ actual operating parameters. The temporal-spatial gas distribution inside tunnel after blasting is predicted as well. This theoretical model is proven to be suitable for other ventilation scenarios in tunnel under construction. The accuracy of the theoretical model is verified by 4 tunnel on-site measurements data. Taking a tunnel project as illustration, this model is used to evaluate the air-supply effectiveness of the ventilation system during the construction period. With the increasing air supply length, the flow rate delivered to the working face is predicted. The maximum air supply length of fan 1# is determined under different fan’s rotating speed and air duct leakage situation. For three installation locations of fan 2#, the increased air supply length, the fans’ operating parameters and fans’ power are further compared. The rational installation location of fan 2# is given out. Under different air duct leakage situation, the temporal and spatial distributions of CO concentration inside the tunnel after blasting are predicted. The effect of air duct leakage situation on the safe re-entry time for the workers is given out. This research can provide theoretical support for engineers to evaluate the effectiveness of pressed-in ventilation system during the construction period.