Reduced pressure effects on smoke temperature, CO concentration and smoke extraction in tunnel fires with longitudinal ventilation and vertical shaft

Nowadays, high-altitude tunnels at reduced pressure are emerging in the world's road network. A better understanding of the temperature and CO concentration distribution as well as the shaft extraction performance during tunnel fires with combined longitudinal ventilation and shaft exhaust under reduced ambient pressure is particularly crucial for the tunnel fire protection design. Series of tunnel fire simulations were carried out with ambient pressures of 50 kPa-101 kPa, fire heat release rates (HRRs) of 3 MW and 10 MW, and longitudinal ventilation velocities of 0 m/s to 2 m/s. The results show that the reduced ambient pressure enhances the difference in vertical and longitudinal distribution of CO concentration and smoke temperature, while the longitudinal ventilation gradually weakens this difference in longitudinal distribution. The natural exhaust shaft reduces the difference in vertical distribution but increases the difference in longitudinal distribution. As the reduction of ambient pressure, the smoke extraction efficiency of the shaft will increase slowly. Longitudinal ventilation can enhance the shaft's smoke extraction performance, but the general smoke extraction efficiency of the shaft deteriorates with the increasing longitudinal ventilation velocity. This study provides a fundamental reference for tunnel natural ventilation system under different ambient pressures, particularly in high-altitude tunnel fires.