*2.8. Mixture Model*

A mixture model is a simplified multiphase model regularly used to simulate homogeneously mixed flows if the coupling between the phases is strong. This model was selected to monitor the concentration of a tracer gas inside the dead-end tunnel [26]. In general, the most problematic gases after blasting are CO and NOx (NO + NO2), of which NO2 is the most critical. This is because NO quickly oxidizes to NO2, which is the most toxic and diffuses with greater difficulty.

All simulations were initialized at a concentration of 5% in the volume of this gas. Different flow rates of fresh air (0% NO2) were simulated at the inlet to compare them with the aerodynamic simulation results. Ventilation times not greater than 20 min were considered because longer re-entry times after blasting significantly reduce tunneling productivity. All simulations were conducted for scenario 4 and with fresh air duct velocities less than 20 m/s.
