[en] Regarding mine ventilation in a tunnel with one mining face, the turbulent flow structure and Mean age of air (MAA) of fresh air discharged from the ventilation ducts are numerically studied using the commercial CFD code, ANSYS CFX 17.0. The characteristics of the three-dimensional turbulent transport and fluid age are considered to provide new insight to design ventilation ducts. For the optimal design of ventilation ducts, parametric studies are conducted. In particular, the effects of duct angles, distances between the duct outlet and mining face, and fresh air velocities are numerically evaluated with respect to the MAA. Flow streamlines, velocity vectors, MAA, eddy viscosity, and total pressure in the tunnel of a mine with ventilation ducts are presented. The variation in the MAA with the angle, distance, and velocity is considered for the optimal design of ventilation ducts. Overall, the numerically predicted results describe the optimal designing points of a ventilation duct in the working area of a mine with heading faces by using the MAA