Numerical research of the influence mechanism of fan diameter and jet velocity on tunnel ventilation efficiency

Full-jet longitudinal ventilation is widely used in urban tunnels for its structural simplicity and high cost-effectiveness, yet its overall efficiency is constrained by high energy consumption. This study systematically investigates the influence of jet fan diameter and outlet velocity on ventilation performance. A three-dimensional numerical model of a typical urban tunnel was developed using ANSYS Fluent. Parametric simulations were conducted by varying jet velocity from 20 m/s to 38 m/s and fan diameter from 500 mm to 1600 mm. Jet flow evolution and the underlying mechanisms governing ventilation efficiency, including pressure development, air entrainment, and cross-sectional flow uniformity were rigorously analyzed. Results indicate that for a 500 mm diameter fan, increasing jet velocity from 20 m/s to 38 m/s elevates the ventilation pressure coefficient from 16% to 66%. At a fixed velocity of 38 m/s, enlarging the diameter from 500 mm to 1600 mm further improves the pressure enhancement coefficient from 66% to 73%, with fan pressure gain exhibiting a quadratic relationship with diameter. The analysis demonstrates that higher jet velocities enhance both momentum thrust and ambient air entrainment, thereby strengthening the longitudinal airflow driving force. Concurrently, larger fan diameters widen the jet diffusion angle, producing a more uniform cross-sectional airflow with reduced stratification. These findings clarify the distinct yet interdependent roles of jet velocity and fan diameter in jet behavior and ventilation efficacy, offering quantitative guidance for fan selection and energy-efficient longitudinal ventilation design.