Jet Fan: Smoke Ventilation in a Tunnel

Description

In tunnel ventilation, longitudinal ventilation is the approach that offers the lowest installation and operating cost. The most common method uses jet fans, whose relatively high-velocity discharge induces airflow along the tunnel. As the air leaving the fan diffuses, it transfers energy to the tunnel airflow and raises the static pressure. Exhaust ducts then remove the hot gases and smoke produced by vehicles and by any fire from the tunnel volume; in some cases the extraction capacity may limit the longitudinal velocity in the tunnel during normal operation. Every tunnel ventilation system relies on tunnel jet fans, large axial fans, or a combination of the two to provide both pollution-control ventilation and smoke ventilation during a tunnel fire.

This project presents a 3D simulation of a tunnel in which several cars are moving. Hot gases are emitted from the vehicle exhausts, and three pairs of fans mounted at the top of the tunnel extract the pollution by suction. By driving a flow in the direction of the fans, the pollutant — which rises because it is less dense than air — is carried out of the tunnel.

Geometry & Mesh

The domain is a 3D rectangular tunnel measuring 60 m in the X direction, 10 m in Y, and 9 m in Z. Four cars are positioned at different locations along the tunnel, and three pairs of fans are mounted on the ceiling, spaced 25 m apart. The geometry was created in SpaceClaim, and meshing was performed in ANSYS Meshing using tetrahedral elements. After importing the mesh into Fluent, the elements were converted to polyhedral, which significantly reduced their number to approximately 1,200,000.

Methodology

Several assumptions underpin the simulation: a pressure-based solver is used, the energy equation is enabled, the model is steady, and the effect of gravity is included. The jet fans are the core of the setup — they establish the longitudinal airflow that drives the smoke and pollutants toward the extraction points and out of the tunnel.

Conclusion

On completion of the solution, two-dimensional contours of pressure, velocity, and the volume fractions of pollution and air were obtained, with the contours captured at every fifth iteration. The static pressure can be seen varying across different parts of the tunnel as a result of the fan action, and the streamlines trace the pollution leaving the vehicle exhausts and following its exact path toward and through the fans. Overall, the simulation demonstrates how a jet-fan longitudinal ventilation system establishes the airflow needed to control smoke and pollutants along a tunnel — the central function of the fans in maintaining safe tunnel conditions.