Jet Fan for a Car Explosion Ventilation in a Tunnel

Jet Fan Application in a Tunnel Considering a Car Explosion, CFD Simulation ANSYS Fluent

Description

This project simulates a car explosion within a tunnel using ANSYS Fluent, modeling a scenario where a car ignites due to a gasoline leak combined with airflow contact while crossing through the tunnel's interior. The resulting explosion ignites a fire and releases carbon dioxide into the enclosed tunnel space, where the gas accumulates and cannot disperse into open air, posing a serious hazard to other vehicles and passing pedestrians. To address this, tunnels are equipped with jet fans mounted on the roof, which draw carbon dioxide and other pollutants upward and expel them outward, serving as the primary defense against dangerous gas buildup during such an event.

The 3D geometry was designed in SpaceClaim, representing the interior of a tunnel with a car modeled on the floor and a jet fan mounted on the roof. A specific volume at the car's rear was defined as the explosion source, alongside a designated region at the jet fan's inlet representing the fan itself. The domain was meshed in ANSYS Meshing using an unstructured grid totaling approximately 2,700,000 cells.

Methodology

This model assumes the combustion reaction has already occurred, so rather than modeling combustion itself, the simulation focuses solely on the resulting carbon dioxide emission. The Species Transport model was used to define the air and CO2 species, with airflow entering the tunnel at 300 K and 1.5 m/s. At the explosion source volume, a CO2 emission rate of 10,000 kg/m³·s and a thermal energy source of 1000 W/m³ were defined to represent the combustion byproducts. The jet fan itself was modeled using a fan boundary condition with a pressure jump of 1,000,000 Pa, which drives the surrounding air to be drawn into and expelled through the fan.

Several assumptions were applied throughout: a pressure-based solver was used, the simulation was run under steady-state conditions, and gravitational effects were excluded.

Conclusion

Results include 2D and 3D contours of pressure, velocity, temperature, and the mass fractions of air and CO2. The results confirm that the jet fan system functions as intended — the explosion at the car's rear generates significant heat and releases CO2, which is then drawn upward and extracted through the jet fans, effectively preventing the gas from spreading throughout the tunnel's interior.

To evaluate the jet fan's performance more rigorously, the simulation was repeated under identical conditions but with the fan boundary condition removed from the jet fan inlet. Without the jet fan operating, the results show CO2 filling the tunnel's interior unchecked, underscoring the jet fan system's critical role in maintaining a safe environment during a tunnel fire event.