At Qomaira, we recently conducted a performance study. We examined a tunnel ventilation system under both normal operation and emergency fire conditions. The aim was to analyze the interaction between jet fans, tunnel geometry, and air/smoke behavior in critical scenarios.
🎯 Project Goal
We need to evaluate whether the current fan layout and airflow capacity meet the required performance levels for tunnel safety. This is especially important during a 25 MW fire event involving a bus.
💨Input parameters
Provided by the HVAC team:
- Make up& Exhaust fans.
| Parameter | Flow rate / fan | Quantity/ side | Flow rate/ side |
| Make-up fans | 24141 cfm | 5 | 120706 cfm |
| Exhaust fans | 28401 cfm | 5 | 142007 cfm |
| Parameter | Static pressure/ Fan | Quantity/ side |
| Jet Fans | 282 pa | 14 |
Emergency operation
- In the event of a fire, make-up and exhaust fans increase their variable flow rate accordingly.
- The heat release rate (HRR) for fire analysis was set at 25 MW
Assumptions:
Simulations are based on provided drawings and are performed on one side of the tunnel.
For 1st scenario: No vehicles or temperature analysis included
For 2nd scenario: Location of the bus is in the center of the tunnel.
Geometry Observations:
We encountered issues obtaining clear drawings for CFD analysis, making it difficult to work effectively. Additionally, the geometry indicated that neither side of the tunnel is connected. This is not advisable for tunnel design and safety.
🔧Setup:
First simulation:
- Inlet flow rate = 70 kg/s (120706 CFM)
- Jet fans pressure jump 282 Pa
- Make up and exhaust fans pressure jump = 500 Pa
Second Simulation:
- Inlet flow rate= 85 Kg/s ( 147500 CFM)
- Jet fans pressure jump 282 Pa
- Make up and exhaust fans pressure jump = 500 Pa
- Smoke mass flow rate 105 kg/s (172880 CFM)
- Heat generation of 25 Mw.
🔍 Key Simulation Results:
For Normal Scenario
Jet Fan Flow Obstruction: The First and last sets of fans are blocked by the tunnel walls
→ Reduces system efficiency significantly
Mean Air Velocity: 0.546 m/s on car level
❌ Below NFPA 502 lowest of 1.5 m/s
Fig.1: Velocity contours on mid horizontal plane.
Fig.2: Velocity Average on car level.
Fig3: Velocity contour on car level
Fig.4: Velocity mid vertical plane
For 2nd Scenario Results:
Back-layering of Smoke: Smoke travels upstream toward the traffic direction
→ Caused by insufficient flow from jet fans
Temperature Around Bus: Reached dangerous levels, indicating a potential threat to passengers and responders
Smoke Propagation: Visualized in volume fraction and streamline plots
Air Velocity & Pressure: Lower than required to push smoke downstream effectively
Fig.5: Pressure contours on mid vertical plane.
Fig6: Streamlines from smoke.
Fig.7: Volume Fraction of smoke inside the Tunnel.
Fig.8: Volume Rendering of Temperature inside the Tunnel.
Fig.9: Temperature contour inside the Tunnel on car level.
Fig.10: Temperature contour in mid Vertical Plane.
Fig.11: Velocity of air at the car level.
Fig.12: Vectors of velocity on the fan level.
Conclusion:
· Tunnel Geometry and Jet Fan Placement:
Issue: The current tunnel geometry obstructs the airflow to the first and last sets of jet fans.
Recommendation: Adjust the geometry of the tunnel to ensure smoother airflow. Reposition the first and last jet fans to avoid any obstacles. This change will improve the overall efficiency of the ventilation system.
· Mean Tunnel Velocity:
Issue: The mean velocity of 0.546 m/s is below the NFPA 502 recommendation of 1.5m/s.
Recommendation: Increase the airflow rate to achieve the recommended mean velocity. This might involve upgrading the jet fans or adding additional fans to boost the overall air movement within the tunnel.
· Back-layering in Fire Cases:
Issue: Significant back-layering during fire events indicates that the current jet fan flow rate is insufficient to clear the smoke.
Recommendation: Increase the flow rate of the jet fans. This will help effectively manage and remove smoke during a fire. This involves higher-capacity fans or more frequent maintenance checks to ensure optimal performance.
· Temperature Concerns Around the Bus:
Issue: The temperature around the bus is dangerously high, posing a threat to human and vehicle safety.
Recommendation: Install quick and easily accessible evacuation ladders throughout the tunnel. This ensures that individuals can evacuate quickly in the event of an emergency.
Contractor Recommendations
- Jet Fan Placement Issue: The first and last pairs of jet fans are not positioned optimally. Recommendation: Reposition these jet fans to improve ventilation effectiveness and airflow distribution.
- Tunnel Drawings: Clear and final drawings of the tunnel should be provided, including both side and top views.
Recommendation: These drawings must clearly illustrate whether both sides of the tunnel are open to each other. This clarity is necessary to ensure accurate design. It also supports safety assessments.
✅ Final Thoughts
This project revealed that tunnel ventilation systems must be revisited to meet fire safety standards. Fan placement, airflow velocity, and tunnel geometry all play vital roles. By implementing the recommended improvements, this tunnel can achieve safer evacuation conditions. It can also manage smoke better. Additionally, it will fully comply with NFPA 502.