1. Introduction

In infrastructure such as road tunnels, railway/metro tunnels, and underground parking garages, air quality control, smoke evacuation, and safety compliance are pressing concerns. Traditional passive or ducted fans are being replaced by jet fans, which offer stronger airflow, more flexibility, and lower long-term cost. This article—focused on the keyword jet fan—discusses how jet fan systems perform, their design parameters, and what sets them apart.

2. What Is a Jet Fan?

A jet fan is a high-speed axial-type ventilation device, mounted directly inside tunnels—on the ceiling or walls. It produces a concentrated, high-velocity airflow (jet) that entrains surrounding air, creating a full-length longitudinal airflow without ductwork. This makes jet fans compact, scalable, and well-suited for retrofits or new constructions.

SHUANGYANG FAN project - Guangdong Kaichun Expressway                                              SHUANGYANG FAN project - Yunnan Yanglin Tunnel

3. Jet Fan Working Principle

Jet fans operate on fluid dynamics principles:

1. Air Ejection & Momentum Transfer
   A strong jet is expelled, carrying momentum into the tunnel air and inducing a directional current—creating a thrust measured in Newtons (N).

2. Entrainment Effect
   As the jet travels, it entrains ambient tunnel air, dramatically amplifying the ventilated volume—multiplying the actual air volume moved.

3. Pressure Gradient Creation
   Aligned jets over the tunnel produce a consistent pressure gradient, moving air from inlet to outlet portals.

This creates an effective tunnel-scale airflow. Different from ducted fans needing channels to guide airflow, jet fans use their momentum to circulate air across the full tunnel length.

4. Key Applications & Why Jet Fans Outperform Alternatives

4.1 Vehicle Tunnels (Highway, Railway, Metro)

• Road tunnels: Vehicle emissions (CO, NOx) build up without forced ventilation. Jet fans manage pollutants while maintaining clear visibility for drivers.

• Rail/metro tunnels: Trains create a piston effect, helping airflow but insufficient alone. Jet fans augment this for consistent ventilation.

4.2 Underground Parking Garages

Emissions (CO, NO₂) can accumulate rapidly. Jet fan-based systems deliver controlled, sensor-enabled dilution fandowns—efficient and energy saving.

4.3 Emergency Smoke Control

Jet fans are critical during tunnel fires. In longitudinal ventilation, they blow smoke downstream of the fire, keeping upstream tunnels clear for evacuation and rescue.

5. Jet Fans vs. Traditional Ventilation

6. Design & Dimensioning Considerations

6.1 Thrust and Pressure Balance

The thrust needed (T_total) must overcome friction, portal losses, traffic piston effects, and meteorological influences. Using pressure terms (Pa) and tunnel face areas, net thrust ensures airflow.

6.2 Critical Velocity for Fire Conditions

To prevent back-layering (smoke flowing upstream against traffic), airflow velocity (v) must exceed a critical velocity (v_c, typically 2.5–3.5 m/s).
Design includes analytical and CFD modeling (e.g., NFPA 502 equations), ensuring v ≥ v_c under fire scenarios.

6.3 Entrainment & Installation Factor

Effective thrust is impacted by installation geometry. Aero deflectors can optimize entrainment and minimize losses.

6.4 Placement & Travel Direction

Fans are spaced along the tunnel axis. Multi-unit arrangements ensure redundancy. In fire situations, upstream installation ensures continued extraction.

7. Smart Control & Sensor Integration

Modern systems use sensors (CO, NO₂, visibility) for real-time control—turning fans on/off as needed.
Automation panels manage these fans, especially for smoke conditions, reducing energy use and enhancing safety.

8. Fire Safety & Compliance

8.1 Smoke-Free Evacuation.

By directing smoke downstream, jet fans maintain escape routes.

8.2 Temperature Certification

Fans must operate under high temperatures in case of fire emergency (the highest requirement is 400 °C for 2 hr, certified under EN 12101-3).

8.3 Back-Layering Mitigation

Design ensures critical velocity is met: multiple jets, spacing, and power allow sink-free upstream zones.

9. Economic, Operational & Environmental Benefits

9.1 Lower Capital & Operational Cost

Jet fans eliminate ductwork, reducing material and labor cost. Compact units simplify installation.

9.2 Energy Savings

Entrainment-enabled airflow maximizes efficiency—compared to ducted fans, jet systems use less power per cubic meter ventilated.

9.3 Enhanced Safety → Liability Reduction

Smoke control and emergency readiness aid compliance and may reduce insurance risk.

9.4 Modularity & Retrofit Flexibility

Jet fans can be incorporated gradually or adjustably—ideal for evolving infrastructure.

10. Implementation Roadmap

1. Assessment: tunnel length, traffic direction, vehicle types, baseline airflow.

2. Preliminary sizing: compute necessary airflow velocity and thrust to exceed critical v_c.

3. Layout design: position multiple jet fans, considering geometry, redundancy, fire zones.

4. System sizing: select appropriate fan units (thrust rating, certification, noise ratings).

5. Modeling & validation: use CFD or deterministic methods.

6. Control design: integrate sensors, automation panels, manual overrides.

7. Certification & testing: ensure compliance with international standards under fire tests.

8. Maintenance & commissioning: scheduled performance audits and smoke trials.

11. Future Trends in Tunnel Jet Fans

• Nozzles improvement offers higher thrust efficiency gains without increased power draw.

• Variable-speed and reversible fans support bidirectional ventilation.

• Hybrid systems will combine jet fans and intermittent supply/exhaust fans for optimal energy use.

• Smart control platforms will manage multi-zone ventilation automatically.

• Fire-resistant composites and coatings will improve fan lifespan and temperature tolerance.

12. Summary

Core Advantages of Jet Fans in Tunnel Applications:

• No ductwork, high airflow, scalable installation

• Proven safety in emergency evacuation and smoke control

• Energy-efficient, sensor-driven operation

• Strong ROI and regulatory alignment

Note: The data is for reference only, please consult relevant companies for details. And the content of the article cannot avoid omissions and errors. Welcome to propose corrections.