How Jet Fans Drive Longitudinal Tunnel Airflow
A longitudinal tunnel jet fan drives the tunnel air through momentum exchange. Its discharge speed alone cannot describe the installed result. Nearby walls and the next fan can change how much of that momentum reaches the surrounding tunnel stream.
Follow the air through the process
Start with a local high-speed jet
The fan accelerates air through its own bore. That local discharge occupies only part of the tunnel cross-section; it is different from the average velocity farther away.
Let the jet exchange momentum
As the discharge spreads, it draws surrounding air into motion and contributes to longitudinal tunnel flow. The overall path and resistance of the tunnel still determine the operating result.
Account for nearby surfaces
A jet close to the ceiling or sidewall interacts strongly with that surface. The associated friction reduces the useful installed contribution compared with an ideal unrestricted discharge.
Examine the next fan inlet
At short spacing, a downstream fan may ingest much of an upstream jet before it has shared momentum with the surrounding air. More fans in a short distance do not automatically provide the expected additional tunnel thrust.
Evaluate spacing and clearance together
The layout must account for tunnel air velocity, wall roughness, fan geometry and the available structural envelope. Qualitative illustrations cannot set a universal clearance or longitudinal spacing.
Airflow function
- Accelerate the local air stream through the fan assembly.
- Transfer momentum through jet mixing to support tunnel ventilation.
- Operate as part of an installed layout whose useful thrust must be assessed.
What can change the result
- A discharge runs too close to a neighbouring surface and loses useful momentum.
- The next fan captures the upstream jet before sufficient mixing.
- Free-discharge data are treated as the complete installed tunnel result.
Start with the complete air path
Share your layout and the information you already have. Unknown values can be identified before a configuration is selected.
Match the fan to this air path
A tunnel jet exchanges momentum with the surrounding air. Wall clearance and the next fan inlet affect the installed result. Discuss LONGWELL axial components where the assembly design permits a component solution, using the complete tunnel layout and duty.
Where it sits
Support component selection within a separately engineered tunnel ventilation assembly.
Why consider this configuration
A tunnel jet exchanges momentum with the surrounding air. Wall clearance and the next fan inlet affect the installed result. Discuss LONGWELL axial components where the assembly design permits a component solution, using the complete tunnel layout and duty.
Check the result in your equipment
Application questions
Is outlet velocity the same as tunnel air velocity?
No. The outlet jet is local and relatively fast. Tunnel velocity describes a much larger cross-section after interaction with the surrounding air.
Can I add more fans close together to get proportional thrust?
Not necessarily. A downstream inlet can capture the upstream jet before it mixes, changing the useful installed contribution. The fan arrangement needs to be assessed as a system.
What clearance should every tunnel use?
There is no universal value. The complete fan and silencer geometry, tunnel section, surfaces, spacing and operating conditions determine the layout.
Does normal ventilation selection cover smoke-control duty?
No. Emergency duty requires its own specified complete assembly and qualification. The component discussion on this page concerns the stated ventilation design.
Discuss the component and assembly duty
Begin with a tunnel section and proposed fan positions, or an existing component model. Include the normal ventilation duty, clearances, spacing and complete assembly requirements.
A layout, installation photos or your existing model is enough to start the discussion.