"This fan delivers 5000 m³/h" is an incomplete statement. A fan operates where its own curve intersects the system resistance curve. If the system resistance is higher than expected, the same fan delivers far less flow — and that is not a fault of the fan.
Two curves, one intersection
The fan curve comes from the manufacturer and is measured on a standard rig: ANSI/AMCA 210 (published concurrently as ANSI/ASHRAE 51) defines the laboratory conditions under which a fan's flow, pressure, power and efficiency are determined [1].
The system curve, on the other hand, is your design, and in the turbulent regime the pressure loss rises with the square of the flow rate. What CFD is for here is exactly that curve: compute the pressure loss at a few flow rates, build the system curve, then intersect it with the fan curve.
The catalogue condition is not the real installation
On a standard test rig the approach and discharge to the fan are clean. In a real installation they usually are not, and the difference is known as system effect:
- An elbow close to the fan inlet delivers a distorted profile and shifts the fan curve downward.
- Without enough straight length at the discharge, the fan cannot use its own diffuser and the static pressure regain is lost.
- On fans built into a cabinet, the geometry of the inlet box alone can cost several points of fan efficiency.
None of these losses is in the catalogue. Modelling the fan together with its real installation geometry is what makes system effect visible.
Modelling decision: how to represent the fan
Pressure jump / fan-curve boundary condition. The blade geometry is not resolved; a flow-dependent pressure rise is imposed on a surface. If you are investigating system resistance, this is the correct and cheap choice.
Resolving the blades (frozen rotor or sliding mesh). Required if the fan's own efficiency, noise or blade loading is the question. See the note on rotating machinery.
A frequent mistake is imposing a fixed flow rate at the inlet while investigating system resistance. That amounts to assuming the fan curve is infinitely steep: flow does not fall as blockage rises, and system effect never appears. See the boundary condition note.
The difference in smoke exhaust and jet fans
In fire-rated fans the operating point shifts because the density of the medium changes with temperature: smoke at 400 °C is roughly half as dense as air at 20 °C. The same fan at the same speed carries far less mass flow. Fan selection has to be made for the hot condition.
References
- ANSI/AMCA Standard 210 / ANSI/ASHRAE Standard 51 — Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating. Air Movement and Control Association International / ASHRAE.
- Ferziger, J. H., Perić, M. & Street, R. L. (2020). Computational Methods for Fluid Dynamics, 4th ed. Springer. DOI: 10.1007/978-3-319-99693-6