Disturbed flow conditions

To achieve catalogue performance in respect of flowrate and developed pressure, it is essential that the velocity profile of the air presented to the fan is fully developed, symmetrical and free from swirl. It is also essential to have a sufficiently straight duct on the discharge to permit the recovery of velocity pressure and its conversion into useful static pressure.

Similar considerations are also applicable to any noise ratings. One of the main noise sources has been observed to be due to turbulence and non-uniform inflow. Figure 14.23 shows outlet sound spectra for a specific fan fitted with different bends at varying distances from its inlet. Objects downstream of a fan will affect the noise generated by causing a non-uniform flow. This will in turn result in non-steady loading of the blades. The effect, however, is usually less severe than that for obstructions upstream.

Disturbed flow conditions

Octave band frequency — Hz Overall

E (63-8000)

Figure 14.23 Effect of upstream bend on axial fan noise at outlet

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