A study on acoustic noise and flow rate from two tandem tubeaxial fans

Stacking two tubeaxial fans in series can simultaneously achieve fan redundancy, high airflow delivery and high packaging density. However, the impact of packing two fans together on overall noise emissions is far from well defined and much less well understood as compared to noise emission from isolated fans. This study demonstrates that spacing between two series fans has a significant impact on overall acoustic noise emissions. Using sound power measured for two tightly bolted 120 mm Nidec fans as a reference, a noise reduction of 5 dB can be achieved by separating two fans 40 mm apart when the air channel between two fans is enclosed. The overall sound power can further be reduced by nearly 3 dB when the channel between two series fans is open to ambient. The impact of open/closed channel and spacing on overall volumetric airflow is also investigated for normal operation and various fan failure scenarios. Flow degradation becomes obvious only in certain fan failure scenarios with the biggest impact observed for downstream fan failure with its fan blades locked at the maximum spacing.