Sound Radiation of a Smoke-Removal Fan System

The aeroacoustics of a smoke-removal fan system used by firemen is studied. It consists in a 21" fan with straight blades and a classical constant tip clearance directly mounted on a thermal engine with a protecting grill upstream and a short volute downstream. Acoustic measurements have been done on the full system and without the grill and the volute. Scaling laws have been obtained to characterize the nature of the acoustic sources. Overall sound is found to be caused by two separate sources: a monopolar one mostly at low frequencies with significant tonal content caused by the thermal engine, and a dipolar one at mid and high frequencies with both tonal and broadband contributions caused by the fan system. A method to separate rotating and stationary noise sources has also been generalized and applied to yield the separate spectra of each noise source. The contribution of the different sound sources and of the different components of the system is also discussed depending on the operating condition.