On using multiple-jet nozzles to suppress industrial jet noise.

This work investigates the efficacy of multiple-jet nozzles in reducing exposure to noise radiated from high-speed jet flows that are frequently used for cleaning and other purposes. A single-jet nozzle is replaced by one with multiple and smaller outlets with the same combined exit area. Noise generated by multiple-jet nozzles will shift to higher frequencies due to the smaller openings; thus, a portion of the sound energy is moved to the ultrasonic range and the audible noise levels will be reduced. In this study, nozzle outlet sizes of 2.2 mm, 2 mm, 1 mm, and 0.7 mm are tested at various flow rates. Results show that the noise-abating effect of using smaller outlets is significant. Although industrial jet flows, in general, have multiple noise-generating mechanisms, the multiple-jet approach is effective for flows ranging from the subsonic to the highly choked. Noise spectra show that the reduction of noise level in the audible range is related to frequency shifting. The comparison of overall sound pressure levels generated by 1-mm and 0.7-mm outlets shows that even a raise of frequency by 40% can have a notable effect. As for the concern that the multiple-jet nozzles may suffer a drop in thrust, as long as the total exit area is fixed, thrust loss generally is not serious. One potential hazard of the multiple-jet nozzle is that posed by noise at frequencies higher than 10 kHz or ultrasound. More investigations in this field are suggested.