A Comparative Study of Heated Single and Equivalent Twin Jets

Abstract

This paper compares the flow characteristics and noise generated by a single axisymmetric nozzle and parallel and canted twin nozzle configurations. The twin nozzles were designed such that they had a cumulative exit area equivalent to that of the single nozzle. PIV images obtained for the jets operating at cold conditions reveal that the parallel jets begin to merge at x/D = 4 (D = single nozzle diameter) while the canted jets begin to merge at x/D = 2. Of note is that for x/D > 12, the merged canted jets appear to have a jet width that is similar to that of the single jet. Acoustic measurements were made using heated jets operating at comparable conditions. As expected, the parallel twin nozzles produced less noise, up to 3 dB, than the single nozzle at all of the angles examined here. Canting the twin nozzles together reduced the noise suppression benefits and the resulting jet noise was similar to that of the single jet at some angles, depending upon the twin jet orientation. A dependence on the orientation of the jets was also exhibited by the frequency spectra for both the parallel and canted twin jets in both the peak radiation and normal directions. The eect of water microjet injection on the twin jet configurations is also discussed and reductions of up to 7 dB have been observed.