Core and fan streams' mixing noise outside the nozzle for subsonic jet engines with internal mixers

摘要

An experimental investigation was completed to determine the acoustic benefits of three Internal Exhaust Gas Mixers (IEGM's). The IEGM's had 12-lobes, 20-lobes, and 24-lobes. Data presented is for ideally expanded jet velocities of approximately 1330 ft/sec (jet Mach number, Mj = 1.0) and for flight Mach numbers from 0.20 to 027. Acoustic data, LDV data and Schlieren images indicate the existence of a core and fan mixing region located within approximately 1.5 nozzle diameters downstream of the nozzle exit. This 'residual mixing' region exists because of incomplete mixing within the nozzle. The Effective Perceived Noise Level (EPNL), an indicator of the perceived noisiness of an aircraft, is sensitive to the residual mixing frequencies. The residual mixing noise levels at these frequencies are several decibels above the simple jet noise and the IEGM's internal noise levels. The residual mixing noise seems to cause the 24-lobe IEGM's total noise to be approximately 1.5 PNdB above that predicted for a simple jet. Consequently, improvements to the IEGMs are needed to increase EPNL reductions. The improvements should be directed toward eliminating the residual mixing by completely mixing the core and fan streams within the nozzle. Once the residual mixing is eliminated, the next noise floor may be the internal noise that an acoustically lined exhaust nozzle may help reduce.