Hot-Jet Noise Test of a Revised Notched Nozzle

Abstract

Jet noise remains a significant noise component in modern commercial aero-engines. A high-speed flow mixing with the surrounding air constitutes noise sources behind the nozzle. One noise-reduction technology is a mixing device attached to the nozzle. Several fixed-geometry mixers such as chevrons have been studied by both computational and experimental approaches. The authors have previously proposed a notched nozzle with dents allocated along the nozzle lip and discussed its ability to reduce the noise level. The revised notch was expected to suppress the broadband jet-mixing noise as well as additional noise at higher frequencies. However, further assessments are required before proceeding to a large-scale engine test in an outdoor environment. First, the influence of the gas temperature on acoustic results must be tested because the temperature affects the mean jet velocity and sound propagation. As the preliminary noise test in the previous paper was limited to the cold-jet condition, far-field noise data under the hot-jet condition should be investigated. Second, the aerodynamic performance must be evaluated. Data on the flow rate and thrust would help in considering the aerodynamic performances between the baseline, notched, and chevron nozzles. This study focuses on noise tests for the finer-notched nozzle under the hot-gas condition. A small jet engine for model jet planes was employed to generate a high-temperature jet. An engine test stand was designed to monitor the engine performance data, consisting of the pressure and temperature at several positions, the fuel flow rate, and the thrust. The hot-jet test with and without the mixing device served as a compact and flexible test for aerodynamic evaluation of the nozzle. The noise test results under the hot-jet condition with this rig showed that the noise reduction characteristics of the finer-notched nozzle are different from those of conventional mixers.

Overview of Jet Noise Measurement at JAXA

“…This hot-jet test rig is capable of measuring the thrust as well as far-field sound pressure levels. A combination of a model jet engine and an anechoic wind tunnel 22,23) enabled noise measurement under heated gas. A turbojet engine with a maximum thrust of 140 to 160 N is used as a hot gas generator.…”

“…To satisfy this requirement, JAXA conducted static engine tests. 24,25) A turbojet engine with a nozzle exit diameter of 260 mm was employed as a jet noise demonstrator. This demonstrator engine was operated in an outdoor environment at JAXA's Noshiro Testing Center.…”

Enhancement of Propulsion Performance Through Jet Noise Reduction Technologies: A Review

“…The enhanced mixing leads to the suppression of broadband peak components of jet noise with little effect on the engine performance [21]. A 5% notched (6-notched) nozzle means that the depth of the dent is 5% of the exit diameter of the baseline nozzle [22]. A 6-notched nozzle was attached to the nozzle end of a turbojet engine.…”

“…This tendency is apparently related to a mixing feature that causes a moderate mixing process within a longer distance from the nozzle. The noise reduction level was slightly increased toward the side direction, which should reduce the perceived noise level (PNL) at a lateral point during takeoff [22].…”

Jet mixing noise suppression by means of a claw mixer: Revision of the nail configuration to improve the high-frequency acoustic property

“…The LES on the near field of the nozzle was applied to other similar mixer, notched nozzle Ishii, et al, 2011b). The redesigned geometry and configuration of the notched nozzle showed improved acoustic performance in the subsequent experiment (Ishii, et al, 2012a). Similar to the case of the notched nozzle, the authors tried to revise the nail geometry and configuration according to LES.…”