Turbojet Engine Noise Reduction With Mixing Nozzle-Ejector Combinations Page: 4 of 34
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NACA TN 4317
Previous work on the effect of ejectors on the noise characteristics
of an engine with a conical convergent nozzle (ref. 10) showed that the
ejector and simple nozzle did not sufficiently alter the mixing process
and provided only a 2 decibel noise reduction. However, model tests of
combined mixing nozzles and ejectors reported in reference 11 showed
that the noise-suppression properties of mixing nozzles could be supple-
mented by ejectors of moderate length. A combined investigation of the
acoustic characteristics and thrust and drag penalties of several full-
scale noise-suppressor configurations (ref. 12) included a mixing nozzle
with an ejector, which also showed promising results. An extension of
this work into the transonic regime is reported in reference 13.
The purpose of the work at the NACA Lewis laboratory reported herein
was to study the noise characteristics, the engine performance, and the
primary-secondary jet-mixing characteristics of full-scale engines
equipped with ejectors and exhaust-mixing nozzles. Two ejector diam-
eters and a range of ejector lengths and ejector to nozzle-spacing
distances were investigated. Pressure and temperature surveys at the
ejector exit were made and velocity profiles were obtained.
APPARATUS AND PROCEDURE
Engines and Test Stands
Two axial-flow turbojet engines with rated sea-level thrusts of
approximately 9000 and 5000 pounds were used. The larger engine (9000-lb
thrust) is given the designation "engine A" and the smaller engine the
designation "engine B" for the data presentation and discussion to
follow. Engine A develops its rated thrust at an exhaust-nozzle pressure
ratio near 2.3 and at a relatively low exhaust-gas temperature (approxi-
mately 9000 F). Engine B operates at a lower exhaust-nozzle pressure
ratio of 1.7 at rated conditions, but at a higher exhaust-gas temperature
(12750 F). .As a result, both engines have effective jet velocities of
approximately 1750 feet per second at rated thrust.
Engine installations. - Engine A was mounted in an outdoor thrust
stand as shown in figure 1. The centerline of the engine was 8 feet
above ground level. A large bellmouth inlet section was used to provide
undistorted airflow at the compressor inlet, and a large screen at the
bellmouth entry prevented ingestion of foreign material. An acoustic
panel of perforated metal and glass fiber material backed with cement
board was placed in front of the engine to reduce compressor noise annoy-
ance in nearby buildings.
Engine B was one that had been installed previously in the thrust
stand for the acoustic study reported in reference 8 and, therefore, the
acoustic and performance characteristics were well established. For this3
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Coles, Willard D.; Mihaloew, John A. & Callaghan, Edmund E. Turbojet Engine Noise Reduction With Mixing Nozzle-Ejector Combinations, report, August 1958; (https://digital.library.unt.edu/ark:/67531/metadc57100/m1/4/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.