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8. The introduction of a single stage low pressure
compressor (fan) significantly reduces the
compressor noise because the overall turbulence
and interaction levels are diminished. When the bypass
ratio is in excess of approximately 5 to 1, the jet
exhaust noise has reduced to such a level that the
increased internal noise source is predominant. A
comparison between low and high by-pass engine
noise sources is shown in fig. 19-4.
9. Listed amongst the several other sources of
noise within the engine is the combustion chamber. It
is a significant but not a predominant source, due in
part to the fact that it is ’buried’ in the core of the
engine. Nevertheless it contributes to the broadband
noise, as a result of the violent activities which occur
within the combustion chamber.
METHODS OF SUPPRESSING NOISE
10. Noise suppression of internal sources is
approached in two ways; by basic design to minimize
noise originating within or propagating from the
engine, and by the use of acoustically absorbent
linings. Noise can be minimized by reducing airflow
disruption which causes turbulence. This is achieved
by using minimal rotational and airflow velocities and
reducing the wake intensity by appropriate spacing
between the blades and vanes. The ratio between
the number of rotating blades and stationary vanes
can also be advantageously employed to contain
noise within the engine.
Noise suppression
202
Fig. 19-4 Comparative noise sources of low and high by-pass engines.
11. As previously described, the major source of
noise on the pure jet engine and low by-pass engine
is the exhaust jet, and this can be reduced by
inducing a rapid or shorter mixing region. This
reduces the low frequency noise but may increase
the high frequency level. Fortunately, high
frequencies are quickly absorbed in the atmosphere
and some of the noise which does propagate to the
listener is beyond the audible range, thus giving the
perception of a quieter engine. This is achieved by
increasing the contact area of the atmosphere with
the exhaust gas stream by using a propelling nozzle
incorporating a corrugated or lobe-type noise
suppressor (fig. 19-5).
12. In the corrugated nozzle, freestream
atmospheric air flows down the outside corrugations
and into the exhaust jet to promote rapid mixing. In
the lobe-type nozzle, the exhaust gases are divided
to flow through the lobes and a small central nozzle.
This forms a number of separate exhaust jets that
rapidly mix with the air entrained by the suppressor
lobes. This principle can be extended by the use of a
series of tubes to give the same overall area as the
basic circular nozzle.
13. Deep corrugations, lobes, or multi-tubes, give
the largest noise reductions, but the performance
penalties incurred limit the depth of the corrugations
or lobes and the number of tubes. For instance, to
achieve the required nozzle area, the overall
diameter of the suppressor may have to be
increased by so much that excessive drag and
weight results. A compromise which gives a
noticeable reduction in noise level with the least
sacrifice of engine thrust, fuel consumption or
addition of weight is therefore the designer’s aim.
14. The high by-pass engine has two exhaust
streams to eject to atmosphere. However, the
principle of jet exhaust noise reduction is the same
as for the pure or low by-pass engine, i.e. minimize
the exhaust jet velocity within overall performance
objectives. High by-pass engines inherently have a
lower exhaust jet velocity than any other type of gas
turbine, thus leading to a quieter engine, but further
noise reduction is often desirable. The most
successful method used on by-pass engines is to
mix the hot and cold exhaust streams within the
confines of the engine (fig. 19-5) and expel the lower
velocity exhaust gas flow through a single nozzle
(Part 6).
15. In the high by-pass ratio engine the
predominant sources governing the overall noise
level are the fan and turbine. Research has produced
a good understanding of the mechanisms of noise
generation and comprehensive noise design rules
exist. As previously indicated, these are founded on
the need to minimize turbulence levels in the airflow,
reduce the strength of interactions between rotating
blades and stationary vanes, and the optimum use of
acoustically absorbent linings.
Noise suppression
203
Fig. 19-5 Types of noise suppressor.
Noise suppression
204
Fig. 19-6 Noise absorbing materials and location.
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