Applied Acoustics 65 (2004) 441–455
www.elsevier.com/locate/apacoust
Technical note
Design of an acoustic enclosure with duct
silencers for the heavy duty diesel engine
generator set
Hyeon-Don Jua,*, Shi-Bok Leeb, Weui-Bong Jeongb,
Byung-Hoon Leeb
a
Department of Computer Industrial Application, Chinju College, Chinju, 660-759, South Korea
b
School of Mechanical Engineering, Pusan National University, Pusan, 609-735, South Korea
Received 15 October 2002; received in revised form 1 May 2003; accepted 21 October 2003
Abstract
Diesel engine generator sets in heavy industry plants and residential/official buildings can
cause serious noise problems. In this paper, a low noise diesel engine generator set is developed
through constructing an acoustic enclosure with ventilation duct silencers that effectively
block the acoustic flow but guarantee good thermal flow. Acoustic design of the enclosure,
which is initially layout by rule of thumb, is evolved systematically through numerical reanalysis procedure, based on indirect boundary element method (IBFM) with a commercial
acoustic analysis code. The cooling performance of the acoustically determined enclosing
structure is checked and confirmed through numerical heat flow analysis. The acoustic and
cooling performances of the developed low noise diesel engine generator set are confirmed by
the experiment.
# 2003 Published by Elsevier Ltd.
Keywords: Acoustic enclosure; Ventilation duct silencer; Indirect boundary element method (IBEM)
1. Introduction
Diesel engine generator sets are widely used as main electric power supplying
equipment in many industrial plants and facilities in official/residential buildings,
especially in the situation of abrupt electric outage. In this case, the components of
* Corresponding author. Tel.: +82-55-751-8149; fax: +82-55-761-7407.
E-mail address: [email protected] (H.-D. Ju).
0003-682X/$ - see front matter # 2003 Published by Elsevier Ltd.
doi:10.1016/j.apacoust.2003.10.007
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the diesel engine generator set, such as the diesel engine, generator, radiator fan, and
engine exhaust, etc., would appear as main noise sources in the plants or buildings.
Most conventional diesel engine generator sets have simple covers only to protect
the components and guide the flow of cooling air, and would generate very high
level of noise.
In this paper, an acoustic enclosure system for a heavy duty diesel engine generator set is designed to reduce the noise level below A-weighted sound pressure
level (SPL) of 80 dB from the high level exceeding 100 dB at a specified work
position.
The enclosure system adopts some ventilation duct silencers that effectively block
and dissipate the noise flow but guarantee good thermal outflow. The design process
consists of two phases, one of which is acoustic phase and the other is thermal one.
First, the acoustic design is performed through numerical analysis procedure using
an acoustic analysis code SYSNOISE, based on the indirect boundary element
method (IBEM). To prepare real acoustic source conditions necessary in analyzing
the acoustic system, the sound intensities of the components of the generator set, i.e.
diesel engine, generator, and radiator fan are measured and used in the calculation
of the sound power. Second, the acoustically determined enclosure system is
inspected in terms of cooling performance given as a specified temperature difference
between at inside and outside of the enclosure. A commercial program FLUENT is
used in this design phase.
2. Measurement of noise characteristics and sound power
Noise characteristics of the generator set should be identified to produce effective
and concrete measures to control the noise. The sound powers of the noise sources
need to be used in the numerical analysis.
The sound powers of the main noise sources, the diesel engine [1], generator and
radiator fan, are measured by sound intensity method. A sound intensity probe with
two microphones (B&K 4183) of 12 mm distance is used [2]. A measuring grid wall
with a grid size of 200 mm height and 200 mm width is installed at 0.75 m in front of
the centerline of the generator set as shown in Fig. 1(a) and another measuring grid
wall with a grid size of 290 mm height and 290 mm width at the front of the
radiator.
Fig. 2(a–c) present the results of 1/3 octave sound intensity spectrum analysis. The
symbol ^ in any bar in Fig. 2 indicates the intensity of the sound reversing to the
source. The units of sound intensity and A-weighted sound power are W/m2 and dB,
respectively. Fig. 2(a) presents the sound intensity spectrum at the grid position in
front of the engine exhaust manifold located around the cross point of the 4th row
and 4th column grid lines on the measuring wall. The spectrum reveals that a
dominant noise reaching the A-weighted sound intensity level of 99 dB arises in the
1/3 octave band centered at 1000 Hz. The overall A-weighted sound power radiated
from the engine is 108 dB. In Fig. 2(b), showing the sound intensity spectrum at the
grid position in front of the generator cooling fan located around the cross point of
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Fig. 1. Grid wall for sound intensity measurement and a typical diesel engine generator set without any
special anti-noise measures.
the 6th row and 2nd column grid lines on the measuring wall for the generator, the
1/3 octave band centered at 500 Hz appears to be the most troublesome frequency
band, in which A-weighted sound intensity level from the generator cooling fan
reaches up to 102 dB. On considering that the blade rotation frequency of the cooling fan of the generator is 450 Hz, it is apparent that the high noise in this band
is due to the blade rotation. The overall A-weighted sound power of the generator is
110 dB. Fig. 2(c), giving the sound intensity spectrum emitted from the radiator fan
located around the cross point of the 5th row and 4th column grid lines on the
measuring wall for the radiator, reveals that the 1/3 octave hand centered at 200 Hz,
which encompasses the blade rotation frequency 217 Hz, of the radiator fan, is most
problematic, generating A-weighted sound intensity of 112 dB. These experimental
facts provide some important guides for us to adopt ventilation ducts with silencing
capability of low frequency noise. But straight ducts are ways for the noise to pass
out through and act as effective noise transmitters. In order to interrupt noise flow
effectively, the ducts should be bent in some angles.
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Fig. 2. 1/3 octave-band sound intensity spectra from the three main noise components: (a) engine exhaust
manifold, (b) generator cooling fan, (c) radiator fan.
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2.1. Initial design of the acoustic enclosure and ventilation duct silencers
Our acoustic enclosure design is performed for a heavy duty diesel engine generator set with a high electric power supplying capacity above 320 kW, the specification of which is given in Table 1. The main idea in our design, different from
conventional diesel engine generator sets covered with simple enclosures, is to install
the ventilation ducts and impose silencing function on them.
Initial spatial configuration of the generator system with an acoustic enclosure
and ventilation ducts is laid out by the rule of thumb, paying attention to the
maintenance feasibility and space utility, to conform to the following requirements
and specifications.
1. The acoustically designed generator set should satisfy that length is smaller
than 5.0 m, width 1.5 m, and height 4.0 m.
2. The A-weighted SPL at the maintainer work position of 1 m away from the
enclosure side wall, 1 m above the ground and 0.75 m behind the enclosure
front wall should be lower than 80 dB.
3. Temperature difference between at the inside and outside of the acoustic
enclosure should be lower than 10 C.
The shape and structure of enclosure are laid out so as to block up effectively
broad band noise, especially high frequency noise, and supply cooling air with an
intake duct and evacuate heated air with an outtake duct sufficiently. The enclosure
acoustically encompasses the internal components, i.e. a diesel engine, generator,
radiator, radiator fan, electric control unit and two ventilation ducts.
Table 1
Specification of a heavy duty diesel engine generator set
Diesel Engine
Maker, model no.
Net power output
Rated speed
Generator
Maker, Model No.
Rated output
Frequency/rpm
Radiator
Muffler
Core size (mm)
Cummins NTA-855-G3
Stand-by 515 HP
Continuous 460 HP
1800 rpm
Newage Stamford
MHC 634-2G
576 kW(720 kVA) at 80 C
60 Hz/1800 rpm
Radiated heat
Pin pitch
12001200141
7 Row
275,000 kCal/h
3.0 mm
Sound Attention
Overall length
Body diameter
40 dB
2187 mm
460 mm
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Fig. 1(b) shows a typical diesel engine generator set (length of 4.6 m, width of 1.5
m and height of 3.0 m) designed without paying attention to noise-reduction, having
the electric power capacity the same as our design generator set. The noise level of
this generator set appears to be A-weighted 102 dB at the maintainer position. The
main contribution of the high level noise comes from the noise radiated directly
through the ventilation outlet of the enclosure. From this observation, we judge that
the conventional diesel engine generator enclosures are not effective on the control
of the low frequency noises that compose the main portion of the overall level. So
substantial reduction of the noise level can only be accomplished through adopting
some means to control the low frequency noises. Here, we try to cope with this
problem by adopting two square-type ventilation ducts, one of which is the intake
one and the other is the outtake one, and assigning a silencing function to them.
To obstruct the free passage of the low frequency noise, the outtake ventilation
duct is bent by 90 near mid length. The vertical part of the outtake ventilation duct
is put in the enclosure and the upper part outside above the enclosure, and is bent
once more by 90 near the outlet. Splitting a square duct into two ducts with two
separation plates makes the intake duct and the tipper part of the outtake duct. One
of the plates has some inclination angle to the duct length direction, to use the
space effectively. They are placed just above the enclosure. The inclination angle
of the separation plate has large influence on the noise transmission characteristics of the outtake duct [3].
Now this ventilation duct acts as a duct silencer. With the same reasoning, the
intake ventilation duct is bent twice by 90 . Fig. 3 shows the acoustic enclosure,
ventilation duct silencers and the flow path of cooling air. The cooling air enters into the
inlet of the intake duct silencer, flows into the internal space and subsequently passes
Fig. 3. Acoustic enclosure with ventilation duct silencers.
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over the heat sources, such as the generator, diesel engine, and radiator fan. The
heated air goes outside through the outlet of the outtake duct silencer. The cutoff
frequency [4] of the outtake duct silencer is designed to be higher than the blade
rotation frequency of the radiator fan. Only a small portion of the noises in the
frequency range lower than the cutoff frequency is diffracted out into the air.
The initial basic design is evolved by numerical reanalysis procedure to accomplish systematically the second and third requirements.
3. Design improvement by numerical acoustic analysis
In our system, the noises pass through the ventilation duct silencers or radiate
through the acoustic enclosure, and so form both internal acoustic scattering and
external acoustic radiation problems. Here we rely on the indirect boundary element
method (IBEM) that can deal with both internal and external acoustic fields simultaneously with relatively less computation effort [5]. The boundary condition for
the enclosure surface is assumed to be zero double-layer potential and the engine,
generator and radiator fan are considered as acoustic spherical sources.
Generally, several design parameters can be adopted to select the best choice of
noise reduction system. But, with our purpose to develop a low noise generator set
with a SPL below 80 dB with low cost and time, we choose only one critical design
parameter in our basic design, i.e. the inclination angle of the separation plate
between the intake and outtake ducts. The plate was adopted to suppress the low
frequency noises by prolonging the outlet duct length. In implementing the plate, the
inclination angle complicatedly influences the acoustic field structure in our comlex
shape duct. The noise level variations occur due to the variation of the acoustic field
structure. The variation of the noise reduction performance due to the inclination
angle of the plate is calculated from the numerical reanalysis and the best inclination
angle is identified. From the search results given in Table 2, the inclination angle of
about 30 degrees is assumed to be most favourable.
Fig. 4 shows the A-weighted SPL contours at the frequencies 125 and 500 Hz in
the middle cross-section of the upper outtake duct along the length direction. Also,
Fig. 5 gives the results in the middle cross-section of the intake duct along the length
direction.
Still, some portion of the noise in the frequency range lower than the cutoff frequency, 230 Hz, of the duct silencer, is transmitted out by diffraction as shown in
Fig. 4(a). The contour lines of A-weighted SPL in the frequency range greater than
the cutoff frequency appear to be dense near the walls of the enclosure, which means
Table 2
SPL variations according to the dividing plate angle
Angle ( )
27
28
29
30
31
32
33
34
SPL (dBA)
93.9
115.0
83.1
83.5
83.7
83.9
83.7
87.4
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Fig. 4. A-weighted SPL (dBA) contours in the middle cross-section of the upper outtake duct at two main
troubled frequencies: (a) 125 Hz, (b) 500 Hz.
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Fig. 5. A-weighted SPL (dBA) contours in the middle cross-section of the upper intake duct at two main
troubled frequencies: (a) 125 Hz, (b) 500 Hz.
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that the noise in this frequency range is greatly reduced in the ducts. The noise
through the outtake duct silencer is radiated into the sky without substantial influence on the system maintainer location. Fig. 6 shows the A-weighted SPL contour
lines at 1 m in front of the wall of the enclosure at the frequency 500 Hz. The contributions of two frequency components 125 Hz and 250 Hz are little. It informs that
the SPL is lower than 80 dB at the system maintainer position of 1 m distance from
the control panel side wall, 1 m height front the ground and 0.75 m distance
from the front wall.
4. Cooling performance check by numerical analysis
Since most noise sources in the genetator set also play as heat sources, the acoustical design of the enclosure system should be examined in terms of cooling performance [6,7]. The cooling performance of the designed enclosure model is checked by
numerical thermal flow analysis with a commercial code FLUENT [8].
Fig. 3 also presents the thermal boundary conditions at the inlet and outlet of
cooling air. First, the velocity of cooling air at the inlet is assumed to be constant,
and the flow direction at the outlet not to vary. Second, pressure conditions at the
inlet and outlet are set with the total pressure, i.e. sum of ambient and dynamic
pressure, and the ambient pressure, respectively. At the inlet, the flow velocity into
Fig. 6. A-weighted SPL (dBA) of 500 Hz at 1 m in front of the enclosure wall.
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the enclosure is 16 m/sec, the dynamic pressure 128.6 Pa and temperature 38 C. The
boundary conditions of the enclosure wall and the plates consisting the internal
structure are assumed such that the enclosure is adiabatic and the internal plates
have the thermal conductivity of 73 W/(m C). And no-slip flow condition for all
the walls and plates are forced. The surface temperatures of the diesel engine and
generator are set constant as 100 and 60 C, respectively.
Fig. 7. Temperature contours ( C) along: (a) the intake cooling air path, (b) the outtake cooling air path.
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Fig. 7(a) and (b) show the temperature contours along the intake and outtake
cooling air path, respectively. These are the analysis results with only forced convection
considered. As shown in the figures, the temperature contours concentrate densely
around the heat sources, and lie sparsely in the other space. These observations show
that the temperature varies rapidly only near the heat sources, but does not change
much along the cooling air path and so the cooling air temperature is not varied
substantially during the travel from the inlet to the outlet. From this, it can be
judged that the acoustically designed enclosure with ventilation ducts satisfies the
specification of temperature difference, lower than 10 C, between ambience and
enclosure inside.
5. Experimental confirmation of the design
The designed low noise generator set is constructed and placed in a yard fenced by
a wall made of absorbing materials to make a free acoustic field. As shown in Figs. 8
Table 3
Comparison of analyzed and measured SPLs
Location
1
2
3
4
5
6
7
Measuring height (m)
(from ground)
1
1
2
2
3.7
3.7
3.7
SPL [dBA(A)] : analysis
SPL [dBA(A)]:
experiment
125 Hz
250 Hz
500 Hz
Overall value
45
45
46
45
53
46
45
65
68
65
68
68
69
65
76
76
73
73
75
75
78
76.3
76.6
73.6
74.2
75.8
76.0
78.2
Fig. 8. SPL measuring points.
81
77
81
74
78
75
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and 9, and Table 3, the sound pressures are measured at several specified points on a
measuring plane in front of the generator set.
Table 3 compares the Sound Pressure Levels between numerically analyzed and
measured. The small differences between the analyzed and measured values at the
locations 1 and 3 seem to occur due to the neglect of the noise radiated through the
Fig. 9. 1/3 octave-band sound pressure spectra at two main locations: (a) location 1, (b) location 3.
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chinks of the doors in the boundary condition and the noise above the frequency 500
Hz in the analysis. At the location 3, the A-weighted SPL from the measurement
shows the highest value, 76 dB, at 1.6 kHz, the noise of which frequency may go out
easily through the door chinks. Henceforth, if the frequency range extends to 2 kHz
and the door chinks are considered in the numerical analysis, the analyzed values are
expected to be much closer to the measured values. At locations 5 around the inlet, 6
and 7 around the outlet, the analysis anticipates relatively well the measured values.
The relatively large difference at the location 5 between them seems to be due to the
noise of the exhaust gas pipe located near to the position 5. The position 2 is where
the control panel is installed and the system maintainer does his job and so is the
most important location in the noise reduction design. At this position, the analyzed
value closely predicts the experimental one, A-weighted 77 dB, which is less than the
design target level 80 dB.
Resistance temperature detectors are installed in the upper space just above the
engine, at the inlet and outlet. Fig. 10 shows the measured temperatures. The
ambient temperature of the generator set varies from 17 to 28 C. The temperature
difference between in the ambience and in the enclosure inside is measured as about
2 C.
6. Conclusion
An acoustic enclosure system with ventilation duct silencers for the heavy duty
diesel engine generator set was designed through an acoustic analysis procedure
based on the indirect boundary element method (IBEM). The noise characteristics
Fig. 10. Measured temperatures of the upper space above the engine, the inlet and the outlet.
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of the system required for the analysis and design guide were identified with sound
intensity measurement. The acoustically designed enclosure and duct system was
confirmed to guarantee smooth circulation of the cooling air through the numerical
thermal and flow analysis.
First, the acoustic enclosure system with ventilation duct silencers was initially
designed by rule of thumb and improved numerically to satisfy A-weighted SPL
lower than 80 dB at a specified system maintainer position and temperature difference lower than 10 C between in the ambience and the enclosure inside. Finally, the
developed low noise generator set was approved by the experiment.
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