Experimental investigation of the noise reduction of a plug fan by leading-edge
serrations
Ignacio ZURBANO-FERNÁNDEZ(1) (2) , Alain GUÉDEL(1) , Mirela ROBITU(1)(1) CETIAT, France(2) Univ Lyon, École Centrale de Lyon, LMFA, France, ignacio.zurbano-fernandez@cetiat.fr

Abstract
In previous years, leading-edge serrations have been successfully used as a noise-reduction technique applied toairfoils. More recently, this technique has also been applied to axial fans. In this study, sinusoidal leading-edge
serrations have been designed, optimised and integrated on a plug fan (centrifugal fan without volute). The outcome of RANS simulations of the non-serrated baseline impeller has been used to deﬁne an assumed optimum
serration geometry in terms of noise reduction. This optimum geometry has been used as a reference to manufacture three impeller prototypes of various serration geometries, deﬁned by their amplitude 2h and wavelengthλ.
This paper presents the results of the test of the prototypes in a reverberant room, where noise and air performance measurements are done simultaneously. For each impeller, ﬁve fan operating points are presented and,for each point, sound power level is measured at inlet and outlet of the fan. This allows us to assess the effectiveness of serrations at these operating points while checking the impact on the fan efﬁciency. The results arecompared with the non-serrated baseline fan.
Keywords: Fan Noise, Plug fan, Serrations, Leading-edge noise

1

INTRODUCTION

Fan noise is becoming a growing concern in different applications, such as air-conditioning, ventilation or electronic devices. The impact of environmental concerns and new regulations have made noise reduction a competitive issue in today’s global market. New technologies have been emerging recently, based on acoustic control,
such as porous materials, leading-edge and trailing-edge serrations. Leading-edge serrations have proven to bean efﬁcient means to reduce turbulent interaction noise on airfoils. This can be explained with different noise
reduction mechanisms, such as destructive interference of the scattered surface pressure (1, 2), cutoff effect due
to the oblique edge (3) and, indirectly, stall delay (4, 5). For instance, a noise reduction of 3 dB over 1 kHz
was measured on a NACA 65 airfoil (6). The use of complex leading-edge serrations, with double wavelength,
chopped-peak, slitted-root and slitted-V, produced greater noise reductions than single wavelength serrations,with a maximum reduction of 10-15 dB (7).
Serrations were added to the leading edge (LE) of the blades of an axial fan (8), achieving a reduction in
low-frequency broadband and tonal components. Both single and double-sine serrations on the LE of an axial
fan were also tested (9), with the former being more efﬁcient than the latter (and therefore contradicting resultsfrom airfoils (7)). Serrations were added to the LE of an axial fan with a turbulence generating grid (5). A
maximum noise reduction of 13 dB was achieved for a certain conﬁguration (but it also increased noise up to
3 dB at other operating points). The impact on the noise reduction of serrations have been studied for airfoils
and axial fans but, to our knowledge, it has not yet been investigated for centrifugal fans.
The objective of this study is to design optimal serrations for a plug fan and to evaluate the impact of this
implementation on the noise and air performance of the fan.

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2

IMPELLER DESIGN

Sinusoidal leading-edge serrations were designed and optimised for a plug fan, i.e. a backward-curved centrifugal fan without volute. As baseline for the design of serrations, we used a fan with seven blades of constant
thickness of 2 mm and an outer diameter of 360 mm (Table 1), which was manufactured by Ziehl-Abegg SE.
Table 1. Fan parameters
Designation
Number of blades zb
Blade thickness e [mm]
Inlet diameter Di [mm]Outlet diameter De [mm]Outlet span L [mm]Rotational speed n [rpm]

Value
7
2
224
360
98
1440 / 720

A sinusoidal shape has been used, deﬁned by two parameters: wavelength λ and amplitude 2h (Figure 1).
λ

θ

2h

Figure 1. Serrations parameters
To calculate the optimum serration angle θO , the model proposed by Chaitanya et al is used (10). It is deﬁned
as a function of the turbulence integral length scale, L (Equation 1):
θO = tan−1 (

2h
)
L

(1)

The turbulence integral length scale has been estimated from CFD simulations of the fan. The model chosen was
3D steady RANS, using Multiple Reference Frame (MRF) approach and the k − ε turbulence model. Overall,
9.39M unstructured polyhedral volumes were used, with 5.22 M of them in the rotating domain, an impeller
surface meshing of 0.5mm and a boundary layer of 5 layers.
The estimation of the turbulence length scale is done with
3/2 k

L = Cμ

3/2

ε

(2)

where Cmu is a constant of the turbulence model (0.09 in our case) and k and ε are the constants of the k − εmodel.
The equation has been applied over a rectangular area situated upstream of the leading edge, at a distance of
10% of the chord C. This area has been centred on the projection of the arc which deﬁnes the blade (Figure2). The results have been averaged over the area, yielding a value of L = 7.8mm for the best efﬁciency point(BEP), from which we can deduce an optimum wavelength λO = 15.8mm ≈ 16mm.

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Figure 2. Area for the visualization of the turbulent length scale (left) and outcome of the simulation (right)
There is no optimum criterion for the serration amplitude. However, empirical results usually show that thebigger it is, the better, as long as a compromise with the aerodynamic performance is kept. A value of 1/6of the chord has been taken, for it is similar to other values available in the bibliography (1, 5). In our case,
2h = 1/6C = 22mm. The original blades of the plug fan have been modiﬁed with leading edge serrations as deﬁned above. In order to perform a parametric study of the serration geometry, in terms of 2h and λ , two otherprototypes have been designed. In one case, λ has been doubled while keeping the same 2h. In the other, 2h
is twice as big while maintaining the same λ . The parameters for the three impellers are summarized in Table 2.
Table 2. Fan parameters
Designation
L8H11
L16H11
L16H22λ [mm]
8
16
16

h [mm]
11
11
22

λ /2h
0.36
0.73
0.36

The manufacturing of the prototypes, by Ziehl-Abegg, was similar to that of the baseline fan. The only modiﬁcation is the removal of material to carve the serrations on the blade. This has been done with a laser cutting
tool, and it slightly reduces the blade surface area, and thus may have an impact on the lift and therefore on
the fan curve. This potential degradation of the fan performance may easily be taken into account in the comparison of the acoustic results of the different blade geometries. Figure 3 shows the three impeller prototypes.

Figure 3. Impeller prototypes: L16H22 (left), L16H11 (middle) and L8H11 (right)

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3

EXPERIMENTAL SETUP

All three impeller prototypes and the baseline fan have been tested in a double reverberant room according totest category A: non-ducted at inlet and outlet (Figure 4). The fan is mounted on a support, on the partition
between the two rooms of different sizes, the bigger room being on the inlet side. The auxiliary fan allows
adjusting the operating point of the test fan. The ﬂowrate is measured with a multi-nozzle chamber while thefan pressure is obtained according to ISO 5801 (11) with pressure rings in both reverberant rooms. The fansound power levels are determined in both rooms following ISO 13347-2 (12), using a rotating microphone in
the big room and 3 ﬁxed microphones in the small one to make a spatial average of the sound pressure ﬁeld
in each room.

Figure 4. Top view layout of the test facility (left) and view of the outlet pressure chamber with the fan
assembly (right)
For each prototype, the same assembly (base, inlet nozzle, motor as in Figure 4 right) has been used, with
only the impeller being replaced. The motor is connected to a variable-frequency drive (VFD) which allows tochange the speed from 1440 rpm (100% of the nominal speed) to 720 rpm (50% of the nominal speed). To
generate turbulence at the inlet, a grid has been manufactured. It has been made by laser cutting of a 3 mm
thick PMMA sheet. It is a square-mesh array of bars which are 10 mm wide spaced 50 mm. The grid has aporosity of 36%, and is manufactured with a circular shape to cover the inlet nozzle while keeping the stiffnessof the sheet. It is installed directly on the inlet nozzle. The width of 10 mm will generate turbulence with an
integral length scale of around 8 mm according to Equation 18 from (13).

4

RESULTS AND DISCUSSION

The fan aerodynamic curves measured both with free inlet and grid are shown in Figure 5 (left). All the results
have been converted to a rotating speed of N = 1440 rpm and an air density of ρ = 1.2 kg/m3 , which are very
close to the test values. The results for 720 rpm have not been presented for the sake of simplicity. With free
inlet, for all the serration conﬁguration there is a pressure decrease with respect to the baseline impeller at low
ﬂowrate, and a moderate pressure increase at high ﬂowrate. In terms of pressure, the L16H11 impeller is thebest, followed by L8H11 and L16H22. The same hierarchy (Figure 5 right) is observed for the overall static
efﬁciency (ηeds = p f s · qv /Ped where Ped is the electric power of the motor and VFD). The fan curves measured
with the inlet grid show a slightly different tendency, as the curves for the serrated impellers are lower than thebaseline at low ﬂowrate, but not higher than the baseline at high ﬂowrate. This suggests that the grid induces a
fan system effect, in addition to a pressure drop.

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Static pressure-airflow, free inlet

Static efficiency-airflow at 100%
45%

350

40%

Efficiency ηeds (%)

Static pressure pfs(Pa)

400

300
250
200

150
100
50

0
500

1500

2500

35%
30%
25%
20%
15%

10%
5%
0%

3500

500

Airflow qv (m³/h)
Baseline, free inlet
L8H11, free inlet
Baseline, grid
L8H11, grid

1000

1500

2000

2500

Airflow qv (m³/h)

Baseline, free inlet
L8H11, free inlet
Baseline, grid
L8H11, grid

L16H11, free inlet
L16H22, free inlet
L16H11, grid
L16H22, grid

3000

3500

L16H11, free inlet
L16H22, free inlet
L16H11, grid
L16H22, grid

Figure 5. Fan static pressure (left) and overall static efﬁciency (right)
The overall sound power level in dBA gives a general idea of the acoustic performance of the serrated impellers
for different operating points. As it can be seen in Figure 6, for a given impeller with free inlet the noise tends
to increase away from the BEP (which corresponds to a ﬂow qv ≈ 2200m3 /h). In terms of noise reduction, theL16H11 impeller offers the best performance, followed by L16H22 and L8H11. For the grid conﬁguration, thenoise always increases with the ﬂow, with a maximum difference of about 10 dBA at 2800 m3 /h.

Sound power level-flow at outlet
90

83

88

81

86

79

84

LWA (dBA)

LWA (dBA)

Sound power level-flow at inlet
85

77

75
73
71

82
80

78
76

69

74

67

72

65

70
500

1000

1500

2000

2500

3000

Airflow (m³/h)
Baseline, free inlet
L8H11, free inlet
Baseline, grid
L8H11, grid

L16H11, free inlet
L16H22, free inlet
L16H11, grid
L16H22, grid

3500

500

1000

1500

2000

2500

3000

3500

Airflow (m³/h)
Baseline, free inlet
L8H11, free inlet
Baseline, grid
L8H11, grid

L16H11, free inlet
L16H22, free inlet
L16H11, grid
L16H22, grid

Figure 6. Overall sound power level, inlet (left) and outlet (right)
Third octave band sound power spectra at inlet plot are shown in Figure 7 (the outlet results will not beshown as they are very similar). Despite a noise increase around 80-100 Hz, the L16H11 impeller generates
an important noise reduction between 125 and 1000 Hz. The L16H22 impeller shows a similar behaviour, butwith lower noise reduction. Finally, the spectrum of the L8H11 impeller is the closest to the baseline. All
the serrated impellers induce a small noise increase over 1000 Hz. It has been observed that, as qv increases
this difference between the baseline and the serrated impellers at high frequency is reduced. At ﬂowrate above
2600m3 /h a noise reduction is observed with serrations, with values between 3-7 dB at maximum ﬂow.

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Sound power level at inlet, qv=2200 m3/h
7065

Lw (dB)

60
55
50
45

Frequency (Hz)Baseline-free inlet

L16H11-free inlet

L8H11-free inlet

L16H22-free inlet

Figure 7. One-third octave sound power level at inlet for all the impellers, qv = 2200m3 /h
The impeller with the best noise performance, L16H11, is compared to the baseline, both with and without grid.
In Figure 8, for qv = 2200m3 /h i.e. at BEP, we can clearly see the noise increase induced by the grid, speciallyabove 1000 Hz. Furthermore, we also observe that the serrations have no effect when the inlet turbulence
intensity is high, as the spectra of the baseline and L16H11 impellers overlap. This has also been observed forthe other impellers, and suggests that the chosen design criteria seems to be effective for the free inlet case (forwhich it was optimised) but not when the inlet turbulence level is ampliﬁed.

Sound power level at inlet, qv=2200 m3/h
7570

Lw (dB)

65

60
55
50
45

Frequency (Hz)Baseline-free inlet

Baseline-grid

L16H11-free inlet

L16H1-gridFigure 8. One-third octave sound power level at inlet for the baseline and L16H11 impellers, qv = 2200m3 /h,free inlet and inlet grid conﬁgurations

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Figure 9 shows the narrowband sound pressure spectra of all the impellers, for Δ f = 8Hz. This allows to analyse and identify the most important peaks. The blade passing frequency and the ﬁrst two harmonics are easily
identiﬁed (BPF, 2BPF and 3BPF). There is an important peak at 296 Hz, which amplitude is reduced by the
L16H11 and L16H22 impellers. Another peak at 576 Hz is also modiﬁed with the serrated impellers, being
completely cancelled with the L16H11 impeller. Finally, a peak at 768 Hz is not changed at all by the different
serration geometries. The peaks at 576 and 768 Hz correspond to 24 and 32 times the rotating frequency (24Hz), but a further analysis is required to identify their origin.

BPF Peak 24N
32N
2BPF
3BPF

Figure 9. Narrowband sound pressure spectra at fan inlet for all the impellers, qv = 2200m3 /h

5

CONCLUSIONSA design and optimization of leading-edge serrations for a plug fan has been carried out in order to reduce thenoise. Three impeller prototypes with serrations were manufactured and tested to investigate their aerodynamic
and acoustic effects. Measurements were carried out under free inﬂow condition and with a grid generating
turbulence. Experimental results show that the impeller with the optimum design conﬁguration yields the best
results in terms of air performance and acoustics. It seems that noise reduction appears to be more sensitive toserration wavelength than to amplitude.
Overall, leading edge serrations tend to reduce noise under 1000 Hz but can increase the noise at higher frequencies, depending on the operating point. The serrations reduce the broadband noise, but they also reduceor even cancel some peaks. The installation of a grid at the fan inlet completely cancels the effect of the serrations. This tends to conﬁrm the validity of the design criteria, based on RANS simulations on the baseline
impeller.

ACKNOWLEDGEMENTS
The authors wish to thank Michel Roger from École Centrale de Lyon for his guidance and help throughout the
project; Hervé Bingan, Jean-Hugues Salazar and Benoît Savanier, from CETIAT, for their contribution to thetests; Hervé Miller, from CETIAT, for his help with the simulations; and Frieder Lörcher and Walter Angelis,
from Ziehl-Abegg SE, for providing the fan prototypes.
The present work has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie SmartAnswer project (grant agreement No 722401). It has also received funds from CETIAT’s industrial members which are fan manufacturers.

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