Large axial fans are used in underground ventilation systems at
subway stations and high-speed railway stations. These axial fans
generate high noise level up to 110 dB (A) due to high pressure at high
flow capacity. It is needed to attach silencers to reduce the high noise
level generated from the axial fans. In this thesis, a circular type pod
silencer with annular two-layered air-passages has been developed to
reduce a high noise level generated from axial flow fans for the
underground ventilation. The noise characteristics of the given axial fan
with 100 horsepower were determined by noise measurement. The noise
consists of two components such as discrete frequency noise component
at blade passing frequency (BPF) due to rotating impellers and
broadband noise component due to turbulence in inflow and exhaust jet
mixing. Main contribution into the high noise level is due to the discrete
frequency noise component. In order to effectively reduce the noise level
of the axial flow fan, the circular-type pod silencer with annular
two-layered air-passages has been modelled. The finite element analysis
(FEA) has been implemented to predict the transmission of the silencer.
The design parametric study has been performed in order to identify
sensitive design variables of the silencer such as air gap thickness, pod
diameter and sound absorbing material thickness. The design
optimization have been performed to design the circular-type pod silencer
with annular two-layered air-passages. The design optimization led to
the design of circular type pod silencer with annular two-layered
air-passages with a length of 4.3 m, a diameter of 2.4 m, a pod diameter
of 0.8 m, two air gap thickness of 0.2 m and a 0.2 m thickness of sound
absorbing material made of glass wool with density of 48 . According
to the optimized design, the circular type pod silencer with annular
two-layered air-passages has been manufactured. In order to test the
silencer performance, the noise was measured when the axial flow fan
was attached to the silencer and when it was not attached. The noise
measurement was performed at 1.5 m away from the center of silencer
and from the axial flow fan, respectively. Experimental data showed the
nose reduction 19dB(A) from 107 dB(A) of overall noise level. It has
been compared with the noise reduction 23dB(A) predicted by the
optimization design process. Additionally the design of the silencer for
the axial fan with 100 horsepower was modified and extended to fit the
silencer suitable for an axial fan with 260 horsepower. Because the hub
diameter of the axial fan with 260 is 1.094 m which is larger than the
0.8 m diameter of the axial with 100 horsepower, an extended duct was
designed so that it was located in the space between the hubs of the
axial fan and the pod of the circular-type pod silencer with annular
two-layered air-passages. The duct consisted of a perforated tube filled
with sound absorbing material inside to increase the noise reduction
effect. The performance of the silencer with the extended duct was
tested by measuring the noise when the axial flow fan was attached to
the silencer with the extended duct and when it was not attached.
Experimental data showed the nose reduction 26dB(A) from 113 dB(A)
of overall noise level. It has been compared with the predicted noise
reduction 25dB(A). Those results in this thesis showed that the designed
circular-type pod silencer with annular two-layered air-passages can be
effectively used to reduce the noise generated from the large axial fans
in underground ventilation systems at subway stations and high-speed
railway stations.