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Study on Noise Generation in a High Speed Centrifugal Fan
Abstract
This study presents the investigation of aerodynamics and aeroacoustics of centrifugal fan using commercial computational fluid dynamics (CFD) code. The unsteady turbulent flow of the fan is simulated with Detached Eddy Simulation (DES) and the acoustics sources are computed based on the pressure fluctuations. The Ffowcs Williams and Hawking model is used to predict the tonal noises in aeroacoustics simulation.
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This paper summarizes the research work done by various experimenters during the last fifteen years. Only those investigations are included in which an attempt was made to reduce the generated sound by modifying the fan itself. Noise attenuation by applying any kind of silencer is not the subject of this paper. Most of the work described was aimed at reducing the blade passage sound, but in some cases a decrease of the broad band noise was also obtained. Three different centrifugal impeller designs are dealt with: multi-vane impellers with forward curved blades, impellers with backward curved blades, and impellers with radial blades.
SUMMARY This paper investigates the tonal noise radiated by a subsonic blower fan of an HVAC system. Numerical simulations have been performed and compared to measurements. The numerical simulation is based on the aero-acoustic analogy where the unsteady flow is first computed using STAR-CD and then passed to LMS SYSNOISE to compute the acoustic radiation. The measurements have been performed in the semi-anechoic room of Denso Thermal System Italia. Good correlation has been found between the numerical simulation both in terms of level and spatial distribution.
The objective of this research was to identify which aspects of the fluid dynamics are associated with noise generation in
centrifugal turbomachinery. Research emphasis was placed on the generation of noise at frequencies other than the blade passage
tones. In order to avoid noise generated by the interaction of the discharged flow and stationary objects outside of the impeller,
experiments are performed on a centrifugal impeller without diffuser and casing. With this discharge configuration, the radiated
noise spectra are shown to be dominated by harmonically related broad humps at low frequency. These were proven to be generated
by the interaction of a coherent unsteady flow structure rotating around the impeller discharge and the trailing edges of
the impeller blades.
The aeroacoustic characteristics of a centrifugal fan for a vacuum cleaner and its noise reduction method are studied in this
paper. The major noise source of a vacuum cleaner is the centrifugal fan. The impeller of the fan rotates at over 3000 rpm,
and generates very high-level noise. It was revealed that the dominant noise source is the aerodynamic interaction between
the rotating impeller and stationary diffuser. The directivity of acoustic pressure showed that most of the noise propagates
backward direction of the fan-motor assembly. In order to reduce the high tonal sound generated from the aerodynamic interaction,
unevenly pitched impeller and diffuser, and tapered impeller designs were proposed and experiments were performed. Uneven
pitch design of the impeller changes the sound quality while the overall sound power level (SPL) and the performance remains
similar. The effect of the tapered design of impeller was evaluated. The trailing edge of the tapered fan is inclined. This
reduces the flow interaction between the rotating impeller and the stationary diffuser because of some phase shifts. The static
efficiency of the new impeller design is slightly lower than the previous design. However, the overall SPL is reduced by about
4 dB(A). The SPL of the fundamental blade passing frequency (BPF) is reduced by about 6 dB(A) and the 2nd BPF is reduced about 20 dB(A). The vacuum cleaner with the tapered impeller design produces lower noise level than the previous
one, and the strong tonal sound was dramatically reduced.
Key WordsVacuum Cleaner-Aeroacoustic Noise-Centrifugal Fan-Low Noise-Performance-Specific Noise Level
Prediction of noise generated by centrifugal fans is much more complex than prediction noise generated by axial fans. A complete, aerodynamic and aeroacoustic, investigation of the tonal noise of a high rotational speed centrifugal fan is proposed in this paper. The studied fan is made up of an impeller, a diffuser and a return channel. The purpose of this work is to understand the nature of noise generated within this type of machine. An aeroacoustic model based on the Ffowcs Williams and Hawkings equation is used to predict dipole and monopole tonal noises in the frequency domain. Showing the importance of the monopole source in this kind of fans constitutes the main contribution in these research tasks. A numerical simulation of the fluid flow validated by experiments, enables to obtain the fluctuating forces and normal velocity on the impeller and diffuser blades needed for the aeroacoustic computation.
Among all household appliances, the vacuum cleaner is the most powerful source of noise. The built-in suction unit, consisting of an electric motor and a centrifugal blower, represents the main source of noise; this is partially aerodynamic, mechanical and electromagnetic in origin. The contribution of the particular origin of noise generation to the total noise level depends on the geometry of the suction unit and operating conditions. The operating point of a suction unit built into a vacuum cleaner depends on the amount of dust particles in the dust bag and is constantly changing during operation until it reaches a zero flow rate. There are differences in noise generation at different operating conditions as well as between the suction units having a blower without and with a vaned diffuser. In this part of the paper, an overview of aerodynamic, mechanical and electromagnetic noise origins at different operating conditions is given.
Principal of Turbomachinery
- R K Turton
Numerical noise prediction: application to radial fans, in INTER-NOISE
- Yilmaz Dogana
- Faruk Bayraktar
- Y Kenan
- Sanliturkd