*individual record*

Noise radiated from high speed centrifugal compressors is becoming a significant design factor. The dominant noise components of a centrifugal compressor can be identified at its blade passing frequencies (BPFs), of which each is a strong tonal noise. Popular noise control methods of a centrifugal compressor includes building of sound insulation enclosure, wrapping of noise insulation materials, mounting of vibration absorbers, and etc. These noise control methods are more effective for broadband noise reduction. Thus, they are generally ineffective to reduce a tonal noise such as the BPF components of the centrifugal compressor. In this thesis, a quarter-wavelength resonator array installed at the outlet pipe of a single-stage centrifugal compressor is studied, which is aimed to reduce the tonal noise of the compressor.

In order to optimally design the quarter-wavelength resonators, numerical simulations are performed by using a commercial software package, COMSOL Multiphysics - Acoustics Module. The optimization procedure is to optimally select geometric design parameters of the quarter-wavelength resonator array such as the diameter and depth of each resonator, and the spacing between the resonators in the axial and circumferential directions. The next procedure is to investigate the effect of four different configurations (i.e., staggered and slanted resonators and the resonators mounted on an expansion pipe and a curved pipe) on the noise reduction performance. Then, the analyses of flow effects on the noise reduction performance of the resonator array are conducted both numerically and experimentally. The high speed air flow induced by the compressor results in wavenumber shifts of noise waves, thus influencing the tonal noise reduction performance of the resonator array. The numerical flow effect analyses are conducted by using commercial software packages, ANSYS FLUENT and ACTRAN. In particular, these software package are used to conduct hybrid aeroacoustic simulations, in which fluid flow and aeroacoustic noise analyses are conducted separately, based on the theory of the Lighthill's aeroacoustic analogy.

The numerical and experimental results show that the optimally designed quarter-wavelength resonator array can be used to achieve nearly 10 dB noise reduction. From these results, it is also shown that a relation between the Mach number of the airflow and the maximum performance frequency shift is represented as a quadratic curve of ?f = 1918M^(2) - 24.21M where M is the Mach number and ?f is the frequency shift.

- Kim, Yong-Joe Associate Professor