Wan-Ho Jeon

A numerical study on the flow and sound fields of centrifugal impeller located near a wedge

Abstract Centrifugal fans are widely used and the noise generated by these machines causes one of the serious problems. In general, the centrifugal fan noise is often dominated by tones at blade passage frequency and its higher harmonics. This is a consequence of the strong interaction between the flow discharged from the impeller and the cut-off in the casing. However, only a few researches have been carried out on predicting the noise because of the difficulty in obtaining detailed information about the flow field and considering the scattering effect of the casing. The objective of this study is to understand the generation mechanism of sound and to develop a prediction method for the unsteady flow field and the acoustic pressure field of the centrifugal impeller. A discrete vortex method is used to model the centrifugal impeller and a wedge and to calculate the flow field. The force of each element on the blade is calculated by the unsteady Bernoulli equation. Lowsons method is used to predict the acoustic source. In order to consider the scattering and diffraction effects of the casing, Kirchhoff–Helmholtz boundary element method (BEM) is developed. The source of Kirchhoff–Helmholtz BEM is newly developed, so the sound field of the centrifugal fan can be obtained. A centrifugal impeller and wedge are used in the numerical calculation and the results are compared with the experimental data. Reasonable results are obtained not only for the peak frequencies but also for the amplitudes of the tonal sound. The radiated acoustic field shows the diffraction and scattering effect of the wedge.

Aeroacoustic characteristics and noise reduction of a centrifugal fan for a vacuum cleaner

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.

Development and application of fan noise prediction method to axial and centrifugal fan

Numerical predictions of fan noise have not been studied extensively. This is due to the scattering effect of the fan casing, duct and the difficulty in obtaining aerodynamic acoustic source. New method to predict the fan noise and performance is developed and used to calculate various fan noise problems. A vortex method is used to model the fan and to calculate the flow field. Acoustic pressures are obtained from the unsteady force fluctuations of the blades using an acoustic analogy. But the acoustic analogy can be applied only in the free field in general. In order to consider the solid boundary effects of the casing, the newly developed Kirchhoff-Helmholtz BEM (Boundary Element Method) is introduced. With the above-mentioned method, the flow field and sound field of centrifugal and axial fan were calculated. Reasonable results are obtained not only for the peak frequencies but also for the amplitudes of the tonal sound. Also, in the predicted sound field, we can see the scattering effect of duct and casing.Copyright

A study on the flow and aeroacoustic characteristics of the sirocco fan of OTR (Over The Range)

Aeroacoustic characteristics of sirocco fan used in Over-The-Range (OTR) has been analyzed in this paper. A microwave hood combination over the gas range is short for the OTR. The flow phenomena of the double-sided sirocco fan was analyzed numerically and experimentally by using commercial code and three dimensional PIV for flow visualization. Also, microphone array is used in order to understand acoustic characteristics of OTR. Two dimensional unsteady flow and acoustic simulation is tried to qualitatively estimate the effects of tonal noise and broadband noise on the overall sound pressure level. It is found that tonal sound is generated from the strong interaction between the impeller and cutoff while broadband sound is generated from the strong secondary flows along the scroll surface. To reduce the noise level, the V-shape cut-off was applied to improve the sound quality by reducing tonal noise. So the peak noise at BPF (Blade Passing Frequency) was almost reduced. The shape of flow-guide to suppress the secondary flow over the scroll surface was carefully checked. It is found that this affects flow pattern at the fan exit and reduces the broad band noise. Through this numerical and experimental study, the sound pressure level was lowered by 4dBA compared to that of the previous fan at the operating point.

Experimental study of noise reduction and improved cooling fan performance in a PDP TV

The present experimental study addresses fan system noise reduction and improvement in cooling performance in a plasma display panel (PDP) television (TV). The broadband noise in a PDP TV is closely related to system losses. The system losses, mainly due to rear case of the TV near the fan, are reduced by increasing the number of vent holes on the case, preventing secondary leakage flow between the fan and the case and modifying the rear case shape. The discrete noise is mainly related with the inflow conditions therefore, the removal of structures that distort inflow results in a discrete noise reduction. Additionally, the fan rotating speed is reduced because of the increased flow rate and reduced flow fluctuation, which is obtained from the reduction of system loss (resistance). 7.3dB(A) noise reduction and a 10 % increase in flow rate are achieved. The main concepts behind realizing noise reduction are prevention of recirculation flow around the fan and reduction of system loss.

An Analysis of the Unsteady Flow-Field and Aerodynamic Sound of a Turbo Fan used in a Vacuum Cleaner

A new method to calculate the aeroacoustic pressure of a centrifugal fan that is used in a vacuum cleaner has been developed. The centrifugal fan consists of the impeller, the diffuser, and the circular casing. Due to the high rotating speed of the impeller and the small gap distance between the impeller and diffuser, the centrifugal fan makes very high noise levels at BPF and its harmonic frequencies. In order to calculate the sound pressure of a centrifugal fan, the unsteady flow field data is needed. This unsteady flow field is calculated by the vortex method. The sound pressure is then calculated by acoustic analogy. In this paper, only dipole term is considered in the equation. The noise generated by moving impeller and stationary diffuser is calculated separately. The predicted acoustic pressures agree very well with the measured data. The difference between the two is less than 4dB