Hua Ouyang

Investigation of air-flow fields and aeroacoustic noise in outdoor unit for split-type air conditioner

Predictions and measurements of the three-dimensional structure of flow fields inside an outdoor unit of a split-type room air conditioner are presented. The three-dimensional incompressible Navier-Stokes equations in a moving reference frame are solved on tetrahedral meshes. A sliding mesh technique is utilized to characterize unsteady interactions. The accuracy of the predicted flow fields is evaluated by comparsion to measurement results taken with a laser particle image velocimetry (PIV). Results show that the outdoor unit has a complex vortical flow field near blade tips. Its formation starts on the blade tip suction side and decreases in magnitude on the pressure side. With the computed unsteady viscous flow solutions, the discrete frequency noise radiated from the unit is predicted. The numerical results are compared with the measured radiated noise levels.

Investigation of Unsteady Flows and Noise in Rotor-Stator Interaction with Adjustable Lean Vane

Abstract The paper is a study of the noise reduction effects of the stator lean angle. Firstly, a numerical study has been carried out to investigate the unsteady characteristics of the flow induced by the rotor-stator interaction. To obtain the rotor‟s wake, the distribution of the RMS pressure and the pressure disturbance coefficient, the compressor with an adjustable stator lean angle has been numerically studied. Then the compressor was experimentally tested at 11 different lean angles to obtain the free field noise spectra. Both the unsteady flow property and noise characteristics are discussed on the basis of the Computational Fluid Dynamics (CFD) results. The results indicate that (1) the pressure fluctuation of stator is the main source of the interaction noise; (2) the phase of rotor‟s wake can be tuned by the lean angle of the stator; (3) the distribution of the first harmonic of tone noise decides the distribution of total Sound Pressure level (SPL); and (4) stator with positive lean angle has better noise reduction than negative one and the lean angle should exceed 10° for better noise reduction.

Design and characterization of a small aeroacoustic wind tunnel

Experimental investigation of sources of sound is required to fundamentally understand the noise generation mechanisms. In order to investigate the sound from fluid-structure interactions, a low-noise wind tunnel has been designed and implemented. Speeds up to a test section velocity of 81 m/s are possible based on a square nozzle of dimension 180×180 mm2, thus the maximum Reynolds number is near 1×106. The wind tunnel is driven by a separate, low noise level centrifugal fan controlled by a variable frequency drive. Design and construction considerations for the major tunnel sections, i.e. inlet, anechoic chamber, diffuser, muffler, are described. Aerodynamic and acoustic calibration of the tunnel is discussed in relation to design criteria. Initial testing demonstrated that a sufficiently low background noise level has been obtained. Moreover, the suitability to flow induced cavity noise is explored with a cavity mode. The paper concludes with suggestions for improvements and provides an outlook on future applications.

Experimental and numerical investigation of noise generated by rotor blade passing an exhaust grille.

This paper considers the noise generated by interaction between the rotor blade and the exhaust grille. Acoustic measurements were first performed in a semi-anechoic chamber to investigate the characteristic of the noise. An approach that combines a Reynolds averaged Navier-Stokes calculation with the stochastic noise generation and radiation method is used to identify noise sources. With the computed flow solutions, the broadband noise radiated by the grille is evaluated based on a semi-empirical noise model. The numerical results are compared with the measured noise levels and good agreement is obtained.

Investigations of rotating instability and fluctuating tip clearance flow in a low-speed axial compressor

To investigate the relationship between rotating instability and fluctuating tip clearance flow, experimental tests as well as numerical studies are conducted on a 1.5 stage low-speed axial compressor rig. The rotating instability is detected through casing mounted dynamic pressure sensors. The phase-locked and root-mean-square pressure contours show that the interaction of tip clearance flow with the neighboring blade is the most likely cause of casing flow field fluctuation. From the calculated results of entire-annulus cascade, it is found that the tip clearance vortex structures oscillate periodically at low flow rate conditions. That leads to intense fluctuation of the casing flow field. Further, phase lag of pressure fluctuations exists between two neighboring passages, which induces a rotating pressure pattern of multiple frequencies. The frequency characteristics of this rotating pressure pattern caused by fluctuating tip clearance flow is coincident with the frequency of rotating instability. This suggests that the fluctuating tip clearance flow is the major contributor to the initiation of rotating instability.

An analytical investigation of stator lean on rotor–stator interaction noise

This paper concentrates on the noise reduction effect of stator lean in rotor–stator interaction. The compressor with a serial stator-blade lean angle has been employed to acoustically test and numerically calculate. The experiment results show that stator-leaned positive has better effect on noise reduction than leaned negative; the tone noise is determinant on total sound pressure level, and the lean angle of the stator should exceed 10°. Based on the results of unsteady calculation about the compressor, the amplitude of the unsteady loading of stator decreases with the increase in the lean angle. Leaned positive stator has lower unsteady force and loading than leaned negative. The phase parameters q and pcle of wakes are almost proportionate to the stator-blade lean angle. The distribution of q and phase close-leading edge (pcle) shows that the compressor with 25° and −20° stator-blade lean angles has the maximum and minimum phase variation, respectively.

Investigation on Aeroacoustics and Aerodynamics of the Stator Lean Effect for Rotor-Stator Interaction

The rotor-stator interaction in a 1.5 stage compressor with different lean angle of downstream stator blades was investigated by aeroacoustic and numerical study. The aeroacoustic performance with narrow spectra, the sound pressure level distribution and the unsteady loading of the compressor were tested and analyzed. The RMS pressure, the axial thrust and torque of the downstream stator blade distribution revealed the unsteady characteristic of rotor-stator interaction at different downstream stator blade lean angle. The positive lean stator had better noise constraint than the negative lean stator. However, the RMS pressure and the unsteady force increased with the increase of the lean angle when the stator had positive lean angle. The time-spatial contours of the axial velocity demonstrated the effect of time delay for the rotor wake intersection with the leading edge of the stator blade. And the phase lag distribution of the wake presented the detail distribution of the wake phase along the span. The downstream stator blade lean was effective to obtain a significant reduction for the unsteady force and get large phase lag for the rotor’s wake of rotor-stator interaction. Leaned positive blade had better benefits than negative for the noise reduction and the phase lag of the wake.Copyright

Experimental investigation of acoustic oscillations in a heat exchanger in an air-conditioning indoor unit

The behavior of noise generated by flow through the heat exchanger of indoor unit at low-speed free stream is studied by aero-acoustic performance measurements. First, the aero-acoustic performance experiment has been carried out for different velocities and angles. The characteristics of noise are investigated. It was found that the noise can be generated at particular conditions which are considered as the most important factors that influence the aerodynamic performance of the heat exchanger. Moreover, analysis was carried out to identify the oscillation modes in cavities. Second, a modified semiempirical formula for the model is presented matching the experimental results so that the model of noise can be applied at the low-speed free stream.

Casing Pressure Measurements and Numerical Simulations of a 1.5 Stage Axial Compressor: Tip Leakage Flow and Rotating Instability

Experimental and numerical investigations on the unsteady casing flow field in a one-and-half stage low speed axial compressor have been carried out. By using fast response pressure transducers instrumented on the rotor casing, the pressure time series were acquired at different operation points from throttle wide open to near-stall operation point. The pseudo-spatial pressure contours, phase-locked averaged and root-mean-square pressure contours and power spectrums of unsteady pressure signal have been achieved. The CFD simulations were conducted to help understanding the features of tip leakage vortex.The rotating instability has been detected throughout an operation range from small flow rate point to near stall point. The frequency characteristic of rotating instability according flow rate was discussed. Based on the pattern of RIF varying with flow rate, the developing process of rotating instability according to flow rate could be divided into two stages, referred as early-developing stage and fully-developed stage. By analyzing the correlation between rotating instability and casing flow field, it was discovered that the origination and development of rotating instability was closely related to the fluctuation induced by tip leakage vortex.Copyright

Analysis of circumferential skew effect on rotor/stator interaction noise reduction

The rotor/stator interaction noise as a type of the dipole sound source is generated from the pressure fluctuation on the stator vanes. The noise is one of the main noise sources in multistage turbomachines such as axial compressors and fans. In the paper, the effect of circumferential skewed angle on the rotor/ stator interaction tone noise reduction was studied. The Ffowcs-Williams and Hawkings (FW-H) acoustics model showed that the amplitude of pressure fluctuation on the stator vanes was the main impacting factor of the interaction noise when the sound source was compact. Consequently, the pressure fluctuation on the stator vanes was calculated using computational fluid dynamics (CFD) method to study the relationship between the circumferential skewed angles and the amplitude of pressure fluctuation at BPF (blade passing frequency) and its harmonics. The research results concluded that the test fan with circumferential backward skew rotor blades and backward skew stator vanes could reduce the amplitude compared against other test fans. The sound pressure levels (SPLs) of the interaction tone noise calculated using the FW-H acoustics model also supported the conclusion