Research On The Vortices Flow And The Aerodynamic Noise Prediction Of The Air-Conditioner's Fan

Compared to the general power fan, the fan used in the air-conditioner system has a different configuration, a particular internal flow phenomena and flow mechanism. Research on the vortices flow and the aero-acoustic performance inside this fan is scarce presently. Therefore, in this dissertation, the vortices flow inside the blade passage of the half-ducted axial flow fan installed in the outdoor unit and the multi-blade fan installed in the indoor unit of split room air conditioners, the optimization design of the configuration of half-ducted axial flow fan rotor and shroud, and noise prediction of the open axial fan are analyzed by numerical simulation and experimental research to explore the internal flow mechanism and seek some approaches to improve the fan's aero-acoustic performance.The formation and development of the tip vortex in the rotor tip region of the half-ducted axial flow fan installed in the outdoor unit of split room air conditioners is analyzed numerically, from which the distributions of the trajectory of tip vortex along axial, circumferential and radial direction are revealed. Formation of the tip vortex starts from the blade tip's suction side near the rotor's leading edge, due to rolling-up of the intense shear layer flow between the main axial flow and the suck-in inward flow. It disappears near the rotor's exit finally. The tip vortex passes through the blade passage in a curve reversed towards the blade's rotational direction. Its trace is partial to the tangential direction as it enters the aft part of the blade passage covered by the shroud, simultaneously, its trace in the radial direction turns from the outward to the inward direction. Moreover, the influence of the operating flow rates and shroud's width on the tip vortex's trajectory is discussed. The operating flow rates have a significant effect on the axial position of the tip vortex's trace, while effect on the radial position is negligible. The effect of the shroud's width on the tip vortex's trajectory is notable. For a fan with a wide shroud, the trace of the tip vortex moves upstream, while it moves downstream for a fan with a narrow shroud. The tip vortex region is smaller along the radial direction in a fan with a wide shroud than that with a narrow shroud.The internal flow pattern inside the blade passage of the multi-blade fan installed in the indoor unit of split room air conditioners is analyzed numerically, and the vortices flow characteristics at various axial and circumferential locations are compared in detail. The calculation results show the existence of massive separations in the blade passages inside the fan. Even in the main flow region near the back plate of the rotor, the separated flow at the leading edge on the blade suction surface occurs in about two thirds of the blade passages along circumferential direction. While near the shroud side of the rotor, it occurs in almost all of the blade passages. The influence of the eccentric inlet on the flow field distribution is also discussed, which indicates that the vortices flow will be weakened and the fan's performance will be improved when the fan's inlet is installed eccentric to the inner volute properly.The influence of the angle of skewed-swept blade, the axial location of the rotor leading edge relative to the shroud, the width of the shroud on the internal flow inside the half-ducted axial flow fan and the fan's performance is discussed. The results disclose that the vortices flow is improved, and the reverse flow area just upstream the shroud is diminished when large forward skewed-swept angle is applied. Both the axial location of the rotor leading edge relative to the shroud and the width of the shroud have important influence on the reverse flow area. On the basis of these, the configuration of the rotor and the shroud are optimized. The experiment results demonstrate that the fan's noise is reduced by installing the new optimized four-vane skewed-swept rotor by 0.5~1.3 dB (A) without any negative influence on the aerodynamic performance of the fan. The noise reduction of 0~1.7 dB (A) and the flow rates increase of 7% are obtained by installing the newly optimized three-vane skewed-swept rotor. When the new optimized rotor matchs the newly optimized shroud, the fan's noise reduction is up to 1.9 dB (A).The instantaneous vortices flow characters inside two different open axial flow fans are studied, and the pressure fluctuation on the blade surface is obtained using large-eddy simulation. The far field aerodynamic noise is predicted using the FW-H equations, and the spectra of the noise radiated from various rotor surfaces are discussed. The calculation results reveal the location of the tip vortex core changes as a function of time in the open axial flow fan. Both the tip vortex and the leading edge separation vortex induce pressure fluctuation at the corresponding position on suction surface, which forms the main noise source. The dominant source for low-speed axial flow fan's noise is the broadband noise. The overall noise level can be reduced by weakening the vortices flow.The present work is financially supported by the National Natural Science Foundation of China under the grant No. 50176012.