Influence of Blade Number on Aerodynamic Noise of Propeller Fans for Outdoor Unit of Air-Conditioner

D1 - Computational Aeroacoustics

In this study, flow fields in half ducted propeller fans for outdoor unit of air-conditioner have been calculated by a finite element method-based Large Eddy Simulation (LES) with the aim of investigation for influence of blade number on aerodynamic noise. 2-blade and 4-blade propeller fans were intended. Noise level of the 2-blade propeller fan was smaller than the 4-blade by 5.1dB experimentally. Dynamic Smagorinsky model was used as sub-grid scale model. Computational grid consisted of three parts that were inlet part, propeller part, and outlet part. The grid is composed of hexahedral elements. Numbers of grid elements for inlet and outlet parts were approximately 2 million and 5.5 million elements, respectively. Numbers of grid elements of fan neighborhood part for the 2-blade and 4-blade propeller fan were approximately 5.5 million and 10.6 million elements, respectively. Aerodynamic noise was calculated by Curle's equation based on pressure fluctuation calculated by LES. The calculated static pressure rise and shaft power showed reasonable agreement with the experimental results. The calculated time-averaged velocity distributions downstream of the blades were also compared with experimental results by hotwire anemometry. The velocity distributions showed reasonable agreement between the calculated and experimental velocity profiles. But the calculated velocity fluctuations downstream of the blades were overestimated the experimental results. Calculated noise level of the 2-blade propeller fan were smaller than that of the 4-blade. Calculated difference of overall noise level was 5.7 dB. Moreover, we confirmed that the tip vortex and leading edge separation vortex had a great influence on half-ducted propeller fans. The tip vortex trajectory and the blade pitch of the 2-blade propeller fan were longer than those of the 4-blade propeller fan. These suppressed the interaction between the tip vortex, the ring, and the adjacent blade. The 2-blade propeller fan was therefore more silent than the 4-blade propeller fan.