Influence of Blade Sweep on Aerodynamics and Acoustics of Low-Pressure Axial Fans

2.3 Design for Low-noise Fans

In this paper three different low-pressure axial fans are experimentally and numerically investigated. All fans are following the same design process using different values to influence the degree of blade sweep being applied. In previous investigations forward sweep was applied to successfully reduce sound emissions. Observations showed: the larger the maximum sweep angle the larger the reduction of noise. The reduction of the section lift coefficient was compensated by an increase in chord length accepting a small loss in efficiency. The observed effect is now being investigated in a more intensive measurement program focusing on tip clearance flow.
Fan No. 1 is considered as a reference fan. Fans No. 2 and 3 differ in the shape of the radial sweep angle curve; No. 2 reaching for a very extreme value at blade tip, No. 3 reaching for the same value at blade tip as the reference but with a bulkier curve shape at blade mid-height.
All fans are experimentally investigated in an on-site fan test chamber. In addition to basic fan characteristic measurements the wake flow field is measured using a five hole probe. For further investigations on the tip vortex phase-locked wall pressure measurements are carried out. These measurements are then used to validate the numerical RANS-simulations that give a more detailed view on the complex three-dimensional flow phenomena near the blade tip. Externally conducted acoustic measurements show the sound power level on suction and on pressure side as well at different operation conditions. The attempt is made to find a correlation between used design parameters, tip-clearance flow field and the effect on overall sound emission.