ON THE AEROACOUSTIC PROPERTIES OF A PRANDTL-FAN

Axial fans are used in many machines and equipment in close proximity to people. Therefore, acoustic optimization of the fans is essential to protect the well-being of people and to ensure a quiet environment. The sound emission of fans is determined, among other factors, by the selected load distribution of the blades and the resulting angles of attack and chord lengths. A bell-shaped load distribution developed by Prandtl in 1933 was applied to a benchmark fan and investigated aeroacoustically. A total of three axial fans were numerically simulated for flow and acoustics at the design point. One of these fans is an international benchmark fan, the second is a fan with the same isoenergetic load distribution but a different chord length, and the third fan has the Prandtl load distribution and the same modified chord length. Based on the simulations performed, it was shown that a larger chord length leads to an improvement in the aerodynamic and acoustic properties of the axial fan with isoenergetic load distribution. This is due to the reduced blade tip overflow and the associated reduced pressure fluctuations at the blade surface. The bell-shaped load distribution increases the efficiency of the fan, but also increases the total sound emission by 1.5 dB compared to the axial fan with constant chord length and isoenergetic load distribution. This is due to the increased interaction between the blades, as the blade tip vortex hits the following blade leading edge more frontally. This increase occurs despite a reduction in the strength of the blade tip vortex. Compared to the benchmark fan, the changes in chord length and Prandtl's load distribution resulted in a sound reduction of 3.9 dB and an increase in efficiency of 4%. It was shown that the modifications can be used for future aeroacoustic optimization of axial fans.