In fan blade design, a main goal is to minimize the noise emissions made by the fan. Thus, different design modifications need to be examined regarding their noise emissions. The Acoustic Camera is a helpful tool in this task. "Investigations on noise sources on a contra-rotating axial fan with different modifications" , a paper published by R. Krause, C. Friebe, M. Kerscher and C. Puhle at fan2018, shows how the Acoustic Camera was applied to examine design modifications of axial fans.
Axial fans are the most common type of fans and are used in many fields including wind tunnels and cooling towers. They work by the continuous rotation of an axial impeller. Similar to aircraft propellers, the gas flows in an axial direction, parallel to the shaft about which the blades rotate. Due to this working principle, swirls occur at the trailing edge of the blades. These swirls may have unfavourable influence on subsequent devices, e.g. higher pressure drop or lower heat transfer coefficient. Thus, they are unwanted and should be minimized.
Design parameters for fan blades include power, flow rate, pressure rise and efficiency. Static pressure rise is one evaluation criterion for the fan efficiency. To rise the efficiency, the dynamic pressure of the swirls should be converted into a static pressure rise. One common way to increase the efficiency is to install contra-rotating fans (CRF), because they provide high power density and high efficiency. Nevertheless, this type is also known for problematic noise behaviour because of the interaction of both wheels with different directions of rotation. This problematic noise behavior makes a CRF the perfect testing object for sound analyses of fans using the Acoustic Camera.
During the project, different modifications of the blade design were tested regarding their influence on decreasing the noise emission. Therefore, a CRF with different blade couples was used to measure the noise emission. Because the blades are rotating during operation, a special measurement technique is required for this type of measurement.
To localize the noise sources, the Acoustic Camera was used with a so-called rotational beamforming algorithm. This special algorithm allows for the detection of sound sources on the rotating blades by using a virtual rotation of the microphones. To investigate the influence of different blade designs, the trailing edges of blades were modified for different measurements. Serrations were used for modification, which shall lead to vortices at higher frequencies.
Different noise sources could be localized with the Acoustic Camera and rotational beamforming. Even in case of contra rotating impellers, the sound sources could be assigned to the specific impeller.
Please find the corresponding paper here. It presents first results of which modifications lead to noise reduction, as well as an outlook for the future work and usage of this measurement technique.
All relevant articles can be be downloaded in our publications archive.
