Influence of the Rotational Speed onto Compact Axial Fans Broadband Noise at Constant Design Point

2.3 Design for Low-noise Fans

From an industrial point of view, fan size and particularly fan diameter is the major design constraint. Once this parameter is set, fan design consists in choosing the optimal couple between rotation speed and blade loading in order to achieve the target operating point. The influence of the rotation speed onto fan noise has been known for a long time. Moreover, recent studies clearly showed that blade loading affects fan broadband noise. The current knowledge can be resumed as follows: at too high speed, quadrupole related noise (turbulence, jet) will prevail and noise will rise. At too low speed, the blade loading will get high and the blade boundary layer very thick. Flow detachment or even stall is likely to happen, leading to excessive noise too. An optimum thus has to be found. Based on this conclusion, a two steps study has been carried out. The first step implied extensive CFD analysis and was presented in a previous conference. The second step aims at validating the numerical results and will be presented. A design point has been selected, and five geometries were developed for five different fan rotation speeds, from 7500 rpm to 10000 rpm.
The design rules as well as the CFD study will be summarized in the first part. Methods and major results will be presented. The numerical study did highlight an optimal rotation speed for the selected design rules. In a second part, the challenges met during the prototype realization phase will be discussed, and the technical choices made will be justified. Among others, CFD generated profiles often get very thin trailing edges, which are great challenges to manufacture. The experimental conditions will then be described. Finally, the experimental results will be presented and the comparison between CFD predictions and measurements will be discussed.

Literature: Patrick Buchwald, Damian M. Vogt, Julien Grilliat, Wolfgang Laufer, Michael B. Schmitz, Andreas Lucius and Marc Schneider, "Aeroacoustic Analysis of Low-Speed Axial Fans With Different Rotational Speeds in the Design Point", Paper No. GT2017-64417, ASME Turbo Expo 2017, Charlotte, North Carolina, USA, June 26–30, 2017