Acoustic Performances of Irregular Blade Spacing Fans Tested on an Excavator

On mobile machinery, fans are often the dominant source of external noise, especially with the reduction in noise generated by internal combustion engines. The gradual arrival of electrified machines will not necessarily solve the problem, as the need for cooling the combustion engine will be transferred to the batteries or accessories, and fan noise is likely to emerge even more with a quieter engine. Initial tests have shown that irregular fans have little impact on the overall A-weighted level but do reduce the level of blade passing frequency components. Manufacturers in the sector have expressed a desire to know the acoustic performance of fans with uneven blade spacing blades on a machine, among other noise sources. For the purposes of the demonstration, 4 excavators were assessed and the one radiating most noise on blade passing frequencies was selected. Two irregular fans with different angular distributions were tested and compared with standard fan noise. The paper presents the results of experiments carried out on these machines. The tests were done in a configuration close to that of ISO 3744 (sound power measurement). The first step was to evaluate the contribution of the fan to the total noise of the machine. The result depends of course on the machine, but the contribution is in all cases important, even very important as regards the external noise, it is negligible for the noise in the cabin. The investigations were continued on a machine for which the fan noise has strong tonal components. Two Irregular Blade Spacing Fans were then designed using simple modelling to be compared with the original fan. Measurements were made with the three fans in identical configurations at different engine and fan speeds. They confirm a first result previously acquired on a test bench, which is the conservation of the overall A-weighted level. The 1/3 octave spectra are also slightly modified. The impact is important on the discrete frequencies for which we observe very different distributions of the acoustic energy between the characteristic frequencies of the engine and the fans. The influence on 3 psychoacoustic criteria is also examined. In parallel, the influence of the Flexible Blade Extension which modify the shape of the spectra (decrease in broadband noise but increase in discrete frequencies) is analysed. Perspectives are then provided for further investigation of this subject, both in terms of experimental and simulation aspects.