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With the aim of analysing the efficiency of leading edge serrations under realistic conditions, an experimental rig was developed where a ducted low-speed fan is installed that allows to gather data of both, aerodynamic and aeroacoustic nature. Turbulent inflow conditions were generated via biplane-square grids, resulting in turbulence intensities of different magnitude and of high isotropic character that were quantified by use of hotwire measurements. The fan blades were designed according to the NACA65(12)-10 profile with interchangeable features and an independently adjustable angle of attack. Altogether, five different parameters can be analysed, namely the serration amplitude and wavelength, the angle of attack, the inflow turbulence and the rotational speed. In addition, the blade design allows for a variation of the blade skew, sweep and dihedral as well. The presented work focusses on validating and optimising the test rig as well as a detailed quantification of the turbulent inflow conditions. Furthermore, first aerodynamic and aeroacoustic results of fan blades with straight leading edges are compared to those of serrated leading edges. The aerodynamic performance was found to be mainly affected by the serrations as a function of the serration amplitude. Aeroacoustically, a clear sensitivity towards different incoming turbulence intensities and serration parameters was detected, showing significant broadband noise reduction below 2 kHz with an overall noise reduction of ΔOASPL = 3.4 dB at maximum serration amplitudes and minimum wavelengths.
Heavy-duty centrifugal fans require high reliability and first-class performance. Besides, extreme conditions and harsh environments are often encountered, such as in the papermaking process, in steel or cement plants or the chemical and petrochemical industry. Therefore, the design of high-performance heavy-duty industrial fans requires robust yet efficient solutions. The previous work indicates a high aerodynamic and aeroacoustic sensitivity concerning the specific position of the volute cutoff (tongue). This effect will be further investigated, not by directly changing the orientation of the cutoff, but by varying the position of the impeller relative to a fixed volute casing. The initial evaluation is done through a numerical study of three influencing parameters, which allow the aerodynamic dependencies to be modeled using low-layer artificial networks. Subsequently, extensive experimental studies were carried out to validate the aerodynamic dependencies and also to incorporate information on the aeroacoustic performance. The obtained results show that the operating point represents the key factor in determining the optimal positioning, with qualitatively comparable dependencies found for both tested fans. From an aeroacoustic point of view, the determined optimal configuration does not necessarily coincide with the observed aerodynamic desires, so careful analysis and a reasonable compromise are required, motivating for a multi-objective optimization process.
Leading edge serrations are identified as an effective passive treatment for reducing fan broadband noise due to high turbulent inflow conditions. This paper aim to investigate the isolated effect of serrated applications in a rotating frame, covering the aerodynamic and aeroacoustic performance. With this purpose, a serration design, previously analyzed in the rigid domain, is transferred to the rotating frame, following a successive approach in form of a continuous increase of the fan blade number. This is considered as a feasible way to isolate the serration effects and to provide information on fan blade interaction and possible masking effects. Comparing blades with straight and serrated leading edges by analyzing the spectral noise reduction and the overall level results in deep insights in the underlying noise reduction mechanisms. Furthermore, analysis of phase differences by means of the wall pressure fluctuations leads to the identification of rotating flow phenomena, non-synchronized with the rotor speed. The results obtained indicate an efficient noise reduction by the serrations in the vicinity of the design point. By use of the presented successive approach, noise reduction phenomena observed with the full rotor could be identified to be of either aeroacoustic or aerodynamic nature. A reduced noise is observed for the full rotor case, showing a reduction of blade interaction effects. At reducing flow coefficients, an improved stall margin of the serrated rotor is identified that also affects the aeroacoustic signature.