Contents lists available at ScienceDirect Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust Centrifugal fan with inclined blades for vacuum cleaner motor Jurij Prezelj, Tadej Novaković ⁎ University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana, Slovenia ARTICLE INFO Keywords: Centrifugal fan Impeller Inclined blades Aeroacoustics Psychoacoustics Noise ABSTRACT Centrifugal fans have several technical applications. Their aerodynamic performance is well-optimized nowa- days, but the same does not apply to their acoustic performance. Noise control studies of centrifugal fans are often focused on designed operating conditions or operating conditions close to the onset of instability. This paper seeks to find an alternative geometry of the centrifugal fan impeller which would improve noise control of centrifugal fans in a wide range of operating conditions. In this paper, the term noise control refers to noise level reduction and additionally to manipulating the psychoacoustic properties of noise. The experimental work and numerical calculations focus on the centrifugal fan in vacuum cleaners. The experimental work, based on numerical simulations, is described to investigate the influence of the triangular cross section on the flow channel, formed by two inclined blades in opposite directions, on aerodynamic properties and the psychoacoustic performance of the impeller. The study demonstrates that impellers with a triangular flow channel achieve an aerodynamic performance which is comparable to that of standard impellers. The results also show that impellers with inclined blades deliver superior results in psychoacoustic metrics compared to impellers with upright blades. 1. Introduction The noise generated by vacuum cleaners is the most recognizable noise source among all household appliances with a suction unit as the dominant source of noise. The suction unit is an original equipment manufacturer product, assembled from an electric motor, an impeller and a protective cover, forming a vaneless diffuser and airflow chan- nels. Quieter and less annoying vacuum cleaners can be designed only by installing quieter and less annoying suction units; performing noise control measures on the suction unit itself is therefore inevitable. The dominant source of noise in the suction unit is its impeller and this study therefore focuses on noise control of the impeller. In this paper, the term noise control refers to noise level reduction and additionally to manipulating the psychoacoustic properties of noise. The suction unit generates noise due to its vibrating surfaces (vi- bration-induced noise) and due to pressure pulsations in the airflow (aerodynamically generated noise). The level of vibration-induced noise and the level of flow-induced noise are comparable to the same type of noise levels in large fans. In the case of small and medium-sized fans, which are the main focus of this paper, aerodynamically generated noise is dominant [1]. The level of vibration-induced noise in a suction unit is typically at least 10 dB lower than the level of aerodynamically generated noise, making the latter the dominant source of noise in any suction unit [2]. Aerodynamically generated noise has both tonal and broadband characteristics. Discrete or narrowband tonal noise is cor- related with rotation and the blade passing frequency (BPF), along with its higher harmonics. Turbulent noise is characterized by a broadband frequency spectrum [3]. Tonal noise is usually the dominant element of the total noise level and the main disturbing aspect of noise. Tones are produced as a result of regular cyclic motion of the fan’s blades with respect to a stationary observer, and by interacting with adjacent structures [4,5]. Flow in- teractions between the impeller and the volute casing cause periodic pressure fluctuations on the impeller’s solid walls and casing. These periodic pressure fluctuations are the sources of aerodynamic tonal noise radiation, mathematically described by dipole sources, as well as periodic structural vibrations resulting in tonal noise [6]. Reduction of the tonal noise level with BPF is essential for overall noise control and for reducing the irritating character of suction unit noise. Scientists have long been concerned with this problem and sev- eral diverse solutions have already been proposed. Neise summarized the methods for noise reduction of the centrifugal fan and proposed the following: (1) increasing cut-off clearance, (2) increasing the radius of the cut-off edge’s curvature, (3) increasing the angle of inclination between the impeller’s blades and cut-off edge, (4) staggering the blades of double inlet or double row impellers, (5) mounting wire meshes along the inner and outer circumference of the radial blade row, (6) irregular blade spacing, (7) mismatch between the acoustic https://doi.org/10.1016/j.apacoust.2018.05.010 Received 28 November 2017; Received in revised form 29 March 2018; Accepted 6 May 2018 ⁎ Corresponding author. E-mail address: tadej.novakovic@fs.uni-lj.si (T. Novaković). Applied Acoustics 140 (2018) 13–23 0003-682X/ © 2018 Elsevier Ltd. All rights reserved. T