Experimental development of a measurement technique to resolve the radial distribution of fan aero-acoustic emissions S. Bianchi a) , A. G. Sheard b) , I. R. Kinghorn b) , A. Corsini c) and F. Rispoli c) (Received: 24 January 2008; Revised: 29 September 2008; Accepted: 29 September 2008) This paper presents an experimental technique developed to assess the performance of a family of axial fans for compact cooling units. It is used to study fan rotor noise sources to enable the effectiveness of design modifications to be assessed. By varying the radial positions of a near field microphone, correlations with a far field microphone are used to study noise source signatures along the blade span. The correlations reveal distinctive acoustic signatures that are described in detail. The methodology has been shown to be effective in identifying: (i) the blade tip feature with the best acoustic performance; and (ii) other significant noise sources along the blade span. © 2009 Institute of Noise Control Engineering. Primary subject classification: 11.4.1; Secondary subject classification: 72 1 INTRODUCTION Fans that operate in the cooling systems of industrial heat exchangers and in building heating and ventilation are a source of noise pollution. The link between acous- tic emissions and the aerodynamic features of the rotor flow has been studied extensively; in particular, the work of Wright 1 , Cumpsty 2 and Holste and Neise 3 has enhanced understanding of the aero-acoustics of turbo- machines. According to the works of Cumpsty 2 and Holste and Neise 3 the amount of power radiated into noise by a rotor is generally small compared with the aerody- namic power released to the air flow. Noise is radiated by forces, volume displacements and nonlinearities which are either unsteady or periodic in their effects. According to the origin of the noise produced, the source mechanisms considered in this study can be divided into three categories: inflow turbulence noise, impeller noise and tip flow noise (Fukano et al. 4,5 and Sharland 6 ). The first source originates from the unsteady loading due to the ingested turbulence fluctuations. Inflow turbulence was seen to be an important noise source over a range of frequencies in the work of Wright 1 for low speed fans and Cumpsty 2 for high speed turboma- chinery. Their analysis also focused on the humped nature of the low frequency spectrum, below the BPF, as being due to the large scale components of incoming turbulence. These large scale components give nearly periodic disturbances as they are swept through the rotor plane. As far as the self-generated noise from the impeller, as recognized as far back as 1976 1 , it is produced by the pressure fluctuations from the turbulent boundary layer on the blade and by the shedding of vortices from the trailing edge. This confirms earlier observations by Sharland 6 , stating that the local forces set up from the random fluctuation of a moving flow on a surface, may well act as acoustic sources and so, at least in the first instance, it is reasonable to consider the turbulent boundary layer inside a fan as a possible source of noise. Moreover, the work of Cumpsty 2 argued that the shear layer, interacting with the rotating blades should lead to additional noise in some manner. The same lifting surfaces that create the shear layer produce a dipole-like source that rotates with the blade. When the dipole rotates with the machine the repetitive pattern of the fluctuating pressure passing a given point (as a stationary strut) produces the narrowband tone known as the BPF. This tone and its harmonics are the hallmark of this kind of rotating machinery. When looking at the tip region of a fan blade, it is recognized as the most significant noise source, as stated in dedicated studies on the far field emission of this source (Longhouse 7 , and Fukano and Jang 8 ). Marcinowski 9 , who was the first to present a study of noise associated with tip dynamics, demonstrated that increases in broadband noise levels occur with increas- ing tip clearance, with the largest changes being appar- a) Dipartimento di Meccanica e Aeronautica, Sapienza Uni- versity of Rome, Via Eudossiana 18, Roma, 00184, ITALY; email: bianchi@dma.ing.uniromal.it b) Fan Technology, Fläkt Woods Ltd, Axial Way, Colchester, Essex, C04 5ZD, UK. c) Dipartimento di Meccanica e Aeronautica, Sapienza Uni- versity of Rome, Via Eudossiana 18, Roma, 00184, ITALY 360 Noise Control Eng. J. 57 (4), July-Aug 2009