Beamforming studies on basic models of low-speed axial fan blade sections

Abstract

The paper reports on establishing a beamforming dataset, relying on the phased array microphone (PAM) technique, on the broadband noise of rectilinear basic models of low-speed axial fan blade sections. A cambered plate profile is in the focus of the studies, in comparison with a flat plate and a traditional airfoil profile. At low Reynolds numbers, cambered plate blade sections have a potential to perform aerodynamically better than profiled airfoil sections. Thus, the established dataset aims at contributing to the design background of aerodynamically efficient, low-noise, low-Reynolds-number fans. The wind tunnel-PAM configuration enables the acoustic investigation over the plane being normal to the spanwise direction at midspan. An illustrative study on representative cases selected out of the dataset demonstrates the capabilities and limitations of the presently available experimental and evaluation method. Spatially simultaneously resolved information is presented on the signatures of systematically investigated classes of blade section noise. KEYWORDS beamforming, blade noise, cambered plate blade, low-speed axial fan, phased array microphone Abbreviations BG background LE leading edge PAM phased array microphone TE trailing edge 1. INTRODUCTION AND OBJECTIVES Axial flow fans characterized by blade sections of chord Reynolds numbers of Re £ 10 5 are termed herein as "low-Reynolds-number fans". Such fans are of relatively small size and/or rotor speed, e.g. cooling fans for computers (Huang, 2003) and for electric motors (Borges, 2012), or refrigerator fans (Gue et al., 2011). Axial fans are often required to provide a prescribed flow rate and/or total pressure rise even at moderate diameter and/or rotor speed. These requirements are in accordance with the constraints of limited available space and/or speed-the latter being constrained e.g. by a directly-driving electric motor-in industrial applications. For these fans, the flow coefficient and the total pressure coefficient may extend over F » 0.4 and Y » 0.6, incorporating operational ranges throttled significantly below the design flow rate (e.g. Corsini and Rispoli, 2004). Such fans are termed herein as "high-specific-performance fans". Their blade sections are often of high aerodynamic load-i.e. high lift coefficient-, high camber, and are often exposed to high a angles of attack. Cambered plate blades of circular arc camber line are often applied in axial fans, enabling a relatively simple and low-cost manufacturing technique. A further reason for choosing cambered plate blades appears in low-Reynolds-number fan applications. For Re £ 10 5 , a cambered plate section may perform aerodynamically better than a profiled airfoil section. The potential benefits are the following: higher maximum lift coefficient CL, and higher maximum lift-to-drag ratio CL/CD. Illustrative examples are given for such favourable trends by Carolus (2003), referring to Albring (1978). A comprehensive explanation is given herein on the basis of the experimental data and discussi...

Lift and drag force measurements on basic models of low-speed axial fan blade sections

“…was aligned manually, with the use of the built-in protractor. The calculation of the reference area, A, and the lift, C L , and drag, C D , coefficients are shown in equations (6) and (7). As C L and C D are functions of the angle of attack, an uncertainty due to the setting was calculated from the derivative of the measured values, and was included in the estimation of overall absolute uncertainty, U CL and U CD of the quantities.…”

“…16 The comparative characteristics of the profiles are given in Table 3. For the cases from references, 7,9 the aspect ratio (AR) is 1.5. For the cases from Wallis, 16 the AR can be considered infinite, since the measurement data were corrected to represent an infinite AR.…”

“…Thus, the present authors' long-term aim is to reduce the acoustic emission along with the aerodynamic losses. The aerodynamic measurements linked to preliminary acoustic studies 7,8 are presented in this paper.…”

Establishment of a Beamforming Dataset on Basic Models of Low-Speed Axial Fan Blade Sections

“…Measurements corresponding to Re mid are presented herein. Reference [26] gives an overview on the experiments related to Re max . The present paper is confined to discussing some arbitrarily selected specific results regarding the cambered plate and the RAF6-E airfoil, as these two profiles are representations for common low-speed axial fan blade geometries.…”

“…According to [27], St can vary between 0.8-1 for profile vortex shedding. Such approximation has been refined on the basis of the experiences gathered in evaluating the beamform results for Re max , documented in [26]. For blunt trailing edge vortex-shedding, St is approximated as 0.2 [22].…”