Tip Leakage Flow and its Implication on the Acoustic Signature of a Low-Speed Fan

2.1 Noise Generation Mechanisms

Fans and blowers are omnipresent in daily life. They are utilized in large scale Heating, Ventilation and Air-Conditioning (HVAC) systems, automotive cooling systems as well as small scale systems such as electronic cooling systems for home computers. They represent as many sources of noise disturbances.
Meanwhile, the health impacts of environmental noise are fast becoming a growing concern. A recent report from the World Health Organization concludes that ”If negative effects on sleep are to be avoided the equivalent sound pressure level should not exceed 30 dBA indoors for continuous noise”. This is a challenge for fan and blower manufacturers. A new generation of both efficient and quiet air moving systems is needed. For that purpose, a better understanding of the noise generation mechanisms in low-speed fans and blowers is required.
Within this scope, a consortium named Ultra High-Efficiency Quiet (UHEQ) Fans has been funded between Michigan State University (MSU), University of Notre Dame (ND) and Université de Sherbrooke (UdeS). The first phase of this consortium, from 2010 to 2015, focused on the collection of experimental data sets. The present phase of the UHEQ consortium aims at using advanced numerical tools together with the available experimental database in order to give a strong understanding of the noise generation mechanisms in blowers and fans.
The fan investigated in this study, dubbed Rotating Controlled Diffusion Blade (RCDB), is a research fan that has been subject to an extensive experimental investigation using the Axial Fan Research and Development (AFRD) test facility at MSU. The AFRD facility consists in a large plenum of 3x3x2 m large, the test fan being mounted in an opening on the ceiling wall. The fan has a diameter of 740 mm, and a 3 mm annular tip clearance with the shroud surface; the blade span is 135.2 mm and the controlled diffusion (CD) profiles forming the blades have a constant chord of 133.9 mm along the span. The RCDB was designed to be modular and is able to accommodate anywhere from 2 to 9 blades. The design flow rate of the fan is 2.31 kg/s and the design rotational speed is 437 RPM. Available experimental data from the AFRD facility includes wall-pressure measurements on the fan blades along the chord for several radial positions. Hot-wire anemometry and PIV measurements were also conducted to provide an in-depth description of the flowfield encountered downstream of the fan. More recently, aeroacoustic measurements were also carried out within the non-anechoic AFRD facility.
For the purposes of this study, the 3-blade RCDB configuration within the full AFRD facility was simulated using a lattice-Boltzmann method (LBM) code to further investigate the entire flowfield around the fan. Three operating points covering the entire stable operating range are covered and a detailed aeroacoustic analysis is carried out in order to especially investigate the role of tip leakage flow on the acoustic footprint. A detailed analysis of the trajectory and of the unsteady behaviour of the tip-leakage flow is performed. The main flow features and their connections with typical acoustic signatures are explained.