Numerical Optimization of the Tonal Noise of a Backward Centrifugal Fan Using a Flow Obstruction

A1 - Tonal Noise Control with Flow Obstructions

Broadband and particularly tonal content of the noise radiated by fans can be perceived as a strong annoyance for people staying in their vicinity and can affect working conditions of operators. As a consequence, fan noise reduction represents a real challenge and an important business criterion for fan and integrated system suppliers. The origin of tonal noise is mainly related to the uniformity and unsteadiness of the inlet flow and to rotor-casing interactions and occurs at the Blade Passing Frequency (BPF) f0 and harmonics 2.f0, 4.f0,… which frequency values depend on the rotation frequency and the number of blades of the fan.

In this study, our interest is focused on a passive noise control device so-called flow obstruction significantly reducing the fan BPF noise. This device is calibrated meaning that its shape, position and orientation with respect to the fan are optimized. The obstruction is located upstream of a centrifugal fan with a heat exchanger placed in between is order to load the fan as in real hood conditions. Some measurements performed at various operating conditions on the CETIM fan aeroacoustics bench test show that for an optimal position and orientation of the obstruction, the BPF noise is reduced by 10-13 dB. These experiments will be presented and detailed in the final version of the paper.

The goal of this paper is to use a CFD/CAA approach for modeling this problem and numerically determine the optimized position of the obstruction. Simulations results are also used to provide a better understanding of the physical mechanisms involved in the noise reduction. The CFD/CAA code PowerFLOW 4.3 based on the Lattice Boltzmann Method (LBM) is used. This flow and acoustics solver is transient, explicit and compressible and the full fan and bench geometries are included in the simulation domain. The fan is truly rotating during the simulation using a Local Reference Frame (LRF) approach. The flow-induced noise contribution radiated from the turbulent flow is simulated during the transient flow simulation and no coupling to another acoustics propagator solver is required. Simulations are carried out at two operating conditions with and without the obstructions and the noise reduction from the obstruction recovered. Additional post-processing of the transient flow and acoustics fields show the effect of the obstruction on the radiated noise and will be presented in the final paper.