Measurement and analysis of radiated sound from a low speed fan with a large tip gap

Abstract

The wake flow field and radiated sound from a low speed axial fan is studied experimentally. The fan geometry uses controlled diffusion blades and is designed with a low aspect ratio (0.9). The fan is installed with a large tip gap, approximately 10% of the blade span. The radiated sound field is analyzed using a known trailing edge noise formulation. First, the model is compared to an experiment of a single airfoil in a wind tunnel to assess the predictive capabilities. Second, measurements of the fan are made at two different blade loading conditions. Hot wire measurements are made in the near wake of the fan to assess the extent of the tip leakage flow for each condition. The radiated sound fields are compared with the trailing edge noise theory. Use is made of the wake measurements as an input to a surface pressure model. When the fan is operated with the optimal blade loading, the influence of the tip leakage flow is found to be of secondary acoustic impact. When the fan is operated at a high loading condition for the blades, a more significant leakage flow develops and is found to be responsible for the dominant radiated sound.

Time-resolved PIV measurements of a tip leakage flow

“…1 In recent years, beside the research reported in Jacob et al. 1 that addresses the tip leakage flow of an isolated non-rotating airfoil, most studies are concerned with tip leakage noise in fans: several authors studied low speed fans, 2 fans near surge conditions, 3 rotating instabilities, 4 or fans with tip/casing treatments. 4 In fact, few papers are concerned with tip flows of isolated non-rotating blades, and since the experiments of Jacob et al.…”

“…They are not only the cause of aerodynamic losses but are also suspected to be efficient sound sources. For all these reasons, rotor tip clearance flows have been a subject of interest in turbomachinery and aeroacoustic research for a long time as discussed in Jacob et al 1 In recent years, beside the research reported in Jacob et al 1 that addresses the tip leakage flow of an isolated non-rotating airfoil, most studies are concerned with tip leakage noise in fans: several authors studied low speed fans, 2 fans near surge conditions, 3 rotating instabilities, 4 or fans with tip/casing treatments. 4 In fact, few papers are concerned with tip flows of isolated non-rotating blades, and since the experiments of Jacob et al 1 and related computations, there have been no further studies in such academic configurations.…”