Computational study for noise reduction and characteristic of unsteady flow field/flow-induced noise generated in a small radial fan

This study is an attempt to characterize unsteady flow field and predict flow noise produced by a radial fan installed in a system that has a tiny gap between the fan casing and upper wall (similar conditions found in actual products). The large-eddy simulation turbulence model was used to simulate unsteady flow conditions, and an impeller was rotated 13 times in total to sufficiently develop unsteady flow and obtain data for Computational aeroacoustics (CAA). Unsteady flow field due to the radial fan structure and narrow flow channel in a system was verified, and a location wherein flow properties rapidly changed over time due to the complicated flow field was identified. The noise spectrum obtained through CAA was compared with that measured through experiment. Both tonal noise components consisting of blade passing frequency with its harmonic frequencies and broadband noise were matched. The major source of noise of the radial fan was on the inner surface of the casing. However, the sources of noise in the upper and lower sides of the casing were not equivalent to each other due to the interaction between the flow coming through an inlet located in the upper part of the casing and the flow discharged through an outlet installed at its side, as well as the interaction between impeller rotation and cut-off. Sources of noise located at the upper casing are caused by the flow around impeller tips, whereas sources on the lower casing are related to the interaction of the flow between the impeller and casing. A low noise model with a modified impeller tip was proposed, and its noise reducing effects were evaluated. The modified model reduced overall sound pressure level by 0.8 dB compared with the base model.