Control of tip-vortex noise of axial flow fans by rotating shrouds

Noise and performance tests were conducted on a low tip speed, half-stage, axial flow fan such as is used in automotive applications. The purpose of the tests was to determine the characteristics, relative importance, and the methods of control of the tip clearance noise of the fan. The fan was 356 mm in diameter and had eight equally spaced blades. The noise measurements were made in both free field and reverberant field environments and the fan backpressure and speed were varied during the tests. An acenaphthene coating on the blades was used to determine the regions of laminar and turbulent flow and smoke was used to visualize the blade-tip flow patterns. Tip clearance noise was shown to contribute up to 15 dB in overall noise level over the entire fan operating range for tip clearances of the order of 3–4% of the blade chord and larger. Large clearances result in a significant (∼10%) drop in fan peak efficiency. Essentially, tip clearance noise is due to interactions of the blade tip vortex with the trailing edge of the blade and with the leading edge of the following blade. A properly contoured, rotating ring-shroud attached to the fan tips was found to be a practical method of controlling tip clearance noise. For example, with a clearance of 6·5% of the chord length, the noise level with the ring-shroud was as much as 12 dB lower than in the fixed shroud case with the same clearance.