Yasuo Takamatsu

The effects of tip clearance on the noise of low pressure axial and mixed flow fans

Abstract Results are presented of a systematic experimental investigation of the effects of tip clearance on both noise and performance of four commercially representative fans (three low pressure axial flow and one mixed flow) in a circular duct, both with and without coincidence of the fan and duct axes (fan/duct eccentricity). It is shown that with eccentricity significant pure tone noise is generated due to blade tip/duct wall interaction, with a plane wave mode component. Reducing tip clearance both improves performance and reduces noise, not only at the maximum efficiency operating point but also in an appreciable low flow rate operating region. With tip clearances small enough to optimize performance and minimize noise fan/duct eccentricity must be kept to a very small value, implying strict precautions in fan and duct manufacture, installation and service conditions.

Noise generated by low pressure axial flow fans, III: Effects of rotational frequency, blade thickness and outer blade profile

In a previous paper [1], practical formulae which correlate the noise sound power of a low pressure axial flow fan to the design parameters of the impeller were derived analytically by introducing a simple model of the flow past the fan blade. The validity of these formulae was examined experimentally in respect to the effects of such parameters as number of blades and chord length on sound power [2]. In this report, the experimental results in respect to the effects of the fan rotational frequency and blade thickness at the trailing edge are described and these results are compared with the theory. The agreement of the theoretical and the experimental results is generally quite satisfactory. Finally, it is shown that the outer profile of the blade has considerable effects on both noise and aerodynamic characteristics of fans: that is impellers with blades swept forward are much superior to those with blades swept backward.

Noise generated by low pressure axial flow fans, II: Effects of number of blades, chord length and camber of blade

Abstract In a previous paper [1], a practical formula which correlates the sound pressure level of turbulent noise originating from an axial flow fan to the design parameters of the impeller was derived analytically by introducing a simple flow model. The validity of this formula is examined in respect to the effects of some of these parameters, such as number of blades and chord length, on the noise level. Experiments are also described on the effect of blade camber, to examine the applicability of the formula adopted for the evaluation of the wake width of the blade in the calculation of theoretical values, because the wake width plays a very important role in estimating the sound power. The agreement between the experimental and theoretical results is good unless separation occurs on the blade surface. Finally, it is shown that the measured sound pressure level data for most of the test impellers fall on a single curve when a dimensionless parameter, named the DT ratio (which represents the ratio of the wake width to the pitch of the blade row), is used as the abscissa.