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.