Effect of Inflow Turbulence on Noise in Low Speed Centrifugal Fans - A Frequency Domain Approach

摘要

High axial velocities and low radial velocities at the inlet section of the impeller have been found to cause a separation region over approximately one-third of the impeller width. TIx high turbulent intensities in this region are a strong function of the axial distance into the inlet and circumferential position. The two fluctuating velocity components a’ and v’ are always partially correlated but the extent of coherency varies with location. This partial conelation indicates that the two components are interacting and collectively form a complex source region. The broadband energy levels in the velocity power slwctra are higher in the separation region than regions further into the inlet, by two orders of magnitude, showing higher levels of turbulent energy. The partial coherence functions between the fluctuating velocity components and SPL have been found to be very low at all the locations due to the interactions of the turbulent fluctuations with one blade passage not being correlated with the interactions of other blade passages. In fact, the radiated SPL is a cumulative effect of turbulence in many such passages. Farther, low coherency is also doe to the fact that the separation region is a complex, interactive source region instead of several independent warces of noise. The coherent input power spectrum calculated at a particular location in the fan inlet yields a measure. of the energy in the velocity fluctuations that affects the SPL. Therefore, locations having higher turbulence arc shown to be sources of broadband noise. These results imply that the broadband noise could be reduced If the size and strength of the separation region could be reduced, the turbulence in the inlet decreased, and the flow through the scroll and cat-off made more uniform. It was, therefore, logical to consider the use of a suitable flow turning device or an inlet guide vane (IGV) to direct the flow smoothly through the impeller blades. The performance of the IGV has been evaluated in another paper by Vadari et al. 161.