Characterization and control of small axial-flow fan noise

Abstract

This thesis analyzes both aerodynamic and acoustic characteristics of small axial-flow cooling fan and proposes methods for noise abatement. The work starts with fan sound scaling law which is expressed as ("W" _"b" ) ̃" = " ("c" _"b" ) ̃" × " "M" ^"n" where ("W" _"b" ) ̃ denotes the dimensionless broadband noise power and M is the Mach number based on tip velocity. This scaling law is achieved based on the analysis of numerous noise data measured at different working conditions. Both n and ("c" _"b" ) ̃ changes with different system resistance coefficients ("c" _"si" ) ̃. In terms of a standard single fan without stator vanes, n is between 5 and 6 before stall inception while ("c" _"b" ) ̃ is almost proportional with ("c" _"si" ) ̃. The value of n goes beyond 6 after stall and simultaneously ("c" _"b" ) ̃ suffers from a significant rise. Fan scaling law implies that the most effective noise abatement method is decreasing fan speed and the fan with more stages might radiate lower noise at identical operation point. The latter is confirmed by comparing the noise from a single with a 2-stage fan, and again between 2- and 3-stage fans. Contra-rotating fan is observed to radiate lower broadband noise than the similar-sized two-stage fan because of its lower speed. Improving the tip leakage flow is a concrete method to enhance fan aerodynamic performance so that its speed can be reduced. Reduction of tip clearance is experimentally proved to be beneficial to fan’s performance. In addition, two other methods for reduction of noise due to fan tip leakage flow are offered: casing treatment and blade modification. For casing treatment, porous casing is found to outperform casing grooves; it postpones stall inception and reduces pressure loss in the stall region. For a single fan, as much as 10 dBA noise reduction is experimentally achieved with porous casing in the otherwise stalled flow region. Numerical simulation using CFX for the three dimensional, transient flow reveals that both the total pressure loss and the strength of the tip vortex decrease, so that the blockage due to tip leakage flow is alleviated. The porous boundary keeps the generation of casing-boundary-layer-separation vortex and there is momentum transport through the fluid-porous interface, which leads to a weaker tip leakage vortex. For blade modification, end-plate and end-bending are found to be ineffective for small axial fan. For the reduction of interaction noise, screens are experimentally proved to reduce the sharp unsteady lift response on the blade caused by the upstream velocity deficit. Theoretical work combines the indicial response theory and the influence of the screen on the non-uniform flow, leading to the conclusion that there is an acoustically optimal flow resistance coefficient K = 2.4 through the screen. Finally, the screen is sandwiched between two stages in a newly designed C-R fan with stator vanes at its outlet, and porous casing is also applied. Compared with a benchmark two-stage fan, a 3 dBA total noise reduction is obtained for the medium and high loading operation points.