The cross-correlations of fluctuating lift with radiated sound in cooling fans

초록

The broadband and discrete sources of sound in small propeller cooling fans were investigated to understand the turbulent vortex structures from multi-bladed fans using an ANSI test plenum for small air-moving devices (AMDs). The noise measurement method uses the plenum as a test apparatus to determine the acoustic source spectral density function based in acoustic similarity laws at each operating conditions similar to real engineering applications. Small propeller fans showed a hysteresis in their performance characteristics reflecting rotating instability. Source spectra of propeller fan without a plate showed strong peaks centered at about 0.7, 1.4 times blade passage frequency. This from of rotating instability dominated the low frequency noise production, but with a decrease in normalized sound pressure level due to a sharp drop in performance. Propeller fan with an impinging plate had broadband source characteristics without any hysteresis in its performance. The sound power levels with respect to the rotational speeds were measured to reveal the mechanisms of stall and/or surge for different loading conditions and geometries, for example, fans installed with an impinging plate. The fan noise mostly comes from dipole sources closely related to the vortex structures involved. The dominant time scale and r.m.s. velocity scale for fluctuating lift forces of dipole source type are strongly dependent upon organized vortex structures in each mode and affect the values of exponents in sound power law. The rotating stall in the small propeller fan with a bellmouth guided can be explained by an azimuthal mode of unsteadiness due to leading edge separation in the pressure side interacting in phase with organized tip vortices in the suction side of the blade upstream. It creates a blockage in the passage and the reduction in the flow rate. Cross-correlation techniques can be used to distinguish among the various forms of rotating stall against unwanted noise. Correlat