An Experimental Study of Flow Structures in Small Cooling Fans Using Acoustical Similarity

Abstract The broadband and discrete sources of sound in small propeller and centrifugal type 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 on acoustic similarity laws at each operating condition similar to real engineering applications. The characteristics of fans such as the head rise vs. volumetric flow rate performance were measured using a performance test facility. The sound power spectrum is decomposed into two non-dimensional functions: an acoustic source spectral distribution function F(St, ϕ) and an acoustic system response function G(He, ϕ). Here St, He, and ϕ are the Strouhal number, the Helmholtz number, and the volumetric flow rate coefficient, respectively. The autospectra of radiated noise measured for the fan operating at several volumetric flow rates, ϕ, are analyzed using acoustical similarity. The rotating stall in a small propeller fan with a bell-mouth guide is mainly due to a leading edge separation. It produces a blockage in the passage and reduction in flow rate. The sound power levels were measured with respect to the rotational speed to reveal the mechanisms of stall and/or surge for different loading conditions and geometries, for example, fans installed with an impinging plate. Lee and Meecham (1993) studied the effect of large-scale motions, like impinging normally on a flat plate, using Large-Eddy Simulation (LES) and Lighthill’s analogy. Using cross-correlations of the hot-wire and microphone signals, the dipole and quadrupole sources in the fans tested are shown to be closely related to vortex structures.