Noise source analysis for two identical small axial-flow fans in series under operating condition

The noise signature of two identical small axial-flow cooling fans in series, under the operating condition of 120 Pa back pressure to simulate its usual working condition, is analyzed and the BPF analysis method is used to decompose the total noise into various noise source components. Acoustic directivity measurements are conducted for two different inlet flow conditions: free/unobstructed inlet and partial admission by several kinds of flat plates all blocking 30% inlet area. The inlet obstacle increases the broadband noise and the tonal noise of the upstream fan while it has little effect on the tonal noise of the downstream fan. In addition, the tonal component from the downstream fan dominates over that from the upstream fan for unobstructed inlet while the dominance by the downstream fan is reversed for partial admission. Furthermore, the aerodynamic and acoustic effects of a flow straightener placed between the two fans and in front of the upstream fan in an attempt to relieve the adverse effect of the inlet flow distortion are studied as well. The flow straightener between the two fans plays an important role in straightening the non-axial flows from the upstream rotor before impinging upon the downstream rotor. The broadband noise in the case with partial admission is reduced by 3.5 dB by the flow straightener in front of the upstream fan although the tonal noise radiated by the upstream fan is not relieved effectively.