Une possibilité de prise en considération de critères acoustiques pour le calcul aérodynamique d'un ventilateur axial

A possibility of taking acoustic criteria into consideration in the aerodynamic calculation for an axial fan

Docteur de troisième cycle ingénieur Société METRAFLU

Abstract

The use of axial flow fan is more suitable than that of centrifugal blower, as soon as large mass flow with low pressure fise must be achieved. Nethertheless, the noise level of the first type of these machines is 10 to 20 dB higher at the same operation point. The Metraflu and Neu companies have join their efforts to developp a theoretical study [1] and the build up of a quiet axial fan. The design technique is based on the numerical resolution of the fIuid equilibrium equations. The obtained velocity triangles are used to define the blade shape [6]. The contribution of stationary and unstationary forces is examined. Unstationary forces are analysed with the hypothesis of a quasi-stationary aspect of the problem. The noise component due to stationary forces can be explained by the formulations of page 616. It can be seen that noise reduction can be obtained with a reduction of the terms ΩT, ΩD. As pressure rise is defined, the term ΩD is constant. Only the ΩT term could contribute to the noise reduction. ΩT ~ UΔPS ~ ΔPS ΔPT/ΔVu ΔPT = ΔPS so ΩT ~ Cst /ΔVu The noise reduction could be obtained if important deflections are imposed (associated with low rotation speed). The noise level of a new fan built with this concept is 5 dB lower than that of a reference fan known to be already quiet.