A Study on Unsteady Flow and Aerodynamic Noise Generation in a Multi-Blade Blower

Abstract

A aerodynamic noise problem in an automotive air-conditioning multi-blade blower was investigated numerically. The preceding measurements of the noise from a multi-blade blower demonstrated that the noise having a peak at 60 and 110 Hz was generated when it was operated at a low flow rate (φ=0.09), whereas the noise broadly ranged from 400 to 800 Hz was detected for a high flow rate (φ=0.22). In order to investigate the aerodynamic mechanism that generates these noises, flow in the multi-blade blower was numerically studied with large eddy simulation. The results showed a large pressure fluctuation at the tongue in the low flow rate. Observing the flow near the tongue, it was found that a re-circulated flow from the scroll terminal collided with specific point on the tongue surface and an excited oscillatory flow was issued periodically. Due to this flow behavior, it was postulated that the noise at 60 and 110 Hz for the low flow rate condition was induced by the flow impingement against the tongue related to the self-excited oscillation. In contrast, for the high flow rate, a prominent pressure fluctuation was found at the tongue and the blade surface of the shroud side. The flow visualization indicated that vortices shed from the trailing edge, interacted with a main stream and collided with the tongue. In addition, vortices in the shear layer which was formed behind the bell-mouth intermittently convected into a passage of the blades. These vertical interactions are considered to cause a large pressure fluctuation at the tongue and between the blades, thereby attributed to be the noise source in the high flow rate condition.