Influence of Diffuser on Aerodynamic Noise of a Forward Curved Fan

In order to clarify the influence of a diffuser on the charactersfics of a forward curved fan, the influence of the bare ratio and the outlet angle on the characteristics of the fan were measured through an experiment performed with an actual fan as well as through a numerical simulation, respectively. The mechanism of the discrete fre- quency noise generated by the separated flow of the diffuser was analyzed. The optimized bare ratio was ap- proximately 17%. The flow separated inside of the diffuser generated discrete frequency noise owing to the inter- action between the reversed flow from the diffuser and the impeller rotating at the blade passing frequency. The diffuser outlet angle influenced the pressure ratio more than that by the bare ratio. Furthermore, it was confirmed that restraining the separation in the diffuser effectively decreases the fan noise.