Analysis of vortical flow field related to aero-acoustic sound in an air conditioning system by wall pressure measurement and CFD (2^nd Report, blade passing frequency in a blower)

Abstract

This paper presents an elucidation of three-dimensional flow phenomena that induce an aero-acoustic sound of Blade passing frequency (BPF). Three-dimensional separated and vortical flow fields in an air conditioning system have been investigated by Experimental fluid dynamics (EFD) analysis using an unsteady wall pressure measurement system and by Computational fluid dynamics (CFD) analysis using a Reynolds-averaged Navier-Stokes (RANS) simulation as described in the first report of the present study. Generally, the aero-acoustic sound of BPF is one of the main aero-acoustic sounds in air conditioning systems. Based on the EFD analysis, the wall pressure fluctuation of BPF is found to be not only located near the scroll tongue region but also widely located around the scroll casing wall. It was found that there is no correlation between the sensor positions in the circumferential direction and the pressure fluctuation level of BPF. Based on the CFD analysis results, it was found that an attachment line on the center of the scroll casing wall develops by interacting with secondary flow from the multi-blade fan. A pair of secondary vortex structures in the scroll casing longitudinally develops in the circumferential direction. As the pair of secondary flow structures with high vorticity develops in the circumferential direction and interacts with the scroll casing, wall pressure fluctuation of BPF is generated even though it is located far away from the scroll tongue.