Analysis of vortical flow field related to aero-acoustic sound in an air conditioning system by wall pressure measurement and CFD (1^st report, resonance in a duct)

Abstract

This paper elucidates three-dimensional flow phenomena that induce aero-acoustic noise at certain frequencies. Three-dimensional separated and vortical flow fields in an air conditioning system are investigated by Experimental fluid dynamics (EFD) analysis using an unsteady wall pressure measurement system with 30 high-respond pressure transducers and by Computational fluid dynamics (CFD) analysis using a Reynolds-averaged Navier-Stokes (RANS) simulation with a k-ε turbulence model. The EFD investigation revealed that the regions with high wall pressure fluctuation are located near the scroll tongue and on the bottom of the diffuser casing wall. A wall pressure fluctuation of 0.056 Blade passing frequency (BPF) was observed at sensor A near the scroll tongue, and a wall pressure fluctuation of 0.173 BPF was observed on part of the evaporator casing wall. The CFD investigation revealed that the reverse flow is generated by interaction between the scroll tongue and multi-blade fan. The reverse flow develops strongly over a wide range in the multi-blade fan near the scroll tongue. Furthermore, there is also a longitudinally separated vortex near the bottom of the diffuser casing. The reverse flow and longitudinally separated vortex interact with the wall casing. For this reason, the scroll tongue and the bottom of the diffuser casing had high pressure fluctuations in the EFD investigation. By considering aero-acoustic factors, an aerodynamic sound of 0.056 BPF can be explained by Helmholtz resonance phenomenon, which is generated by high pressure fluctuation due to the reverse flow near the scroll tongue. An aerodynamic sound of 0.173 BPF can be explained by the standing wave phenomenon, which is generated by high pressure fluctuation due to the longitudinally separated vortex on the bottom of the diffuser casing wall.