Acoustics comfort is a key point for the ground transportation market and in particular in the automotive area. A significant contributor to the noise levels in the cabin in the range 200Hz to 3000Hz is the HVAC (Heating, Ventilating, and Air Conditioning) system, consisting of sub-systems such as the air intake duct, thermal mixing unit, blower, ducts, and outlet vents. The noise produced by an HVAC system is mainly due to aeroacoustics mechanisms related to the flow fluctuations induced by the blower rotation. The structure borne noise related to the surface induced vibrations and to the noise transmission through the dash or plastic panels may also contribute but is not considered in this study.
This study presents a digital approach for HVAC aeroacoustics noise predictions related to the ducts and outlet vents. In order to validate the numerical method flow and acoustics measurements are performed on production HVAC systems placed in an anechoic room. The flow is generated using a fan located outside the room and a muffler is used to create a silent incoming flow. The dependency between the volume flow rate and the noise levels is investigated together with the effect of the outlet vents on the aeroacoustics mechanisms. The aeroacoustics simulations are performed using a time explicit, unsteady and compressible method in which flow and acoustics are calculated at the same time. The numerical results are compared to the experimental ones and certain analyses, not easily accessible through measurements, are provided. In particular, duct mode behavior and the influence of the outlet vents on the acoustic radiation are highlighted. The numerical predictions are shown to correlate reasonably well to the test measurements, and the developed method can be used during the vehicle development process to evaluate and optimize the aeroacoustics performance of the HVAC system.