Aeroacoustic Measurement of Automotive HVAC In-Duct Elements

A1 - CEVAS Project (HVAC of cars)

An HVAC is a compact system composed of moving (blower) and fixed (flap, thermal exchangers…) elements designed to provide thermal comfort inside the vehicle. These in-duct elements which interact with air flow generate noise inside car cabin.
Assuming that the aeroacoustic properties of all these elements can be modelized by 2N-ports linear systems, the UTC (Université de Technologie de Compiègne) has developed a test bench to perform the measurements of these properties. The facility satisfies the Valeo Thermal Systems’ requirements in terms of dimensions, duct geometries, airflow rate and frequency range. In addition, aeraulic measurements are conducted using a laser based method: Particle Image Velocimetry (PIV).
The 2N-ports model describes an induct element by its passive and active acoustic properties assuming N propagative modes. The acoustic passive property is represented by its multimodal scattering matrix (reflection and transmission coefficients) and is first estimated with a 2N-source method. The acoustic active property is represented by its N-component source vector and is secondly measured. Then, based on the Nelson & Morphey’s theory for low Mach numbers (M<0.1), the coefficient depending on the Strouhal number, K(St) which characterizes the aeroacoustic source is deduced from the measurements of source vector and element’s pressure loss. The PIV aeraulic measurement method provides mean velocity field around element but also RMS velocity which describes airflow’s turbulence state.
Under CEVAS project, the measurement methods described above are applied to academic elements (diaphragm and separately close diaphragms). For automotive HVAC components (flap, flap + small barrier and blower), the studies are performed in vehicle conditions of airflow rate. The measurement results are used under CEVAS project as input data for acoustic synthesis studies and as numerical simulations validation.
In this paper, the duct flow facility is described and the experimental methods are presented. Then, the acoustic and aeraulic properties of in-duct elements are defined. Finally, the results of the measurements carried-out with academic and industrial elements are presented and analyzed.