Numerical acoustic modelling of a ventilation unit by 3D FEM and application to the design of an ANC feedforward system

Strong insulation in modern buildings and housing requires efficient ventilation systems. A popular solution is a decentralized system with one ventilation unit per room, but the noise emitted by such a unit then has a major impact on the comfort. In this research, it is intended to equip an existing ventilation unit with an active noise control (ANC) system to reduce its low-frequency noise emissions. The ANC system will be hosted in an additional duct deporting the air inlet aperture approximately one meter away from its original position. The one-channel feedforward ANC system has a reference microphone located between the fan and the anti-noise loudspeaker and an error microphone at the duct’s end. The transfer functions have been computed by a 3D FEM solver, between the fan and several reference and error microphone locations. Several combinations of reference/error microphone positions were then tested regarding the possible active attenuation of a white noise emitted by the primary source. Optimal control by the ANC controller was assumed, the optimal filter being computed with the help of the corresponding transfer functions. Theoretical attenuations were obtained and optimal positions were finally defined for the two microphones of the ANC system. The first results are finally discussed in this paper and raise some questions regarding the influence of the noise spectrum on the attenuations obtained.Silenthalpic (C7711

Numerical modelling and characterization of a heat exchanger

Strong insulation in modern building and housing requires efficient ventilation systems. A popular solution is a decentralized set-up with one ventilation unit per room. The noise emitted by a ventilation unit then has a major impact on the room comfort. In order to reduce the heat transfer to the outside, a ventilation unit is typically designed as a double flux-system with a heat exchanger. This exchanger has a noticeable impact on the acoustic behavior of the ventilation unit. It is therefore of interest to study its effect through numerical simulations. The numerical modelling of a realistic heat exchanger is presented in this paper. The exchanger is placed inside its ventilation casing and modelled using a double equivalent fluid homogenization. Unknown homogenization properties are retrieved in two steps. Acoustic transfer functions are first measured experimentally in order to characterize the propagation paths through the heat exchanger. In a second step, an optimization loop is computed with the numerical model of the heat exchanger. This allows to determine the homogenization properties fitting the measured transfer functions for each frequency. Finally the results for the characterized heat exchanger homogenization model are compared against measurement.Silenthalpic (C7711