Humming Noise Mechanism in Automotive Blower
2.7 Installation Effects
Automotive blowers for HVACs units are highly constrained ventilation devices, mainly for the purpose of the integration in the dashboard which require a good compactness. As a consequence, many efforts have been put in the reduction of both the wheel diameter and the scroll width, and the machines in production represent now a good compromise between aerodynamic efficiency, acoustics and packaging.
However, some new trends related to the cabin comfort are bringing an additional pressure loading on the blower, which has to force the air through a series of heat exchangers (heating and cooling), air filters and flaps aimed to redirect the flow. In some circumstance, a spurious additional humming noise is observed in the system and it has been named by product engineers as a “vuvuzela” noise. This particular phenomenon is quite noisy and unpleasant and can be easily identified in the acoustic spectrum with the evidence of wide peaks at the wheel rotational frequency and its harmonics.
Some acoustic tests have been conducted on a test rig at different flow rates and for two different geometries. It shows that the humming noise appears when the wheel is highly loaded, and when interactions between the wheel and the scroll yield to a large amount of recirculation.
Several unsteady simulations have been performed for the same conditions and have allowed a deeper analysis of the flow. They highlight the effect of the recirculation at the top of the wheel, which brings some vortical structures at the wheel entrance and which impacts the blade leading edges. Simulations show the appearance of these vortexes which are spinning at low velocity inside the wheel, and which are creating quasi periodic pressure fluctuations on wheel and scroll surfaces. Additional pressure recordings at several locations confirm the relationship between the wide peaks and the vortex distributions.
These analyses have allowed to identify the phenomena and to bring some technical solutions. This method will be used also to drive the design of the next generation of blowers in order to remove the occurrence of this humming noise mechanism.