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Commercial Vehicles: HVAC System and Blower Noise

Blowing Air Without Blowing Noise Targets

Commercial Vehicle HVAC System
3D isosurfaces of vortex cores colored by velocity magnitude, representing noise sources throughout a vehicle HVAC system. Mannequin and window surfaces colored by instantaneous static pressure.

As commercial vehicle manufacturers seek to reduce the noise levels experienced by passengers, the noise due to the heating, ventilation, and air conditioning (HVAC) system becomes a target for improved acoustic performance. The HVAC system is complex, consisting of a blower and mixing unit coupled to many ducts through which air is transported to various locations, including faces and feet of front and rear passengers, as well as windshield and side glass defrost. The blower must supply sufficient pressure head to achieve desired air flow rates for each thermal comfort setting. Noise is generated due to the blower rotation, and by the turbulent air flow in the mixing unit, through the twists and turns of the ducts, and exiting the registers (ventilation outlets). When designing an HVAC system it is difficult to predict whether noise targets will be met, and to achieve multi-disciplinary performance optimization. Compromises between flow, thermal, and acoustic performance can result in late design changes or degraded passenger comfort. The effects of integrating the HVAC system into the commercial vehicle, which changes the performance relative to the test bench, also may result in degraded performance, driving late design changes.

Commercial Vehicle HVAC Noise Reduction
Instantaneous flow induced by the true rotation of a typical HVAC centrifugal blower (3000 rotations per minute).

  

Commercial Vehicle HVAC Noise Analysis
3D streamlines colored by velocity magnitude through a complete HVAC system including cabin.

  

Commercial Vehicle HVAC Noise Analysis
Sound Pressure Levels radiated by the HVAC ducts and registers at an equivalent driver's ear location: (red) Semi-anechoic room measurements; (blue) PowerFLOW direct predictions;(gray) Facility background noise

TECHNICAL CHALLENGES

Commercial HVAC system noise heard by passengers involves various sources and paths. The blower is a radial fan which generates noise from the interaction of the moving blades with the surrounding air, and the impact of the moving air on nearby static components. This fan noise is acoustically propagated through the complex network of ducts, out of the registers, and into the cabin. The duct and mixing unit flow noise sources are generated mainly by flow separations and vortices resulting from airflow past the detailed geometric features, and are also acoustically propagated through the system. Noise due to the flow exiting the registers depends on the fine details of the grill and its orientation, and the resulting outlet jets which mix with the ambient air and may impact a solid surface like the windshield (e.g. for defrost). Therefore the requirements for the numerical flow-acoustic prediction are challenging and include handling of very complex geometry, prediction of the fan and flow induced noise sources, and their acoustic propagation all the way through the system to the locations of the passengers. Accurate prediction of fan noise has been a key challenge and unsolved problem in the field of aeroacoustics.

EXA SOLUTION

Exa®’s PowerFLOW® software is the only solution to provide accurate numerical noise prediction for fully detailed automotive HVAC systems due to its unique technology that provides accurate prediction of the complex flow structures, corresponding noise sources, and resulting propagated acoustics to the passenger head space locations, including effects of geometric details throughout the integrated system. The transient flow characteristics and acoustics are fully simulated directly within the PowerFLOW solver, including the rotating fan flow and noise using the new True Rotating Geometry capability, as well as direct prediction of acoustic propagation throughout the system without the need to couple to another solver. PowerFLOW enables HVAC engineers to obtain early noise assessment of proposed designs and evaluate potential design options, and/or diagnose and improve noise problems on an existing design. Easy to use analysis and visualization capabilities (with PowerACOUSTICS® and PowerVIZ®) allow identification and insight into sources of noise, including band-filtered pressure analyses to isolate phenomena at specific frequency bands of interest. Predicted spectra at passenger locations can be converted to audio files for comparative listening to the effects of various design options. Complex geometry handling and efficient post-processing enable rapid turnaround time for assessment and design improvement. PowerFLOW also provides accurate HVAC system pressures, flow rates, and thermal mixing behavior—hence it can be used to assess multi-disciplinary design tradeoffs to design the HVAC system with optimal aero, thermal, and acoustic performance using a single model.

EXA SOFTWARE USED FOR THIS APPLICATION

Simulation Preparation: 
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