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Automotive: HVAC System Performance

Evaluating Your HVAC System Through Simulation

HVAC units and distribution systems are an integral part of the cabin climate control function. To ensure the comfort of the occupants, i.e. drivers and the passengers, the right amount of conditioned air at desirable temperature and humidity levels should be delivered to the target locations. Adequate flow delivery is also important for the safe operation of a vehicle that requires proper demist and defrost capabilities.

At the same time, the energy required for the flow delivery should be minimized for better fuel economy, while the adverse effects of the associated acoustic noises should be limited. To achieve these goals, the designs of the ducts and the registers as part of climate control system are carefully evaluated and optimized for greatly varying ambient conditions as part of vehicle development process.


HVAC unit Geometry. Image courtesy of Denso

 


Drawing showing inlet and multiple outlet ducts. Image courtesy of Denso

  

Automotive HVAC System Evaluation
Interior of the HVAC unit showing the thermal mixing. Image courtesy of Denso

TECHNICAL CHALLENGES

Due to the tight packaging in vehicle interiors, the available physical space for HVAC units and distribution systems is very limited. Designers of climate control systems are often required to work around the geometrical restrictions imposed by other vehicle interior components, often without up-front understanding of the impact of potential design changes on the system performance.

In addition, complex physics such as flow turbulence, thermal mixing and radiation play important roles in determining air flow distribution, fan power requirement, and air temperature stratification inside a vehicle cabin. However visualization and measurement of such physical effects are difficult in real vehicle applications.

System optimization in a climatic wind tunnel or through road testing for widely varying ambient conditions requires significant time and effort, particularly when some of the testing has to be conducted for transient conditions. Besides, the evaluation matrix tends to be large due to various operating modes (defrosting/ventilation/bi-level/footwell) of an HVAC system.

EXA SOLUTION

Exa provides an accurate way to predict the flow and the thermal characteristics involved in HVAC units and distribution systems that often have very complex geometries. These include turbulent flow effects and thermal mixing as well as comprehensive heat transfer thru all three modes-conduction, convection, and radiation.

For design evaluation, the pressure drop and flow splits in the distribution systems and the air flow patterns through multiple outlets can be analyzed. In case of thermal analysis, air temperature distribution can be also calculated, including the effect of the heat transfer between the HVAC system components and its neighboring components.  In addition to flow and thermal analyses, aeroacoustic noise analysis involving fans and flows thru the distribution systems such as ducts and registers can be performed as well. (Details of this topic can be found under  the HVAC System and Fan Noise application section.)

The insights and knowledge obtained from the analysis can be applied efficiently in order to optimize the HVAC systems. Since the volume mesh generation is fully automatic in PowerFLOW process, multiple geometric variations (such as different mixing flap positions in a duct simulation) can be evaluated with minimal user efforts by simply exchanging the surface geometry of the relevant part in the case setup.

Features and benefits of Exa’s solution for HVAC units and distribution systems:

  1. PowerFLOW is capable to capture the flow changes due to small differences in geometries, leading to meaningful design evaluations for better flow distributions.
  2. Rotating geometries such as fans and blowers are simulated with either sliding mesh or a multiple reference frame (MRF).
  3. Thermal analysis including all three heat transfer modes: convection, conduction and radiation.
  4. Fully automated two-way coupling process between PowerFLOW and PowerTHERM available in case of comprehensive heat transfer analsys.
  5. When radiation effect is not important, PowerFLOW only option for thermal analysis combined with proper wall boundary conditions (thermal resistance B.C. available) as a lower cost alternative.

EXA SOFTWARE USED FOR THIS APPLICATION

Simulation Preparation: 
Simulation: 
Results Analysis: 

TECHNICAL REFERENCES