Aerodynamic Design and Numerical Simulation Analysis of a Passenger Car’s Defrosting Duct

Many cars are equipped with the same Heating, Ventilation and Air Conditioning (HVAC) system with different outlet flow rates, shapes, sizes and positions etc. according to modern automobile modular and platform design strategies. Consequently, the aerodynamic design of airflow duct has become an important issue of the automobile HVAC system. In this research, the HVAC defrosting airflow canal of a passenger car was designed using Computational Fluid Dynamics (CFD) method. The HVAC defrosting outlets’ sizes and positions were considered as design variables. The mass flow rate distribution and the cabin flow structure were considered as design targets. The steady Reynolds Averaged Navier–Stokes (RANS) method was adopted because it is an efficient way to help select possible good designs in the early stage of new product development without time consuming and huge computational cost of the unsteady methods. Various duct design configurations were evaluated by analyzing the defrosting mass flow distribution of each flow outlet and by visualizing the flow structure near the windshield and the front left side window. The CFD results showed that the total area of the outlets near the rain wipers was a decisive parameter for mass flow distribution in this duct design. The defrosting flow structure near side windows were difficult to be improved only by enlarging the area of the outlet. The effective flow structure was realized by choosing proper angles of the vanes skirt of the outlets to defrost the windshield region. The overall performance of HVAC system, such as defrosting time and cabin temperature distribution, could not only be predicted based on the results of the steady RANS method. It was shown that the important parameters including the mass flow distribution of each outlets and the flow structure near the windshield and side windows could be quickly evaluated from the steady state CFD simulation results in the early design stage.