Cooling Air Management For Hybrid Electric Vehicles Using Combined 3D Aerodynamic & Thermodynamic CFD : For External Automotive Aerodynamics
Abstract: This is a master thesis report in vehicle aerodynamics and cooling air management. The thesis is carried out at China Euro Vehicle Technology (CEVT) AB and is part of the course P7010T, Master Thesis in Space Engineering at Luleå University of Technology (LTU). The thesis has been supervised by Mattias Olander at CEVT and Gunnar Hellström at LTU and was done over 20 weeks during the spring semester of 2019. As the vehicle industry moves from mostly using combustion engines to hybrid and electric power systems the importance of decreasing cooling air drag has increased. Cooling air drag can be around 5-15% of the total drag, and a lot of research has been done over the years on how to decrease it. Cooling drag is defined as the force acting in x-direction from the cooling air flowing through the engine bay. The air is let in through the grilles to cool down the engine and escapes through different outtakes usually below the vehicle and through the wheelhouse. The air loses a lot of energy inside the engine bay as well as it changes in temperature. In this study a method has been developed to include the energy equation in the aerodynamic computational fluid dynamics (CFD) simulation. Research has also been done on which design parameters that affect the cooling air drag and how air ducts could be designed to better transport the flow to and from the radiators without loosing to much in energy. In the first part of this study a method was developed to solve the vehicle aerodynamics with energy equation included. All method development and design parameter tests weredone on the Sport Utility Vehicle (SUV) A model, which is a CEVT concept car. The model was first implemented on a simple symmetric model and then on a full model. It was tested both with a normal steady state solution and a pseudo transient solutions. The pseudo transient solution proved to solve for a faster convergence, although both methods worked well. Therefore the design parameter testing was chosen to be done with the pseudo transient solver.The design parameter testing was done in two steps, first opening and closing different outlets and then trying to implement different cooling air ducts. The first study showed that air through the wheelhouses increases the drag as well as having air entering only inthe upper grille and travelling down through floor and wheelhouse. In the second study,the area between the grilles and cooling package was sealed and inlet ducts were created to control the flow from the grilles to the cooling package. When just adding inlet ducts, the mass flow through the grilles was decreased, but the mass flow through the cooling package was increased due to less separation of the air, which lead to a drag reduction of 0.2%. Other design implementations was to reshape the wheelhouse outlets, therefore a wheelhouse outlet duct was designed. The ducts purpose was to lead the air out of the wheelhouse and behind the tire and exit the vehicle parallel to the free stream flow. The wheelhouse duct is most effective and decreases the drag force by 0.7%. An air duct was also designed to lead the flow after the cooling package fan to the outlets. The ducts purpose was to prevent the air from loosing in energy when rising to the roof of the engine bay, the duct compresses the air and leads it efficiently over the engine with a drag force decrease on 0.3%. The inlet duct, wheelhouse duct and after fan duct was all put together to a thesis design. Due to the higher mass flow through the cooling package the upper grille could be sealed by 9.9% and still allow the same mass flow through the cooling package as for the SUV A. The thesis design for improved cooling air management allowed a decrease of drag force timesarea by 0.9%. In conclusion there is much that can be done to improve the cooling air drag. It is most favorable to have a sealed volume with inlet air ducts before the cooling package, have outtakes aligned with the free stream flow, minimize cooling air to escape through the wheelhouse outlets and to minimize the height of the engine bay as much as possible.
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