Experimental and numerical investigations of underhood flow for vehicle thermal management

Doctoral thesis, 2021

Electrified vehicles are an essential part in reducing emissions from the transportation sector. Their development comes with a variety of new challenges. Most of them revolve around the energy efficiency of the vehicles as it directly relates to the range that can be covered with one charge. In the area of vehicle thermal management, this means that the focus shifts from a pure cooling perspective towards providing a thermal environment in which the electric power components can operate most efficiently. As the required amount of cooling air through the front is lower than in conventional combustion vehicles, an additional benefit can be gained from smaller front openings that reduce the aerodynamic drag. However, this requires a deeper knowledge of the flow physics in the underhood environment in order to utilise the available cooling air efficiently.

Computational Fluid Dynamics (CFD) is an important tool for the investigation of the underhood flow, since it gives the possibility to look at the flow field even in areas where measurement equipment cannot reach. In the first part of this work, the focus is on the simulation of the axial cooling fan. Different methods to simulate an axial cooling fan were compared to each other and to experimental data that was acquired using Laser-Doppler-Anemometry. The commonly used Multiple Reference Frame approach was shown not to be suitable for investigating underhood flow, as the stationary blades leave an imprint on the wake flow. In addition, inhomogeneous temperature distributions experienced an unphysical rotation due to the switch in reference frame. These issues do not occur with the Rigid Body Motion approach, and the simulation results compared well to the measurements.

In the second part of this project, it was investigated how the flow downstream of a fan is affected by different components placed up- and downstream. A simplified underhood rig was designed and constructed to provide a controlled, vehicle-like environment for the measurements and simulations. Two front designs, representative of a hybrid and a battery electric vehicle, were utilised and it could be shown that the upper grille opening that is missing in the battery electric vehicle configuration has a visible impact on the flow field downstream of the fan. Simulating the same configurations in CFD showed some differences to the experimental data. For a second cooling fan, the results were well matched.