Advanced thermal management optimization of a liquid cooled compact electrical engine with air gap winding

"The green revolution in the automotive industry leads to a rising impact of electro-mobility solutions. Next generation electrical engines are characterized through a high specific power density, which is restricted by thermal loads induced by Joule heating in the engines’ windings due to Ohmic resistances. Built-in permanent magnets, sensors and non-metallic materials such as foils, coatings and adhesives are highly limited in their temperature resistance. This makes it a crucial task to increase the cooling efficiency via an optimized liquid cooling concept. A reduction of space, weight as well as a longer lifetime of the engines’ components represent a worthwhile reward. The heat dissipation is restricted by thermal resistances between the winding as heat source, through different layers like thermal interface materials (TIM), the iron core and the cooling channels (CC) as heat sink. This paper treats different approaches regarding an optimized integral liquid cooling concept for an electrical external rotor engine with air-gap winding. Since the approach of optimizing the convective heat transfer is linked to the handling of the turbulence inside the flow, it is based on the trade-off between lower pressure losses along the channels and increased wall-fluid-interaction in the CC near the winding-heads. Here the optimization lies in the adaption of laser additive manufacturing methods. These deliver many degrees of freedom in terms of a redesigned cooling channel inlet/outlet geometry. The overall turbulence can be reduced and be limited to thermally stressed areas requiring an increased convective heat transfer. Newly designed channel-built-in fins deliver further support in terms of heat transfer into the CC and result in a smoothed temperature field inside the stator. Coupled numerical FEM-simulations of conductive and convective heat transport and fluid flow accompanied the additive manufacturing design process of the engine step-by-step. These simulations were validated through active IR-thermographic measurements, allowing to separately evaluate each step with its possible error sources in form of later emerging hot-spots. Previously carried out investigations inside this working group identified the TIM as a main contributor to the overall heat resistance between source and sink. The detection of heat conductivity, heat capacity and layer thickness of said TIMs is important for the numerical model implementation, therefore uncharacterized TIM-adhesive-lamination properties were determined using Laser-Flash-Analysis and Differential-Scanning-Calorimetry. To the knowledge of the authors, a variable that has been hardly investigated so far, is the influence of the material’s surface roughness the TIM is applied to. For getting a better understanding of how the heat resistance of the TIM-metal-compound is affected at different states of surface roughness, a series of experiments were performed supported by design of experiment (DOE). The detected influence of different input parameters on the heat resistance has been considered during following production steps. Finally, the influence of the modified CC geometry on the temperature field and the pressure losses are presented compared to non-modified and conventional milled geometries. The influence of surface roughness on the heat transfer of TIM-compounds is discussed and an outlook on a modified engine design for improved cooling performance is given. "