A Thesis Submitted to the School of Graduate Studies in Partial Fulfillment of the Requirements for the Degree of Master of Applied Science in Mechanical Engineering / In the wake of the rising global demand for more electric transportation, aerospace electrification is becoming a highly active research area as commercial fully electric aircrafts are becoming a reality. The transportation electrification industry is challenged to develop powerful, safe, and compact-sized machines that can replace fossil fuel powered engines in aircrafts. Axial Flux Permanent Magnets (AFPM) machines are currently being intensively developed as a great candidate for this purpose due to their inherently higher power density compared to other machine electric machines topologies. The efforts of further increasing AFPM machines power density add more thermal challenges as intensive cooling is required at a relatively small machine package to avoid machine failure. One of the most concerning failure modes in these machines is power output reduction due to overheating of the rotor-mounted permanent magnets or even complete failure due to irreversible demagnetization. This research discusses the design process of an integrated rotor air cooling system for a 100 kW AFPM machine designed for an electric aircraft propulsion system. The embedded cooling system allows the rotor to be self-cooled at a sufficient cooling rate while minimizing the impact on machine efficiency due to windage power losses. The presented design process includes several stages of cooling enhancement including the addition and fine-tuning of rotor fan blades and rotor vents design. These enhancements are done by studying the air flow over the rotor surfaces in conjunction with heat transfer through Conjugate Heat Transfer (CHT) Computational Fluid Dynamics (CFD) analyses. In an initial study, different rotors with different combinations of rotor cooling features are studied and their thermal performance is compared. The results show that using rotor embedded fan blades in throughflow ventilated rotor geometry offers the best performance balance, achieving sufficient rotor cooling rate within a reasonable increase of windage power loss. A parametric study is performed to improve the rotor blade geometry for a higher ratio of heat transfer to windage losses. Another study is performed where the rotor and the enclosure geometries are fine-tuned simultaneously to reduce the negative effect on rotor heat transfer imposed by the enclosure. The final geometry of the rotor enclosure assembly is generated based on the research results and the design is integrated into the final machine prototype to be tested. / Thesis / Master of Applied Science (MASc) / Axial-flux permanent magnets (AFPM) machines are gaining the transportation electrification industy attention as a greener alternative to combustion engines in aircraft propulsion systems due to their high power and torque density. The intense endeavors of the current research to further improve AFPM machines power densities brings thermal design challenges to ensure the safe operation of the machine. Rotor permanent magnets failure due to demagnetization as a result of overheating can impose a great risk to the machine operation and safety. Accordingly, special attention should be paid to rotor thermal management. This research discusses the design process of an integrated rotor air cooling system for an AFPM machine designed for an electric aircraft. The machine mechanical and thermal design parameters are used to set an initial rotor design with different rotor cooling features based on literature findings. Rotor fan blades and air vents are selected as the main rotor cooling features for the design. Several design iterations are then made to fine-tune the rotor geometry targeting low operating temperature of the permanent magnets at a low cost of windage losses. The thermal performance of the different designs is assessed and compared to each other using conjugate heat transfer (CHT) computational fluid dynamics (CFD) analyses. Safe operating temperature of the magnets is achieved at an acceptable windage losses value with the final design, and it is selected for prototyping.

The thesis work described in this report is about simulation of the cooling of an electrical machine rotor. Limitations and simplifications were made on the CAD model of the rotor with the purpose of reducing the simulation time, for it to then be used for CFD-simulations using STAR-CCM+. This was done to see the temperature, as well as its distribution, in the model. By changing various parameters, one at the time whilst the rest were kept at their assigned standard values, the changes could be analysed and thereafter compared. The tests include smaller geometry changes, parameters of the coolant and its flow, parameters for the airgap and the materials in the laminates and the material around the magnets, as well as changes in loss values. The simulations for geometry changes involving the magnets and their surrounding material resulted in minor temperature increases. An inner rotor radius increase gave a relatively large temperature decrease (although this change would be more difficult to make in practice). Most of the mean values of the temperature changes in the regions of the model were within 10% from the standard simulation used. Increased thermal contact resistance between the Bakelite and the laminates, and increased losses had the worst impact on the cooling. Meanwhile the changes in coolant parameters (as well as the its inlet temperature and mass flow) and reduced losses had the best impact on the cooling. Generally, the temperature distributions looked similar for the different simulations. There were more differences in the distributions for the simulations with changed material properties or thermal contact resistance. / Examensarbetet som beskrivs i denna rapport handlar om simulering av kylningen av en elmaskinsrotor. Avgränsningar och vissa förenklingar gjordes på en CAD modell av rotorn för att reducera simuleringstiden. Sedan användes CFD-simuleringsprogrammet STAR-CCM+ för att simulera temperaturfördelningen i modellen. Genom att ändra olika parametrar, en åt gången medan resten hölls vid sina bestämda standardvärden, kunde förändringarna undersökas och sedan jämföras. Det som testades inkluderar bland annat mindre geometriförändringar, parametrar för kyloljan och dess flöde, parametrar för luftgapet och materialen i laminaten och runt permanentmagneterna, samt förlustförändringar. Geometriförändringarna som gjordes för magneterna och det omgivande materialet gav små temperaturökningar. En ökad inre radie på rotorn gav relativt stor temperaturminskning (men denna ändring vore svårare att genomföra på rotorn i praktiken). Majoriteten av medelvärdena av modellregionernas temperaturförändringar var inom 10% från standardsimuleringen som användes. Ökat termiskt kontaktmotstånd mellan Bakeliten och laminaten samt ökade förluster hade sämst påverkan för kylningen, medan förändring i kylvätskans egenskaperna (samt inflödestemperaturen och massflödet) samt minskade förluster hade bäst inverkan. Generellt så såg temperaturfördelningarna lika ut för de olika simuleringarna. Det var främst när materialegenskaperna eller det termiska kontaktmotståndet ändrades som fördelningen såg annorlunda ut.