## 4 Thermal Design Investigations with the Validated CFD Model
### Effects of Spacing between Windings
Here, convective heat transfer emerges as the primary differential in cooling performance. However, maintaining the same coil MMF necessitates an increase in the current density as the copper volume diminishes, leading to a rapid increase in energy loss density as the channel widens, which counteracts the benefits of the improved convective heat transfer. The optimal channel width identified in this case is 1.5 mm, yielding a 33.7 K temperature rise of the hotspot, marking a 30% and 10.4% improvement over the original design with and without slot liners, respectively. When the channel width extends beyond 1.5 mm, the temperature rises significantly due to the escalating loss density. Clearly, the first approach, maintaining the SFF while increasing the CFF, results in lower temperature rises for all channel widths. It is important to note, though, that practically increasing the CFF while maintaining the SFF (Concept 1) poses technical challenges, whereas decreasing the SFF while keeping the CFF constant (Concept 2) is more straightforward. Notably, effective cooling does not necessitate wide channels between coils. A 1.5 mm channel width, accounting for approximately 14% of the slot width, suffices for maximal effect in both scenarios. For research purposes, the first concept, which shows superior performance, is chosen for further investigation. Figure 12 presents the variation in area-weighted average surface heat transfer coefficients with increasing channel widths. The removal of the slot liners between the windings resulted in a mean surface heat transfer coefficient of 52 W/m\({}^{2}\)/K, which rose to approximately 77 W/m\({}^{2}\)/K when the air channel width between the windings increased to 1.5 mm. No significant change can be observed when the channel width is beyond 1.5 mm, consistent with the variation in the hotspot temperature rise. Further expanding the air channel beyond this point is not recommended, as there is no significant thermal improvement. Figure 12: Area-weighted average surface heat transfer coefficients varying with increasing air channel widths. The distribution of the surface heat transfer coefficients is presented in Figure 13, with the typical winding-winding channel widths ranging from 0.5 mm to 1.5 mm. It can be observed that, in terms of the 0.5 mm channel width, the winding surface towards the downwind-side end windings presents much worse heat transfer coefficients than that near the upwind-side end windings. The highest heat transfer coefficient at the upwind side is more than 100 W/m\({}^{2}\)/K, while the lowest near the downwind side is less than 10 W/m\({}^{2}\)/K. This is because the air slows down as it flows axially down the narrow 0.5 mm channel. The improvement is significant when increasing the winding-winding channel width to 1.0 mm and 1.5 mm. Generally, the lower part of the winding has a better heat transfer coefficient than the higher part, due to the airflow adjacent to the stator tooth tip. The turbulence intensity has a significant impact on cooling performance, as presented in Section 4.2. At the wind speed of 10 m/s and with the 1.5 mm optimal winding-winding channel width, the variation in surface heat transfer coefficients at a turbulence intensity of 40% is compared with that at a turbulence intensity of 0.5%. The significant improvements due to the higher turbulence intensity appear near the upwind-side end winding. It can be observed that the heat transfer coefficients near the upwind-side end winding at the 40% turbulence intensity are almost doubled compared to those at the turbulence intensity of 0.5%, leading to an area-weighted average surface heat transfer coefficient of 85 W/m\({}^{2}\)/K, which is 10% higher than that of the case at 0.5% turbulence intensity.