## 4 Thermal Design Investigations with the Validated CFD Model
### Effects of Spacing between Windings  
The strategy to enhance heat transfer involves adjusting the spacing between individual winding coils while preserving the same magneto-motive force (MMF). This adjustment can be realised by increasing the coil fill factor (CFF), reducing the slot fill factor (SFF), or implementing both modifications. Initially, the windings exhibit an average circumferential width of 5.0 mm, with a coil fill factor (CFF) of 0.548 and a slot fill factor (SFF) of 0.434, denoting a standard winding configuration. This analysis focuses on two distinct approaches to augment the spacing between windings:  
* Concept 1: Keeping the SFF constant while augmenting the CFF. This adjustment maintains the copper cross-sectional area as the inter-coil channel expands, enhancing the thermal conductivity of the coil without altering the coil's loss for a given MMF. * Concept 2: Preserving the CFF constant and diminishing the SFF, achievable through reducing the wire diameter. This maintains the thermal conductivity of the winding constant while increasing the loss density in the copper for a constant MMF. For this, a 0.25 mm thick Nomex-410 slot liner, with a thermal conductivity of 0.139 W/m/K, was utilised. In certain machines, the slot liner encases each coil entirely and is therefore positioned near the slot's centre. Here, the significance of the slot liner's presence between the windings was assessed regarding its effect on heat transfer, especially when cooling is facilitated via airflow down the slot's centre. At an air speed of 10 m/s and a turbulence intensity of 0.5%, eliminating the slot liners between the windings resulted in a temperature decrease of 12 K inside the winding and 10 K at the tooth tip, equating to improvements of 25% and 20.8%, respectively. This enhancement is attributed to the Nomex paper's extremely low thermal conductivity, which limits conductive heat transfer from the copper losses and obstructs convective heat transfer between the winding and the cooling medium. Consequently, the Nomex layers between the windings are omitted in further analyses, establishing a 0.5 mm air channel (comprising two 0.25 mm layers) as the baseline for subsequent investigations. Figure 11 illustrates how varying the width of the inter-coil air channel influences the cooling efficiency at an air speed of 10 m/s and a turbulence intensity of 0.5%. A comparison of the results achieved with a 0 mm channel and those achieved with a 0.5 mm channel--representing windings with and without slot liners, respectively--shows identical temperature rises at these channel sizes, marking the \"true\" initial condition from a 0.5 mm channel width. In the scenario where the SFF is constant and the CFF is increased, as the channel width enlarges, there is a corresponding change in the equivalent anisotropic thermal con  
Figure 11: Effects of increasing the air channel width between windings upon the hotspot, maintaining either a constant slot fill factor or a constant conductor fill factor. ductivities due to the altered slot composition. Consequently, as the air channel widens, the thermal conductivity and coil surface cooling improve, leading to a temperature decrease. Interestingly, beyond a 1.5 mm width, no substantial further reduction in temperature is observed, indicating a limit to the benefits of increased compression. The most significant temperature rise reduction of the hotspot, 28.7 K, is 19.3 K lower than in the original uncompressed windings with slot liners, and 8.9 K lower than in the uncompressed windings without slot liners, showcasing improvements of 40% and 24%, respectively. These improvements are attributed to an enhanced conductive heat transfer from the increased thermal conductivity of the combined winding blocks and improved convective heat transfer from the greater air channel widths. Alternatively, maintaining a constant CFF results in the coil retaining anisotropic thermal conductivities equivalent to those of the original winding, since the copper percentage remains unchanged.