## 4 Thermal Design Investigations with the Validated CFD Model
### Overall Improvement Analysis  
Following the insights detailed in the preceding sections, adjustments were implemented in the initial design to enhance its performance. By increasing the coil fill factor while keeping the slot fill factor constant and introducing a 1.5 mm separation between the coils, along with a turbulence intensity of 40%, the revised model is evaluated against the standard design. It is crucial to note that any performance enhancements must be assessed at a consistent airflow velocity to accurately highlight the design's benefits. In the enhanced design, the peak temperature is reduced by 21 K compared to the original model, resulting in a 44% improvement. This enhancement is attributed to the increased efficiency in both the convective and conductive heat transfer. Importantly, these advancements are achieved without the need for complex modifications and are maintained at a consistent MMF.  
In the context of industrial design, understanding the intricacies of heat transfer mechanisms, specifically conduction and convection, is of paramount importance. To facilitate this comprehension, a thermal equivalent circuit has been distilled from the optimised design, which is explained in Figure 14. In this representation, the variables \(\mathbf{P}\) (W), \(\mathbf{R}\) (K/W), and \(\mathbf{T}\) (K) signify the user-defined heat power, equivalent thermal resistance, and local temperature, respectively. Furthermore, the subscripts \(\mathbf{\mathrm{w}}\), \(\mathbf{\mathrm{s}}\), and \(\mathbf{\mathrm{{}_{A}}}\) correspond to the winding, stator, and ambient conditions, respectively. It should be noted that \(\mathbf{\mathrm{R_{S}}}\) encompasses the thermal resistance attributable to conduction within the stator, inclusive  
Figure 13: Contours of surface heat transfer coefficients with the channel widths of (**a**) 0.5 mm, (**b**) 1.0 mm, and (**c**) 1.5 mm at a wind speed of 10 m/s and 0.5% turbulence intensity. The flow direction is from left to right.  
of slot liners. The thermal resistances \(\mathbf{R_{\mathrm{SA}}}\) and \(\mathbf{R_{\mathrm{WA}}}\) are indicative of the convective heat transfer from the stator laminations to the ambient environment and from the winding to the ambient environment, respectively. These resistances are modifiable through the manipulation of convective parameters such as air velocity and turbulence intensity. Quantification of these thermal resistances can be achieved through the employment of mean heat transfer coefficients.  
In the simplified thermal equivalent circuit, the transfer of the input heat power is divided into two routes: one is the convection route, which is through the winding surface to the ambient, and the other is the conduction route, which is through the winding to the stator and subsequently to the ambient. The total surface heat flux through the convection route is presented in Figure 15. The upwind side of the winding presents a total surface heat flux higher than 3000 W/m\({}^{2}\), while the downwind side only shows that the heat flux can be lower than 1000 W/m\({}^{2}\) in the area where the velocity of the air is extremely low.  
The transferred heat power through the conduction and convection routes can be determined by integrating the corresponding total surface heat flux. For this specific case, 63% of the transferred heat power is mitigated through the convection route and 37% is mitigated through the conduction route.