###### Abstract  
This paper investigates innovative methods for enhancing heat transfer efficiency in high-power permanent magnet electrical machines. The objectives are to quantify the effects of increasing the air speed, increasing the turbulence intensity, and introducing the spacing between windings on cooling performance. The cooling of stator windings is studied through experimental wind tunnel testing and Computational Fluid Dynamics (CFD) modelling. The CFD model is validated against wind tunnel measurements to within 4 Kelvin (K). The results demonstrate that each enhancement method significantly improves the cooling capability. Increasing the air speed from 10 m/s to 40 m/s reduces the winding hotspot temperature by 34%. Introducing a high turbulence intensity of 40% leads to a 21% lower hotspot temperature compared to 0.5% turbulence intensity. Creating a 1.5 mm spacing between coils also substantially improves convection and conduction heat transfer. Overall, combining these optimised design parameters yields over a 40% reduction in hotspot temperature compared to the original design. This research provides practical guidance for maximising heat transfer efficiency in high-power permanent magnet machines, without increasing complexity. The findings will lead to higher machine efficiency, reliability, and longevity for aerospace and other applications. s 2024 14 2658. 14. 14. 2658. 14.  
high-power-density motor design. Historically, thermal design has often been considered secondary to electromagnetic performance within this field [4]. However, the generation of high power at high efficiency with minimal size and mass is heavily reliant upon high electric loading, rendering thermal design a critical factor. Furthermore, a reduction in the operating temperature can significantly extend the machine's lifetime, as an increase of 10 \({}^{\circ}\)C can halve the insulation life [5]. Consequently, it is essential to employ effective thermal management strategies to minimise internal temperatures and thermal cycling. Thermal management is a persistent challenge in high-power-density electrical machines, with inefficient heat dissipation potentially limiting their performance and lifespan [6; 7]. Stator windings, integral to machine function, are particularly receptive to thermal stress [8]. Permanent magnet (PM) motors, known for their efficiency, face additional thermal concerns due to the sensitivity of the magnets to high temperatures [9]. Diverse thermal management strategies have been explored, including conventional air cooling, liquid cooling, and emerging techniques with phase-change materials [10]. While air cooling offers simplicity, optimising airflow and heat transfer within complex winding geometries remains an active research area [11]. Direct air cooling is a compelling alternative to liquid cooling systems, which often introduce complexity and potential failure points. One key parameter for optimising air-cooling efficiency is freestream turbulence intensity. Studies in turbine blade film cooling have demonstrated that increasing the turbulence intensity can significantly improve cooling effectiveness [12; 13; 14]. The mechanisms of film cooling, where injected air forms a protective layer, share similarities with convective cooling over complex surfaces such as stator windings. This suggests that turbulence intensity may offer similar benefits within electrical machines. This concept has been subsequently applied to the cooling performance of natural wind on solar PV panels, resulting in a substantial 40% enhancement by merely increasing the freestream turbulence intensity [15; 16]. These successes make it worthwhile to investigate the potential benefits of freestream turbulence intensity on the cooling of newly designed winding configurations. Computational Fluid Dynamics (CFD) plays a vital role in understanding thermal behaviour and designing effective cooling solutions [17; 18].