Additively Manufactured Winding Design for Thermal Improvement of an Oil-Cooled Axial Flux Permanent Magnet Machine

This article proposes a new additively manufactured winding with integrated heat sinks to improve the thermal performance of oil-cooled yokeless and segmented armature (YASA) axial flux permanent magnet machines. The heat sinks featuring a pin-fin structure are integrated into the two sides and top of the winding to increase the heat transfer area and convective heat transfer coefficient, thus improving the thermal performance. Computational fluid dynamics is employed to evaluate the thermal performance of the proposed winding, which is further compared with that of the state-of-the-art rectangular winding. Besides, the influence of pin spacing in the streamwise direction, tilt angle, flow rate, and resistances on the thermal performance and pressure drops of the proposed winding is investigated. Finally, prototypes of the proposed winding and the counterpart rectangular winding are manufactured to verify the numerical analyses. The experimental results show that the winding temperature of the proposed winding can be reduced by 27.6 °C compared with that of rectangular winding.


I. INTRODUCTION
E LECTRIFICATION is a main enabler for decarbonized transportation.To achieve the "Net Zero" target in future decades [1], ambitious roadmaps have been drawn up globally, which translate into step-change performance requirements for electrical machines in terms of their power density level, Hui Tan is with the China-EU Institute for Clean and Renewable Energy, Huazhong University of Science and Technology, Wuhan 430074, China (e-mail: tanhuitongxue@163.com).
Digital Object Identifier 10.1109/TTE.2023.3282213from 8 to 30 kW/L [2], [3].The power density of stateof-the-art machines is primarily limited by the ability to minimize power losses and maximize heat transfer to maintain operation within the material property boundaries.In recent years, the axial flux permanent magnet (AFPM) machines have drawn ever-increasing attention in the transportation sector due to their inherent high torque density and high efficiency [4], [5], [6].Among AFPM machines, the yokeless and segmented armature (YASA) AFPM machines have been the research hotspots due to their superior power density and more efficient active material utilization [7], [8].Meanwhile, this "multiplate" arrangement brings significant challenges in the thermal management of the machine.The stator segments of YASA AFPM machines are between the two rotors without a common yoke structure as a heat dissipation channel.This also leads to additional difficulty in integrated cooling system design, thus becoming a key barrier in further boosting the power density of YASA AFPM machines.
To enhance the thermal performance of YASA AFPM machines, the well-adopted cooling methods include air cooling [9], [10], [11], [12], water cooling [13], [14], [15], and oil cooling [16], [17], [18].Air cooling is beneficial for its low cost, simplicity, and high reliability.Based on the unique segment stator structure, fins can be inserted into the space between the adjacent coils or at the coil ends to improve the thermal performance [11], [12].Even though the thermal performance is improved by using these novel structures for the air-cooled machines, it still cannot meet the power density requirement for some applications.To further improve the cooling performance, more efficient water cooling methods can be used [19].Chen et al. [13] applied a water jacket in contact with the inner ring of the winding, while Zhang et al. [14] arranged some water pipes at the outer ring of the winding.These techniques are supposed to achieve limited thermal performance improvement due to the insufficient heat dissipation area.In order to further strengthen the cooling performance, Chang et al. [15] applied U-shape water-cooling pipes that are integrated inside the fins around the segment coils to reduce thermal resistance.It is reported that the power density has been improved by 38% using this method.Compared with water cooling, oil cooling is a more effective cooling option, as oil can be in direct contact with windings to transfer heat much more efficiently without concerns about degrading insulation level [20].Hence, direct oil cooling attracts more attention in ultrahigh-power-density applications [16], [17], [18].Camilleri et al. [17] proposed a compact and integrated stator oil-immersed cooling structure for a YASA AFPM machine with the peak power density of 6.7 kW/kg.Besides, they further integrated a heat sink with the coil, which reduces the maximum winding temperature by 87 • C and increases current density by 140% compared with traditional structure [18].The work has shown great potential in improving the thermal performance using a novel winding cooling structure, whereas this structure is difficult to manufacture, which has limited its applicability.
In order to enhance the thermal performance of machines while overcoming manufacturing problems, additive manufacturing (AM) technology offers great possibilities [21], [22], [23].Based on AM technology, complicated structures with advanced designs of machine components, such as windings [24], [25], [26], [27] and permanent magnets [28], [29], can be manufactured in a highly flexible and customized way.Among them, the AM winding attracts more attention in terms of its great potential in improving the slot filling factor [24] and thermal performance [25], [26], [27].Some studies have already been carried out to improve the thermal performance for the radial-flux machines using AM technology [30], [31].For the YASA AFPM machines with special structure and cooling configuration, the capability of AM technology in improving thermal performance remains to be further investigated.
In this article, a new additively manufactured winding with integrated heat sinks is proposed to improve the thermal performance of a YASA AFPM machine, which is configured with a unique oil cooling structure.The heat sinks are featured with a pin-fin structure and are integrated into the winding surface, which can extremely increase the heat dissipation area and stir up fluid turbulence, thus improving the thermal performance.The remaining content of this article is structured as follows.The studied oil-cooled YASA AFPM machines and the proposed winding structure will be introduced in detail in Section II.In Section III, computational fluid dynamics (CFD) models of the proposed winding and the rectangular winding are established.Besides, the thermal performance and flow condition of the proposed winding and the rectangular winding will be detailedly investigated and analyzed.In addition, the influence of pin spacing in the streamwise direction, tilt angle, flow rate, and resistances on the thermal performance of the proposed winding is studied.In Section IV, the experimental validation of the proposed winding and CFD model will be presented and discussed.Finally, Section V concludes this article.

A. Structure and Parameters of Studied YASA AFPM Machine
In this section, the structure and parameters of the studied YASA AFPM machine with a unique cooling structure will be introduced.The main parameters are shown in Table I.The studied machine is targeted for aerospace propulsion applications with a continuous power density of 4.8 kW/kg.For the studied machine, the stator is between the two rotors, which makes it difficult to dissipate the high winding losses to the ambient.Since the main losses are located in the stator, the power density will be limited if the stator cooling is not sufficient.
To improve the stator cooling, a unique stator-immersed oil cooling structure is proposed, as shown in Fig. 1.The cooling structure is designed based on the special mechanical support, which fixes 24 coils along the stator circumference.Oil first enters the cold oil chamber from an external inlet and then is distributed to the stator chamber through 12 internal inlet holes.After the oil flows into the stator chamber, it flows radially between the adjacent coils and flows circumferentially on the top of the coil to absorb the stator losses.Then, it gathers with the oil from the adjacent branch and flows between the adjacent coils to further absorb the heat.After that, it flows out through 12 internal outlet holes that are alternately arranged with the 12 internal inlet holes and, finally, collects in the hot oil chamber and flows out through an external outlet.The proposed cooling structure integrates with the active magnetic components and the mechanical parts, which makes it much more compact and lightweight.Besides, the direct contact with the oil and stator support also greatly reduces the thermal resistance between the winding and coolant, thus significantly improving the thermal performance.

B. Proposed Winding Structure With Integrated Pin-Fin Heat Sinks
Based on the proposed cooling structure, a new winding structure adopting AM technology is proposed to further improve the thermal performance from the point of view of winding design, as shown in Fig. 2. To improve the thermal performance of winding, staggered and cylinder pin-fin heat sinks (PFHSs) with 5.5-mm spacing are added to the two sides and top of the winding [as shown in Fig. 2(b)].The pin-fin diameter is equal to the axial thickness of each turn (0.81 mm), and the height is the maximum to almost hit the adjacent winding and sleeve.As there are four stator support cylinders to fix the coil, only 11 turns in the middle are configured with PFHSs on both sides as a practical consideration.Each turn has seven or eight layers of pin-fins on each side, which are 45 • tilt upward considering the processability of AM technology.At the top of the winding, apart from the two turns at both ends, each turn has six or seven staggered PFHSs that are vertically upward.Besides, the winding cross section area is increased as much as possible to decrease the coil resistance by utilizing the space at the corners, as shown in the blue circle of Fig. 2(a).The windings are assembled with the other parts, as shown in Fig. 2(c).The stator shoes made of SMC are used to reduce the slot opening while fixing the coil.Based on the above designs, the heat dissipation area can be significantly increased, and the turbulence intensity can be enhanced due to the extended pin-fins, thus improving the thermal performance.
The PFHSs have been widely used in the cooling of electronic components [32], [33].For the pin-fin structure, the heat transfer coefficient h fin can be calculated by dimensionless number as follows [34]: where C 1 is a constant that depends on the longitudinal and transverse spacing, array of the pins, and thermal boundary conditions.Nu d is the Nusselt number of PFHSs, and k f is the thermal conductivity of PFHSs.Re d is the Reynold number of PFHSs.Pr is the Prandtl number where S L is the pin spacing in the streamwise direction.S T is the pin spacing in the spanwise direction as shown in Fig. 3. Based on the above formulas, the relationship between thermal resistance and fluid velocity is shown in Fig. 4. It can be seen that the thermal resistance of a single plate is almost 15 times that of PFHSs under the same fluid velocity due to PFHSs' larger heat transfer area and convective heat transfer coefficient (CHTC).Compared with the in-line array, the staggered array has lower thermal resistance under the same heat transfer area.That is attributed to the fact that the flow disturbance of the staggered array is more intensive compared with that of the in-line array when fluid flows in a curved channel with alternating contraction and expansion.Therefore, the thermal performance can be significantly improved by using staggered PFHSs compared with a single plate.It should be noted that the pin-fins also increase the pressure drops due to the decreased flow area, increased flow velocity, and local turbulence.

A. Basic Equations and CFD Models
In this section, CFD is employed to evaluate the thermal performance and flows of the proposed winding and rectangular winding.The rectangular winding in AFPM machines is proven to be superior to cylindrical winding [35].Therefore, the proposed winding is compared with the rectangular winding manufactured by the winding mold from the flat wires to prove its superiority.The foundations of CFD are the mass conservation equation, the momentum conservation equation, and the energy conservation equation.Based on the CFD, complex geometries and fluid domains can be accurately computed.
The CFD establishes differential equations in discrete microelements and solves them using Navier-Stokes equations with high accuracy in the 3-D laminar and turbulent flow.To solve complicated 3-D turbulence, Reynolds-averaged Navier-Stokes (RANS) equations are used to reduce computational cost as follows [36]: where Ū and u are the fluid time average and the fluctuating velocity, respectively, P is the pressure, µ is the dynamic viscosity of the fluid, S i j is the mean strain-rate tensor, and −ρ ū′ j u ′ i is the symmetric Reynolds stress tensor with six components.
Due to the complicated structure of the proposed winding, the flow characteristics are inherently complex.The fluid flows in alternating channels of contraction and expansion, leading to complicated fluid impact and separation phenomena due to the PFHSs.Therefore, the choice of the turbulence model is crucial.Among the turbulence models, the high Reynolds number k-ε model is only valid in the fully developed flow region.In contrast, the k-ω model is more accurate in solving fluid regions near the wall [37].Based on them, the shear stress transport (SST) k-ω model combines the k-ω model near the wall and the k-ε model for the free flow to get accurate results and save time [38], which has good performance for swirling flows without requiring damping and improves separation flow prediction [36].Therefore, in these CFD models, the SST k-ω model is adopted.
The CFD model of the proposed winding with its main boundary conditions is developed, as shown in Fig. 5.To reduce simulation time, the full machine model is simplified, and only a single pole piece (including a winding, a stator core, a stator support, and two endcaps) is studied based on a reasonable consideration of circumferential symmetry for the machine.Since the stator shoes are not located on the main heat dissipation path, to reduce the manufacturing complexity of the cooling specimen, the stator shoes are Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.not considered in this study.To simulate the temperature of winding, both solid parts and fluid domain are modeled due to conjugate heat between solid parts and oil.The mass-flowinlet boundary condition is applied to the inlet with inlet oil at 30 • C and 1.6 L/min.Moreover, the pressure-outlet boundary condition is set at the outlet with 0 Pa, which is the reference for upstream pressure.With the power loss of the stator core neglected in the pole piece model, which only carries dc current, the copper losses of the proposed winding and rectangular winding are 590.3 and 586.3 W, respectively, based on the same input current (220 A) due to the different designed resistance (10.5 and 9 m at room temperature).The cross section area of the proposed winding is smaller than that of the rectangular winding due to thicker insulation (the same total axial thickness as that of rectangular winding), resulting in higher resistance.
With the winding temperature being the research focus, reasonable considerations on the thermal conductivities of every component in this model, especially the winding part, are important, which are directly related to thermal resistances.Since the winding is a heterogeneous body including both copper and insulation material, the modeling of winding is complex and time-consuming; thus, some equivalence is required.In the CFD model, the winding is represented by a solid constituted of the compound, which has different thermal conductivities in different directions, as shown in Fig. 6.In the actual prototypes, there is a gap between the winding and the endcaps, which is filled with epoxy to restrict the flow paths.To simplify the CFD models, the winding is directly in contact with the endcaps.To consider the heat transfer between winding and endcaps, the contact thermal resistances are added to the contact surfaces between the winding and the endcaps.In the CFD models, the contact thermal resistance is inserted by setting an equivalent epoxy gap (2 mm) on the contact surfaces between the winding and the endcaps.Using rectangular wires, slot fill can be increased, and resistance can be decreased.As for the proposed winding model, based on the rectangular winding model, the PFHSs are added to increase the heat transfer area and enhance turbulence to increase CHTC.As gaps between turns of winding increase the heat transfer area between winding and oil, some grooves are added to obtain more precise results.Furthermore, windings are further divided into several small parts, and each part   [37] has its own local coordinate system as per their anisotropic thermal conductivities to increase the reasonability of the CFD simulation.
The equivalent thermal conductivity of winding can be calculated using Hashin and Shtrikman (H+S) as follows [39]: ) where k c and k i are the thermal conductivities of copper and insulation material, respectively, and v c and v i are the volume ratios of copper and insulation material, respectively.The equivalent thermal conductivities of the proposed winding and rectangular winding in tangential, radial, and axial directions are 295.99/295.99/1.32 and 338.07/338.07/2.33W/m/ • C, respectively.The detailed properties of materials applied in the CFD models are listed in Table II.The cooling medium in the studied YASA AFPM machine is aviation lubricating oil.The relationships between the oil properties and temperature are shown in Table III.As the CHTC is sensitive to oil properties, the oil properties in the CFD model are adjusted to be temperature-dependent function according to Table III to get more accurate results.
In addition, high-quality mesh is the calculation foundation of CFD.In the CFD model, the 3-D mesh is developed and optimized by Fluent Meshing.To improve mesh quality and decrease the number of mesh, tetrahedral elements with a minimum size set at 0.1 mm and a maximum size set at 3 mm are applied in the solid domain and the fluid domain due to their good adaptability in complex geometry.The skewness and aspect ratio are usually used to evaluate the quality of the 3-D mesh, which are usually required below 0.95 and 15, respectively.Besides, a three-layer boundary layer mesh with a growth rate of 1.1 is employed in the interfaces between the solid domain and the fluid domain, which is essential in the complex flow condition.The value of dimensionless wall distance y+ from the first centroid to the wall is used to measure whether inflation mesh is refined properly, which is better less than 1 in the SST k-ω model and can be calculated by formulas ( 8) and ( 9) where y is the distance between the mesh and the solid wall, ν is the friction velocity, and τ ω is the wall shear stress.The mesh of the proposed winding is shown in Fig. 7.A meshindependent analysis has been done to ensure the accuracy of results, and the total number of elements in the final model is 65 million.

B. Simulation Results
Based on the mesh and boundary conditions mentioned in Section III-A, the streamline and velocity of the proposed winding and rectangular winding are shown in Fig. 8.The maximum oil velocity (2.2 m/s) of the proposed winding model is equal to that of the rectangular winding model.Compared with the proposed winding model, the maximum oil velocity of the rectangular winding model occurs at the top of the winding due to drastic area contraction.As a limitation of processing capability, the rectangular winding usually features a large rounded corner when bending, which leads to the reduced fluid area at the top of the rectangular winding model.Therefore, the oil velocity at the top of the proposed winding model is lower than that of the rectangular winding model.In the meanwhile, the maximum oil velocity of the proposed winding model occurs at two sides of winding due to PFHSs' disturbance.Besides, the oil velocity on two sides of the proposed winding is higher than that of the rectangular winding model.In addition, the average oil velocity of the proposed winding model is almost the same as that of the rectangular winding.It can be seen from Fig. 8 that oil flow bypasses and impacts the staggered PFHSs to form severe turbulence and interactive wake in the proposed winding model, whereas oil flow is laminar in the rectangular winding model.Compared with laminar, turbulence has higher fluid velocity and better thermal performance under the same flow rate due to the heat exchange of different fluid layers.To further study the thermal performance of the proposed winding, the surface CHTCs of the proposed winding and rectangular winding model are investigated, as shown in Fig. 9.It can be seen that the overall average CHTC of the proposed winding model is higher than that of the rectangular winding model.For the proposed winding model, the CHTC around PFHSs is maximum, and CHTC increases along fins from top to bottom.This is due to the fact that local turbulence stimulates heat transfer of different oil layers around PFHSs, and higher oil velocity around PFHSs increases the Reynolds number.For the rectangular winding model, CHTC remains almost the same on two sides of winding as the laminar oil flow and the maximum value occur at the top of winding due to higher oil velocity.Besides, the CHTC variation is mainly caused by area variation.(111.8 • C).For the two models, winding temperatures both increase from top to bottom of winding.The maximum winding temperature of the rectangular winding model occurs at the bottom of winding in two ends due to zero local oil flow, whereas the maximum winding temperature of the proposed winding model occurs near the outlet.Besides, for the proposed winding model, minimum temperature occurs at the top of PFHSs due to the higher CHTC and heat transfer area.Compared with the proposed winding model, winding temperatures on two sides of winding are much higher in the rectangular winding model.On one hand, the PFHSs of the proposed winding model provides about 1.75 times the heat transfer area of the rectangular winding model.On the other hand, PFHSs induces turbulence, which features a higher Reynolds number and higher CHTC.
The pressures drops of the two models are shown in Fig. 11.It can be seen that, for the proposed winding model, the pressure drops are mainly located in channels at the winding two sides, which are mainly caused by the PFHSs, whereas, for the rectangular winding model, the pressure drops are mainly located in channels at the winding top.It is attributed that the winding top channels are smaller due to the arched shape of the winding on the top.The curvature of the arch is limited by the mechanical properties of the rectangular winding at the corners, which is not a problem, if AM technology is used.In total, the pressure drop of the proposed winding (26.0 kPa) is not significantly higher (22%) than that of the rectangular winding.
In summary, the main simulation results are listed in Table IV.The resistance of the proposed winding is 16.7% higher than that of rectangular winding at room temperature.Besides, the system loss of the proposed winding is almost the same as that of rectangular winding due to the lower average winding temperature.Under this condition, the maximum winding temperature of the proposed winding can be reduced by 33.2 • C compared with the rectangular winding.Meanwhile, the pressure drop is increased by 22% as a penalty.

C. Effect of Parameters
Based on (1)-( 3), pin spacing in the streamwise direction and spanwise direction, and the number of rows in the streamwise direction all affect the thermal performance of PFHSs.However, for the proposed winding structure, the pin spacing in the spanwise direction is equal to the spacing between adjacent turns.Besides, the pin spacing in the streamwise direction is inversely proportional to the number of rows in the streamwise direction.Therefore, only the pin spacing in the streamwise direction and the tilt angle are investigated in this section.
The effect of the pin spacing of PFHSs in the streamwise direction is investigated by CFD, as shown in Fig. 12.It can be seen that the temperature of the winding increases, while the pressure drops decrease as spacing increases.This is because the wake downstream may interact with the PFHSs in the next row for intensive PFHSs, which enhances the turbulence intensity.The enhanced turbulence intensity, together with the increased heat dissipation area of PFHSs, reduces the thermal resistance, thus decreasing the winding temperature.Meanwhile, the intensive PFHSs also leads to decreased flow area, increased flow velocity, and local turbulence phenomena, which causes the pressure drop to increase.
Moreover, different tilt angles are selected to study the effect on PFHSs.The results are shown in Fig. 13.It can be seen that the winding temperature almost does not change (less than 1%) due to the almost same heat transfer area and heat transfer coefficient.Besides, the pressure drops increase first and then decrease with the tilt angle increasing, and it reaches a maximum value when the tilt angle is −7.5 • (perpendicular to winding).On the one hand, the tilt angle affects friction loss through the fiction loss factor.When PFHSs are perpendicular to the winding, the friction factor  reaches the maximum value.On the other hand, the tilt angle also affects local loss.The local loss factor is the maximum when PFHSs are perpendicular to winding due to the most serious fluid area variation.In total, the pressure drops are the maximum when the tilt angle equals −7.5 • and the pressure drop curve is almost symmetry about −7.5 • .
The process of the AM technology directly affects the resistance of the proposed winding, thus affecting the winding temperature.To investigate the influence of the process of the AM technology on the thermal performance of the proposed winding, winding temperatures based on different resistances are compared, as shown in Fig. 14.It can be seen that the winding temperature of the proposed winding is sensitive to the winding resistance.The higher the resistance of the proposed winding, the higher the proposed winding temperature.

IV. EXPERIMENTAL VALIDATION AND ANALYSIS
To validate the effectiveness of the proposed winding and the CFD models, two coils, including the proposed one and the conventional rectangular one, are prototyped.As copper has high reflectivity to the typically applied laser with a long wavelength and has high thermal conductivity to dissipate heat quickly, it cannot absorb and accumulate enough power to melt well, which makes it difficult to manufacture using the typical laser powers by AM technology [40].In this article, the proposed winding is additively manufactured by Trumpf TruPrint 1000 green edition with a green laser of TRUMPF, as shown in Fig. 15(a).
The tested coil resistance of the proposed winding prototype is 13.24 m at room temperature, which is 26% higher than the designed resistance (10.5 m ).The increase in the resistance may be attributed that the compactness of the printed copper on the coil surface is not high, and the copper on the surface got oxidized due to the unprotected processing from the oxygen, as shown in Fig. 15(a), in which the winding surface is much coarse and gets black.To ensure insulation strength, a thicker insulating layer compared to rectangular winding is required due to the coarse surface.Due to a large number of turns, the larger coil surface area also magnifies the defects of AM.With the printing parameters and corresponding processing techniques improved, the conductivity of the printed copper can be further improved.After the printing, a high-temperature insulating paint is sprayed on the surface to insulate the coil, as shown in Fig. 15(b).The insulation coating called "SCOTCHCAST BRAND ELEC-TRICAL Resin 260 C-free" is supplied by 3M Company.The insulation powder is absorbed into the winding surface under the electrostatic field.Then, the winding with the insulation material is heated to solidify the insulation powder to form the insulation layer on the winding surface.
The coil prototypes are further assembled with the stator core, the support, and a transparent case to form the model shown in Fig. 5, as shown in Fig. 16.Besides, two temperature sensors (PT100) are inserted into winding through slots of the stator core to measure the temperature of the coil, as shown in Fig. 16(c).The experimental setup is used to verify the thermal performance of the proposed winding, as shown in Figs. 17 and 18.The oil flow rate can be adjusted by controlling the variable frequency pump integrated into the integrated oil tank and radiator.The oil flow can be kept around the setting value through proportional-integral (PI) feedback adjustment method.The inlet oil temperature is set at 30 • C by the integrated fuel tank and radiator.Two temperature sensors (PT100) are applied to measure the temperature of oil in the inlet and the outlet.Two pressure gauges are used to measure the pressure of the inlet and the outlet, whose range is 0.6 MPa and accuracy is ±3 Pa.Besides, the flowmeter is located at the inlet to measure the flow rate of oil.The dc current is injected into the winding to supply winding loss by the dc power supply.
The experimental results of the rated condition (220 A, 1.6 L/min) are shown in Table V.Compared with the rectangular winding, the hot spot temperature (temperature of "Winding down") of the proposed winding is reduced by 27.6 • C. At that time, the current density of the proposed winding and rectangular winding are 37.9 and 33.5 A/mm 2 , respectively.In addition, the hot spot temperature of the proposed winding reaches 145.7 • C under 50 A/mm 2 and 1.6 L/min.It can be seen that the pressure drops of the proposed winding are slightly lower than that of the rectangular winding.
To validate the CFD models, the simulation winding temperatures under different current levels and the flow rates of the proposed winding model and the rectangular winding model are compared with the measure ones, as shown in Fig. 19.It can be seen that the simulation temperatures agree well with the measured ones.The error of the winding temperature between the experiment results and simulation results is less than 8%.The small error indicates that the CFD models are accurate to describe the thermal performance of the proposed winding and the rectangular winding.The small difference is mainly due to imprecise thermal sensor locations, inaccurate winding loss, and some extra gaps in prototypes where oil flow can pass through.Furthermore, the simulated and experimental results of pressure drops without injecting current (inlet temperature is 30 • C) are shown in Fig. 20.It can be seen that the pressure drop of the rectangular winding is slightly higher than that of the proposed winding.When the flow rate is 1.6 L/min, the experimental pressure drop of the rectangular winding is about 9.7% higher than that of the proposed winding.The error of the pressure drop between the experiment results and simulation results is less than 11%.The error is mainly due to CFD model simplification and measurement errors.
The above analysis shows the great potential of the proposed winding concept for high-power-density electrical machines applied in transportation.For mass production occasions, such as electrical vehicles, the proposed AM winding may increase the production time and cost, which is rather important and has to be considered in the area.However, in some applications, such as racing cars and more electrical aircraft, where the limitation of production time and cost are looser, the AM technology can serve as a good alternative to extend the boundaries of motor output performance, thus meeting the ultimate requirements of motor output performance in occasions.

V. CONCLUSION
This article proposes a new winding structure using AM technology to improve the thermal performance of YASA AFPM machines.Based on the unique oil cooling structure of the studied machine, the proposed winding integrates PFHSs that are located on the two sides and the top of the winding to increase heat transfer area and CHTC.The flow characteristics, CHTC, pressure drop, and temperature distribution of these two windings are comparatively and comprehensively investigated by CFD.Besides, the influence on the thermal performance of the proposed winding of pin spacing in the streamwise direction, tilt angle, inlet flow, and resistances is discussed.The simulation results show that the maximum temperature of the proposed winding is reduced by 33.2 • C compared with the conventional rectangular winding with the same pure copper conductivity and current input.Two coil prototypes have been manufactured to validate the proposed AM winding concept and CFD models.It shows that, even though the resistance of the proposed winding is increased by 26% than the designed value due to the AM challenges, the maximum temperature of the proposed winding can be still reduced by 27.6 • C compared with the rectangular winding.The effectiveness of the proposed winding has demonstrated the great potential of AM technology in improving the machine's thermal performance, thus enabling the everdemanding density requirement to be met for electrified transportation.

Manuscript received 22
February 2023; revised 26 April 2023; accepted 27 May 2023.Date of publication 7 June 2023; date of current version 16 March 2024.This work was supported in part by the National Natural Science Foundation of China under Grant 52007069, in part by the National Natural Science Foundation of China under Grant 51991382, and in part by the Maritime Technology Innovation Center Foundation of China under Grant JJ-2020-712-01.(Corresponding author: Xinggang Fan.)

Fig. 1 .
Fig. 1.Cooling structure of the studied YASA AFPM machine.(a) Cross section of the oil-cooled stator.(b) Structure of the oil-cooled stator.

Fig. 10
shows that the maximum winding temperature of the proposed winding model (78.6 • C) is decreased by 33.2 • C compared with that of the rectangular winding model

Fig. 12 .
Fig. 12. Temperature and pressure drops of the proposed winding with different spacings.Note that the inlet temperature is 30 • C, and the flow is 1.6 L/min.

Fig. 13 .
Fig. 13.Temperature and pressure drops of the proposed winding with different tilt angles.Note that the inlet temperature is 30 • C, and the flow is 1.6 L/min.

Fig. 14 .
Fig. 14.Winding temperature under different resistances of the proposed winding.Note that the inlet temperature is 30 • C, and the flow is 1.6 L/min.

Fig. 20 .
Fig. 20.Simulation results and experimental results of the pressure drop without input current.Note that the inlet temperature is 30 • C.

TABLE I MAIN
PARAMETERS OF THE STUDIED MACHINE

TABLE II PROPERTIES
OF MATERIALS APPLIED IN THE CFD MODELS

TABLE III OIL
PROPERTIES VARIATION WITH TEMPERATURE [ • ]