Case Study No: FTLCFD04
Keywords: Electric motor cooling, rotating oil jet, CFD, AMR, Conjugate heat transfer, VOF
Introduction
Efficient thermal management is vital for sustaining the performance and lifespan of high-speed electric machines. In electric vehicle traction motors, intense heat generation within rotor magnets and stator windings can lead to magnet demagnetization, insulation failure, and reduced efficiency. To mitigate this, advanced direct liquid cooling using Automatic Transmission Fluid (ATF) offers an effective and compact solution for localized heat removal.
This CFD study investigates rotating jet impingement cooling in a high-speed electric motor, focusing on ATF jets that strike rotor surfaces and magnet regions to form dynamic cooling films that enhance convective and conductive heat dissipation. The simulation captures complex multiphase flow behavior jet breakup, film formation, and centrifugal spreading that governs the overall cooling performance.
ATF serves as both a coolant and dielectric medium, providing efficient heat removal while maintaining electrical insulation. Its favorable thermophysical properties enable lightweight, high-performance thermal management suitable for modern electric machines.
Computational Domain
The computational domain was constructed to represent the internal cavity of an electric motor, including the rotating shaft, magnets, housing, and surrounding air . The domain is subdivided into two primary regions:
- Rotating domain – enclosing the rotor, hollow shaft, and ATF jet nozzles.
- Stationary domain – comprising the surrounding air and the motor casing.
Interfaces between these regions were defined as sliding interfaces to capture transient fluid exchange. The total domain volume was sufficiently large to allow ATF droplet breakup, film formation, and drainage without artificial confinement effects. Figure 1 illustrates the modeled computational domain, showing the clearly defined rotating and stationary regions of the motor assembly
Figure 1 : Computational Domain
Mesh
A high-quality unstructured poly-hexacore mesh was generated in ANSYS Fluent Mesher to capture the complex flow interactions caused by the rotating oil jets. Near-wall regions and fluid–solid interfaces were resolved with inflation layers to accurately predict film development and wall heat flux.
The mesh comprised approximately 1.6 million elements, with refinement zones dynamically adjusted using Adaptive Mesh Refinement (AMR) based on gradients of volume fraction. Refinement was automatically triggered in key flow regions such as:
- Jet impingement and splash zones
- Film front propagation areas
- Thin-film breakup and recirculation regions
Figure 2 illustrates the initial non-adapted base mesh, while the accompanying video in Figure 3 further demonstrates the dynamic adaptation process during the simulation, highlighting mesh refinement in regions of active jet impact and film motion
Figure 2 : Mesh without Adaption
Figure 3 : Dynamic mesh adaption process
Numerical Model
A transient, incompressible multiphase simulation was carried out using the VOF model to capture air–ATF interface dynamics. The SST k–ω turbulence model was employed due to its robustness in predicting near-wall behavior and rotational effects.
The simulation accounted for:
- Conjugate heat transfer (CHT) between fluid and solid regions (rotor, magnets, shaft, housing)
- Gravity, influencing film drainage and liquid accumulation
The sliding mesh technique resolved rotor motion, and the VOF scheme was coupled with AMR for dynamic mesh refinement.
| Region | Variable |
| Rotating Jet Inlets (hollow shaft) | Mass flow |
| Temperature (Prescribed inlet temperature) | |
| Magnet region (solid) | Solid domain |
| Magnet — solid/fluid interface | Heat flux |
| Sump outlet (lower outlet / drain) | Pressure outlet |
| Top relief outlet (vent / pressure-relief) | Pressure outlet (vent) |
| All other surfaces | Wall |
Table 1: Boundary conditions
Results
The simulation effectively captured jet impingement, liquid film spreading, and dripping flow regimes across rotating surfaces shown in Figure 3 . The AMR-based VOF model accurately resolved the transient interface evolution, providing detailed insights into liquid wetting patterns, local heat transfer coefficients, and temperature distributions across the rotor magnets and shaft.
Monitored quantities included:
- Average magnet surface temperature
- Wetted area fraction
Figure 3 : Motor oil cooling
Figure 4 represents the temperature contour showing localized cooling zones beneath the impingement regions and thermal gradients across the rotor body. The ATF jets formed thin, stable cooling films which spread circumferentially due to centrifugal forces, promoting uniform temperature reduction.
Figure 4: Thermal profile with localized cooling
Conclusion
A high-fidelity CFD simulation of ATF-based rotating jet cooling for electric motor rotors was performed using the VOF multiphase model coupled with Adaptive Mesh Refinement (AMR).
Key outcomes:
- The model successfully resolved impinging jets, film spreading, and transient drainage on rotating surfaces.
- Conjugate heat transfer between rotor solids and ATF captured local and average temperature reduction with high accuracy.
- AMR enhanced the fidelity of liquid–gas interface prediction while maintaining computational efficiency.
- Results demonstrate the potential of rotating jet impingement cooling as a robust and compact thermal management strategy for high-speed electric drives.
References
- “Poubeau, A., Vinay, G., Alves, B.M., Bai, X., Viot, P. (2024). Numerical simulations of direct liquid cooling of the end-windings of an electric machine. International Journal of Heat and Mass Transfer, 235, 126162. https://doi.org/10.1016/j.ijheatmasstransfer.2024.126162
- Viot, P., Bai, X., Alves, B.M., Vinay, G. (2021). Heat transfer in electrical machines cooled by oil sprays. SAE Technical Paper Series. Bai, X., Alves, B.M., Viot, P., Vinay, G. (2021). High-fidelity CFD modeling of oil jet impingement in EV motors. IFPEN Transports Energie Reports.
- Vinay, G., et al. (2019). Direct oil cooling of electric motors: Experimental and numerical investigations. IFPEN Technical Notes
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