Electric motor or e-motor cooling is a vital aspect of the thermal management system for vehicles. Efficient cooling leads to significant boosts in performance and contributes towards the use of smaller motors for a given application or increasing the range of batteries. As a result, it is crucial to meet the space, weight, and cost targets of the project.
Making use of CFD simulations can deliver great insights into the e-motor cooling process to optimize the design stage. However, running comprehensive simulations for e-motor cooling can be rather costly in terms of computational power.
This is a great opportunity to apply the new Continuous Particle Size (CPS) feature which allows fine particles for regions where we have a large amount of heat transfer, and efficiently increases the particle size of the fluid in the regions of lower interest. Details about Continuous Particle Size (CPS) can be found in the article: “Continuous Particle Size (CPS) and Adaptive Refinement“. As a result, the number of particles required in the simulation is minimized and the computational effort can be reduced.
Apart from this, a volumetric heat source is necessary to best simulate the heat generated within the components of the e-motor. Up until now only surface thermal boundary conditions were available in PreonLab.
Now with version 5.2 and PreonLab’s new feature called heat field, a user-defined heat source can be generated within a volume. The heat source can be applied to the domain and controlled spatially via an embedded geometry or even a point cloud resource.
A point cloud resource object in PreonLab is a cloud of sampling points that can be assigned with different user-defined values such as heat source strengths.
Figure 1a depicts the temperature distribution in a sphere resulting from a uniform heat field, Figure 1b showcases the use of an embedded spherical geometry to control where the heat field is applied (red particles), and Figure 1c shows the concept of applying the heat field via a point cloud resource object.
For the examples in this article, we consider an e-motor with hairpin-style copper windings as is shown in Figure 2. The temperature within the windings can reach up to 120°C.
Figure 2: Main components of an e-motor.
Three types of cooling are investigated in this article: Jet cooling from the rotating shaft, jet cooling from injection points along the stator, and jacket cooling.
The e-motor is cooled with oil as a coolant for the jet cooling method and water or a mixture of water and ethylene glycol as the coolant for the jacket cooling method. This is done because we do not want the water to come into contact with the components of the e-motor directly.
Furthermore, the oil is already available in the gearbox and can be used for cooling the e-motor without much additional effort.
Note: For demonstration purposes, only a wedge-shaped section of the e-motor was simulated for all three types of cooling investigated in this article.
Figure 3: Inlets for oil cooling within the rotating shaft.
Similarly, the copper windings can also be cooled by oil which is emitted out of injection points along the stator. In the following section, we look at the simulation results for this type of cooling.
Using CPS, a larger range of particle sizes can be used for the simulation. As can be seen from Figure 4, it is possible to use particles with a size as low as 0.125 mm at the impingement area and a particle size of 0.25 mm near the windings.
Once the oil flows away from the windings, the particle size is increased to 1 mm. Consequently, the simulation time can be reduced from 26 hours (achieved using 2 levels of refinement with PreonLab version 5.1) to 23 hours on 64 CPUs.
Figure 4: Different particle sizes used for oil during simulation with CPS.
Conjugate heat transfer:
An important aspect is the analysis of the conjugate heat transfer between the oil and the windings. For this purpose, a solid solver is added to simulate the heat transfer within the copper windings. The conjugate heat transfer can be observed in the following video:
In addition to the two types of jet cooling discussed so far in this article, the e-motor could also be cooled by jacket cooling, where the coolant flows through channels without coming into direct contact with the internal components of the e-motor, as is shown in Figure 5.
The heat field comes in handy during this investigation as well to generate heat not only within the windings but also within the stator. This is necessary to model the iron losses which occur within the ferromagnetic core.
Figure 5: Jacket cooling: The channel through which the water flows is colored blue in the image.
Based on all the information gathered via analysis of the three types of cooling, the design engineer can vary the cooling system parameters such as the type of coolant and the cooling pattern which includes the size, location and number of oil inlets and perform further simulations.
Apart from the effectiveness of cooling, information about the oil infiltration in the air gap between the rotating shaft and the stator can be vital for design engineers.
When it comes to optimizing the design of the e-motor, this oil infiltration needs to be minimized.
The air gap is considerably smaller than the other components of the e-motor and we need very fine particles at this location to resolve the flow properly during simulation. This can be observed in Figure 7.
Due to the large size ratio of the particles possible with the CPS feature, the infiltration of oil in the air gap can be analyzed as part of the same simulation along with the jet cooling simulation.
Without CPS, this would be rather difficult in terms of computational effort.
Figure 7: Oil infiltration in the air gap between the rotating shaft and the stator.