1. Introduction
In recent years, following the increase in worldwide awareness towards environmental issues such as climate change, stringent regulations have been established for greenhouse gas standards to achieve carbon neutrality. Accordingly, the development and commercialization of environmentally friendly electric vehicles have been accelerating, and the need for the research and development of core technologies for environmentally friendly power sources in electric vehicles has increased [
1]. However, compared to the batteries used in internal combustion engines, the batteries used in electric vehicles are not sufficiently developed to produce large outputs; one of their limitations concerns battery thermal management for consistent performance [
2,
3]. Additionally, there is a decrease in mileage due to factors such as the weight of added parts. The in-wheel motor technology, which can reduce the vehicle weight, can be used to solve this problem. In an in-wheel motor, the suspension, steering, and braking components as well as the drive motor of the vehicle are built into the wheel; each motor is independently driven through distributed actuation [
4]. In the case of the conventional inline motor method, significant power loss occurs when the power is delivered from the engine to the wheel. In contrast, the in-wheel motor method can reduce power loss with a minimized power delivery system and is advantageous for saving internal space. Furthermore, better steering performance compared to that of the inline motor method, which controls the drive shaft, can be achieved [
4]. It is argued that the use of in-wheel motors increases the unsprung mass, leading to more discomfort of the passengers compared to the conventional powertrain system. Separate research is ongoing to eliminate the unsprung mass from the in-wheel motor for the sustainable development of in-wheel motors [
5]. Considering the above facts, for the sustainable development of in-wheel motors, an effective thermal design is also necessary to regulate the temperature, which significantly affects the performance and durability of the in-wheel motor [
6,
7].
Cooling methods used for regulating the in-wheel motor temperature in electric vehicles are classified as air-cooling, oil-cooling, or water-cooling methods. In the air-cooling method, the heat generated by the motor is radiated through direct contact with external air. While driving, the heat generated from motor operation is dissipated through the heat exchange between the external housing surface encompassing the in-wheel motor and the surrounding air. A cooling groove on the housing surface, which is used for maximizing the heat exchange with the ram air, is crucial for these cooling methods, in which the air acts as a heat transfer medium [
8]. The air-cooling method, which is the simplest among the three cooling methods, is adopted when the motor power is small or when there are difficulties in connecting the coolant pipes. However, the air-cooling method has limitations in controlling the amount of generated heat, which rapidly increases when the in-wheel motor rotates at a high speed as the vehicle drives. The oil-cooling method utilizes the pump with the oil as a cooling medium to circulate the oil inside and outside of the motor and reduce the heat. Oil has a higher convection coefficient than air. Therefore, the oil-cooling method demonstrates a higher thermal performance than the air-cooling method. It also enables direct spraying on the exothermic component [
9]. Concurrently, cooling oil can be used for lubrication, which allows for an integrated module design with a reduction gear and increases the lifespan of the motor. However, the viscosity of the cooling oil may significantly vary according to the temperature at which a coolant is used to reduce the heat generated by the motor. The water-cooling method can use coolants with a higher convective heat transfer coefficient. Compared to other cooling methods, the water-cooling method exhibits a superior performance. When a coolant is used for the heat dissipation of the motor, the temperature of the permanent magnet and the winding temperature decrease by approximately 26% and 32%, respectively, compared to the case when a coolant is not used [
10].
Various experimental and numerical studies have been carried out using water-cooling channels utilizing different configurations of the heat exchanger. Fasquelle and Laloy [
11] used a water-cooling system applied in a permanent magnet synchronous motor and obtained the desired cooling performance of the synchronous motor. Another three-dimensional numerical study by Rehman and Seong utilized the cooling water jacket for the housing for a three-phase induction motor [
12]. A higher cooling performance was reported for a six-pass cooling jacket with two-port configuration at a flow rate of 10 LPM for the safe operation at the maximum motor output. The oil-cooling channel with three cooling positions was studied for a traction motor of a hybrid electric vehicle in a numerical investigation by Huang et al. [
13]. The authors reported that direct cooling methods were found to depict the smaller temperature rise of the stator. A water loop placed at the bottom of the teeth with two parallel flow paths by Li et al. [
14], unlike the conventional water cooling in their experimental investigation, was found to be a promising method of end-winding cooling for an electric motor. In a numerical analysis, different shapes of cooling ducts such as spiral, U-shaped (one duct), U-shaped (bifurcated), and an axial water jacket were used between the housing and stator to investigate the cooling performance of the induction machine by Satrustegui et al. [
15]. The authors reported a higher pressure drop for the axial water jacket than the spiral water jacket for the same heat transfer. Studies have also been carried out using water–ethylene glycol in the spiral cooling channel of an electric motor by Deriszadeh et al. [
16]. The authors reported that the cooling performance of the motor increased with the increase in ethylene glycol concentration and number of turns for the spiral channel, and the heat transfer coefficient subsequently increased. Furthermore, the increase in the heat transfer coefficient was more significant than the increase in the pressure drop. Yang et al. [
17] tested and simulated oil jacket cooling for an electric motor. It was suggested that the cooling performance of the motor was better at a smaller height and width of the channel but at the cost of a higher pressure drop.
It is evident from the above literature that numerous designs of the cooling channel have been used at different locations of the electric motor. However, more designs need to be identified that deliver a higher cooling performance for an electric motor. In addition, there have been no studies utilizing the different turbulence enhancers (vortex generator) for a water-cooling channel. A higher cooling performance is expected when the coolant proceeds in a turbulent flow by joining the different passages of the water channel with multiple corners, compared to the case when the coolant proceeds via a plain water channel in a laminar flow [
18]. Moreover, to avoid problems associated with insulation or corrosion, direct contact between the motor components and the coolant should be prevented. Therefore, a separate water-cooling channel is thus necessary. In addition, water mixed with ethylene glycol lowers the freezing point of water. The heating of the permanent magnet in the motor leads to its demagnetization and therefore, the magnetic force of the motor decreases. When the temperature exceeds 180 °C, irreversible demagnetization occurs, wherein the magnetic force is no longer restorable [
19,
20]. In addition, the coil insulator melts when its temperature exceeds the temperature corresponding to heat resistance; subsequently, the stator core and coil directly come into contact, which results in serious problems such as motor operation failure. Considering the safety factor according to the various operating conditions of the in-wheel motor, which is the target object of this study, the maximum temperature for the rotating and fixed unit components must be set to 150 °C or lower.
Following the above reasons for the requirement of the cooling of in-wheel motor and considering the aforementioned limitations in the literature, a water-cooling method was applied to a 25 kW interior permanent-magnet-type in-wheel motor for effective heat dissipation in the present study. Furthermore, two types of vortex generators modified from a trapezoidal vortex generator were designed, and their thermal performances were compared and evaluated through a computational fluid dynamics (CFD) analysis. Initially, the cooling channel is designed with single vortex generator for enhancing the turbulence intensity inside flow regime. Later, considering the higher turbulence intensity, the cooling channel is designed with pair type vortex generator. Further, the investigation has been made to evaluate the cooling performance of the water channel considering the ram air effect under high-speed operating conditions of an actual in-wheel motor.
4. Conclusions
In this study, a water channel was designed to water-cool an in-wheel motor, and thermal flow analyses were performed at base and maximum speeds to determine the durability and performance of the in-wheel motor for electric vehicles. Accordingly, the thermal performance and temperature of each component were estimated. A vortex generator was installed inside the water channel to generate coolant vortices and induce wake flows to improve the thermal performance of the in-wheel motor.
(1) The in-wheel motor without a vortex generator, which only possessed a simple water channel, showed high temperatures at the permanent magnet and rotor core under maximum-speed conditions. In particular, the volumetric average temperature of the permanent magnet was 156.9 °C, which involved risks of irreversible demagnetization of the magnet and motor performance degradation. Therefore, the cooling performance of the water channel must be improved to effectively discharge the motor heat.
(2) Upon using vortex generators placed inside the water channels of the in-wheel motors, the temperature of the permanent magnet of the rotating unit in the models with a single-vortex generator and pair-type vortex generator decreased by 3.8 °C and 6.0 °C, corresponding to 4.1% and 6.5% improvements, respectively, in the in-wheel motor thermal performance based on the coolant entrance temperatures.
(3) Under high-speed driving conditions of the vehicle and maximum speed conditions of the in-wheel motor, upon considering the ram air effect, the maximum temperature of the permanent magnet decreased by 2.1 °C. The resulting maximum temperature of 148.8 °C satisfied the design limit of the in-wheel motor temperature.