**1. Introduction**

The motor is widely used in ship, municipal, electric power, port handling, and other fields, with broad development prospects and considerable market capacity. In recent years, with the upgrading of power system requirements, the traditional motor with low efficiency and low power to weight ratio has been unable to meet the market demand, which has prompted the need for the research and development of new oil-cooled motors with high efficiency, high power density, low vibration and noise, and strong overload capacity [1,2]. During operation, the motor will produce immense heat, which will reduce the operating efficiency of the motor. In order to ensure the efficient operation of the motor, a heat exchanger is often used to enhance heat transfer and cool the motor. Fluid mediums in heat exchangers are diverse, and the oil phase is widely used because of its superior heat exchange effect, low cost, and long service life [3]. However, when the motor is compact, there will still be areas with a high temperature in the motor after the oil phase heat exchange [4,5]. For instance, in an oil-cooled motor, the temperature at the

end of the coil outlet is higher than that of the iron core section, so further improving the heat transfer effect at the end of the coil outlet can enhance the performance of the whole motor [6–8]. Three ways are acknowledged to enhance heat transfer: increasing heat transfer area, increasing average temperature difference, and increasing the heat transfer coefficient [9]. According to the characteristics of coil oil cooling, the feasible heat transfer enhancement method of the coil end can be analyzed, and the vortex generator can be used to increase the heat transfer coefficient [10].

As a passive heat transfer enhancement technology, the vortex generator can produce vortexes to effectively improve the heat transfer rate of the heat transfer system. Because of its economy and convenience, it has attracted extensive attention in recent years. When it was initially proposed, it was mainly used in the field of aerodynamics [11]. Later, Johnson and Joubert [12] studied the heat transfer enhancement effect of the delta wing vortex generator on the air of the heat exchanger, which initiated the application of the vortex generator in the field of heat exchangers. Chai et al. [13] investigated the improvement of heat exchanger performance via the installation of vortex generators, based on the mechanism of the longitudinal vortex destroying the growth of the boundary layer, increasing the turbulence intensity, and producing secondary fluid flow on the heat transfer surface. There are various structures of vortex generators, such as rectangular wing, triangular wing, trapezoidal wing, cylindrical trapezoidal wing, cylindrical triangular wing, cylindrical rectangular wing, and so on [14–17]. Promvonge et al. [18] studied the influence of the vortex generator, combined with a fin and airfoil, on the heat transfer and drag characteristics of the flow passage under the condition of uniform heat flow boundary. The results showed that the heat transfer efficiency and friction loss of the fluid with the fin and airfoil vortex generator were higher than those with a smooth channel. Chen et al. [19] optimized the aspect ratio of the fluid channel and the height of the vortex generator. The results showed that, in a fluid channel with a large aspect ratio, the heat transfer performance could be enhanced while reducing the pressure loss. So far, many scholars have done a lot of research on the size and attack angle of vortex generators [20–25]. Wijayanta et al. [26] used the *k* − *ε* turbulence model to explore the heat transfer and pressure drop characteristics of vortex generators with various attack angles, and found that the maximum increases in Nusselt number and friction coefficient are 269% and 10.1 times higher than those of smooth tubes, respectively. Zhang et al. [27,28] explored the best combination of length, width, and longitudinal distance of the vortex generator, and its total efficiency was 7.2% higher than that without the vortex generator. Ebrahimi et al. [29,30] studied the heat transfer and fluid characteristics in the laminar flow channel installed with the vortex generator. It was noted that the channel of the vortex generator had higher efficiency, the friction coefficient increased by 2–25%, and the Nusselt number increased by 4–30%.

So far, the application of the vortex generator to enhance heat transfer has mainly been used in the heat exchanger, and air is primarily used as the fluid medium. In addition, the current research on the heat transfer of the vortex generator mainly focuses on the attack angle, size, and shape. However, only a few studies roughly explore the influence of the longitudinal distribution mode of the vortex generator on heat transfer, while there is a lack of detailed and orderly analysis and research on the specific longitudinal and transverse distribution mode of the vortex generator. Therefore, it is of great significance to study the arrangement of the vortex generator on the coil in the motor with oil as the fluid medium. With the help of computational fluid dynamics (CFD) software, the effect of different distribution types of the vortex generator on the heat transfer effect can be simulated, and the temperature change at the end of the coil and the pressure loss before and after the installation of the vortex generator can be analyzed. At the same time, the best distribution type can be obtained, and the mechanism of heat transfer enhancement by turbulence at the end of the coil can be revealed. Thus, through these explorations, this paper can provide a feasible idea for the design and application of the motor in industrial manufacturing.
