title: Investigation of Flows in Air Gaps of Electric Motors with Direct Liquid Cooling
Creators: name: Auernhammer, Ariane Clara
Publisher: name: UNSPECIFIED
Year: 2025

description: In this dissertation, the flow in the air gap between a concentric rotor and stator is experimentally investigated by applying advanced laser and torque measurement techniques. Such air gaps are part of motors of electric cars, which are considered a vital part in reducing greenhouse gas emissions in the transport sector. Electric motors require cooling for efficient operation. A common cooling strategy is direct liquid cooling. The flow processes of the two-phase flow generated in the air gap when liquid coolant enters the gap and their effect on the motor’s power loss need to be understood and therefore investigated. Thus, a systematic study is conducted with increasingly complex setups of optically accessible models of an electric motor (Generic and Realistic Model) regarding geometric features, liquid injection, and operating conditions like rotational speeds, volume flow rates, or temperatures.

The single-phase air in the annular gap (radius ratio 0.99, aspect ratio 100) of the simplified Generic Model is investigated using particle image velocimetry (PIV) at rotational speeds from 100 rpm to 2000 rpm (gas Reynolds numbers ReG 52 to 1035). The flow regimes observed are found to develop similar to those described in literature. The single-phase flow determines the initial condition for the subsequent two-phase flow investigation.

In a phenomenological study, a liquid is injected into the air gap of the Generic Model using a lance to model direct liquid cooling. The annular gap and a similar gap with axial grooves in the stator are investigated at different rotational speeds (100 rpm to 2000 rpm) and liquid volume flow rates (0.26 l/min to 0.72 l/min). The rotational speed has the greatest influence on the three classified main two-phase flow regimes.

The two-phase flow in the air gap with grooves of the Generic Model is investigated at a volume flow rate of 0.5 l/min and rotational speeds from 100 rpm to 2000 rpm, using high-speed laser-induced fluorescence (LIF) imaging. LIF, enabled by the fluorescing liquid, allows the distinction of both phases in the flow and the detailed study of the flow regimes. Additionally, torque measurements are performed, from which the power loss due to the injected liquid is calculated. The power loss can be linked to the flow regimes.

The two-phase flow in the air gap of the Realistic Model is investigated by including realistic component geometries, liquid injection system, cooling liquid, and the variation of the rotational speed (2000 rpm to 10 000 rpm, ReG 931 to 4656), volume flow rate (1 l/min to 6 l/min), liquid temperature (20 °C to 80 °C), and operational mode. During operation, a liquid film is formed on the stator. The flow phenomena of this film are analyzed utilizing high-speed LIF recordings. An intensity-based LIF film thickness measurement technique is developed, calibrated, and applied in the air gap. The most influential factors on the flow phenomena and the film thickness are rotational speed, volume flow rate, and, in certain cases, the operational mode. The power loss, calculated from torque measurements, increases with rotational speed, but its magnitude is low compared to other types of losses and the power output of a passenger car’s electric motor.

The analysis of the single and two-phase flow in the air gap provides new insight into the flow processes and the associated power losses. The results are important for the development of electric motors with direct liquid cooling and allow the validation of numerical simulations.