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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. U.S. Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.

Thermal management for electric machines is important as the automotive industry continues to transition to more electrically dominant vehicle propulsion systems. With the push to reduce component size, lower costs, and reduce weight without sacrificing performance or reliability, the challenges associated with thermal management for power electronics and electric machines increase. The transition to more electrically dominant propulsion systems leads to higher-power duty cycles for electric drive systems. Thermal management of electric machines directly improves power density and reliability; however, it is a significant challenge because heat transfer and fluid flow in electric machines are complex. Thermal management is one path to improve the performance, efficiency, cost, and the sizing of electric machines to operate within thermal constraints.

In this study, the results of NREL’s continued work on experimental characterization of the thermal performance of free-surface jets of automatic transmission fluid impinged on a heated target surface are presented. The measured heat transfer coefficients are useful for understanding factors influencing performance of driveline fluid-based cooling systems for electric machines and help designers in developing high-performance, power-dense and reliable machines. Experiments were carried out for different fluid and target surface temperatures (50°C, 70°C, and 90°C for the fluid and 90°C, 100°C, 110°C, and 120°C for the target surface). Impinging jet velocities (0.5 m/s to 7.5 m/s) and the jet position on the target surface (center versus edge) were also varied. The impinging angle was kept at 90° relative to the target surface. It was found that higher target surface temperature increased heat transfer coefficients, namely, increasing surface temperature from 90°C to 120°C enhanced heat transfer coefficient values at higher impinged jet velocities (7.5 m/s) by up to 15%.

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