Experimental and numerical study of heat transfer characteristics of single-phase free-surface fan jet impingement with automatic transmission fluid

Abstract

Due to the ever-increasing power density of the advanced electric machines (motors/generators) in electric vehicles and hybrid electric vehicles, thermal management of electric machines has become increasingly critical to permit high power output, enhanced reliability, and miniaturization. The cooling of electric machines is mostly accomplished by depositing fluids or oils directly onto the machine's coil windings to remove the excessive heat. As an efficient approach of direct cooling, jet impingement has been attracting significant attention from researchers and industry due to the high heat transfer rates and large cooling capacity. In this work, we present a study of fan jet impingement of automatic transmission fluid (ATF) on a target surface with a constant heat flux. Experiments were carried out with ATF temperatures varying from 50 °C to 90 °C and flow rates varying from 1 × 10−5 to 2.5 × 10−5 m3/s to encompass potential operating conditions within the automotive transaxle environment. A numerical model of the test section was developed and validated with experimental results and used to investigate the influence of flow parameters and nozzle geometry on the heat transfer characteristics of the impinging fan jet. Major controlling parameters studied in this work include flow rate, fluid temperature, nozzle-to-target distance, and incidence angles. Average heat transfer coefficients are calculated as the major metric to compare the impact from various controlling parameters. Correlation of flow and heat transfer characteristics of fan jet impingement is explored within the scope of this work. In addition, heat transfer coefficient results of fan jet impingement are compared with the circular jet, which is commonly applied to cool electric machines. At all flow velocities, circular jets yield heat transfer coefficients 20%–25% higher than those obtained from the fan jet impingement.