Experimental Investigation into the Challenges of High-Viscosity Oil Spray Cooling Systems

Abstract

Oil spray cooling is increasingly adopted for direct thermal management of hairpin end windings in electric traction motors, yet high-viscosity oils introduce atomization and coverage limitations that are not well quantified using practical, off-the-shelf hydraulic nozzles. This paper experimentally investigates how lubricant viscosity (via temperature), nozzle type, and inlet pressure jointly influence (i) nozzle inlet parameters (flow rate and pressure evolution), (ii) spray formation quality, and (iii) the trade-off between heat removal and temperature uniformity on a representative hairpin conductor. Flat-fan, full-cone, and hollow-cone nozzles are tested under identical system constraints (1–4 bar, flow limited by the pump) and assessed using spatially distributed thermocouples. Results show that increased viscosity at low temperature reduces flow rate and can distort spray patterns, with the full-cone nozzle most sensitive to viscosity while the flat-fan nozzle remains comparatively robust. At 4 bar, the hollow-cone nozzle L40 yields the most uniform surface temperature but the lowest heat-transfer coefficient, whereas the flat-fan NF0390 and full-cone MPL0.3M provide the best balance between uniformity and heat removal. A simplified nondimensional interpretation is provided to link observed performance to internal flow regime and impact dynamics, enabling practical nozzle-selection guidance for early-stage design.