Evaporation of Thin Lubricant Films under Laminar and Turbulent Boundary Layers

Abstract

The evaporation of engine oil hydrocarbons from the cylinder liner of internal combustion engines is a significant contributor to engine emissions. This evaporation occurs from the thin films deposited by the piston compression ring as it sweeps along the liner. Establishing the magnitude of the liner evaporation with respect to different lubricant and engine conditions is therefore important to both engine performance and oil consumption. The contribution from liner evaporation to oil consumption has been the subject of several experimental studies which show reasonable agreement with liner evaporation models employing one-dimensional heat and mass transfer. However, models normally eschew specification of the convective mass transfer conditions at the liquid interface, opting instead to employ heat-mass transfer analogies for given convective heat transfer conditions. Direct measurement of the evaporation of discrete oil films under turbulent boundary layer flow is therefore a useful comparative tool for evaluating model performance and methodology, especially for full-engine simulations which employ submodels for liner evaporation. This study experimentally investigates the effects of laminar and turbulent boundary layers on the evaporation of thin, liquid oil films. A wind tunnel, incorporating a flow conditioner and test section, is designed and validated in order to produce repeatable boundary layer flows within an aspect ratio 2 rectangular duct (20 mm x 40 mm) with a length of 1 m (~37.5 hydraulic diameters). Particle image velocimetry (PIV) is used to characterize the velocity fields and turbulence statistics within the test section. Reynolds numbers based on hydraulic diameter of 10,650, 17,750, and 35,500 are considered, with grid-generated inlet turbulence being manipulated by wire-meshes and validated by hot-wire measurement. The evaporation of oil films with initial thicknesses of 50 μm and temperatures of 50°C are then measured directly by an analytical microbalance for different exposures under the laminar and turbulent boundary layers with varying levels of near-wall turbulence intensity and shape. Reynolds number is found to have a significant effect on the evaporation rate. Specifically, increasing near-wall velocity gradient is found to increase the rate of evaporation. Varying near-wall turbulence intensity is shown to have little effect for the length scales of the study, implying that the shear velocity and transport within the viscous sublayer are the predominant parameters governing the convection-limited mass transfer.