Shear localisation in the elastohydrodynamic lubrication regime
File(s)
Author(s)
Jeffreys, Stephen
Type
Thesis
Abstract
Understanding the shear behaviour of lubricants subject to high shear stresses and high shear rates is crucial to successfully model elastohydrodynamic friction. A method for determining this is through measuring the flow condition. Three in situ luminescence-based velocimetry techniques have been applied to a point contact with submicron film thickness. The techniques focus on quantifying through-film velocity distributions to understand the validity of flow assumptions used in modelling EHL.
Nanoparticle tracking velocimetry was investigated to see whether the flow behaviour could be determined from nanoparticle velocity distributions. A protocol was developed deriving formal relations between nanoparticle velocity distributions, nanoparticle concentration distributions and through-film fluid flow velocity distributions, obtained using molecular tagging velocimetry (MTV). Nanoparticle velocity distributions were found to be pressure dependent. Results were proved quantitatively that the pressure dependency was consistent with the observed change in the flow condition; as the flow changed from Couette to partial plug flow above a critical pressure.
The work presents for the first time simultaneous measurements of the velocity and temperature in elastohydrodynamic lubricant films. MTV was applied to determine through-film velocity profiles of a polyphenyl ether lubricant film operating in the EHL under high pressure and high shear. In this work, velocity profiles severely deviate from Couette shear and exhibit shear localisation. The flow behaviour is heavily dependent on the thermal conductivity of the contacting surfaces and shear heating, changing the through-film position of localised shear. A novel complementary molecular tagging thermometry technique, based on the principle of phosphorescence lifetime, was developed and applied to confirm through-film temperature gradients. At higher pressures, the flow behaviour is dominated by the normal pressure, which is observed through a strong asymmetric shape resembling a glassy partial plug. The work provides an improved understanding of the parameters leading to flow heterogeneity in elastohydrodynamic lubricant films.
Nanoparticle tracking velocimetry was investigated to see whether the flow behaviour could be determined from nanoparticle velocity distributions. A protocol was developed deriving formal relations between nanoparticle velocity distributions, nanoparticle concentration distributions and through-film fluid flow velocity distributions, obtained using molecular tagging velocimetry (MTV). Nanoparticle velocity distributions were found to be pressure dependent. Results were proved quantitatively that the pressure dependency was consistent with the observed change in the flow condition; as the flow changed from Couette to partial plug flow above a critical pressure.
The work presents for the first time simultaneous measurements of the velocity and temperature in elastohydrodynamic lubricant films. MTV was applied to determine through-film velocity profiles of a polyphenyl ether lubricant film operating in the EHL under high pressure and high shear. In this work, velocity profiles severely deviate from Couette shear and exhibit shear localisation. The flow behaviour is heavily dependent on the thermal conductivity of the contacting surfaces and shear heating, changing the through-film position of localised shear. A novel complementary molecular tagging thermometry technique, based on the principle of phosphorescence lifetime, was developed and applied to confirm through-film temperature gradients. At higher pressures, the flow behaviour is dominated by the normal pressure, which is observed through a strong asymmetric shape resembling a glassy partial plug. The work provides an improved understanding of the parameters leading to flow heterogeneity in elastohydrodynamic lubricant films.
Version
Open Access
Date Issued
2020-07
Date Awarded
2020-12
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Wong, Janet
Spikes, Hugh
Sponsor
Engineering and Physical Sciences Research Council
Shell
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)