1997-10-01

Mixed Lubrication Modelling of Newtonian and Shear Thinning Liquids in a Piston-Ring Configuration 972924

Mixed-lubrication models comprising of Patir and Cheng's [1,2] average Reynolds equation and Greenwood and Tripp's [3] asperity interaction formulations have hitherto been widely used in predicting piston-ring performance. In this paper a number of models have been developed to allow mixed-lubrication of both Newtonian and shear thinning fluids to be simulated.
Lubricating action usually involves two anisotropic solid surfaces of statistically different profiles. Various forms of the average Reynolds equation and the asperity interaction models require parameters representing the composite surface roughness and profile parameters at the contact. Here a strategy for determining these equivalent composite parameters is presented.
Mathematical simulations indicate that when the composite RMS and composite summit RMS roughness of the contact approach the same value, the performance of the mix-lubrication model becomes dominated by the asperity interaction formulation. Consequently the stochastic aspects of the flow as manifested through the flow factors become negligible. This is a significant result as in most hitherto reported works in the literature, the composite and composite summit RMS roughness values are implicitly assumed to be equal.
The developed models have been applied to determine the performance of a single ring for a number of model fluids and real lubricants. On doing so the pertinent physics of mixed-lubrication of shear thinning liquids has been discussed. Once the shear viscosity function of a lubricant is known, its viscous film generating action can be determined. Any further film generating action could be attributed to viscoelastic effects. The comparisons made with measurements for a number of oils indicate that viscoelastic effects may lead to a considerably greater film generating action than that which may be predicted from a purely viscous hydrodynamic action.

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