Effects of Water Contamination of Lubricants on Hydrodynamic Lubrication: Rheological and Thermal Modeling

Even if water has more favorable thermal characteristics than oil, its use in the realm of bearings is still restricted to some rare applications. Moreover, the presence of water in lubricating circuits is seen as not at all desirable. The contamination of oil by water results in dangerous effects on lubrication. In relation to this, the study of thermal enhancement that this contamination can provide is seen to be insignificant. The literature demonstrates the damaging effects of this type of contamination. However, the existing studies are commonly based on an analysis of the results obtained after a bearing failure. The present study evaluates the instantaneous effect of the water-oil mixture on hydrodynamic lubrication for significant levels of water concentration, up to 10% by mass. The aim of the work is to identify the conditions for which the presence of water is detrimental in lubrication. Such conditions could then be avoided and a new generation of lubrication circuits could be designed to be immune against water contamination. Moreover, any positive effects of water contamination on lubrication deserve to be studied, the possibility being that new lubricant concepts might emerge. Thus, the rheological behavior and thermal characteristics of the mixture (density, specific heat, and thermal conductivity) were numerically modeled and simulations of bearings operating with this mixture were performed. The lubrication characteristics were also measured on a tilting pad thrust bearing, similar effects to those obtained numerically being observed. The presence of water has a slight effect on lubrication, which is nevertheless recognized to amount to an improvement in the lubrication characteristics. In fact, it is found that pure oil could be replaced by a water-in-oil emulsion having the same viscosity. In this case, the film thickness and the friction coefficient will be weakly modified, whereas the bearing will run at a lower temperature. From the point of view of safety, this indicates a significant advantage in operating conditions.