Study On Thermorheological Properties And Drag Reduction In Pipes Of Lubricating Grease

Automation and intelligence of mechanical equipments have been deeply developed, thus in the future, and centralized lubrication of mechanical equipments will certainly become the direction of development. However, the flow drag of centralized lubricating system is not yet resolved during grease supplying process, the reduction of which has become the tireless pursuit of human beings. This is mainly because that the lubricating grease is a structural dispersed system and displays semisolid property. It needs to overcome the structural strength and shear resistance during grease shear flow, thus the grease supplying displays high resistance. Therefore, the application of centralized lubricating system is seriously restricted in mechanical equipment systems, especially that the systems are lubricated by high consistence grease. On the other hand, the lubricating grease possesses obvious thermorheological properties. The flow resistance of lubricating grease is decreased at high temperatures, thus the lubricating grease displays better fluidity and pumpability. Aiming at the thermorheological properties and basic methods for drag reduction, this research mainly studies the following aspects.In this study, the basic conception and characterization method of lubricating grease are explained, and the variation of viscoelastic and flow properties is investigated at different temperatures afterwards. Moreover, the rheological parameters which could describe the grease shear flow via mathematical model are obtained by using data fitting method based on the experimental results of flow property and Herschel-Bulkley(H-B) rheology model. Finally, the variation mechanism of thermo-rheological properties is revealed combined with the microstructure and rheological properties of lubricating grease.The theoretical models of grease pipe flow are established based on basic equations of fluid dynamics and constitutive equation of lubricating grease(H-B rheology model). The flow field distribution(flow velocity, shear rate and shear stress) and flow processes(flow rate-pressure drop relationship) are described through those models. And the influence factors and distribution are obtained at last.Combined with the thermorheological experiment results and theoretical models, the effects of thermorheological properties on flow velocity, shear rate and shear stress of grease pipe flow are analyzed. The flow condition of grease pipe flow is discussed, and the previous plug flow assumption is verified by using Reynolds number. Moreover, the effect of thermorheological properties on flow rate is analyzed. Finally, experimental results of flow rate-pressure drop relationship, which is used to verify the theoretical model, is studied by using the test rig of grease pipe flow.Theoretical models of shear rate on the wall and wall slip velocity are established during grease pipe flow with wall slip. Afterwards, the effect of temperature on wall slip is studied during rheological test process, and the rheological parameters at different analysis conditions are obtained by using data fitting method. Furthermore, the effect of temperature on shear rate on the wall and wall slip velocity are analyzed based on rheological test results and theoretical models. Finally, the improvement mechanism of wall slip for grease pipe flow is revealed combined with Lubrication slip model and wall slip analysis results of grease pipe flow.The grease thermal ageing after being heated is simulated by using static thermal ageing method, and the highest operating temperature(120℃) is selected as the experimental temperature. Afterwards, the variation of colloidal property of grease samples is investigated via oil separation experiment. Furthermore, the entanglement of fibrous structure and the variation of chemical structure for grease molecular groups after being heated are studied by using Field Emission Scanning Electron Microscope(FESEM) and Fourier transform infrared spectroscopy(FTIR) techniques. Finally, the tribological performance of each grease sample after being heated at different conditions is researched by using four-ball friction testing machine, and variations of anti-friction/anti-wear and extreme pressure properties are discussed. The heat treatment time, which could maintain the original properties of lubricating grease, is obtained combined with the mechanism discussion of all grease samples during lubrication process. The critical parameters for thermorheological properties being used in drag reduction are obtained via thermal ageing analysis.Research results from this dissertation may play an important role in guiding the thorough study of thermorheological properties and drag reduction in pipes of lubricating grease and they could provide a basic theory support for drag reduction of lubricating grease with high consistence degree in centralized lubricating system.