| Summary: | 博士 === 國立中山大學 === 機械與機電工程學系研究所 === 104 === A mixed-film model with two adsorbed layers on the solid surfaces and an emulsion layer between them is proposed. The modified Reynolds and energy equations of binary mixtures are derived and solved for this model. A series of simulations using this model are carried out. For the isothermal EHL (elastohydrodynamic lubrication), “the critical speed” is firstly identified, based on which the adsorbed layers can be merged or separated. The two adsorbed layers are separated by the emulsion when the rolling speed is greater than the critical speed, and the central film thickness increases along with the rolling speed and supply oil concentration, because more emulsion is entrained into the Hertzian contact zone. When the rolling speed is less than the critical speed, the total thickness of the two adsorbed layers is greater than the central film thickness, and thus they merge to form the continuous oil phase. If the rolling speed continues to slow down, the position of the continuous oil phase moves upstream, and finally an oil pool is formed. Compared with the experimental results, the nano thickness of the adsorbed layer at the entrance can be successfully predicted. However, the adsorbed layer thickness increases with decreasing rolling speed, because there is enough time to replenish the adsorbed film.
For the TEHL (thermal elastohydrodynamic lubrication), the maximum temperature rise occurs at the emulsion layer because the emulsion layer is the main source of heat. The heat source transfers into the upper interface between the adsorbed and emulsion layers with higher speed in a shorter time, so that its temperature rise is less than the lower one. The minimum film thickness for the thermal case is close to the isothermal solution at the average rolling speed less than 3 m/s, but the difference between them increases along with the rolling speed because the viscosity of the emulsion layer decreases with increasing mean temperature rise. This trend is the same as the result of pure oil in thermal EHL. The temperature rise across the adsorbed layer gradually decreases from the interface to the surface. The surface temperature rise increases with decreasing adsorbed layer thickness due to the shorter distance for the conduction heat transfer. Compared with the results of the pure rolling, the decrement of the minimum film thickness for the lower supply oil concentration at high slide/roll ratio is less than that for the higher one, because the viscosity of the emulsion layer with more amount of water phase is decreased slightly besides the lower temperature rise. Compared with the results of the maximum Hertzian pressure ph = 0.284 GPa, the decrement of the minimum film thickness for the lower supply oil concentration at ph = 1.421 GPa is less than that for the higher one due to the same reason.
For the EPHL (elasto-plasto-hydrodynamic lubrication) of cold rolling, there existed a range of roll speeds which provided sufficient friction to perform the rolling process without front tension under the merged conditions and the lower roll speeds. At the high roll speed, the two adsorbed layers were separated by the emulsion layer, and the pressure hill disappeared due to the smaller surface shear stress acting on the strip, so that the rolling was no longer possible unless accompanied by a front tension. The greater the front tension, the greater the velocity of the strip in the work zone, and the thicker the emulsion layer. For the separated case, the velocity of the adsorbed layers along the z axis almost remained constant, and the shear strain rate due to the surface velocity difference between the roll and the strip mainly occurred at the emulsion layer. However, the surface shear stress acting on the strip was affected by the adsorbed layer, so that it was very small. The inlet film thickness increased with decreasing effective modulus of elasticity or increasing front tension.
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