Lubrication mechanisms for oil-in-water emulsions.
This dissertation presents a series of experimental and computational investigations of the lubrication mechanisms for oil-in-water emulsions. The elastohydrodynamic film thickness and traction behaviors of oil-in-water emulsions using three lubrication mechanisms, plate-out, Chiu-type starvation and dynamic concentration, are discussed. The effect of droplet size is considered. Water is found to be present in the inlet region in Chiu-type starvation and make up about 80% of the lubricant at the contact in dynamic concentration. Droplet entrainment in the inlet region is a focus of this dissertation. Both two and three dimensional simulations of the oil droplet movement in the inlet region are conducted. Large droplets segregate to the center of flow, while small droplets have two off- center stable segregation positions and one neutrally stable segregation position at the flow center. Ignoring the interactions between oil droplets, all droplets segregate to the backflow region and are rejected from the contact zone regardless the rolling speed. This is consistent with the direct observations of emulsion flow, performed at industry relevant speeds, using emulsions with three different mean droplet sizes. Three types of oil droplets are observed. "Stay" droplets locate a certain distance from the edge of contact; "penetration" droplets are those attached to the surfaces or merged into the oil pool; and most oil droplets are "rejected" droplets regardless the droplet size and rolling speed. Both EHL point contact and line contact are considered. The effects of droplet size and initial oil concentration on the extent of oil pool are also discussed. From these results, it can be shown that the lubrication efficiency of emulsions depends on the system's ability to attach oil droplets to the flow boundaries. It is suggested that future research should be directed towards pulsed lubrication or the use of other means to perturb oil particles and encourage their attachment to surfaces.