Influence of Pipe Rotation and Reciprocation in Flow of Non-Newtonian Fluids in Eccentric Annular Spaces

Some industrials processes are associated with flow of non-Newtonian fluids in annular spaces created between parallel pipes. Examples are found in oil industry and food industrial processing. Depending on relative position of both axes, a concentric or eccentric annular space is created. In some typical applications the fluid rheology non-Newtonian and models such as Bingham Plastic or Power Law are required for adequate representation of internal deformations of fluid elements when shear stresses are applied. Depending on annulus eccentricity high resistance can be opposed to flow on narrowest section, including the possibility of having static or quasi-static fluid close to the internal annulus walls. In order to remove this static fluid, two different operations are usually proposed: pipe rotation and pipe reciprocation. In this way, less mobile fluid can be put in motion increasing shear stress. These operations are justified by experimental evidence exists. Scale experiments have been done and predictions for flow behavior in large facilities are extrapolated. However, in large facilities, as oil wells are highly pressurized and they are very deep, it is almost impossible to verify if the whole fluid is mobile and no by-pass fluid remains in the narrowest section of annular space. So, Computational Fluid Dynamics constitutes an ideal technique for analyzing this kind of problem. In this paper, though a Computational Fluid Dynamics study we aim to evaluate the efficiency of pipe rotation and pipe reciprocation in static or quasi-static fluids for Bingham Plastic or Power Law fluid. In order to consider realistic scenarios, oil industry typical conditions are considered for fluid density, rheological parameters, flow rates, casing and hole sizes, and annulus eccentricity. The influence of the variables eccentricity and rotation speed, and the use of reciprocation in shear stress at walls, were used as a measure to evaluate efficiency in static fluid removal. The flow regime was considered laminar. Numerical model capability to reproduce accurately flow patterns in these conditions was assured by comparing it with others analytical-numerical solutions for concentric systems. Results show that both operations are effective for helping in static fluid remotion. However, notable increment for efficiency is observed for eccentricities below 60%. In particular, pipe rotation is effective when rotation speed is greater than 20 RPM for eccentricity greater than 40%. Below this limit, pipe reciprocation is more effective than pipe rotation, independently of the rheological model used to represent the fluid.