Summary
A comprehensive theoretical analysis of turbulent flow of a power-law fluid in coiled tubing was conducted with the approach of boundary layer approximation. Equations of momentum integrals for the boundary layer flow were derived and solved numerically. Based on the results of the numerical analysis, a new friction-factor correlation was developed which is applicable to a wide range of flow behavior index of power-law fluid model. The new correlation was verified by comparing it with the published Ito correlation for the special case of Newtonian fluid. For non-Newtonian fluids, there is also a close agreement between the new correlation and the experimental data from recent full-scale coiled tubing flow experiments.
Introduction
Many fluids that are pumped through coiled tubing are typically non-Newtonian fluids, such as polymer gels or drilling muds. Understanding their flow behavior and being able to accurately predict frictional pressure through coiled tubing are essential for better operations design. A recent literature review (Zhou and Shah 2004) indicates that though there are numerous studies on the flow of Newtonian fluids in coiled pipes, there is, however, very little information with regard to the corresponding flow of non-Newtonian fluids.
Among the various approaches of investigating fluid flow in coiled pipes, there is one important method called boundary layer approximation analysis. It is especially useful for high-Dean (1927, 1928) number flows where the effect of secondary flow is largely confined in a thin boundary layer adjacent to the pipe wall (Dean number is commonly defined as: (equation). According to this approach, the tubing cross-section can be divided into two regions: the central in viscid core, and the thin viscous boundary layer. This leads to much simplified flow equations for high-Dean number flows in curved geometry. This approach has been used by a number of researchers, for example, by Adler (1934), Barua (1963), Mori and Nakayama (1965), and Ito (1959, 1969) for Newtonian fluids, and by Mashelkar and Devarajan (1976, 1977) for non-Newtonian fluids.
In a previous attempt, Zhou and Shah (2007) applied the method of boundary layer approximation to solve the laminar flow problem of a power-law fluid in coiled tubing and obtained an empirical friction-factor correlation based on the theoretical analysis and numerical solutions. In the present study, we take the same analysis approach but consider the turbulent flow of a power-law fluid in coiled tubing. A friction-factor correlation for turbulent flow in coiled tubing is developed, and its validity is evaluated with a published correlation (Ito 1959) and recent full-scale experimental data.
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