Numerical Simulation of the Rotating Cage Problem

The rotating cage apparatus is used in the oil industry as a simple and effective test for corrosion. The apparatus generates a large amount of flow induced corrosion and is used by researchers to design effective corrosion inhibitors. While the apparatus is used frequently, the fluid mechanics of the problem is not well understood. Importantly, there is no correct model for the shear stress profile. In this paper we study the unsteady fluid dynamics of the rotating cage problem. We consider an equivalent, idealized, two-dimensional problem for a Newtonian fluid and perform high resolution simulations of the rotating cage using a vortex method. The vortex method is a particle method that is well suited to complex geometries, unsteady flow and capable of high resolution. We present initial results of various simulations performed at different Reynolds numbers. The shear stress is computed and compared with currently used models. INTRODUCTION Oil and gas operations involve the transport of oil and gas at high pressure and flowing at significant speeds. Oil and gas pipelines transport a large amount of material and are usually of very large lengths. The flowing oil and gas can corrode the pipelines. Frequent replacement of these pipelines is expensive in terms of down time and material costs. Oil companies therefore use corrosion inhibitors to reduce the amount of corrosion. These corrosion inhibitors are usually proprietary chemicals that are added in small quantities. They significantly reduce the amount of corrosion due to the flow. If one is to create effective corrosion inhibitors, it is important to be able to test the effectiveness of the inhibitor in the laboratory using a simple experiment in a repeatable fashion. There are many experimental techniques that are well suited to producing high amounts of flow induced localized corrosion (FLIC). The various experimental techniques to do this are critically reviewed in Schmitt and Bakalli [1]. The rotating cage is one such apparatus that is recommended by several researchers as an appropriate test for corrosion inhibitor performance under severe conditions. The apparatus consists of a collection of flat blades (called coupons) arranged along the circumference of a cylinder. This cage of blades is placed inside a cylinder filled with a viscous fluid. Fig. 1 is a sketch of a rotating cage apparatus. The cage is rotated at high speed for several hours. In [2] the test is run for 18 hours and in [3] for 120 hours. The coupons are weighed before the experiment is started. At the end of the experiment they are cleaned and weighed again. The difference in weight divided by the time taken provides an average corrosion rate. The coupons are also inspected visually to provide an idea of the kind of corrosion that occurs. The apparatus can also be used under high pressure and temperature [2]. This approach is commonly referred to as the “high speed autoclave test” (HSAT). The rotating cage has been successfully used in practice to develop a large number of corrosion inhibitors [4,2,3]. The important advantages of the rotating cage apparatus are that it is relatively easy and inexpensive to setup and use, it is easy to reproduce high pressure and high temperature conditions, the coupons experience a very large shear stress and therefore experience severe flow conditions. Corrosion in1 Copyright © 2006 by ASME