Numerical Investigation of Turbulent Flow around a Rotating Stepped Cylinder for Corrosion Study

The Canadian Journal of Chemical Engineering, Volume 81, February 2003 Complex flows with boundary layer separation and reattachment commonly occur in situations of practical interest. In such flows, any accompanying heat or mass transfer is strongly influenced by the fluid dynamics. For example, rates of heat or mass transfer to the wall can have large variations in the reattachment region. In the case of heat transfer, a practical example is local overheating downstream of a heat exchanger pipe fitting. When it comes to mass transfer, accelerated metal loss is often encountered downstream of flow disturbances due to enhanced mass-transfer-controlled corrosion (Postlethwaite, 1986; Mahato et al., 1968; Nesic and Postlethwaite, 1990). This phenomenon is often termed flow-accelerated corrosion or erosion-corrosion and is most severe in complex flow situations, for example, at heat exchanger tube inlets (Elvery and Bremhorst, 1997; Elvery and Bremhorst, 1996), pipe bends, downstream of orifice plates, valves, fittings, weld beads and in turbo-machinery including pumps, turbines and propellers (Postlethwaite and Nesic, 2000). In all of these cases, increased turbulent transport in the near-wall region enhances mass-transfer rates of species involved in the corrosion reaction. Most of the time these are anionic species which are consumed by the corrosion reaction at the metal surface and need to be replenished by the transport from the bulk. In other cases, they are products of protective film (scale) dissolution which are swept away from the surface leading to more rapid film removal and accelerated corrosion attack. In a recent study, a new compact experimental setup was tested for study of erosion-corrosion under disturbed flow conditions, involving a rotating cylinder geometry with two axisymmetrically-mounted sudden steps (Nesic et al., 2000). It is believed that this geometry can become a simple and effective tool for studying erosion-corrosion under disturbed flow conditions and substitute much more complex and expensive flow-loop based systems. Initial characterization of the stepped rotating cylinder electrode was conducted, involving wall heat/mass-transfer measurements (Nesic et al., 2000; Bienkowski, 1998). The measured variation in the heat and mass-transfer rates behind the step suggested that a complex flow pattern was created which would most likely lead to a variation in the erosion-corrosion rate. Due to the small size and curved shape of the flow geometry, detailed measurements of the flow parameters downstream of the step were difficult. Therefore, flow simulations were undertaken and these are reported below.