Unsteady turbulent flow in 90° bend under the wall thinning degradation environment

Abstract

A detailed investigation of fully developed transient flow in a 90° bend has been undertaken. The demineralised water has been considered as the working fluid under the operating conditions of wall thinning degradation mechanism in Indian nuclear power plant (NPP). The large eddy simulation (LES) model was employed to compute the large eddies and sub grid scale model was employed to compute the small eddies (unresolved eddies). The LES uses a spatial filtering approach to separate the large and small eddies instead of time averaging operation. The considered flow is of transient nature, hence, apart from the initial disturbances, the flow is observed to be quite random and continuously oscillating with the mean flow for all time. The swirling nature is captured in terms of streamlines at different cross sectional planes. Also the streamlines were captured at a critical sectional plane for every minute. The swirling nature, in terms of large and small eddies, is observed to be strong in the vicinity of the bend. The swirling strength was calculated and it is observed to be more at the bend as well as at the downstream of bend. The flow instability is analyzed and captured in terms of transient iso-surfaces of axial velocity. Also the most important parameter which is vulnerable to flow accelerated corrosion (FAC) i.e., mass transfer coefficient is calculated and analyzed. These results will help to analyze the transient variations in pipe at the locations which are vulnerable to FAC.