PNNL

Abstract

Supercritical Carbon Dioxide (sCO2) has been proposed as working fluid for power generation cycles owing to its non-toxic, non-flammable properties, high density and low cost. The high-pressure (200-350 bar) and high-temperature (550-750^o C) fluid enables extremely compact and high efficiency turbomachinery designs. However, there is evidence that even fairly pure sCO2 flowing through turbomachinery at high mass flow rates can lead to material erosion. It is hypothesized that material erosion may occur due to large fluctuations in wall shear stresses owing to turbulence and secondary flow patterns in pipe bends. To test this hypothesis, large-eddy simulation (LES) of a single-phase, turbulent flow of sCO2 in a 90^o pipe bend was performed at three different Reynolds numbers (5000, 27000 and 45000). The radius of curvature to pipe diameter ratio was chosen to be 1. First, the isothermal flow was validated against available experimental and numerical data. Shear stress on the pipe wall and the associated power spectral density (PSD) were then obtained. The snapshot proper orthogonal decomposition (POD) was applied to identify the secondary flow motions and the oscillation of the Dean vortices that cause swirl-switching. This swirl-switching was found to have the same oscillating frequency as that of the shear force on the pipe wall and is suggested to be a potential cause of the material erosion.