A 3-D numerical study of turbulent flow of supercritical water flowing upward in two heated parallel channels with constant applied wall heat flux was developed using a RANS model in the Computational Fluid Dynamics (CFD) code ANSYS CFX. Oscillatory flow instabilities were investigated using the standard k-epsilon turbulence model with scalable wall functions. The effects of changes to the grid sizes and the time step size on flow instabilities were studied first. Then the instability thresholds of experimental cases were obtained with the CFX code. For comparison purposes, Chatoorgoon's 1-D non-linear SPORTS code was also used to determine the instability boundary. These two new numerical results were compared with the experimental data and previous numerical results by other investigators. Additionally, the effects on the instability thresholds of changing the outlet plenum volume, the turbulent Prandtl number, the turbulence inlet conditions, the channel outlet K factor, the maximum iterations per time step in the transient analysis, and the order of the transient scheme were examined. (C) 2017 Elsevier Ltd. All rights reserved.