AbstractThe two‐dimensional flow of an internally heated fluid in a circular vessel has been investigated by using a finite‐control‐volume numerical model. It has been found that, when symmetry constraints are imposed along the vertical midplane of the vessel, two distinct flow patterns can he predicted for the same operating conditions, a jet‐momentum‐dominated pattern and buoyancy‐dominated pattern. These patterns occur in an operating region where momentum and buoyancy forces are of comparable magnitude. However, when the symmetry constraints are removed and the full vessel cross section is modeled, only the buoyancy‐dominated pattern is observed. Results for the two cases are described and possible reasons for the differences in behavior