Abstract: Internally ribbed tubes（IRT） are widely used in water walls of ultra-supercritical circulating fluidized bed（CFB） boilers and normally operated under low mass fluxes. Currently a great variety of tube geometries exist in related studies and practical operations of IRT. It’s essential to develop a proper non-dimensional tube geometry parameter, to facilitate the performance comparison and development of general Nusselt correlation for different IRT. This work numerically investigated flow and heat transfer of supercritical water in IRT with various geometries. Results show that different IRT with the same β W have nearly identical heat transfer performance, as a result of very similar distribution of swirl flow in the boundary region of y+ = 30~100. The geometry effect is overwhelmed in most of the range studied, expect in the region of Bo = 10-5~10-4 and β W ＜ 2.58, where an increase in β W leads to an obvious heat transfer enhancement. Finally, it is pointed out that more future work should target to the development of more accurate Nusselt correlation for purely forced convection flow.