Internally ribbed tubes can improve TORC (Trans-critical Organic Rankine Cycle) heat transfer economics and safety by improving the heat transfer coefficient and restricting buoyancy effects in horizontal flows. The heat transfer characteristics of supercritical organic fluids in internally ribbed tubes need to be more thoroughly understood. The present study calculated the flow and temperature fields in a horizontal ribbed tube to analyze the heat transfer enhancement of super critical R134a and the buoyancy effects on the heat transfer. The results show that the heat transfer enhancement near the pseudo critical point is mainly caused by the large specific heat effect in the boundary layer. The buoyancy enhances the radial direction velocities (x and y directions) and reduces the axial speed (z direction) along the top of the horizontal tube. The lower axial speed reduces the turbulent kinetic energy and the top side heat transfer coefficient as a result. The simulation results were also compared with the heat transfer in a smooth horizontal tube and vertical ribbed tube with upwards flow. The results showed that the spiral follow induced by the internal ribs strongly restricts the buoyancy flow. Vertical upwards flow has a higher overall heat transfer coefficient and no circumferential temperature gradients, which improves the heat transfer economics and safety in TORC systems. Hence, vertical flow heat exchangers should be considered in TORC systems due to the improved heat transfer.