Effect of discrete inclined ribs on flow and heat transfer of supercritical R134a in a horizontal tube

In this work, discrete double inclined ribs (DDIR) are introduced into the horizontal flow of supercritical R134a to improve the thermal performance of vapor generators in the trans-critical Organic Ranke Cycle. Effects of operating and geometric parameters are investigated after model validation against experimental data. Results show that DDIR are adaptable to changes in operating conditions because spiral flow maintains the dominant position of forced convection heat transfer. DDIR enhance the PEC of horizontal ST by at least 70% and reduce circumferential thermal non-uniformity by up to 90%. Rib pitch has the most significant effect on heat transfer, followed by rib height and inclination angle. The optimal values for the three geometric parameters are 20 mm, 0.8 mm, and 60°. Finally, the mechanism of rib-induced enhancement is analyzed. Average heat transfer is improved by the strengthened mixing between bulk and near-wall fluid instead of suppressing the buoyancy effect. DDIR-induced enhancement is superior to subcritical cases owing to the integral effect of sharp variations in specific heat at supercritical conditions. [Display omitted] •Discrete ribs suppress thermal stratification and strengthen radial mixing in supercritical horizontal flow.•Heat transfer in discrete ribbed tubes is insensitive to changes in operating conditions.•Rib pitch has a more significant effect on PEC than other geometric parameters.•Superior rib-induced enhancement at supercritical conditions can be attributed to sharp changes in specific heat.