Heat transfer of R134a in a horizontal internally ribbed tube and in a smooth tube under super critical pressure

The present study compared measured heat transfer coefficients for supercritical pressure R134a in a horizontal internally ribbed tube (d_in = 15.5 mm) and in a smooth tube (d_in = 16 mm) for mass fluxes, G, from 100 to 350 kgm⁻²s⁻¹, heat fluxes, q, from 10 to 25 kWm⁻², pressures, P, from 4.26 to 5 MPa and inlet temperatures from 70 to 105 °C. The heat transfer coefficients, buoyancy effects and pressure drop are compared for the various operating conditions. The results show that the tube wall temperature in the internally ribbed tube varied less for the various working conditions than in the smooth tube. Comparisons of the measured heat transfer coefficients for all 4800 experimental data points showed that the heat transfer coefficients in the internally ribbed tube are 4.61 times those in the smooth tube on the top and 1.40 times those in the smooth tube on the bottom. The buoyancy threshold in the internally tube is higher than in the smooth tube for both the Gr_q/Gr_th and [Formula presented][Formula presented][Formula presented] criteria which shows that the buoyancy in the internally ribbed tube is suppressed. Moreover, the pressure drop in the internally ribbed tubes is higher than in the smooth tubes for the same working conditions.