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

Abstract The present study compared measured heat transfer coefficients for supercritical pressure R134a in a horizontal internally ribbed tube (din=15.5 mm) and in a smooth tube (din=16 mm) for mass fluxes, G, from 100-350 kgm-2s-1, heat fluxes, q, from 10-25 kWm-2, pressures, P, from 4.26 to 5 MPa and inlet temperatures from 70-105°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 Grq/Grth and Gr b Re b 2.0 ρ b ρ w x d 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.