An experimental study on the air-side heat transfer coefficient and the thermal contact conductance in finned tubes

The objective of the present experimental study is to evaluate the heat transfer coefficient outside a tube with annular transverse fins, derived from strips of copper mechanically bounded and coupled outside. Water was used as heating medium, in turbulent conditions and flowing at different temperatures inside the tube. Petukhov’s correlation has been selected to calculate water heat transfer coefficient in the tube. The experimental data obtained have been compared with a correlation from literature, and a similar trend has been observed. A fitting of the data provided a correlation for the three tubes of different external diameter (30 mm, 22 mm and 15.6 mm) which agree very well with the experimental values. The thermal contact conductance has been identified as the main reason for the difference between data and original Briggs and Young’s correlation. An estimation of the contact conductance between fins and tubes provided values between 3500 and 11000 W/m2K, slightly increasing with the air Reynolds number (based on the external diameter of the tube), whose range is 2000 to 8000. The thermal contact resistance has been estimated and its importance has been confirmed, contributing for 30 to 50% to the total air-side thermal resistance in the tubes used in the experiments.