Falling film evaporation on serrated fin tubes

This study investigates the falling film evaporation on serrated fin tubes using R134a at 10°C and 20°C saturation temperatures. The serrated fin tube was made by electrical discharge machining. It had 60 annular fins per inch of 0.4 mm fin height, and the tip of each fin had 40 axial cuts. The liquid flowed through a row of circular holes at a mass flow rate per unit length of 0.0075–0.04376 kg·m^–1·s^–1, and heat fluxes between 4.4 and 49.6 kW·m^–2. The test results showed that the heat transfer coefficient of both the smooth tube and the serrated fin tube increased with the heat flux. It also increased slightly with the flow rate before dry-out occurred. The film flow rate had a minor effect on the heat transfer coefficient for smooth tubes. For the serrated fin tube, a lower film Reynolds number resulted in greater falling film evaporation heat transfer coefficients. Based on observations of FC-72 droplets spreading process from high-speed video, the axial cuts improved the fluid distribution uniformity on the serrated fin tube. The serrated fin tube yielded enhancement ratios of up to 10.18 and 6.02 for falling film evaporation of R134a at 10°C and 20°C, respectively, as compared with the smooth tube.