Heat transfer enhancement of helical tubes during pool boiling

This research utilizes experimental measures to investigate the heat transfer mechanism of helical tubes during saturated pool boiling. Experiments were carried out using R-134a and R-513A as working fluids, with saturation temperatures of 4.4 °C and 15 °C and heat fluxes ranging from 5 to 50 kW/$m^2$. The helical tube surface was heated by hot water with ascending heat flux. The helical test tubes used in this study are annealed smooth copper tubes with an outer diameter of 6.35 mm, tube thickness of 0.7 mm, coil diameter of 58.85 mm, number of turns of 3.5, and tube pitches ranging from 7 to 21 mm. Experimental results show that the tube bundle effect also exists in helical tubes as the boiling heat transfer coefficient obtained from helical tubes is significantly higher than that of single straight tubes predicted by the Cooper correlation. Furthermore, the boiling heat transfer coefficients obtained are greatly influenced by the tube pitch and show a tendency to increase with the decrease in tube pitch. In addition, larger and faster-moving bubbles are observed near the upper part of the helical tube due to agitations generated from the bottom part.