This study experimentally investigates electrolytic two-phase natural-convection heat transfer in a pool containing a 20 wt% sodium sulfate solution, using a conical cooling coil as the heat-transfer surface. The experiments were conducted at an average solution temperature of 65 ± 5 °C, a cooling-water inlet temperature of 12.5–13.5 °C, a cooling-water velocity inside the coil of 4–6 m/s, and a gas volumetric flow rate in the solution pool of 0–3.31 NL/min. By analyzing the effects of the cooling-water velocity inside the coil and the motion of electrolytically generated microbubbles outside the coil on the external heat-transfer characteristics of the conical cooling coil, the classical empirical correlation for natural convection in a large fluid volume was modified. The results show that, within the experimental range, the heat-transfer rate and the corresponding external heat-transfer parameters generally increase with increasing internal flow velocity. At a fixed internal flow velocity, increasing the external gas volumetric flow rate enhances heat transfer outside the coil. The relative deviations between the Nusselt numbers calculated using the modified correlation for microbubble-induced two-phase natural convection and the experimental values are within ±20%.

Conical cooling coil; two-phase natural-convection heat transfer; microbubbles; modified correlation for microbubble-induced two-phase natural convection.