Yasser Amini

Enhancing thermal performance in heat exchange systems through active techniques such as bubble injection is critically important for industrial applications and renewable energy technologies. This numerical study investigates convective heat transfer enhancement from an immersed heated cylinder in a water-filled reservoir with a free surface using air bubble injection. It provides, for the first time, numerical relationships for Nusselt number enhancement via sub-millimeter bubble injection, thereby addressing a key gap in predictive models for cylindrical heat sources. The multiphase Volume of Fluid (VOF) model was employed to capture complex fluid–structure interactions. Key operational parameters, including bubble injection velocity (0.1–0.4 m/s) and nozzle diameter (0.2–0.6 mm), were systematically analyzed to assess their effects on thermal performance. Simulations demonstrated that bubble injection substantially perturbs the thermal boundary layer around the heated cylinder, leading to a remarkable 238 % increase in the average Nusselt number compared to natural convection. The analysis established that both nozzle diameter and injection velocity govern the heat transfer characteristics. Furthermore, optimized bubble injection shifts the dominant heat transfer mechanism from conduction to convection, resulting in up to 85 % improvement in bulk fluid temperature uniformity.