Mesoscopic Modeling of Dropwise Condensation under Hunting Instability: A Lattice Boltzmann Approach Based on the Pseudopotential Model

Dropwise condensation is one of the most effective heat transfer mechanisms in refrigeration systems, air-conditioning units, and heat exchangers. Because of its significant role in enhancing heat flux and improving energy performance, understanding its behavior under realistic operating conditions is essential. However, dynamic instabilities in refrigeration cycles, particularly the hunting phenomenon in expansion valves, can generate periodic fluctuations in surface thermal conditions, disrupt steady condensation, and reduce heat transfer efficiency. This study investigates the influence of such oscillations on the dynamics of dropwise condensation over a horizontal cylinder. For this purpose, the condensation process was simulated using a mesoscopic framework based on the lattice Boltzmann method, in which liquid-vapor interactions were represented through a pseudopotential approach, while the temperature field and coupled heat transfer were simultaneously resolved. The results indicate that hunting-induced oscillations, by periodically modifying surface subcooling, significantly alter droplet nucleation, growth, coalescence, and liquid accumulation. It was further observed that the oscillation amplitude and frequency strongly affect the nonuniform distribution of condensed liquid and the variation of local thermal resistance over the cylinder surface. Quantitatively, the heat transfer coefficient exhibited quasi-sinusoidal fluctuations with an amplitude of about 12.5% relative to the steady-state condition. In addition, the liquid accumulation rate in the lower section of the cylinder was approximately 2.6 times that of the upper section because of the combined influence of gravity and the thermal boundary layer. These findings provide a useful basis for the stable design of thermal systems operating under unsteady conditions.