Investigation of dropwise condensation of water at atmospheric and sub-atmospheric pressure through an individual-based model

• DWC of steam at p sat = 4.2–143.3 kPa and Δ T = 5 K is investigated by numerical simulations. • A 35 % heat flux reduction is observed for saturation temperature decreasing from 110 °C to 30 °C. • Low pressure increases thermal resistance reducing heat exchanged by drops by 52%. • Wettability and thermal resistance of the coating strongly affect the heat flux. • Increasing contact angle hysteresis from 5° to 15° reduces heat flux by 15%. Steam condensation at sub-atmospheric pressure is a crucial process in a broad range of industrial applications and energy systems, such as in Rankine cycles and sorption desalination plants. Increasing the effectiveness of heat exchangers is fundamental to improve the efficiency of the whole system and a passive solution is offered by promoting dropwise condensation (DWC) instead of filmwise condensation. The few studies on DWC at sub-atmospheric pressures indicate a reduction of the condensation heat transfer coefficient (HTC) when lowering the saturation pressure. However, the extent of this reduction remains unclear, and the effect of critical parameters, such as surface wettability, coating thermal resistance, and nucleation sites density requires further clarification. This lack of knowledge is here addressed by employing an individual-based model (IBM) that exploits parallel computing to investigate the effect of sub-atmospheric saturation pressure on steam DWC. Firstly, the model is validated against measurements from the literature referring to sub-atmospheric conditions. Then, simulations are performed on a 3 × 3 mm 2 computational domain, varying saturation pressures (in the range 4.2–143.3 kPa) and nucleation sites densities ( N s ) from 10 9 m −2 to 10 11 m −2 . A decrease in HTC of 35 % is found when decreasing saturation temperature from 110 °C to 30 °C, regardless of N s . Such penalization is associated with an increase of the droplet liquid–vapor interfacial (+ 1400 %) and conduction (+ 60 %) thermal resistances. Finally, a parametric study is conducted to unveil the influence of advancing contact angle (from 50° to 130°), contact angle hysteresis (from 5° to 25°), and coating thermal resistance (in the range 0.5–8 K m 2 MW −1 ) on the DWC heat transfer at sub-atmospheric pressure.