Low-pressure steam dropwise condensation on durable PFA-coated horizontal tube: Droplet dynamics in active region

•An ‘active droplet region’ on the horizontal tube which is characterized by droplet dynamics is observed and proposed for the first time.•The effect of vapor pressure on the droplet dynamic behaviors in the active region is studied systematically.•Taking the vapor pressure and circumferential angle into consideration, a semi-empirical for the prediction of droplet departure radius is established. The standard deviation of this model is reduced to less than 15%. The performance of low-pressure steam condensers is vital to the thermal energy utilization efficiency of the power cycle, refrigeration, heat pump, and seawater desalination systems. Dropwise condensation demonstrates better heat transfer performance than traditional filmwise condensation, and its performance is determined by the condensate droplet dynamic behaviors on hydrophobic surfaces. However, the droplet dynamics behavior on the horizontal tube has not been adequately investigated. In this study, a durable PFA-coated copper tube is achieved to sustain stable dropwise condensation at the vapor pressure ranging from 5 to 100 kPa. By optical observation of condensation on the horizontally oriented tube, the droplet dynamic characteristics and the influence of vapor pressure on the droplet dynamics are examined. We show that an ‘active droplet region’ (ADR), which is characterized by frequent droplet shedding and surface renewal promoting more active nucleation, growth, and departure of droplets, exists along the tube circumference. As a result of the varying droplet dynamics, a skew-normal distribution of sliding droplet numbers presents along the tube circumference. The droplets departing within ADR account for 80% of the total departure droplets. Then, the effect of steam pressure on the droplet dynamics behavior in ADR is studied systematically. The average sweeping frequency of droplets under steam pressure of 100 kPa increases by -122% more than that under steam pressure of 5 kPa. The growth rate of droplets and migration speed improve with steam pressure. Thus, the evolution rate of the transient size distribution becomes rapid as the vapor pressure increases. Taking vapor pressure and circumferential angle into consideration, a semi-empirical model for the prediction of condensate droplet departure radius on a horizontal tube is proposed, of which the standard deviation can be reduced to less than 15%. These findings and insights into droplet dynamic behaviors on PFA-coated hydrophobic