Dropwise condensation of atmospheric water vapor is important in multiple practical engineering applications. The roles of environmental factors and surface morphology/chemistry on the condensation dynamics need to be better understood to enable efficient water-harvesting, dehumidication, and other psychrometric processes. Systems and surfaces that may promote faster condensation rates and selfshedding of condensate droplets could lead to improved mass transfer rates and higher water yields in harvesting applications. In the present study, experiments are performed in a facility that allows visualization of the condensation process on a vertically oriented, hydrophobic surface at a controlled relative humidity and surface subcooling temperature. The distribution and growth of water droplets are monitored across the surface at different relative humidities (45%, 50%, 55%, and 70%) at a constant surface subcooling temperature of 15 °C below the ambient temperature. The droplet growth dynamics exhibits a strong dependency on relative humidity in the early stages during which there is a large population of small droplets on the surface and single droplet growth dominates over coalescence effects.At later stages, the dynamics of droplet growth is insensitive to relative humidity due to the dominance of coalescence effects. The overall volumetric rate of condensation on the surface is also assessed as a function of time and ambient relative humidity. Low relative humidity conditions not only slow the absolute rate of condensation, but also prolong an initial transient regime over which the condensation rate remains significantly below the steady-state value.Keywords: dropwise condensation, relative humidity, growth dynamics, droplet distribution,

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During dropwise condensation from the ambient environment, water vapor present in air must diffuse to the surface of each droplet. The spatial distribution of water vapor in the local surroundings of each individual droplet determines the total condensation rate. However, available models for dropwise condensation in humid air assume that such systems of droplets grow either as an equivalent film or that the growth of each droplet is completely isolated; the interactions between droplets are poorly described and, consequently, predictions of total condensation rates may mismatch experimental observations. This paper presents a reduced-order analytical method to calculate the condensation rate of each individual droplet within a group of droplets on a surface by resolving the vapor concentration field in the surrounding air. A point sink superposition method is used to account for the interaction between droplets without requiring solution of the diffusion equation for a full three-dimensional domain containing all of the droplets. For a simplified scenario containing two neighboring condensing droplets, the rates of growth are studied as a function of the inter-droplet distance and the relative droplet size. For representative systems of condensing droplets on a surface, the total condensation rates predicted by the reducedorder model match numerical simulations to within 15%. The results show that assuming droplets grow as an equivalent film or in a completely isolated manner can severely overpredict condensation rates.

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Water vapor present in humid air will condense in the form of many small droplets on a cooled substrate. After nucleation, the diffusion of vapor from the environment to the droplets dominates their growth by condensation, and therefore, all droplets must compete for the vapor available in the surroundings. Models that assume droplets grow in isolation or as an equivalent film poorly capture their interaction during vapor-diffusion-driven condensation and do not correspond with experimental condensation rates. By treating the droplets as point sinks, the interaction between all droplets in a system can be captured by superposing the vapor distributions of each droplet. This paper presents direct comparisons of condensation rates measured in experiments and predicted with a point sink superposition method. The results indicate that it is critical to consider a large number of interacting droplets to accurately predict the condensation behavior. Even though the intensity of the interaction between droplets decreases sharply with their separation distance, droplets located relatively far away from a given droplet must be considered to accurately predict its condensation rate, due to the large aggregate effect of all such far away droplets. By considering an appropriate number of interacting droplets in a system, the point sink superposition method is able to predict experimental condensation rates to within 5%. Diffusion-based models that neglect the interactions of droplets located far away, or approximate droplet growth as an equivalent film, are shown to overpredict condensation rates. exp experiment film film-like growth model iso isolated from all neighboring droplets l liquid PSSM point sink superposition method s at the surface of the drop sub at the substrate sys in the system of droplets  far field