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As a ubiquitous vapor-liquid phase-change process, dropwise condensation has attracted tremendous research attention owing to its remarkable efficiency of energy transfer and transformative industrial potential. In recent years, advanced functional surfaces, profiting from great progress in modifying micro/nanoscale features and surface chemistry on surfaces, have led to exciting advances in both heat transfer enhancement and fundamental understanding of dropwise condensation. In this review, we discuss the development of some key components for achieving performance improvement of dropwise condensation, including surface wettability, nucleation, droplet mobility, and growth, and discuss how they can be elaborately controlled as desired using surface design. We also present an overview of dropwise condensation heat transfer enhancement on advanced functional surfaces along with the underlying mechanisms, such as jumping condensation on nanostructured superhydrophobic surfaces, and new condensation characteristics (e.g., Laplace pressure-driven droplet motion, hierarchical condensation, and sucking flow condensation) on hierarchically structured surfaces. Finally, the durability, cost, and scalability of specific functional surfaces are focused on for future industrial applications. The existing challenges, alternative strategies, as well as future perspectives, are essential in the fundamental and applied aspects for the practical implementation of dropwise condensation. [Display omitted] • Progress of dropwise condensation (DWC) on advanced functional surfaces. • Key components for the performance enhancement of dropwise condensation. • Overview of fundamental understanding and heat transfer enhancement. • Challenges and future prospects in the practical application of dropwise condensation. [ABSTRACT FROM AUTHOR]

• Dropwise condensation with the presence of non-condensable gas is studied. • The distributed point sink method is proposed for the interactions between droplets. • The point sink method and the method of images are further considered for comparison. • The interactions between droplets in the full range can be determined by this method. With the presence of NCG, the vapor diffusion from ambient to the vapor/liquid interface must be considered during dropwise condensation. Historically, modeling dropwise condensation often makes the assumption that the droplet is growing in an isolated way. However, the blocking effect of surrounding droplets will tremendously influence the spatial distribution of vapor, which finally determines a different condensation rate comparing with that by the isolated-droplet growth model. Consequently, an accurate prediction for dropwise condensation must include the blocking effect of neighbors (namely the interaction effect). Some classical methods, including the point sink method (PSM) treating the droplet as single point sink, the method of images (MOI) constructing an infinite series of the point sinks in order to satisfy certain boundary conditions, provide a significant improvement comparing the isolated droplet growth model without considering the interaction between droplets. For capturing the strong interaction during dropwise condensation because of a large number of the droplets and the closer inter-droplet spacing, a distributed point sinks method (DPSM) is proposed. Just like in the method of Green's function with a total mass "sink" responsible for the vapor concentration profile, the condensation droplets are resolved into many mass point "sinks". For guaranteeing the boundary conditions, for each droplet a series of point sinks are spherically distributed inside the droplet using an average manner. The strengths of the point sinks are solved through a matrix formulation which requires certain boundary conditions and the target points of the droplet surface mapped from the point sinks. Considering some simple droplet arrays and a general droplet array in dropwise condensation, the solutions of DPSM are then compared with those using PSM and MOI. Based on the uniqueness theorem, the exactly satisfied boundary conditions state the ability of DPSM in solving the strong interaction. Finally, the DPSM is used to predict the droplet interactions of a characteristic droplet array from dropwise condensation experiments. [ABSTRACT FROM AUTHOR]

• A multi-component multi-phase thermal pseudopotential LB method is developed. • Single droplet condensation with NCG is numerically simulated. • The influence of NCG is depended on the droplet growth stage and its amount. • Smaller contact angle will weaken the NCG accumulation closing the contact line. A multi-component multi-phase thermal lattice Boltzmann model considering vapor-liquid phase change is developed to study droplet condensation with the presence of non-condensable gas. Some tests, including an isolated droplet evaporation, are conducted to verify the capability of this model in simulating multi-component multi-phase flow with vapor-liquid phase change. After that, single droplet condensation considering non-condensable gas is investigated with different mass fraction of non-condensable component and contact angles. The results show that the influence of the non-condensable gas upon droplet condensation heat transfer is depended on the growth stage and the amount of the non-condensable gas. The mass transfer of vapor and non-condensable component will tend to an equilibrium state with the droplet condensation going. Furthermore, for different contact angles, the dynamic behavior of the contact line plays a critical role in the accumulation effect of the non-condensable component. And the heat transfer of droplet condensation is enhanced by the hydrophilic substrate rather than the hydrophobic substrate as expected, no matter adding the non-condensable component or not. In different conditions, the power law, which fits the droplet radius with time, is used to define the growth rate mathematically. [ABSTRACT FROM AUTHOR]

Highlights • Investigation of dropwise condensation from turbulent humid air flow. • The condensate heat transfer resistance is accessed by infrared theromgraphy. • A correlation for the condensate heat transfer resistance is proposed. Abstract A new facility feasible for the investigation of dropwise condensation at a vertical polymer surface (9 × 6 mm, L × H) is described in detail. The droplets are liquefied out of humid air, which flows through a rectangular channel (12 × 32 mm, W × H) and passes the sub-cooled surface of a sensor. The latter one is flush mounted into one of the vertical channel walls beyond a hydrodynamic entry length of 550 mm determining perpendicular heat fluxes by measuring temperature differences. Furthermore the droplet covered surface is observed through the opposite channel wall by an infrared camera. The variable experimental parameters include temperature, water content and flow velocity of the humid air, as well as the surface temperature of the sensor. The scope of the experimental study is to investigate the mean heat transfer resistance of the condensate droplets which form on the substrate surface. The procedure is based on the determination of heat fluxes and the evaluation of the temperature differences between the substrate surface and the liquid/gas interface by means of thermography, both of them carried out synchronously. The experimental droplet heat transfer coefficient, h d , is evaluated as an average value and it considers both heat conduction through the droplets and the augmentation of heat transfer due to inner convection. The parameter variation covers Reynolds numbers between 7900 and 21400, relative humidities between 56% and 95% and air temperatures between 30 °C and 46 °C for a broad range of cooling temperatures. The latter one influences the droplet surface temperature and together with the water content of the bulk, it determines the driving force for mass transfer. h d was found to increase with the Reynolds number and the driving force for mass transfer. However, increasing the latter one beyond a certain value causes massive accumulation of noncondensable gases at the liquid/gas interface inhibiting the vapour mass flux and leading to invariant droplet heat transfer coefficients. The experimental data is correlated providing the basis for the experimental investigation of the heat transfer at the liquid/gas interface which will be done in future. [ABSTRACT FROM AUTHOR]

A single droplet model is developed to describe the droplet growth during dropwise condensation of moist air on a cold substrate. The condensation process is divided by the droplet surface into two parts. The first step, i.e. the processes of mass transfer from the surroundings to the droplet surface, is modeled by the Kinetic theory and the laws of continuum fluid dynamics formulated using the two-region concept (Knudsen layer and continuum region) at any droplet size and at any concentration of non-condensable gas (NCG). The second step, i.e. the heat transfer across the droplet, is governed by Fourier’s law of heat conduction. These three regions (the continuum region, the Knudsen layer and the region inside the droplet) are incorporated by the matching both the mass flow rates and the energy flow rates. From these, the droplet growth rate, the nucleation size of droplet, the temperature at the droplet surface and Knudsen layer interface can be evaluated depending on different conditions. For this, a numerical algorithm is developed to reflect the droplet dynamics sufficiently detailed, including nucleation, growth/coalescence, slide-off/fall-off, re-nucleation. This is applied to the simulation of the entire condensation process by putting the growth rate and minimum radius from single droplet model into the growth algorithm. Additionally, dropwise condensation experiments of moist air in different relative humidity (RH) are carried out for validation of the simulation results. Good agreement is obtained which demonstrates that the present droplet growth model for dropwise condensation of moist air is credible. The current model and experiments also indicate that the diffusion resistance of water vapor in air from the free stream toward the droplet surface has a significant influence for the heat transfer performance of dropwise condensation of moist air. [ABSTRACT FROM AUTHOR]

• Experimental investigation of dropwise condensation heat transfer out of turbulent humid air flow. • Heat transfer measurements are accompanied by visual evaluation and quantification of the droplet pattern. • A novel modeling approach leads to a correlation predicting the experimental data with rather good accuracy. In this investigation dropwise-condensation heat transfer of out of a turbulent flow of humid air is experimentally investigated. The test section consists of a horizontal channel, sized 12 × 32 × 750 mm. The condensation takes placed on a 9 × 6 mm sized section of the vertical channel wall. A model based on the analogy of heat and mass transfer - previously used in the context of filmwise condensation - is adopted in order to describe the data obtained for varying bulk Reynolds numbers (3000–22000), temperatures (36∘C–55∘C) and relative humidities (36%–55%). Modifications are made in order to capture the effect of fog formation within the mixture and the enhancement of the area available for heat transfer induced by the droplets on the substrate. The formulation of the model is intended to be easily adoptable to other geometries, boundary conditions, types and orientations of the substrates. Previous investigations of this group were focused on the thermal resistance of the condensate. Based thereon, the present modelling completes the former approach, resulting in a model capable of predicting the total heat transfer coefficients of dropwise condensation from humid air. The predictions are validated using own experimental data as well as external data. [ABSTRACT FROM AUTHOR]

• Investigations of dropwise condensation out of convective humid air flow. • Predicting method for the droplet growth rate and the overall heat flux. • The reduction and enhancement factors in terms of heat transfer. • At the current conditions, large droplets are more beneficial for heat transfer. In this work, experimental and modeling investigations of convective dropwise condensation from moist air are carried out for the mechanistic understanding of the influence of non-condensable gas. For modeling, different droplet growth models in the presence of non-condensable gas are firstly developed by treating the droplet as an isolated form. During dropwise condensation, the presence of non-condensable gas brings some specific condensation characteristics, including the interaction effect between droplets and convection mass transfer. However, those characteristics cannot be reasonably captured by the droplet growth model. In this work, the distributed point sink method and the droplet Sherwood number are used to evaluate the inter-droplet interaction and convection mass transfer, respectively. Combining those progresses, the droplet growth rate is predicted and the prediction is evaluated by the measurements. Based on the accurate prediction for the droplet condensation rate, a semi-analytical modeling method is released to predict convective dropwise condensation of moist air. It considers the latent heat by condensation and the sensible heat by forced convection. Finally, this work presents the direct comparisons of the overall heat flux measured in experiments and the prediction. Furthermore, mechanistic understanding is discussed simultaneously and conclusively. Throughout this work, we highlight the reduction (the diffusion resistance of water vapor and the inter-droplet interaction) and enhancement (convection mass transfer) factors for dropwise condensation in the presence of non-condensable gas. In addition, in the case of this work, large droplets are more beneficial for heat transfer of dropwise condensation. [ABSTRACT FROM AUTHOR]