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Phenomena of water vapor condensation process without gravity (“” represents cluster; “” represents nuclei; “” represents nano-droplet)
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The model of water vapor condensation on a composite V-shaped surface with multi wettability gradients was built and the condensation process with different gravity was studied by molecular dynamics to find out whether this model could control the condensation mode and accelerate the condensate drainage from the micro-perspective. With the absence...
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In the present paper, a thorough review of the experimental and numerical studies dealing with filmwise and dropwise condensation under microgravity is reported, covering mechanisms both inside tubes and on plain or enhanced surfaces. The gravity effect on the condensation heat transfer is examined considering the results of studies conducted both...
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... Combining these multiscale findings yields a tension: hydrophilic surfaces are good at capturing water molecules but bad at removing them, whereas hydrophobic surfaces are bad at capturing water molecules but good at removing them. Because of this, innovative surface designs have been developed to provide both features by modifying hydrophobic surfaces that have hydrophilic spots or areas (non-uniformly wettable surfaces) [49,57,[59][60][61], nano-texturing the hydrophilic surface [51,53,58,62], and coating the hydrophilic surface with a more hydrophobic surface [56,[63][64][65]. MDS studies have deeply understood the water vapor condensation on these surfaces. ...
... Furthermore, an interesting finding was observed: the droplet movement could be directed using the wedgeshaped surface. The authors extended the study of water vapor condensation on a V-shaped surface and incorporated the effect of gravity [60]. The same objective was proven, i.e., the surface model could accelerate condensate drainage. ...
Water vapor condensation is a key process for many applications. Existing studies in water vapor heterogeneous condensation have used different methods to modify the hydrophobicity of the water vapor condensation surface. One way to modify the surface properties is to use graphene coating. On a macroscale, surface modification using graphene coating can improve water vapor condensation heat transfer of various substrates. However, on the molecular scale, its effect is poorly understood. This work investigated the effect of graphene coating on water vapor condensation using molecular dynamics simulations (MDS). We examined the water vapor condensation on bare copper surfaces considering the effects of initial temperature difference, water model, and surface size, parameters that have not been investigated by previous studies employing MDS for the same water-surface configuration. One water model was then used to simulate the condensation on a copper surface with and without graphene coating. We then investigated the effect of graphene defect, the energy and vibration of graphene atoms, and the interaction between graphene and copper layers. The surface size notably influenced the condensation rate and heat transfer performance. The condensation rate and heat transfer performance were significantly reduced when the copper surface was coated by graphene. The number of water molecules condensed was 1253 molecules/ns on the bare copper surface, compared to 587 molecules/ns on the graphene-coated copper surface. Moreover, the water molecules condensed on the graphene-coated copper surface tended to return to the bulk vapor phase. Other important results are also provided. This study gives an insight into the water vapor condensation on graphene-coated copper surfaces, useful to pursuit the design and optimization of graphene-coated copper surfaces for applications that need efficient water vapor condensation, such as for industrial applications, like thermal, chemical, and nuclear.
In the present paper, a thorough review of the experimental and numerical studies dealing with filmwise and dropwise condensation under microgravity is reported, covering mechanisms both inside tubes and on plain or enhanced surfaces. The gravity effect on the condensation heat transfer is examined considering the results of studies conducted both in terrestrial environment and in the absence of gravity. From the literature, it can be inferred that the influence of gravity on the condensation heat transfer inside tubes can be limited by increasing the mass flux of the operating fluid and, at equal mass flux, by decreasing the channel diameter. There are flow conditions at which gravity does exert a negligible effect during in-tube condensation: predictive tools for identifying such conditions and for the evaluation of the condensation heat transfer coefficient are also discussed. With regard to dropwise condensation, if liquid removal depends on gravity, this prevents its application in low gravity space systems. Alternatively, droplets can be removed by the high vapor velocity or by passive techniques based on the use of condensing surfaces with wettability gradients or micrometric/nanometric structuration: these represent an interesting solution for exploiting the benefits of dropwise condensation in terms of heat transfer enhancement and equipment compactness in microgravitational environments. The experimental investigation of the condensation heat transfer for long durations in steady-state zero-gravity conditions, such as inside the International Space Station, may compensate the substantial lack of repeatable experimental data and allow the development of reliable design tools for space applications.
Vapor condensation on bioinspired hierarchical nanostructured surfaces with hybrid wettabilities has been investigated using molecular dynamics simulations. A series of hierarchical surfaces consisting of nanocylinder arrays with hydrophilic top and hydrophobic nanopillar arrays are constructed. The results manifest that the condensed nanodroplets undergo three states in the whole water vapor condensation process, and the total condensed atom number on surfaces increases with the increase of nanocylinder diameter (D), which indicates that the introduction of hydrophilic nanocylinders is conducive to improving the condensation performance compared with that on the hydrophobic surface patterned with homogeneous nanopillars. However, the nucleation sites on hierarchical nanostructured surfaces are covered by the condensed nanodroplets at the end of condensation, which suppresses the further enhancement of condensation performance. To solve these problems, we add a collection region close to the edge of the nanostructured surface. It is noticed that the condensed nanodroplets can roll into the collection regions gradually during the condensation process, which keeps the nucleation sites on nanostructured surfaces exposed effectively, especially for the cases of 20 Å ≤ D ≤ 40 Å. Moreover, the cluster number, the total condensed atom number, and the condensation enhancement efficiency on hierarchical nanostructured surfaces with collection regions at 20 Å ≤ D ≤ 40 Å are higher obviously compared with those on surfaces without collection regions. Our study demonstrates that the bioinspired hierarchical nanostructured surface with the collection region is beneficial to boost the vapor condensation performance, which can bring new insights into water vapor condensation.
To provide a microscopic view of droplet self-propelled behavior, droplet movement on surfaces with different parameters (wettability gradient, gradient zone angle, conical micro/nano structure) using molecular dynamics simulation (MDS) were studied. The result shows that both surface wettability gradient and the conical structure can induce the droplet movement. The moving velocity increases as the wettability gradient increases, generally a higher velocity prevails for hydrophobic surface at the same wettability gradient. The maximum peak velocity is observed at the conical angle of around 40°, and the velocity does not show much difference at different wettability of the conical structure.
The oscillation characteristics of a single droplet after normal collision with smooth surfaces are numerically investigated by using volume of fluid (VOF) method. An adaptive mesh refinement technique is used to guarantee the high computational accuracy of the interaction between the falling liquid droplet and the solid surface. The results of VOF simulations are consonant with the previously reported experiments. The effects of seven key issues on the oscillation characteristics of a liquid droplet advancing/recoiling onto both hydrophilic and hydrophobic surfaces are studied quantitatively with the help of dimensionless wetting length, average amplitude ratio and average oscillation period. These issues include impact velocity, contact angle, slip length, surface tension, droplet density, liquid viscosity and droplet diameter. Especially, surface and interface effects gradually become a primary factor that controls the droplet behavior as droplet size is reduced, and therefore the oscillation concerning micrometer-size droplet may be damped more rapidly. The presented study can contribute to provide more insight into the wetting kinetics of droplet collision, the interface interaction between the liquid droplet and the solid surface, and the oscillation features of the droplet spreading/receding after droplet impact.
In order to obtain the influence law and microscale mechanism of droplet wetting state in microgravity, the model of nano-droplet on array micro-structured surface with different wettability and gravity was built and simulated by molecular dynamics. In conditions of no gravity, it was found that critical point of wetting state transition was closer to weak wettability surface with the decrease of low range micro-column height. What's more, the droplet contact angle increased first and then decreased with larger micro-column height when h* < 1.5. Under the action of gravity, the droplet Cassie state basically did not appear. The greater gravity made the droplet wetting state transition (from Cassie to Wenzel-Cassie to Wenzel) need lower requirements for surface wettability and micro-column height. Moreover, microgravity environment was conducive to droplets transition from Wenzel to Wenzel-Cassie or Cassie state, which promoting droplet drainage. Finally, a simple, effective and reliable method was put forward to judge the droplets wetting state under different gravity, with the aim of providing a technical support for controlling droplet to be in Wenzel-Cassie state under gravity.
Due to potential applications in numerous fields (self-cleaning, anti-frosting, condensation enhancement, etc.), coalescence-induced droplet jumping has been investigated extensively by numerical simulations and experiments over the past decade. In this paper, the jumping dynamics of coalesced droplets on the microstructured superhydrophobic surfaces is simulated using the lattice Boltzmann model. Effects of gravity, inclined angle and droplet radius ratio on the jumping velocity and energy conversion efficiency are studied. The numerical results demonstrate that jumping droplets on inclined surfaces driven by the tangential gravity can successfully detach from the surface without returning to the original spot. The jumping velocity can be improved by a larger inclined angle. Coalescence-induced jumping of two-mismatched droplets on the horizontal surface can also produce a horizontal velocity that drives them detaching from the surface along x direction or coalescing with other droplets. Both jumping velocity and energy conversion efficiency are reduced by a lower radius ratio and a larger gravitational coefficient. In addition, no jumping behavior can be observed when energy conversion efficiency is less than 2%.
