Benli Peng

Analysis of droplet jumping phenomenon with lattice Boltzmann simulation of droplet coalescence

Droplet jumping from condensing surfaces induced by droplet coalescence during dropwise condensation of mixed steam on a superhydrophobic surface can significantly enhance condensation heat transfer of mixed steam with non-condensable gas. This phenomenon was visually observed and theoretically analyzed in the present paper. The dynamic evolution of droplet and the velocity distribution inside the droplet during coalescence were simulated using multiphase lattice Boltzmann method. The energy distribution released by droplet coalescence was calculated statistically, and the jumping height induced by droplet coalescence on a superhydrophobic surface was predicted based on the energy conservation method. The theoretical predictions obtained by the modified model proposed in this paper agree well with the experimental observations.

Wetting Mode Evolution of Steam Dropwise Condensation on Superhydrophobic Surface in the Presence of Noncondensable Gas

It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface.

Effect of surface free energies on the heterogeneous nucleation of water droplet: A molecular dynamics simulation approach

Heterogeneous nucleation of water droplet on surfaces with different solid-liquid interaction intensities is investigated by molecular dynamics simulation. The interaction potentials between surface atoms and vapor molecules are adjusted to obtain various surface free energies, and the nucleation process and wetting state of nuclei on surfaces are investigated. The results indicate that near-constant contact angles are already established for nano-scale nuclei on various surfaces, with the contact angle decreasing with solid-liquid interaction intensities linearly. Meanwhile, noticeable fluctuation of vapor-liquid interfaces can be observed for the nuclei that deposited on surfaces, which is caused by the asymmetric forces from vapor molecules. The formation and growth rate of nuclei are increasing with the solid-liquid interaction intensities. For low energy surface, the attraction of surface atoms to water molecules is comparably weak, and the pre-existing clusters can depart from the surface and enter into the bulk vapor phase. The distribution of clusters within the bulk vapor phase becomes competitive as compared with that absorbed on surface. For moderate energy surfaces, heterogeneous nucleation predominates and the formation of clusters within bulk vapor phase is suppressed. The effect of high energy particles that embedded in low energy surface is also discussed under the same simulation system. The nucleation preferably initiates on the high energy particles, and the clusters that formed on the heterogeneous particles are trapped around their original positions instead of migrating around as that observed on smooth surfaces. This feature makes it possible for the heterogeneous particles to act as fixed nucleation sites, and simulation results also suggest that the number of nuclei increases monotonously with the number of high energy particles. The growth of nuclei on high energy particles can be divided into three sub-stages, beginning with the formation of a wet-spot, increase of contact angle with near-constant contact line, and finally growth with constant contact angle. The growth rate of nuclei also increases with the size of high energy particles.

Heterogeneous nucleation capability of conical microstructures for water droplets

The presence of microstructures on a substrate has a great effect on the heterogeneous nucleation of water droplets. A circular conical apex and a cavity are adopted as the physical model to represent the typical defects which exist widely on substrates, and classic nucleation theory is used to quantitatively analyze the nucleation capability of different microstructures at different condensation conditions. The results indicate that conical cavities with narrower cone angles can reduce the nucleation free energy barrier as compared with apexes and a planar substrate, yielding a relatively higher nucleation capability. With the vapor pressure and supersaturation increasing, the nucleation rate increases rapidly, and some of the cavities that are originally not preferred for nucleation gradually translate into active nucleation sites. Consequently, the activated nucleation sites are finite for practical substrates under certain nucleation conditions, and the nucleation sites number density can be affected by the condensation conditions and the distribution of micro cavities on the substrate. The analysis also indicated that it is possible to realize spatial control of nucleation sites by the construction of micro cavities, and the nucleation sites number density can be intensified by increasing the amount of micro cavities on the substrate.

Effect of nano structures on the nucleus wetting modes during water vapour condensation: from individual groove to nano-array surface

The geometrical structures of surfaces are important to the formation and growth of nuclei during water vapor condensation and the related heat and mass transfer performances. In the present research, the nucleus wetting modes on individual grooves and nano-array surfaces were investigated by molecules dynamics simulations. The results proposed a criterion that the nucleus wetting modes on a V-shaped groove are determined by the intrinsic contact angle θ and the cross sectional angle β. As the cross sectional angle decreases to β < 2θ − π, the nuclei can suspend in the groove center and the suspending height increases with decreasing β. For the nano-array surfaces, the nucleus can wet the grooves between adjacent nano arrays during the initial nucleation stage as the initial nuclei are very small and the nucleus surface are fluctuating frequently. The wetting mode may change as nucleation continuous and the nucleus can depart from the groove bottom to form a Cassie mode nucleus on a surface with β < 2θ − π. The apparent contact angle of nucleus also increases sharply with the wetting mode transition. Moreover, the dynamic behaviors of nucleating droplets were also observed on a nano-array superhydrophobic surface that meets the criterion. The droplets on this surface can recover the spherical shape after coalescence and the droplet jumping occurs, indicating a lower surface stiction. The results provide an insight of the interfacial phenomenon between the nucleus and the geometrical structures and propose a guideline to construct nano-array surfaces in the aim of promoting the Cassie mode nucleus.

Evolution of transient cluster/droplet size distribution in a heterogeneous nucleation process

The transient nucleation size distribution model was introduced into a water vapor condensation system to investigate the kinetics of the initial condensation stage. It was proven that the growth/decay of clusters was significantly affected by cluster size and contact angles of the condensation surface. As the cluster size increased, the cluster surface area exposed to vapor was also increased, and the attachment/detachment frequencies increased accordingly. As the contact angle decreased to a certain value, the attachment frequency became larger than the detachment frequency, which is beneficial for the growth of clusters. The evolution of cluster/droplet size distribution was also investigated. The results indicated that the transient cluster size distribution of the heterogeneous process translates from a monotonic decreasing to a unimodal distribution with time. Peak value of cluster/droplet population can be observed for a sufficiently long time, and the size distribution curve is found to be close to a lognormal distribution, which is distinctly different from the homogeneous equilibrium distribution. The peak value in the size distribution curve shifts to larger cluster sizes with time, and the absolute value decreases accordingly. It is very similar to the reported experimental results of micron scale droplets, revealing that the subsequent experimental phenomenon at macroscopic scale was the direct result of the further development of the initial cluster/droplet size distribution. The present study investigated the effect of contact angle on the growth/decay of clusters and analyzed the mechanism of the evolution of the cluster/droplet size distribution from the viewpoint of kinetics.

Dropwise Condensation Heat Transfer on Superhydrophobic Surface in the Presence of Non-Condensable Gas

Roughness-induced superhydrophobic surface was applied to promote dropwise condensation (DWC) on a vertical plate in the presence of non-condensable gas (NCG). The DWC heat transfer characteristics were investigated and the wetting behaviors of the condensate droplets were observed visually. The experimental results have shown that the roughness-induced superhydrophobic surface would enhance the heat transfer characteristics of steam condensation in the presence of NCG with high concentration. The underlined mechanism is analyzed in terms of the droplet wetting modes.Copyright

Pulsating and Bouncing off of Dropwise Condensate on Superhydrophobic Surface

The steam dropwise condensation (DWC) characteristics on superhydrophobic plates were investigated experimentally in the presence of a high concentration noncondensable gas (NCG, >80mol%). The behaviors of condensate droplets on the roughness-induced superhydrophobic surface were observed with a photron high speed camera attached to a microscope. Pulsating features are found during droplets coalescence movement. Bouncing off of coalesced droplet was also observed induced by the strong effect of pulsating motion to overcome the pinning effect of the surface micro-nanostructures. Induced by the pulsating effect of droplets coalescence, the droplet can move at a long distance to join a coalesced droplet.Copyright

A Droplet Model in Dropwise Condensation With the Presence of Non-Condensable Gases

A droplet model is proposed with respect to molecular clustering to describe the state of steam molecules before condensing on the cooled solid surface in steam condensation process, and also the model is used to account for the mechanism of steam dropwise condensation in the presence of non-condensable gases (NCG). The mathematical model is presented based on the Dillmann and Meier’s homogeneous nucleation theory. Correction of surface tension term is conducted to match the physical and chemical characteristics of condensation process, and the heat transfer model considering the effect of interfacial effects was used to calculate the mean temperature of clusters. The model predicted results of Gibbs free energy at different subcooling degrees and different saturated temperatures were given. And the ratio of heat transfer coefficient with to that without NCG with different fractions of NCG was also obtained. The results show that the non-condensable gases reduce the condensation rates in the same way as shown by experimental results in the literature. That confirms the validity of the model.© 2010 ASME