Droplet Condensation Research Articles

Substrate thermal conductivity-mediated droplet dynamics for condensation heat transfer enhancement on honeycomb-like superhydrophobic surfaces

The efficiency of dropwise condensation heat transfer has been improved significantly by the development of superhydrophobic surfaces, which are usually fabricated by coating a thin superhydrophobic layer on different kinds of substrates (e.g., different metals/alloys). However, the effects of the thermal conductivity of the substrate on condensation heat transfer over such superhydrophobic surfaces remain unclear. In this paper, we investigated experimentally the condensation heat transfer and droplet dynamics on honeycomb-like superhydrophobic (HCLS) surfaces with hierarchical microporous structures, fabricated by an electrochemical method on four metallic substrates with different thermal conductivities, i.e., copper, brass H96, brass H65 and stainless steel SS316. We observed spontaneous droplet jumping on all these HCLS surfaces at a relatively low degree of subcooling. We also demonstrated the interplay between the substrate thermal conductivity and condensation heat transfer performance during the flooding condensation transition (FCT) by numerical simulations. In the jumping-droplet condensation and flooding condensation regimes, the condensation heat flux and heat transfer coefficient were found to be almost unchanged for different substrates. However, the substrate with a higher thermal conductivity leads to a higher condensation heat transfer coefficient as well as a higher droplet departure frequency at moderate degrees of subcooling, indicating the synergistic effect between the substrate thermal conductivity and droplet dynamics during FCT. The substrate thermal conductivity-mediated droplet dynamics enables a way for further enhancing condensation heat transfer on the HCLS surfaces.

Experimental investigation of frosting process on ice surface in ice rink

Ice rink is a special kind of public building with high energy consumption, in which frost would occur on ice with high humidity ratio. Frost on ice would substantially affect the quality of ice, which needs to be avoided in high level competition. In this paper, the basic process and characteristics of frost formation on ice are obtained by means of experiment. At the same time, the relationship between frosting time and humidity ratio for different stages are discussed. The feasibility of frost thickness growth model is also validated by experiment. Based on the comprehensive consideration of time and grade requirements, the control range of surrounding air humidity ratio could be discussed and expanded. The upper limit of humidity ratio could be extended by 0.5–0.9 g/kg higher than the saturated value corresponding to the ice surface temperature when the target is no obvious condensation droplets on ice for 4 h. The upper limit could be extended by 1.0 g/kg when the target is no obvious frost crystals on the ice for 4 h. This research also reduces the difficulty of humidity control in ice rink with a wider range. It also provides a theoretical basis for reasonable and energy-saving air parameter control requirements in ice rink.

Three-dimensional condensation in a vertical channel filled with metal foam using a pseudo-potential lattice Boltzmann model

In this study, three-dimensional condensation in a domain filled with metal foam is investigated using the lattice Boltzmann method for the first time. A recently proposed two-phase pseudo-potential lattice Boltzmann model is extended to 3D phase-change problems by adding the energy equation with the Peng–Robinson equation of state. The phase-change model is validated using the thermodynamic consistency test, Laplace law, contact angle test, and D2-law. To provide accurate pore-scale analysis, both the flow and energy equations are discretized with the D3Q27 model. Moreover, the metal foam cell structure is reconstructed by implementing the Lord Kelvin model on the mesh nodes. The results are reported in terms of condensation modes, droplet nucleation, growth and removal, heat flux on the walls and mass flow rate of the discharged condensate. Furthermore, the effects of the Jakob number (in range of Ja=0.06 to 0.24) and cell size (S=16,32, and 64) are investigated regarding the condensation process, average heat flux, and mass flow rate. The condensation curve under constant temperature boundary condition is presented for different configurations. It is shown that the model is capable of simulating convection heat transfer along with different condensation modes, including dropwise and filmwise modes. Our results revealed that the onset of nucleation and mode transition occurs at lower Jakob values as the metal foam cell size is increased. Also, it is shown that at a fixed configuration, increasing the Jakob number affects the condensate mass flow rate more than heat flux. The results of this study could be useful in the design of phase-change heat exchangers with metal foam construction.

Top of line corrosion in gas-condensate pipelines

Low alloyed carbon steel is the only viable material of construction for long pipelines transporting unprocessed gas-condensate. The water that condenses is highly corrosive because it contains dissolved acid gases, i.e., CO2, H2S and organic acids like acetic and formic acid. The high velocity gas also contains droplets of water and condensate, and these will deposit if they hit the steel surface. Monoethylene glycol (MEG) injected to prevent ice and hydrates must be considered when predicting the composition and corrosivity of the aqueous phases in the pipeline. The liquids gathering at the bottom of the pipe have a higher heat capacity than the gas, and the temperature at the top of the pipe will be slightly lower than at the bottom. As the produced fluids cool during the transport from the hot wells to the process plant, water will condense on the cold pipe surface and more at the top than at the bottom. The literature on Top-of-line corrosion (ToLC) has grown steadily since the first reported case in 1960. There are also several prediction models for ToLC. This review is an overview of the main factors that cause ToLC and how these are modelled. Mass transfer from the aqueous phase at the bottom to the top contribute to the condensation. Despite the low MEG to water ratio in the gas due to the difference in vapour pressure, the fraction of MEG in the condensing water may be considerable. The concentration of MEG in the aqueous phase at the top depends on the mass transfer from bottom. The same is the case for organic acids. Liquid droplets entrained in the gas may deposit top of line and contribute to the chemistry of the aqueous phase. Models for ToLC must thus not only predict the composition of the condensing phases but also the mass transfer to be able to estimate the corrosion rate.

Hygroscopic Seeding Effects of Giant Aerosol Particles Simulated by the Lagrangian‐Particle‐Based Direct Numerical Simulation

AbstractThis study investigated the microphysical responses to seeding giant aerosol particles and supersaturation fluctuations. A Lagrangian‐particle‐based direct numerical simulation is used to resolve the interactions among individual aerosols, droplets, and the fluctuating supersaturation field within a turbulent, adiabatic air parcel. It is shown that the giant seeding particles exert strong solute effects throughout the entire simulation to alter the subsequent collision–coalescence process, implying the importance of including the solute term in droplet growth. Small‐scale supersaturation fluctuations in adiabatic cloud regions have a negligible influence on aerosol activation and droplet condensation. This is because in regions free of entrainment and/or large‐scale mixing, the weak supersaturation fluctuations can be quickly smoothed out via diffusion and remain relatively small in magnitude (with a standard deviation <). In contrast, the activation in our simulations is determined by the seeding modulation of the parcel‐mean supersaturation.

An airborne study of the aerosol effect on the dispersion of cloud droplets in a drizzling marine stratocumulus cloud over eastern China

Detailed airborne measurements were carried out to explore aerosol-cloud interactions and cloud microphysical properties in a drizzling marine stratocumulus cloud deck over eastern China. Results show that the collision-coalescence of cloud droplets, the condensation of small droplets, and the collision-induced break-up of drizzle were the dominant microphysical processes in the sampled water cloud parcel. The region in the vicinity of the cloud's lateral boundary was spatially divided into sub-regions to better understand aerosol and droplet interactions. Relationships between the relative dispersion (ε) and the cloud’'s microphysical and dynamical characteristics were also examined. A negative relation was found between ε and the cloud droplet number concentration, with ε showing a close relationship with the liquid water content (LWC) and updraft velocity. When LWC was greater than ~0.75 g kg−1, the range of ε values narrowed, and updrafts dominated. By introducing ε in the cloud droplet effect radius (Re) parameterization, we find that ε can further affect indirect forcing by changing the Re distribution for the cloud examined in this study. The dispersion effect (DE) was estimated using the effective radius ratio and the specific cloud water content. An in-depth analysis indicates that DE may offset the Twomey effect by ~12%. Two different methods of estimating the indirect effect (IE) yielded close values (0.084 and 0.077), suggesting that introducing DE into the estimation had a small influence on the IE calculation in the drizzling marine stratocumulus cloud of this study. Note that the estimated IE has a large uncertainty, given the large biases in the cloud properties measured.

Breath figure templated self-assembly of surface-acylated cellulose nanowhiskers confined as honeycomb films

The self-assembly of cellulose nanowhiskers (CNWs) in confined geometries provides a powerful method for the fabrication of novel structures. Herein, ordered honeycomb microporous films were first prepared with surface-acylated CNWs (CNWs-SU) through the breath figure method. Resulting films showed highly porous order over large regions and the iridescent color was only displayed by their rims, which is different from traditional dish-cast CNW films showing the iridescent color over the whole area. This is mainly due to the condensation of water droplets forming three-dimensional (3D) geometry, which forced CNWs-SU to self-assemble into cholesteric architectures in confined geometry and resulted in the iridescent color of the rims after drying. The mechanism was further studied by investigating the critical influencing factors, primarily the concentration of CNW-SU suspensions, the relative humidity of the atmosphere and the surface-attached moieties. In particular, CNW-SU suspensions with a concentration of 3 mg/mL at the relative humidity of 75% preferentially formed honeycomb films with uniform pores. Too low or too high concentrations of CNW-SU suspensions or relative humidity are not preferable for uniform porous films. CNWs-SU with further immobilized octadecane or fluoroalkyl groups on their surface strongly affected the formation of uniform porous films because of higher hydrophobicity and accompanying inhomogeneous condensation of water droplets. This work provides a novel method to study the interactions of CNWs beyond the planar geometry and the formation of uniform porous films solely with CNWs with structural colors open up interesting possibilities for broad application in photonic nanomaterials.Graphic abstract

Shear relaxation governs fusion dynamics of biomolecular condensates

Phase-separated biomolecular condensates must respond agilely to biochemical and environmental cues in performing their wide-ranging cellular functions, but our understanding of condensate dynamics is lagging. Ample evidence now indicates biomolecular condensates as viscoelastic fluids, where shear stress relaxes at a finite rate, not instantaneously as in viscous liquids. Yet the fusion dynamics of condensate droplets has only been modeled based on viscous liquids, with fusion time given by the viscocapillary ratio (viscosity over interfacial tension). Here we used optically trapped polystyrene beads to measure the viscous and elastic moduli and the interfacial tensions of four types of droplets. Our results challenge the viscocapillary model, and reveal that the relaxation of shear stress governs fusion dynamics. These findings likely have implications for other dynamic processes such as multiphase organization, assembly and disassembly, and aging.

Preparation and performance of nanoparticles-based anti-frosting transparent hydrophobic surfaces

Hydrophobic surface has been used for restraining frost deposition on cold non-transparent surfaces. However, anti-frosting surfaces with good transparency, esp. transparent hydrophobic surfaces are receiving more and attention from both industries and researchers. A simple method for preparing transparent hydrophobic surface is thus proposed and used to fabricate anti-frosting hydrophobic surfaces. A series of frosting experiments on transparent hydrophobic surfaces and unmodified glass surfaces were carried out under natural convection conditions. The results show that the maximum contact angle of the modified hydrophobic surface is increased by 39.3°, which means an increase of 44.1% over that of the unmodified plain glass surface. The surface properties are the best when TEOS addition is 10 mL. In the range of visible light, the surface transmittance is greater than 87%. A very special condensate droplets freezing mechanism, i.e. , the ice bridge mechanism is observed. And the frost crystal appearance time on the hydrophobic surface is more than 660 s, which is 540 s longer than that on the unmodified glass surface. The frost coverage of the hydrophobic surface is significantly smaller than that of the unmodified plain glass surface, the speed of frost propagation is only 47.5 μm/s, which is 29.6% lower than that of unmodified surface. Therefore, compared with the unmodified surface, the hydrophobic glass surface can effectively inhibit the growth of frost layer without much loss of its transparency. • Transparent hydrophobic surface was fabricated successfully by gel-sol method. • Visible light transmittance of transparent hydrophobic surface is as large as 87%. • Good anti-frosting performance of the surface is confirmed under high cold surface temperature conditions.

Droplet condensation in the lattice gas with density functional theory.

A density functional for the lattice gas with next-neighbor attractions (Ising model) from fundamental measure theory is applied to the problem of droplet states in three-dimensional, finite systems. The density functional is constructed via an auxiliary model with hard lattice gas particles and lattice polymers to incorporate the attractions. Similar to previous simulation studies, the sequence of droplets changing to cylinders and to planar slabs is found upon increasing the average density ρ[over ¯] in the system. Owing to the discreteness of the lattice, additional effects in the state curve for the chemical potential μ(ρ[over ¯]) are seen upon lowering the temperature away from the critical temperature [oscillations in μ(ρ[over ¯]) in the slab portion and spiky undulations in μ(ρ[over ¯]) in the cylinder portion as well as an undulatory behavior of the radius of the surface of tension R_{s} in the droplet region]. This behavior in the cylinder and droplet region is related to washed-out layering transitions at the surface of liquid cylinders and droplets. The analysis of the large-radius behavior of the surface tension γ(R_{s}) gave a dominant contribution ∝1/R_{s}^{2}, although the consistency of γ(R_{s}) with the asymptotic behavior of the radius-dependent Tolman length seems to suggest a weak logarithmic contribution ∝lnR_{s}/R_{s}^{2} in γ(R_{s}). The coefficient of this logarithmic term is smaller than a universal value derived with field-theoretic methods.

Numerical investigation on entropy generation in the dropwise condensation inside an inclined pipe

AbstractIn this paper, the entropy generation analysis in dropwise condensation in an inclined pipe is carried out. As the vapor passes through the inclined pipe, the flow of the droplets occurs due to the interfacial shear and gravity. Three distinct zones considered in the numerical study include the saturated vapor flowing in the axial direction, saturated liquid traveling through the bottom of the pipe, and the condensate droplet flowing on the inner side of the pipe wall. With the aim of achieving optimal geometrical conditions of the pipe and flow, the entropy generation was calculated for various cases. The results show that the minimum entropy generation of 0.0754 W·K−1 occurs in a configuration with a diameter of 9.5 mm and an inclination angle of 1°. Finally, the Bejan number, irreversibility distribution ratio, and variation according to the flow conditions were presented.

Dropwise condensation heat transfer on nanostructured superhydrophobic surfaces with different inclinations and surface subcoolings

Due to the excellent heat transfer efficiency, dynamic behaviors of coalescence-induced droplet jumping on superhydrophobic surfaces have been extensively investigated to facilitate the technological applications in power generation, refrigeration, and water harvesting. Despite numerous experiments and simulations of vapor condensation, the droplet dynamics and heat transfer performance affected by the combination of initial wetting state and inclination during pure vapor condensation are still poorly understood. Combining visual experiments and lattice Boltzmann simulations, the droplet dynamics, size distribution and heat transfer on the nanostructured surface at various condensing conditions are analyzed. As the subcooling increases from 0.5 to 3.5 K, the jumping frequency of droplets at α = 90° decreases significantly from 173 to 36 cm−2 s−1 and the average droplet diameter is increased by ∼300%. Meanwhile, the jumping diameter ranges from 20 to 280 μm, with a ∼60% reduction in maximum jumping height. The critical sliding diameters at α = 30° are 1.5, 1.4 and 1.5 times higher than those at a 90° inclination as the subcoolings are 5.0 K, 6.5 K and 8.0 K, respectively. Heat transfer coefficient is enhanced by 21.1% at α = 30°, 49.2% at α = 60°, and 72.4% at α = 90°. In addition, the LB results demonstrate that if the energy conversion efficiency is below 2.1%, condensate droplets are stuck to the substrate rather than jumping away spontaneously.

Robust Micro-Nanostructured Superhydrophobic Surfaces for Long-Term Dropwise Condensation.

Design of hierarchical micromorphology represents an important strategy for developing functional surfaces but has yet to be achieved for promising long-term dropwise condensation. Herein, micropapillaes overlaid with nanograss were created to enhance dropwise condensation. By analyzing the nucleation and evolution of the condensate droplets, we elucidated that these hierarchical micro-nanostructures topologized tapered gaps, which produced upward pressure, to achieve spontaneous dislodging of condensate microdroplet out of gaps, and then to trigger microdroplet navigation before finally departing from the surface by coalescence-induced jumping. The high mobility of condensate delayed flooding and contributed to a very high heat transfer coefficient of 218 kW·m-2·K-1. Moreover, these micropapillaes served as forts that protected the nanograss from being destroyed, resulting in improved mechanical and chemical robustness. Our work proposed new examples of topology creation for long-term dropwise condensation heat transfer and shed light on application integration of such promising functional surfaces.

Effects of surface roughness and temperature on non-equilibrium condensation and entrainment performance in a desalination-oriented steam ejector

• Wet steam model was built and effects of surface conditions were fully studied. • Increase of roughness reduces the ejector performance and condensation intensity. • The performance varies within 4% as the wall temperature changes from 30 to 150 °C. • The condensation droplets occur on the inner wall when it has poor adiabatic properties. Ejectors usually act as thermal compressors to recover the superfluous water vapor in multi-effect desalination systems. However, the influences of wall conditions on the steam ejector performances were not fully understood. To investigate the effects of the surface roughness and temperature on non-equilibrium condensation and entrainment performance in the steam ejector, a wet steam model was built by a computational fluid dynamics software. Simulation results indicate that the increase of roughness height reduces the ejector performance and attenuates the non-equilibrium condensation phenomenon significantly. Furthermore, as the wall temperature changes in the range of 30–150 °C, the ejector performance varies merely within 4%; however, condensation droplets which may cause erosion damage occurs on the internal surface when the wall has poor adiabatic properties. Therefore, the wall roughness and temperature should be fully considered to enhance the ejector performance and mitigate potential condensation on the wall.

Short-range exposure to airborne virus transmission and current guidelines

After the Spanish flu pandemic, it was apparent that airborne transmission was crucial to spreading virus contagion, and research responded by producing several fundamental works like the experiments of Duguid [J. P. Duguid, J. Hyg. 44, 6 (1946)] and the model of Wells [W. F. Wells, Am. J. Hyg. 20, 611-618 (1934)]. These seminal works have been pillars of past and current guidelines published by health organizations. However, in about one century, understanding of turbulent aerosol transport by jets and plumes has enormously progressed, and it is now time to use this body of developed knowledge. In this work, we use detailed experiments and accurate computationally intensive numerical simulations of droplet-laden turbulent puffs emitted during sneezes in a wide range of environmental conditions. We consider the same emission-number of drops, drop size distribution, and initial velocity-and we change environmental parameters such as temperature and humidity, and we observe strong variation in droplets' evaporation or condensation in accordance with their local temperature and humidity microenvironment. We assume that 3% of the initial droplet volume is made of nonvolatile matter. Our systematic analysis confirms that droplets' lifetime is always about one order of magnitude larger compared to previous predictions, in some cases up to 200 times. Finally, we have been able to produce original virus exposure maps, which can be a useful instrument for health scientists and practitioners to calibrate new guidelines to prevent short-range airborne disease transmission.

Unraveling the Secrets of Turbulence in a Fluid Puff.

Turbulent puffs are ubiquitous in everyday life phenomena. Understanding their dynamics is important in a variety of situations ranging from industrial processes to pure and applied science. In all these fields, a deep knowledge of the statistical structure of temperature and velocity space-time fluctuations is of paramount importance to construct models of chemical reaction (in chemistry) and of condensation of virus-containing droplets (in virology and/or biophysics) and optimal mixing strategies in industrial applications. As a matter of fact, results of turbulence in a puff are confined to bulk properties (i.e., average puff velocity and typical decay or growth time) and date back to the second half of the 20th century. There is, thus, a huge gap to fill to pass from bulk properties to two-point statistical observables. Here, we fill this gap by exploiting theory and numerics in concert to predict and validate the space-time scaling behaviors of both velocity and temperature structure functions including intermittency corrections. Excellent agreement between theory and simulations is found. Our results are expected to have a profound impact on developing evaporation models for virus-containing droplets carried by a turbulent puff, with benefits to the comprehension of the airborne route of virus contagion.

Mesoscopic lattice Boltzmann simulation of droplet jumping condensation heat transfer on the microstructured surface

Dropwise condensation heat transfer on the micropillared surfaces is modeled using the 2D multiphase lattice Boltzmann method. Dynamic evolution of condensate droplets and heat transfer performance on the vertical micropillared surfaces with different wettability is investigated. The condensate mass and critical departure radius of droplets decrease as the surface wettability weakens. The superhydrophobic surface with the contact angle of 155° exhibits the highest heat flux. The released latent heat of saturated vapor results in a higher temperature at two-phase interface than elsewhere and a greater temperature gradient near the solid-liquid contact area. When the droplet detaches from the surface, the heat flux is reduced greatly. In addition, we concentrate on droplet jumping dynamics induced by merging on the microspined and microgrooved surfaces, especially considering the coupled phase change heat transfer process. Compared to the microgrooved surface, microspined structure is beneficial to improve the jumping velocity of droplets due to restricting the liquid bridge expansion and increasing the counteraction of substrate towards liquid bridge. Smaller width and height of microgroove can reduce the critical departure size as well as increase the jumping velocity.

Numerical simulations of growth dynamics of breath figures on phase change materials: The effect of accelerated coalescence due to droplet motion

We present the growth dynamics of breath figures on phase change materials using numerical simulations. We propose a numerical model which accounts for both growth due to condensation and random motion of droplets on the substrate. We call this model as growth and random motion (GRM) model. Our analysis shows that for dynamics of droplet growth without droplet motion, simulation results are in good agreement with well-established theories of growth laws and self-similarity in surface coverage. We report the emergence of a growth law in the coalescence-dominated regime for the droplets growing simultaneously by condensation and droplet motion. The overall growth of breath figures (BF) exhibits four growth regions, namely, initial , intermediate or crossover , coalescence-dominated regime , and no coalescence regime in late time , where and t are the average droplet radius and time, respectively. The power law exponents are , , , and . Moreover, the surface coverage reaches a maximum value where the third growth regime starts. We also demonstrate that during the growth dynamics of BF, the random motion amplitude δ and its probability p(R) linked to the power exponent γ of droplet radius R have a specific limiting range within which its effect is more predominant.

Cover Feature: Remodeling of the Fibrillation Pathway of α‐Synuclein by Interaction with Antimicrobial Peptide LL‐III (Chem. Eur. J. 46/2021)

Liquid–liquid phase separation plus the droplet formation of proteins has emerged as a key mechanism for intracellular organization, but there is also evidence linking it to a variety of medical conditions. Pathological aggregation of α-synuclein, which is causally linked to Parkinson's disease, can proceed via such droplet condensation. We show that the antimicrobial peptide LL-III is able to interact with both monomers and condensates of α-synuclein, stabilizing the droplet phase and preventing conversion to the toxic fibrillar state. More information can be found in the Communication by R. Oliva, R. Winter, et al. (DOI: 10.1002/chem.202101592).

A Study of Moist Air Condensation Characteristics in a Transonic Flow System

When water vapor in moist air reaches supersaturation in a transonic flow system, non-equilibrium condensation forms a large number of droplets which may adversely affect the operation of some thermal-hydraulic equipment. For a better understanding of this non-equilibrium condensing phenomenon, a numerical model is applied to analyze moist air condensation in a transonic flow system by using the theory of nucleation and droplet growth. The Benson model is adopted to correct the liquid-plane surface tension equation for realistic results. The results show that the distributions of pressure, temperature and Mach number in moist air are significantly different from those in dry air. The dry air model exaggerates the Mach number by 19% and reduces both the pressure and the temperature by 34% at the nozzle exit as compared with the moist air model. At a Laval nozzle, for example, the nucleation rate, droplet number and condensation rate increase significantly with increasing relative humidity. The results also reveal the fact that the number of condensate droplets increases rapidly when moist air reaches 60% relative humidity. These findings provide a fundamental approach to account for the effect of condensate droplet formation on moist gas in a transonic flow system.