Promote Dropwise Condensation Research Articles

In Situ Opto-Hydrodynamic Characterization of Lubricant-Infused Surface Degradation.

Vapor condensation is widely used in industrial systems due to its effective heat and mass transfer when compared to single-phase thermal transport. In particular, dropwise condensation can significantly enhance heat transfer performance due to rapid droplet shedding and promotion of additional nucleation sites for vapor condensation. Recently, lubricant-infused surfaces (LISs) composed of superhydrophobic structures infused with a low surface tension lubricant have been shown to effectively promote dropwise condensation of a variety of fluids by forming chemically and topographically homogeneous low-surface-energy surfaces. However, depletion of the infused lubricant remains a critical limitation to developing durable LISs which can sustain prolonged dropwise condensation. Moreover, the observed degradation is difficult to detect especially during active condensation on the surface. Here, we introduce an optical measurement technique to quantify in situ and in operando lubricant drainage from LISs. The optical method allows for non-invasive, instantaneous, and accurate prediction of the lifespan of LISs. The method implements the analysis of sample transient transparency, with depletion leading to exposure of the structure and increased light scattering. Our work demonstrates the logarithmic relation between the amount of the lubricant remaining in the LIS and the optical transmittance of the LIS, validating our unique technique for estimating the durability of LISs.

Influence of jumping-droplet condensation on the properties of separated flow in an air-cooled condenser tube: An Euler-Lagrange approach

An Air-cooled condenser (ACC), which finds popularity in a steam power plant in arid areas, is usually less efficient as film condensation occurs inside the condenser tube. Recent research is directed towards eliminating the thermally insulating liquid film with the application of novel superhydrophobic surfaces. The self-cleaning property of such surfaces facilitates easy condensate drainage in the form of jumping droplets exposing favourable nucleation sites, thereby significantly promoting dropwise condensation. The present study explores the characteristics of jumping droplet condensation in finite condenser tubes using computational fluid dynamics (CFD). The wall-heat-flux for condensation is modelled here by a uniform suction boundary condition. The strength of suction is quantified by a suction Reynolds number Re s. We mainly focus on the zone corresponding to 2.3 < Re s < 10, where no previous solution exists. In a long horizontal tube, the progressive realization of a self-similar region starting from the developing regions is demonstrated. We examine the characteristics of the developing region based on the sign of the pressure gradient. The results of three-dimensional CFD simulations illustrate the variations of droplet trajectories with the inception size and coordinates of jumping droplets determined locally by the relative contributions of various force components, viz. gravity, axial drag in the vapour core, suction induced radial drag and Saffman lift. The present study also predicts the effect of pipeline inclination on condensate drainage. Ultimately, considering multiple jumps, we found that the maximum condensate emission can be obtained for small droplets (5−15 μm), while medium-sized droplets (20−50 μm) are most advantageous for isolated jumps.

Microscale Confinement and Wetting Contrast Enable Enhanced and Tunable Condensation.

Dropwise condensation represents the upper limit of thermal transport efficiency for liquid-to-vapor phase transition. A century of research has focused on promoting dropwise condensation by attempting to overcome limitations associated with thermal resistance and poor surface-modifier durability. Here, we show that condensation in a microscale gap formed by surfaces having a wetting contrast can overcome these limitations. Spontaneous out-of-plane condensate transfer between the contrasting parallel surfaces decouples the nanoscale nucleation behavior, droplet growth dynamics, and shedding processes to enable minimization of thermal resistance and elimination of surface modification. Experiments on pure steam combined with theoretical analysis and numerical simulation confirm the breaking of intrinsic limits to classical condensation and demonstrate a gap-dependent heat-transfer coefficient with up to 240% enhancement compared to dropwise condensation. Our study presents a promising mechanism and technology for compact energy and water applications where high, tunable, gravity-independent, and durable phase-change heat transfer is required.

Heat transfer innovations and their application in thermal desalination processes

<h2>Summary</h2> The familiarity of mature thermal technologies for brine concentration and crystallization is likely to sustain their widespread deployment despite their high capital and operational (i.e., energy) costs relative to emerging non-thermal alternatives. This critical review and analysis evaluates the potential for recent innovations in manufacturing devices for improved heat and mass transfer to demonstrably reduce the lifecycle costs of these thermal desalination technologies. We identify three promising innovation applications, including novel heat transfer geometries enabled by additive manufacturing, emerging surface engineering methods to promote dropwise condensation, and new polymer-/polymer-composite-based heat transfer components. Based on the estimated cost and performance impacts of these material and process modifications, we find that polymer-/polymer-composite-based heat transfer devices have the largest potential to reduce the unit cost of water in the near term. Throughout the review, we also highlight remaining research gaps and unmet technoeconomic targets that would benefit from future investigation and collaboration between the heat transfer and desalination communities.

Robust silane self-assembled monolayer coatings on plasma-engineered copper surfaces promoting dropwise condensation

Dropwise condensation is well known to result in better heat transfer performance owing to efficient condensate/droplet removal, which can be harnessed in various industrial heat/mass transfer applications such as power generation and conversion, water harvesting/desalination, and electronics thermal management. The key to enhancing condensation via the dropwise mode is thin low surface energy coatings (<100 nm) with low contact angle hysteresis. Ultrathin (<5 nm) silane self-assembled monolayers (or SAMs) have been widely studied to promote dropwise condensation due to their minimal thermal resistance and scalable integration processes. Such thin coatings typically degrade within an hour during condensation of water vapor. After coating failure, water vapor condensation transitions to the inefficient filmwise mode with poor heat transfer performance. We enhance silane SAM quality and durability during water vapor condensation on copper compared to state-of-the-art silane coatings on metal surfaces. We achieve this via (i) surface polishing to sub-10 nm levels, (ii) pure oxygen plasma surface treatment, and (iii) silane coating integration with the copper substrate in an anhydrous/moisture-free environment. The resulting silane SAM has low contact angle hysteresis (≈20°) and promotes efficient dropwise condensation of water for >360 hours without any visible sign of coating failure/degradation in the absence of non-condensable gases. We further demonstrate enhanced heat transfer performance (≈5-7× increase over filmwise condensation) over an extended period of time. Surface characterization data post-condensation leads us to propose that in the absence of non-condensable gases in the vapor environment, the silane SAM degrades due to reduction and subsequent dissolution of copper oxide at the oligomer-substrate interface. The experiments also indicate that the magnitude of surface subcooling (or condensation rate) affects the rate of coating degradation. This work identifies a pathway to durable dropwise promoter coatings that will enable efficient heat transfer in industrial applications.

Quasi-Liquid Surfaces for Sustainable High-Performance Steam Condensation.

Sustainable high-performance steam condensation is critical to reducing the size, weight, and cost of water and energy systems. It is well-known that dropwise condensation can provide a significantly higher heat-transfer coefficient than filmwise condensation. Tremendous efforts have been spent to promote dropwise condensation by achieving a nonwetting state on superhydrophobic surfaces and a slippery state on liquid-infused surfaces, but these surfaces suffer from severe durability challenges. Here, we report sustainable high-performance dropwise condensation of steam on newly developed durable quasi-liquid surfaces, which are easily made by chemically bonding quasi-liquid polymer molecules on solid substrates. As a result, the solid/water interface is changed to a quasi-liquid/water interface with minimal adhesion and extraordinary durability. The quasi-liquid surface with ultralow contact angle hysteresis down to 1° showed a heat-transfer coefficient up to 70 and 380% higher than those on conventional hydrophobic and hydrophilic surfaces, respectively. Furthermore, we demonstrated that the quasi-liquid coating exhibited a sustainable heat-transfer coefficient of 71 kW/(m2 K) at a heat flux of 420 kW/m2 under a prolonged period of 39 h in continuous steam condensation. Such a quasi-liquid surface has the potential to sustain high-performance dropwise condensation of steam and address the long-standing durability challenge in the field.

Polydimethylsiloxane‐Silane Synergy enables Dropwise Condensation of Low Surface Tension Liquids

AbstractDespite decades of research on promoting the dropwise condensation of steam, achieving dropwise condensation of low surface tension liquids remains a challenge. The few coatings reported to promote dropwise condensation of low surface tension liquids either require complex fabrication methods, are substrate dependent or have poor scalability. Here, the rational development of a coating, which is applicable to all conventionally used condenser metals, is presented by combining a low contact angle hysteresis polydimethylsiloxane with a low surface energy silane using atmospheric vapor phase deposition. The siloxane‐silane coating enables the dropwise condensation of fluids with surface tensions as low as 15 mN m−1 in pure vapor conditions. This siloxane‐silane coating enables a 274%, 347%, and 636% heat transfer enhancement during ethanol, hexane, and pentane condensation, respectively, when compared to filmwise condensation on the same un‐coated surfaces. Furthermore, this coating exhibits 15 days of steady dropwise condensation with no apparent signs of coating degradation. This study not only demonstrates the possibility of achieving stable dropwise condensation of low surface tension fluids on scalable, structure‐less surfaces, it also develops design principles for creating facile, substrate‐independent, durable, and scalable omniphobic coatings for a plethora of applications.

Dropwise condensation of acetone and ethanol for a high-performance lubricant-impregnated thermosyphon

A gravity-assisted heat pipe called a two-phase closed thermosyphon (TPCT) is an efficient thermal device that transfers a large amount of heat at a relatively small temperature difference using the phase-change process. Promoting dropwise condensation on a condenser wall of a TPCT promises an increase in the effective thermal conductance of that device. Acetone and ethanol have been widely used for heat pipe applications as working fluids. Still, they pose a limitation in promoting dropwise condensation using typical hydrophobic coatings due to their lower surface energies. Recent works have shown that lubricant-impregnated surfaces (LIS) can promote dropwise condensation of low-surface-tension fluids, but no studies proved the effectiveness of the surfaces in TPCT applications. This study experimentally achieves dropwise condensation of acetone and ethanol within a TPCT using the LIS surface. We demonstrate heat transfer enhancement of a TPCT by examining temperature uniformity, condenser and evaporator heat transfer coefficients (HTCs), and overall and partial thermal resistances. We demonstrated that the LIS condenser increased the condenser HTC by 220% and decreased the overall thermal resistance of the TPCT by 44% compared to filmwise condensation. Our work also revealed a strong dependence of working fluid viscosity on dropwise condensation stability and heat transfer enhancement by comparing the two working fluids, acetone, and ethanol. This work presents the opportunity for improving thermal performance in heat pipes and applications that rely on the condensation of low-surface-tension fluids, such as HVAC, refrigeration, and organic Rankine cycle.

Elucidating the Mechanism of Condensation-Mediated Degradation of Organofunctional Silane Self-Assembled Monolayer Coatings.

Dropwise condensation is favorable for numerous industrial and heat/mass transfer applications due to the enhanced heat transfer performance that results from efficient condensate removal. Organofunctional silane self-assembled monolayer (SAM) coatings are one of the most common ultrathin low surface energy materials used to promote dropwise condensation of water vapors because of their minimal thermal resistance and scalable synthesis process. These SAM coatings typically degrade (i.e., condensation transitions from the efficient dropwise mode to the inefficient filmwise mode) rapidly during water vapor condensation. More importantly, the condensation-mediated coating degradation/failure mechanism(s) remain unknown and/or unproven. In this work, we develop a mechanistic understanding of water vapor condensation-mediated organofunctional silane SAM coating degradation and validate our hypothesis through controlled coating synthesis procedures on silicon/silicon dioxide substrates. We further demonstrate that a pristine organofunctional silane SAM coating resulting from a water/moisture-free coating environment exhibits superior long-term robustness during water vapor condensation. Our molecular/nanoscale surface characterizations, pre- and post-condensation heat transfer testing, indicate that the presence of moisture in the coating environment leads to uncoated regions of the substrate that act as nucleation sites for coating degradation. By elucidating the reasons for formation of these degradation nuclei and demonstrating a method to suppress such defects, this study provides new insight into why low surface energy silane SAM coatings degrade during water vapor condensation. The proposed approach addresses a key bottleneck (i.e., coating failure) preventing the adoption of efficient dropwise condensation methods in industry, and it will facilitate enhanced phase-change heat transfer technologies in industrial applications.

Highly thermally conductive Ag/SiO2 superhydrophobic coating for accelerated dropwise condensation

Due to the ability to promote dropwise condensation, superhydrophobic coatings are widely used to increase the heat transfer performance of functional surfaces. A higher thermal conductive coating is always required for maximum heat transfer. However, the traditional polymer or ceramic based coatings have poor thermal conductivity and weak adhesion, which will hinder the heat flow across the surface. As a candid attempt to improve the heat transfer, here we propose a smart method to fabricate a highly thermally conductive Ag/SiO2 superhydrophobic coating. The coating is obtained through screen printing Ag paste on Al surface followed by spray coating hydrophobic SiO2 nanoparticles and sintering at 600 °C. The Ag paste acted as a binder to hold the SiO2 particles and increased the thermal conductivity of the Ag/SiO2 coating up to 83.32 W/m⋅K, which was 40 times higher than that of the polyurethane/silica (PU/SiO2) coating. Because of the higher thermal conductivity, Ag/SiO2 coating showed quicker dropwise condensation compared to PU/SiO2 coating. Dropwise condensation of Ag/SiO2 coatings was also confirmed and compared with other metals, Al, Ag, Cu, and SUS. This work would give a solution to the existing design limitation of a coating when it is applied for heat transfer applications.

Effect of layer-by-layer assembled carbon nanotube coatings on dropwise condensation heat transfer associated with non-condensable gas effect

The purpose of this study is to investigate the effect of layer-by-layer (LbL) assembled carbon nanotube (CNT) coatings on dropwise condensation heat transfer under steam–air mixture conditions. To promote dropwise condensation, an additional hydrophobic coating was added to the LbL-assembled CNT coatings, which resulted in a 16% enhanced condensation heat transfer coefficient (HTC) compared to that of the bare surface under saturated steam conditions. Despite the existence of the nanostructures on the CNT-layered surface, the larger droplet size was observed from the high-speed images than that of the bare surface due to flooding condensation phenomenon. Considering that the HTC was enhanced without smaller droplet size, it is evident that this enhancement was mainly attributed to the reduced constriction resistance driven by the high thermal conductivity of the CNT layers. A new HTC model for dropwise condensation incorporating constriction resistance was developed to better predict the measured condensation HTC. Although the enhancement of the condensation HTC on the LbL-assembled CNT coatings was diminished with the air mass fraction, this work reports a new perspective to enhance the dropwise condensation heat transfer performance by controlling the constriction resistance using a highly thermally conductive coating.

Technoeconomic analysis of thermoelectric power plant condensers with nonwetting surfaces

This paper investigates the dynamic performance and economic benefits of using nonwetting tube surfaces fabricated by electrodeposition process in thermoelectric power plant condensers. Nonwetting surfaces enhance performance by reducing fouling resistance and promoting dropwise condensation of steam on the shell side. Using a thermal resistance network of a shell and tube condenser, detailed parametric studies are presented to investigate the effects of design parameters and operating parameters on the annual condenser performance and its impact on the net electric energy output of a representative 550 MW coal-fired power plant. A cost model is presented to evaluate the technoeconomic benefits of the enhanced condenser design in a unified manner, in terms of a new levelized cost of condenser (LCOC) metric. The model is coupled with a numerical optimization method to identify optimal nonwetting condenser tube designs that minimize LCOC. The results show optimal cooling water flow velocity of 2.5 m/s and tube diameter of 0.032 m (0.025 m) for nonwetting (plain) condenser tube surfaces that exhibit dropwise (filmwise) condensation. Condensers with nonwetting surfaces are shown to reduce LCOC by a factor of 2.5 and 1.7, respectively, compared to plain titanium and aluminum-brass tubes.

Dropwise condensation of low surface tension fluids on lubricant-infused surfaces: Droplet size distribution and heat transfer

The use of lubricant-infused surfaces and slippery liquid-infused porous surfaces to promote dropwise condensation of low surface tension fluids has received increasing attention in recent years due to the high condensate droplet mobility and low droplet contact angle hysteresis enabled by these coatings. Though multiple studies have focused on developing models to predict heat transfer during water vapor condensation on hydrophobic lubricant-infused surfaces, a lack of understanding exists for condensation of low surface tension fluids, where the surface becomes wettable to the condensate. In this study, we develop a theoretical model to predict the heat transfer during dropwise condensation of low surface tension fluids (ethanol and hexane) on lubricant-infused surfaces. Using numerical analysis, we develop a relationship between droplet Nusselt number and Biot number to accurately predict individual droplet heat transfer. The droplet size distribution of ethanol and hexane were experimentally determined, with result showing that the size distribution density depends on both the working fluid properties and droplet contact angle behavior. To validate our model, experiments were conducted by condensing pure ethanol and hexane vapor on different diameter tubes coated with lubricant-infused surfaces. From the experimental measurements, two correlations which characterized the distribution density of ethanol and hexane were developed. Using the droplet distribution correlations as input parameters, our developed model predicts the heat transfer coefficients for low surface tension fluid condensation more accurately when compared to the existing models, where the maximum deviation between the prediction and experimental results is less than 15%.

A Review of Research on Dropwise Condensation Heat Transfer

Dropwise condensation is considered to be an effective method of enhancing heat transfer due to its high heat transfer performance. However, because the effect of dropwise condensation is affected by many complex factors, there is no systematic review summarized on the law of dropwise condensation heat transfer by scholars. In this paper, the main methods and problems of promoting dropwise condensation were reviewed based on the dropwise condensation mechanism and theoretical model. The three different hypotheses about the mechanism of dropwise condensation and the heat transfer model of dropwise condensation based on the hypothesis of nucleation sites were summarized. The methods for promoting dropwise condensation and the problems that influence dropwise condensation heat transfer are introduced in this paper. The research showed that many researchers focused on how the surface fabricated forms dropwise condensation rather than whether it enhances heat transfer. In this paper, we point out that the droplet shedding rate is the key to enhancing dropwise condensation heat transfer. Much more research on droplet formation mechanisms and theoretical models of different surfaces is supposed to be carried out in the future.

Robust Silane Self-Assembled Monolayer Coatings on Plasma-Engineered Copper Surfaces Promoting Dropwise Condensation

Robust Silane Self-Assembled Monolayer Coatings on Plasma-Engineered Copper Surfaces Promoting Dropwise Condensation

Wetting and corrosion characteristics of thermally sprayed copper-graphene nanoplatelet coatings for enhanced dropwise condensation application

Water vapour condensation is frequently used as an effective means of transferring heat using dropwise condensation on non-wetting surfaces. The rate of heat transfer can be enhanced with dropwise condensation on non-wetting hydrophobic and superhydrophobic surfaces when compared to filmwise condensation on a wetting surface. Since low surface energy materials are mostly polymeric with low durability and poor thermal conductivity, copper-graphene nanoplatelet (Cu-GNP) composite coatings are developed in this work using atmospheric plasma spray (APS) and high-velocity oxy-fuel (HVOF) as versatile and scalable surface engineering techniques. It is demonstrated that by adjusting the deposition parameters, it is possible to preserve the GNPs in the coating structure. After treating with a stearic acid solution, the coatings demonstrate superhydrophobicity and extreme water repelling behavior with a water contact angle of 164° and sliding angle of 1° Raman analysis confirmed the presence of GNP in the coating with minimal defects. It is shown that the HVOF Cu-GNP coating developed in this work can improve the corrosion resistance up to 89% when compared to the uncoated Cu surface. This coating can potentially promote dropwise condensation while offering enhanced corrosion stability.

Modeling and optimization of hydrophobic surfaces for a two-phase closed thermosyphon

Promoting dropwise condensation on condensers has the potential to significantly improve cooling and heat recovery performance of two-phase closed thermosyphons (TPCTs). If it is well designed, the dropwise condensation can provide an order of magnitude higher heat transfer coefficient than filmwise condensation of conventional metal condensers. The thermal design of an ideal dropwise condensing surface is so important in the rationale for the advantage of dropwise condensation in the TPCTs. However, the effect of the hydrophobic promoter thickness on the thermal characteristics of the TPCT and its optimal value are still unclear. Here, we develop a unified model framework for the TPCT with the dropwise condenser and investigate how much the dropwise condensation can improve the thermal performance of the TPCT. As the hydrophobic promoter for the TPCT, we proposed a nanoscopically smooth Teflon-based hydrophobic coating on theoretical insights, experimentally evaluated its condensation heat transfer, and then developed its dropwise model with high accuracy by combining a recent dropwise model with the optimized parameters. Subsequently, the developed model was unified with the TPCT model to investigate the thermal characteristics of the TPCT system and the effect of the promoter thickness. The model results show that the Teflon-coated condenser with a thin thickness (≤ 10 nm) can significantly reduce the overall thermal resistance of the TPCT by up to 4.5 times compared to filmwise condensation. However, an increase in film thickness limited the performance enhancement of the TPCT due to the dominant thermal resistance through the film. By considering both the durability and the thermal resistance of the TPCT, we suggested the optimal Teflon film thickness as approximately 60 nm. This work provides physical insights and guidelines to obtain ideal TPCTs combined with dropwise condensers.

Polymer Infused Porous Surfaces for Robust, Thermally Conductive, Self-Healing Coatings for Dropwise Condensation.

Hydrophobic coatings with low thermal resistance promise a significant enhancement in condensation heat transfer performance by promoting dropwise condensation in applications including power generation, water treatment, and thermal management of high-performance electronics. However, after nearly a century of research, coatings with adequate robustness remain elusive due to the extreme environments within many condensers and strict design requirements needed to achieve enhancement. In this work, we enable long-lasting condensation heat transfer enhancement via dropwise condensation by infusing a hydrophobic polymer, Teflon AF, into a porous nanostructured surface. This polymer infused porous surface (PIPS) uses the large surface area of the nanostructures to enhance polymer adhesion, while the nanostructures form a percolated network of high thermal conductivity material throughout the polymer and drastically reduce the thermal resistance of the composite. We demonstrate over 700% enhancement in the condensation of steam compared to an uncoated surface. This performance enhancement was sustained for more than 200 days without significant degradation. Furthermore, we show that the surfaces are self-repairing upon raising the temperature past the melting point of the polymer, allowing recovery of hydrophobicity and offering a level of durability more appropriate for industrial applications.

Solution‐processed graphene oxide coatings for enhanced heat transfer during dropwise condensation of steam

AbstractHydrophobic coatings have the ability to promote dropwise condensation on metal surfaces. In this work, reduced graphene oxide coatings are fabricated by dip coating copper substrates into a graphene oxide colloidal suspension, followed by thermal reduction. The formation of reduced graphene oxide is demonstrated by Raman and X‐ray photoelectron spectroscopy characterizations, while scanning electron microscopy, atomic force microscopy and dynamic water contact angle analyses are employed to verify the morphology and the wettability of the coatings. The prepared coatings demonstrate applicability in the promotion of dropwise condensation of pure steam, with an 8‐fold enhancement of the heat transfer coefficient compared to film‐wise condensation. In addition, durability of the reduced graphene oxide coatings of more than 100 hours in the tested condition is reported.

Experimental investigation of condensation heat transfer characteristics and corrosion resistance on coated tube surfaces

The condensation heat transfer characteristics and corrosion resistance of copper, Ni-P-Cu, polytetrafluoroethylene (PTFE), Ni-P, and Ni coated tube surfaces were investigated. The results indicate that condensation heat transfer coefficient of Ni-P-Cu, PTFE, Ni-P, and Ni coated tubes grows by 36.8%, 29.3%, 19.6%, and 7.5% than that of copper tubes, respectively. The phase structure of Ni, Ni-P, Ni-P-Cu, and PTFE coated tube surfaces is mixed crystal structure(nanocrystals-based), mixed crystal structure (amorphous-based), amorphous structure and crystal structure, respectively. Compared with Ni coating and Ni-P coated tube surfaces, the condensation droplets outside Ni-P-Cu and PTFE coated tubes are smaller in size, more densely distributed, and fall off more quickly, which can significantly promote dropwise condensation. The Ni-P-Cu coated tube surfaces achieve optimal condensation heat transfer. The corrosion speed of copper, Ni, Ni-P, Ni-P-Cu, and PTFE coated tubes are 86.5, 42.6, 18.2, 10.7, and 6.1 mg/dm2d per day, respectively. The PTFE coating tubes have the optimal corrosion resistance. The Ni-P-Cu, and PTFE coated tube surfaces have the best condensation heat transfer characteristics and corrosion resistance, and can be well used in the recovery of waste heat from low temperature flue gas. The multiple linear regression of the experimental data was carried out to obtain the experimental correlation formula for Nusselt number of convective condensation composite heat transfer for different coated tubes. The relative error between the predicted value and the experimental value is within ?15%.