Filmwise-to-Dropwise Condensation Transition Enabled by Patterned High Wetting Contrast

2015 ◽  
Vol 137 (8) ◽  
Author(s):  
Youmin Hou ◽  
Miao Yu ◽  
Xuemei Chen ◽  
Zuankai Wang
Keyword(s):  
Liquid Film ◽  
Droplet Growth ◽  
Pinning Force ◽  
Dropwise Condensation ◽  
Ambient Conditions ◽  
Spherical Droplet ◽  
Design And Optimization ◽  
Morphology Transition ◽  
Hydrophobic Coating ◽  
Condensed Water

Recent advances in condensing surfaces with hybrid architectures of superhydrophobic/hydrophilic patterns allow us to decrease the nucleation energy barrier and spatially control the water condensation. However, the condensed water is susceptible to the large pinning force of the hydrophilic area, leading to an ultimate flooding. Here, we demonstrate a hierarchical nanostructured surface with patterned high wetting contrast to achieve a natural transition from filmwise-to-dropwise condensation, which reconciles the existing problems. The energy-dispersive X-ray spectroscopy (EDX) indicates that the fluorinated hydrophobic coating conformably covers the nanostructures except for the tops of micropillars, which are covered by hydrophilic silicon dioxide (FIG 1), resulting in an extreme wetting contrast. Condensation on the hybrid surface was observed in the environmental scanning electron microscope (ESEM) and ambient conditions with controlled humidity. Water preferentially nucleates on the top of micropillars and exhibits a rapid droplet growth (FIG 2). The enhancement is attributed to the filmwise-to-dropwise transition induced by the unique architectures and wetting features of the hybrid surface (FIG 3). The water embryos initially nucleate on the hydrophilic tops and quickly grow to a liquid film covering the whole top area. Since the superhydrophobic surrounding confines the spreading of condensed water, the localized liquid film gradually transits to an isolated spherical droplet as it grows. Remarkably, the condensate morphology transition activates an unusual droplet self-propelling despite the presence of abundant hydrophilic patches. It is important to note that such coalescence-induced jumping is dependent on the size of hydrophilic patches, that is, for larger hydrophilic patches, the energy released by coalescence may not overcome the increased droplet pinning, resulting in an immobile coalescence (FIG 4). The droplet departure ensures the recurrence of filmwise-to-dropwise transition, thus prevents the water accumulation in continuous condensation. These visualizations reveal the undiscovered impact of heterogeneous wettability and architectures on the morphology transition of the condensed water, and provide important insights into the surface design and optimization for enhanced condensation.

SINGLE DROPLET GROWTH MODEL FOR DROPWISE CONDENSATION CONSIDERING NON-CONDENSABLE GAS

2018 ◽  
Author(s):  
Shaofei Zheng ◽  
Ferdinand Eimann ◽  
Christian Philipp ◽  
Ulrich Gross
Keyword(s):  
Growth Model ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Single Droplet

scholarly journals Effect of the Hybrid Hydrophobic-Hydrophilic Nanostructured Surface on Explosive Boiling

Coatings ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 212
Author(s):  
Ming-Jun Liao ◽  
Li-Qiang Duan
Keyword(s):  
Liquid Film ◽  
Coating Thickness ◽  
Bubble Nucleation ◽  
Onset Temperature ◽  
Dynamics Simulation ◽  
Hydrophilic Surface ◽  
Pillar Width ◽  
Nanostructured Surface ◽  
Explosive Boiling ◽  
Hydrophobic Coating

The influence of different wettability on explosive boiling exhibits a significant distinction, where the hydrophobic surface is beneficial for bubble nucleation and the hydrophilic surface enhances the critical heat flux. Therefore, to receive a more suitable surface for the explosive boiling, in this paper a hybrid hydrophobic–hydrophilic nanostructured surface was built by the method of molecular dynamics simulation. The onset temperatures of explosive boiling with various coating thickness, pillar width, and film thicknesses were investigated. The simulation results show that the hybrid nanostructure can decrease the onset temperature compared to the pure hydrophilic surface. It is attributed to the effect of hydrophobic coating, which promotes the formation of bubbles and causes a quicker liquid film break. Furthermore, with the increase of the hydrophobic coating thickness, the onset temperature of explosive boiling decreases. This is because the process of heat transfer between the liquid film and the hybrid nanostructured surface is inevitably enhanced. In addition, the onset temperature of explosive boiling on the hybrid wetting surface decreases with the increase of pillar width and liquid film thickness.

A Heat Transfer Model of Dropwise Condensation Underneath a Horizontal Substrate

2011 ◽  
Vol 199-200 ◽  
pp. 1604-1608
Author(s):  
Yun Fu Chen
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Contact Angle ◽  
Fractal Model ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Transfer Model ◽  
Dropwise Condensation ◽  
Small Contact Angle ◽  
Horizontal Substrate

For finding influence of the condensing surface to dropwise condensation heat transfer, a fractal model for dropwise condensation heat transfer has been established based on the self-similarity characteristics of droplet growth at various magnifications on condensing surfaces with considering influence of contact angle to heat transfer. It has been shown based on the proposed fractal model that the area fraction of drops decreases with contact angle increase under the same sub-cooled temperature; Varying the contact angle changes the drop distribution; higher the contact angle, lower the departing droplet size and large number density of small droplets; dropwise condensation translates easily to the filmwise condensation at the small contact angle ;the heat flux increases with the sub-cooled temperature increases, and the greater of contact angle, the more heat flux increases slowly.

Investigation Concerning the Aerothermodynamic Performance Perturbation of a 19MW Intercooled Air Compressor

2000 ◽  
Author(s):  
Inam U. Haq ◽  
Ali H. Al-Jameel ◽  
Khalid N. Al-Khalidi
Keyword(s):  
Critical Role ◽  
Cooling Water ◽  
Extended Period ◽  
Evaluation Study ◽  
Plant Production ◽  
Ambient Conditions ◽  
Heavy Load ◽  
Air Compressor ◽  
Compressor Surge ◽  
Condensed Water

This paper investigates the performance problem of a large capacity multistage centrifugal air compressor when operated continuously for an extended period of time without overhauling. The compressor flow constitutes fifty percent of the plant production and plays a critical role in meeting the annual contract capacity and can not tolerate performance instability due to high intercooler temperatures, ambient conditions and, fouling of the internal components. During a recent harsh summer operation, the compressor was undergone to surge many times and prompted to initiate a performance evaluation study to identify the cause(s) of surge and the extent of performance deterioration. High cooling water supply temperature and ambient conditions crippled the performance of the compressor. Engineering analysis identified the excessive accumulation of condensed water in the water chamber of the second intercooler as the most logical reason of the compressor surge during the instances of high ambient conditions i.e. relative humidity and temperature greater than 80% and 35°C, respectively. During heavy load of condensed water, the blockage and insufficient size of the condensate drain caused build up of water level in the water collecting chamber which offered instantaneous hindrance to airflow to the next section and, hence, led to the compressor surge. During normal ambient conditions, the overall performance of the compressor was found satisfactory when compared with the commissioning after a long term of continuous operation without maintenance.

Experimental investigations on atomization characteristics of dual-orifice atomizers part II: Optimization method application

2020 ◽  
pp. 095765092095996
Author(s):  
Xiongjie Fan ◽  
Cunxi Liu ◽  
Fuqiang Liu ◽  
Qianpeng Zhao ◽  
Jinhu Yang ◽  
...  
Keyword(s):  
Liquid Film ◽  
Mass Flow ◽  
Cone Angle ◽  
Optimization Method ◽  
Experimental Investigations ◽  
Structure Parameters ◽  
Spray Cone Angle ◽  
Spray Cone ◽  
Design And Optimization ◽  
Expansion Angle

In this paper, the optimization method we obtained from dual-orifice atomizers previously is used to design and optimize new dual-orifice atomizers, whereas there are some differences between the new dual-orifice atomizer and dual-orifice atomizer used in Part I. For example, the mass flow is much smaller, there is an expansion angle at pilot nozzle to regulate pilot stage spray cone angle, and there is no recess length between main nozzle and pilot nozzle. Influences of structure parameters on mass flow, spray cone angle and liquid film fusion and separation are investigated, which are consistent with the expectation. Structure parameters that meet performance requirements of dual-orifice atomizer are analyzed. In addition, a new phenomenon has been found is that liquid film oscillation appears with the increase of Δ P, which should be avoided during the design and optimization of new atomizers. Pilot liquid film oscillation will influence the development of dual-orifice liquid film. Pilot swirling groove depth and expansion angle of pilot nozzle are key parameters that influence liquid film oscillation. Conclusions in this paper can be used to guide the design and optimization of new dual-orifice atomizers.

scholarly journals Near Field Condensation

2020 ◽  
Author(s):  
Xiao Yan ◽  
Feipeng Chen ◽  
Chongyan Zhao ◽  
Yimeng Qin ◽  
Xiong Wang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Population Density ◽  
Near Field ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Droplet Transfer ◽  
Droplet Sizes ◽  
Departure Size ◽  
Contact Line Pinning

Abstract Dropwise condensation represents the upper limit of condensation heat transfer. Promoting dropwise condensation relies on surface chemical functionalization, and is fundamentally limited by the maximum droplet departure size. A century of research has focused on active and passive methods to enable the removal of ever smaller droplets. However, fundamental contact line pinning limitations prevent gravitational and shear-based removal of droplets smaller than 250 µm. Here, we break this limitation through near field condensation. By de-coupling nucleation, droplet growth, and shedding via droplet transfer between parallel surfaces, we enable the control of droplet population density and removal of droplets as small as 20 µm without the need for chemical modification or surface structuring. We identify droplet bridging to develop a regime map, showing that rational wettability contrast propels spontaneous droplet transfer from condensing surfaces ranging from hydrophilic to hydrophobic. To demonstrate efficacy, we perform condensation experiments on surfaces ranging from hydrophilic to superhydrophobic. The results show that near field condensation with optimal gap spacing can limit the maximum droplet sizes and significantly increase the population density of sub-20 µm droplets. Theoretical analysis and direct numerical simulation confirm the breaking of classical condensation heat transfer paradigms through enhanced heat transfer. Our study not only pushes beyond century-old phase change limitations, it demonstrates a promising method to enhance the efficiency of applications where high, tunable, gravity-independent, and durable condensation heat transfer is required.

Electrowetting-Based Coalescence of Droplets During Dropwise Condensation of Humid Air

2019 ◽  
Author(s):  
Enakshi Wikramanayake ◽  
Vaibhav Bahadur
Keyword(s):  
Heat Transfer ◽  
Electric Field ◽  
Electric Fields ◽  
Applied Electric Field ◽  
Growth Dynamics ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Condensation Rate ◽  
Humid Air

Abstract Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement.

A DSMC Model on Droplet Clusters to Explore the Limiting Factors of Modified Surfaces Promoting Enhanced Dropwise Condensation

2015 ◽  
Author(s):  
Hector Mendoza ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Cross Section ◽  
Limiting Factors ◽  
Future Research ◽  
Computational Domain ◽  
Dropwise Condensation ◽  
Cold Wall ◽  
Ambient Conditions ◽  
Droplet Distribution ◽  
Droplet Sizes

Condensation is a physical process that occurs when a vapor is cooled and/or compressed to its saturation limit. Condensation becomes important in a variety of engineering applications such as in heat exchangers used for distillation purposes. In such instances, higher condensation efficiencies are desirable. Research to improve condensation has focused on dropwise condensation as it has been shown that it can be significantly more efficient than filmwise condensation. Recent investigations of dropwise condensation on nanostructured surfaces suggest that enhanced dropwise condensation can be attained as the average droplet sizes are reduced for clusters growing through dropwise condensation. This, in turn, significantly enhances the heat transfer coefficients of dropwise condensation. This paper summarizes a computational model developed to explore the mechanisms leading to this enhanced dropwise condensation. A Direct Simulation Monte Carlo (DSMC) approach is used here to investigate the mechanisms and limitations of enhanced dropwise condensation for these surfaces aiming to reduce the average droplet sizes of condensation. For computational purposes, several idealizations are assumed by the model, which include: (1) The condensation droplet clusters are assumed to have uniform size, corresponding to an average droplet size observed in actual dropwise condensation scenarios; (2) Due to the assumed uniform droplet distribution, symmetry can be observed from the droplet cluster, so a small but symmetrical cross section of the droplet distribution is used for the computational domain; and (3) Supersaturated steam condensing on a cold wall is assumed for most of the simulations. The mechanisms at play that are deliberately explored are: (1) The effects of surface wettability by using a model that considers droplet conduction variations with varying contact angle; (2) The changes of interfacial resistance with droplet curvature by introducing a surface tension model based on the Tolman length; and (3) The dynamic interactions between neighboring droplets by choosing our computational domain to be a symmetrical cross section that encompasses surrounding droplets in an appropriate fashion. The ambient conditions that were investigated were: (1) Varying atmospheric pressure; (2) Varying amounts of wall subcooling for the droplets; (3) Varying accommodation for water molecules condensing on the droplet; and (4) The introduction of air into the assumed supersaturated steam condensing on the cold wall. To investigate the overall and combined effects of the aforementioned mechanisms on enhanced dropwise condensation through reduced droplet sizes, the simulations were run for droplets with radii between 1 micrometer down to 5 nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer with decreasing droplet sizes breaks down as droplet sizes become smaller due to more prominence of the mechanisms hindering condensation for the reduced droplet sizes. As the model breaks down, a peak heat transfer is reached, and heat transfer is further reduced as the average droplet sizes continue to decrease. The predictions of this particular DSMC model are compared to previous work investigating similar effects. The implications of our observations and potential impact to current and future research in the area is discussed in detail.

An Analysis of Pressure Swirl and Pure Airblast Atomization

1990 ◽  
Author(s):  
Chien-Pei Mao ◽  
S. G. Chuech ◽  
A. J. Przekwas
Keyword(s):  
Liquid Film ◽  
Drop Size ◽  
Physical Structure ◽  
Operating Conditions ◽  
Spray Angle ◽  
Nozzle Geometry ◽  
Ambient Conditions ◽  
Breakup Length ◽  
Wide Range ◽  
Sheet Breakup

The present investigation examines the fundamental aspects of airblast atomization in both the breakup and drop dispersion regimes. Experiments were conducted using a high-magnification 4×5 camera and a Phase/Doppler particle analyzer to evaluate the spray characteristics and atomizer performance. The primary parameters of interest are: liquid film breakup length, spray angle, drop size and trajectory. Observation of wave formation and propagation along the sheet surface was made to provide guidance in formulating mathematical models. Effects of air flow and nozzle design on atomization were examined for a wide range of flow conditions. Computational analysis was also utilized to predict the sheet breakup and subsequent drop behavior. This model considered a swirling sheet interacting with the surrounding air streams. The governing equations were formulated in a curvilinear coordinate system conforming to the film boundaries. Primary breakup is based upon linear stability analysis. The present model is capable of predicting the variations in thickness, trajectory, velocity, and angle of a liquid film as a function of nozzle geometry, operating conditions, fluid properties, and ambient conditions. Secondary breakup and drop history calculations were also included in the model to provide local drop size spectra. Agreement between the experimental and predicted breakup length and angle was excellent. The predictions of drop size, trajectory and other parameters were qualitatively correct. The present investigation demonstrated a realistic approach for simulating the breakup process and described the physical structure of a pure airblast atomizer spray.

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